Layout 1 INTRODUCTION Estuaries are among the most biologically productive and economically valuable ecosystems in the world. The hydrological regime and geomorphological features of es- tuaries make such ecosystems good shelters, feeding and breeding grounds for many marine fish species at various stages of their life cycles (Beck et al., 2001; Pihl et al., 2002; Harrison and Whitfield, 2006; Kostecki et al., 2010; Selleslagh et al., 2015). Estuaries provide essential and complex habitats to sustain different freshwater and ma- rine fish stocks. The estuarine complexity provides juve- nile fishes with adequate protection from predation while attaining high growth rates supported by a high primary productivity (Kostecki et al., 2010; Whitfield, 2016). Nev- ertheless, the ecological status of most estuaries is threat- ened due to modifications made by anthropogenic activities. Estuaries are subjected to increasing human pressure through urbanization, overfishing and upstream catchment disturbances resulting from deforestation, water abstraction for reservoir or multi-dams construction and water diversion for irrigation activities (Abrantes et al., 2014). The physical, chemical and biological characteristics of estuaries are dynamic both on temporal and spatial scales (Selleslagh et al., 2015). The main drivers of such fluctuations are hydrological regimes, encompassing freshwater discharges from the upstream, coastal wave ac- tivity and tidal water flow at the mouths of the estuaries (Gillson, 2011; Hoeinghaus et al., 2011). The interference of such hydrological regime such as upstream water ab- straction for irrigation and damming for hydropower gen- eration causes retention and thus reduction of nutrients, sediment (that serve as an ecological niche to some species), quantity and quality of organic matter reaching the estuary (Han et al., 2016). Variations in the hydrolog- ical regime impact habitat availability and diversity, pri- mary production, trophic interactions and consequently prompt responses in food web structure and functioning (Gillson, 2011; Whitfield, 2016; Donázar-Aramendía et al., 2019). More so, the regulation of the flow rate of freshwater reaching the estuaries greatly influences the diversity of basal organic matter fueling the estuarine food webs (Abrantes et al., 2013). Juvenile marine fish with predatory behavior constitute a great part of estuarine fish communities. They are highly targeted by local or artisanal fisheries while still at juvenile and sub-adult stages, as they attain large body sizes before reaching maturity (Shaw et al., 2016; Mwijage et al., 2018a). As such, they have lucrative market price when compared to the small fish that feed at lower trophic levels. Predatory fish and other higher order consumers coexist ARTICLE Diet and isotopic metrics of predatory and prey fish in two estuaries with different degrees of anthropogenic disturbances: the case study of Wami and Pangani rivers in Tanzania Theresia John Lyasenga1, Alistidia P. Mwijage2, Dativa Joseph Shilla3, John Andrew Marco Mahugija4, Lydia Gaspare1, Daniel Abel Shilla1, Prosper Laurent Mfilinge1 1School of Aquatic Sciences and Fisheries Technology, University of Dar es Salaam; 2Tanzania Fisheries Research Institute (TAFIRI), Dar es Salaam; 3Department of Chemistry, Dar es Salaam University of College of Education (DUCE); 4College of Natural and Applied Sciences, Department of Chemistry, University of Dar es Salaam, Tanzania ABSTRACT Diverse anthropogenic activities including alteration of hydrological regime and agricultural development in the upstream of the river catchments modify the structural components of communities and ecological roles of species in estuarine ecosystems. The present study compared the diet, carbon (δ13C) and nitrogen (δ15N) isotope ratios and Layman community metrics of predatory fish and their prey-fish between two estuaries with different degrees of anthropogenic disturbances. The Layman community metrics were estimated following the Bayesian approach. It was revealed that the diets of the predators Arius africanus, Epinephelus coioides, Sillago sihama and Pomadasys argenteus, and their isotopic values including their main prey, the fish Valamugil buchanani, were significantly different between Wami and Pangani estuaries (PERMANOVA, Pseudo-F≥3.1, p=0.05). The com- parison test of isotopic niche width at estuary level showed that the isotopic niche of Wami estuary was slightly narrower (3.90- 6.94 at 95% CI) than that of Pangani (5.70-9.69 of 95% CI). The comparison of the δ15N values and range of species between estuaries suggest shifts in trophic position of the species in Wami estuary relative to that of Pangani. Furthermore, the Layman metric indices of trophic diversity and redundancy of Wami estuary were substantially smaller, when compared to that of Pangani estuary. These findings indicate that, even though the Wami estuary stands under a conservation status, the extent of disturbances in the upstream is potentially high and enough to induce the ecological changes at the base of the food web downstream, giving rise to subsequent effects transmitted to higher-order consumers. As a result, the ecological redundancy and ecosystem complexity of Wami are possibly compromised relative to that of Pangani estuary. No n- co mm er cia l u se on ly T.J. Lyasenga et al.84 in estuaries as they exert reciprocally positive indirect ef- fects by regulating the populations of their respective preys, and in that way, prevent competitive exclusion of each other’s preys (Sanders et al., 2018). They also link pelagic, benthic and detritivore food webs in estuarine ecosystems through predator-prey interactions (Rooney and McCann, 2012). Because of the array of ecological roles they play, by studying these predators, one can get insights into the role of predatory fish for maintaining the structure and functioning of estuarine ecosystem. The Pangani and Wami River estuaries are situated about 55 kilometers apart in the coastal northern part of Tanzania. The Pangani estuary is managed by Pangani Water Basin Office Authority, and thus it is an open access estuary. The Wami estuary is part of Saadani National Park (SANAPA) and is therefore a protected estuary but the whole river is managed by Wami-Ruvu Water Basin Office Authority. However, the upstream part of Pangani and Wami rivers are subjected to different anthropogenic activities that may be affecting the ecology of estuarine fish and other biota in different ways. In the upstream por- tion of Pangani catchment, water is mainly abstracted by multi-damming for hydroelectric power generation and irrigation (Shaghude, 2016; Mwijage, et al., 2018a). The retained sediments and organic particles in the reservoirs and the elevated load of nutrient inputs arising from these activities affect different ecological processes downstream the rivers including estuaries. Yet, water upstream of Wami estuary is widely used for fishing and irrigation in rice paddies and commercial sugarcane plantations (Eeden and Koppen, 2016). There is an indication that the multi-damming activi- ties in Pangani catchment have reduced the nutrients load- ing downstream (Selemani et al., 2017) and caused some predatory fishes to largely rely on the estuarine and ma- rine organic matter sources (Mwijage et al., 2018a). In the Wami estuary, the basal primary food sources sustaining the components of the estuarine food webs is unknown. Despite the fact that the Wami estuary receives elevated nutrients and suspended particles from the upstream, ac- cording to Kiwango et al. (2015), the estuary is still eco- logically healthy. Generally, the anthropogenic disturbances of Tanzanian estuaries and coastal ecosystem is high (see, e.g., Semba et al., (2016) and Shaghude (2016)), but little is known about the implications of such disturbances on the trophic niche, and other related dy- namics, of fish assemblage and, consequently, on the en- tire estuarine ecosystem. This paper aims at getting more insight into the above- mentioned perspectives, by comparing the diet and other trophic dynamics of fish species at the different degrees of anthropogenic disturbances present in Wami and Pan- gani estuaries. Stable isotopes and stomach content analy- ses are employed in complementarity to describe the trophic structure in numerous ecosystems. Stable isotopes ratios of carbon (δ13C) and nitrogen δ15N can be used as a robust tool to evaluate changes in the diet of native species in relation to the expansion of invasive species (García et al., 2020). Stable isotopes are also used to analyze the trophic niche, dietary overlap and other trophic dynamics of food web components across varied ecosystems (Lay- man et al., 2012; Jackson and Parnell, 2016; Gutmann and Britton, 2018; Wang et al., 2018; Zhang et al., 2019). Through the Bayesian framework, Layman metrics are es- timated to enable evaluation of the numerous human-in- duced impacts on food webs, including ecological redundancy, resilience, diversity and trophic stability of ecosystems (Rooney and McCann, 2012; Matich et al., 2017; Donázar-Aramendía et al., 2019). In this study, we examine the degree of dietary and isotopic niche flexibility of predatory fish and their phyto- detritivore prey-fish in the adjacent estuaries of Wami and Pangani rivers. Specifically, our study intended to (i) as- sess the extent of similarity or overlaps in diet of preda- tory fish; ii) estimate isotopic values and trophic niche of predatory fish and their phyto-detritivore prey-fish popu- lations; and (iii) elucidate the status of complexity, re- silience and redundancy of two estuarine ecosystems using Layman metrics. In presenting and discussing our results, we provide critical information to further under- stand the effects of the anthropogenic activities on fish communities and the resulting impacts on the Tanzanian river ecosystems as well as refine management ap- proaches for sustainability of riverine and coastal re- sources in general. METHODS Study area The study sites were based in the two macro-tidal es- tuaries of semi-diurnal type adjacent to each other, located in the northern coastal part of Tanzania, namely, Wami and Pangani estuaries (Figure 1). Both estuaries are per- manently open, connected to the sea. The Wami estuary is part of the Saadani National Park (SANAPA) which was established in 2005 and thus, it is strictly protected. The tidal range in this estuary reaches up to 4 m during spring tides and the influence may extend up to 8 km up- stream (Kiwango et al., 2018a). During the wet season, the freshwater plume of Wami River extends for several hundred meters into the Indian Ocean during both low and high tides. Only the first five kilometers from the shore are occupied by mangroves. The estuary shows turbidity ≥400 NTU and suspended sediment concentrations of <100 mg L-1 during the dry season, and >800 mg L-1 dur- ing the wet season (Kiwango et al., 2018a). A salt wedge is common in the offshore areas of Wami, several hundred No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 85 meters from the mouth of the river in a northeasterly di- rection. The Pangani estuary is located further north of Wami estuary (Figure 1) with a wave amplitude of about 3.5 m during the spring tides and 3.0 m at neap tides (Mwijage et al., 2018a). The estuary extends to about 20 km from the estuary mouth (Mwijage et al., 2018a). It has an extensive mangrove forest interspaced with coconut/palm trees. Pangani estuary experiences strong mixing of fresh and saline water. The hydrological regime is mainly regulated by the multi-reservoirs upstream of the catchment. Moreover, both estuaries serve as breeding and nursery grounds for marine fish, prawns, and birds, and they are known as the most productive areas of prawns for Tanzania. Wami estuary, to a larger extent, hosts a larger number of hippopotamus and crocodiles compared to those occurring in the Pangani estuary. Field sampling procedures of fish Fish samples were collected in Wami and Pangani River estuaries in July 2019 during the daytime. July is a dry-season month and within the peak of the southeast monsoon. Thus, field sampling was conducted after the peak of the rainy season which usually occurs in May; this allowed to reduce seasonal biasness by capturing isotopic values that would reflect dietary sources that were con- sumed by fish during the wet and dry seasons. This is based on the premise that isotopic values of consumer tis- sues indicate their long-term assimilated diet of at least three months. Fish species were caught by using monofil- ament gill nets of multiple mesh sizes and seine net with the dimensions of 15 m length, 1.5 m width and mesh size 0.5 mm. From the total landing, five abundant predatory fish species, namely Arius africanus (Günther 1867), Ep- inephelus coioides (Hamilton, 1822), Sillago sihama (Forsskål, 1775) and Pomadasys argenteus (Forsskål, 1775), and their main potential prey such as the phyto-de- tritivore Valamugil buchanani (Bleeker, 1853), were se- lected to study their trophic niches flexibility. The selection was based on what predatory fish species of ma- rine affinity are abundant throughout the seasons (Bianchi, 1995; FIU-GLOWS, 2016). Notably, marine fish species inhabiting Wami estuary include A. africanus, V. buchanani, Chanos chanos, Am- bassis gymnocephalus, Hilsa kelee, Thryssa spp., Leiog- nathus equulu, Pomadasys spp., Epinephelus spp. and Tetraodontidae, (Anderson et al., 2007; FIU-GLOWS, 2016). However, the diversity and abundance of these species decrease sharply as the distance from the mouth of the river to the upstream increases (FIU-GLOWS, 2016). In Pangani estuary, fish species with a marine ori- gin commonly seen include C. chanos, A. gymnocephalus, Gerres filamentosus, V. buchanani, Lutjanus argentimac- ulatus, sillago sihama, Epinephelus spp. and species from the families of Sphyraenidae, Hemiramphidae, Gobiidae and Carangidae (PWBO/IUCN, 2008; Bianchi, 1995; Mwijage et al., 2018a). During the sampling, the predatory fish species E. coioides and A. africanus were caught in both estuaries along with their main prey, i.e., V. buchanani. Both S. si- hama and P. argenteus were abundantly caught in Pangani estuary, while P. argenteus was greatly caught within the plume setting at short distance from the mouth of the Wami estuary. Since the two above-mentioned species are classified as zoobenthivore under the feeding mode func- tional grouping (Elliott et al., 2007), their trophic require- ments are equivalent. Due to that, we decided to select them as another pair of comparison between estuaries. To get enough samples for stomach content analysis, addi- tional samples of P. argenteus were bought from the local market immediately after being landed by local fisher- men. All fish species caught were stored in ice boxes con- taining dry ice immediately after collection. Later, total length and weight were recorded, and all fish samples frozen at -20°C on the same day and transported to the Figure 1. Map of Wami and Pangani river estuaries showing the study area. No n- co mm er cia l u se on ly T.J. Lyasenga et al.86 laboratory at the University of Dar es Salaam, College of Agricultural Sciences and Fisheries Technology for fur- ther analyses. Fish stomach content analysis In the laboratory, stomachs of all fish species caught were extracted and subjected to content analysis. The fish stomachs were dissected under a microscope to enable the determination of the frequency of occurrence (%FO) and percentage volumetric contribution of each dietary item to the total stomach volume (%V). This was done follow- ing the point method described by Hyslop (1980). The diet items were identified at the possible low taxonomic level. Each food content in the stomach was allotted one of the number of points - 0, 1, 2, 4, 8 and 16 - based on its vol- ume, from the smallest to the largest value, respectively. Then percentage volumes within each stomach analyzed were then calculated to give the percentage composition in a diet of individual fish species. Also, vacuity index or the stomach emptiness index (VI) of each species was es- timated using the equation given by Euzen (1987): VI = (the number of empty stomachs of species i /total number of the stomachs of species i examined) × 100. By using the dietary volume data, the standardized Levin’s (1968) diet niche breadth (Ba) was estimated for each species in all estuaries following Hurlbert (1978). The following formula was used: where Ba is the standardized trophic niche breadth, Pij is the proportion of food category j in the diet of the species i and n = total number of food items in the diet of species i. The Ba values range between 0, when only one resource is used, and 1, when all resources are used equally. In ad- dition to that, the dietary overlap (O) of the predatory fish species in each estuary were estimated by using Pianka’s index of niche overlap (Pianka, 1973), which considers that the different dietary resources are equally accessible to all species. The formula used was: where Pij and Pik are the proportions of diet category i comprised in the diets of j and k, respectively. Both diet niche breadth (Ba) and overlap (O) were considered low when the values fall within 0-0.39 range, intermediate when they are within 0.4-0.6, and high when ranged from 0.61-1. Overall interspecific variations in the diet of the fish species between and within the two estuaries were as- sessed by main and pairwise permutation multivariate analysis of variance (PERMANOVA). The multivariate analysis, including cluster analysis that produces dendro- grams to measure the similarity in diet, and the PER- MANOVA tests were run based on the Bray Curtis simi- larity matrices made from the square root transformed volumetric percentage dietary data using PRIMERv6 with PERMANOVA + add-on module statistical packages (An- derson et al., 2008). Before conducting any statistical test, diet data were subjected to normality and homogeneity of variance tests, the assumptions were not met. Thus, non- parametric method was opted and because of the multiple variables, the multivariate analyses of variance tests were selected. Stable isotopes and trophic niche indices analyses For each fish species collected for stomach content analysis, representative samples were selected randomly to extract dorsal white muscle tissues for stable isotope analy- sis (SIA). The sample size (n) and length size (total length - TL - in cm) of each species were different: A. africanus from Wami = TL ranged 16.6-29.9 cm (n=16); A. africanus of Pangani = TL ranged 13.5-44.2 cm (n=16); E. coioides of Wami = TL ranged 15.3-30.5 cm (n=6); E. coioides of Pangani = TL ranged 15.6-32.0 cm (n=6); P. argenteus of Wami = TL ranged 31.3-41.0 cm (n=5); and S. sihama of Pangani = TL ranged 15.0-25.6 cm (n=8). The extracted muscle tissues were oven-dried at 60°C and ground to a powder form. A subsample of each sample was weighed to the nearest 0.9 mg and placed into tin capsules for δ13C and δ15N analysis. The δ13C and δ15N compositions of each sam- ple were determined simultaneously using a dual pumped Sercon model 20-20 Continuous Flow Isotope Ratio Mass Spectrometer (CF/IRMS or EA/IRMS) linked to a Thermo model EA1110 Elemental Analyser (EA). The equipment utilizes dual reaction tubes (combustion/reduction), a mag- nesium perchlorate drier and a Carbosieve G separation col- umn. Stable isotope results were expressed in the delta notation (δ) relative to the known standards, Vienna Pee Dee Belemnite (VPDB) for 13C/12C and atmospheric nitro- gen for 15N/14N ratios. The δ13C or δ15N were calculated using the following formula: δ = [(Rsample/Rstandard) – 1] x 1000 where R is either 13C/12C or 15N/14N. These isotope analyses were carried out at the OEA Labs Limited in the United Kingdom. Before statistical analysis of isotopic results, these latter were evaluated to gauge if the lipid contents were likely to affect the analy- sis using the C:N ratio (Post et al., 2007). The C:N ratios of all species were below 3.5, thus, in accordance with Post et al. (2007), the levels of lipids in tissues of fish samples were not enough to skew isotopic results. To test the variations in isotopic values among fish species and between estuaries, the isotope data were subjected to main and pairwise PERMANOVA tests. Two fixed factors, No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 87 species and estuaries, were considered. The tests were run based on the Euclidian distance resemblance matrix made from the normalized isotopic data. Furthermore, to enable quantitative comparisons of the trophic niche of fish communities within and between the two estuaries, the standard ellipse area corrected for small sample size (SEAc), total convex hull area (TA), and other five Layman metrics were estimated using the Stable Isotope Bayesian Ellipses in R (SIBER, Jackson et al., 2011) package version 4.0.5 (R Core Team, 2021). Standard ellipse area corrected for small sample size and TA and were estimated as quantitative proxies of the iso- topic niche width that measure total trophic diversity or niche area. However, these two metrics differ, because SEAc is less sensitive to outliers and sample size, as it is corrected for small sample sizes, than the values of TA. For statistical comparison of SEAc between the commu- nities investigated, the posterior distribution of the Bayesian standard ellipse (SEAb) and the 95% credible intervals (CIs) were calculated (Jackson et al., 2011). To test whether the isotopic niche space of fish population of Wami estuary is larger, smaller or similar to the compa- rable population of Pangani, the 95% CI of SEAb and TA of those targeted pairs of fish species (see previous para- graphs) were compared. When the CI of distinct pairs of species did not completely overlap, the trophic niche of the comparable populations was significantly different. By following Jackson et al. (2011), the probability that SEAb or TA of species of Wami was smaller than that of Pangani was also quantified. The proportions of overlaps for the comparable groups were also calculated using Bayesian Overlap and compared as they had to match with the density plot for the 95% prediction ellipsoids. The overlaps estimated aimed at describing how much of each species’ niche overlapped with others between and within the estuaries. As per Layman et al. (2007), other metrics estimated using SIBER package were δ13C range (CR), δ15N range (NR), mean distance to the centroid (CD), mean nearest neighbor distance (M-NND) and standard deviation (SD) of the M-NND (SD-NND). These Bayesian-derived esti- mates allow statistical comparisons between communities without any restriction on the number of the groups within the communities (Layman et al., 2012; Donázar-Ara- mendía et al., 2019). The CR was used to indicate trophic diversity or niche diversification at the base of the food web. The NR was used to infer both trophic diversity and the trophic length, meaning that a larger range infers a wide trophic spectrum. The CD, which is the average Eu- clidian distance of each community component to the cen- troid, was used to infer the average degree of trophic diversity. The two metrics, M-NND and SD-NND were used to evaluate to what extent the trophic redundancy of the fish community vary between the estuaries. The M- NND measures the density of species packing, meaning that species or groups with similar trophic ecologies show smaller values of M-NND and thus represent increased trophic redundancy. The SD-NND metric indicates the evenness of species packing. This means that lower SD- NND is indicative of more even distribution of species, the indictor of higher trophic redundancy as a result of differ- ent groups having more similar trophic ecologies (Layman et al., 2007). The SD-NND also show diversification of trophic niches (Donázar-Aramendía et al., 2019). RESULTS Diet compositions of predatory fish A total of 420 stomachs of four estuarine predatory fish species were examined. Out of that, 332 (79%) con- tained food items that were used for further analyses. Ir- respective of where species were caught, seven dietary items were identified in the stomachs of predatory fish species, namely: prey fish, stomatopods, bivalves and other unidentified mollusks, prawns, shrimps, polychaetes and crabs. Digested materials and remains of hard parts of preys were also seen in some species. Detrital materials and sand particles were observed as accidental items in- gested by A. africanus. At the species level, the values of vacuity index (VI) were considerably high in both estu- aries (Table 1) and the patterns of VI showed that the predatory fish species were more voracious in Wami rel- ative to that of the Pangani estuary. With the exclusion of polychaetes observed in the stomachs of both A. africanus and S. sihama, as well as detrital materials found only in the stomachs of A. africanus, all other preys were common in all four preda- tory fish species under investigation, albeit with different volumetric contribution (Table 1). Arius africanus of Wami estuary fed mainly (by volume) on bivalves (27%) followed by other mollusks (26%) and small fish (19%), while A. africanus of Pangani estuary ingested large amounts of bivalves (36%), other mollusks (31%) and crabs (9%). Stomatopods and shrimp prey were rarely seen in the stomachs of A. africanus from Wami estuary. Fish prey, prawns and crabs were the three most con- sumed preys equally by E. coioides in the Wami estuary, whereas prawns, fish, and stomatopods occupied large percentage of diet for individuals caught in Wami estuary (Table 1). The diet of P. argenteus of Wami estuary was mainly dominated by prawns and to some extent by fish and mollusks, while S. sihama of Pangani estuary ingested large amounts of fish, shrimps, and substantial amounts of polychaetes (Table 1). Despite grouping the prey types at higher taxonomic level, it was noticed that Valamugil spp. were dominant among the fish preys encountered in the stomachs of all the fish examined. No n- co mm er cia l u se on ly T.J. Lyasenga et al.88 Multivariate analyses of dietary composition in predatory fish The dendrogram prepared based on the volume of prey items showed that the diet items consumed by the popula- tion of A. africanus of Wami and that of the Pangani estu- aries were highly similar for about 75% (Figure 2). It was also revealed that about 50% of the diet of E. coioides of Wami was shared with P. argenteus from the same estuary (Figure 2). The prey items consumed by E. coioides popu- lation of Wami were to some extent different (about 40% similarity) to that of the population of Pangani estuary. The results further highlighted that the preys consumed by Pan- gani-drawn E. coioides and S. sihama were far related (40% Figure 2. Dendrogram for hierarchical clustering of dietary composition (by percentage volume) in Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama collected from Pangani (P) and Wami (W) estuaries. Table 1. Diet composition of Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama found in their stomachs and dietary indexes of each prey item: frequency of occurrence (%F), percentage volumetric contribution (%V), number of stomachs with food/content (n) and vacuity index percentage (%VI). Food items Wami estuary Pangani estuary A. africanus E. coioides P. argenteus A. africanus E. coioides S. sihama (n = 48) (n = 52) (n = 54) (n = 74) (n = 52) (n = 52) %FO %V %FO %V %FO %V %FO %V %FO %V %FO %V Fish 50.0 19.4 51.6 22.4 31.1 11.6 20.8 7.6 64.7 23.2 45.7 33.7 Stomatopods 2.4 1.2 51.6 11.9 13.3 4.0 64.7 19.4 2.9 2.6 Bivalves 42.9 27.0 9.7 1.5 26.7 9.0 58.3 35.7 Other mollusks 40.5 26.0 26.7 10.1 62.5 31.2 35.3 8.8 22.9 13.1 Prawns 51.6 22.1 84.4 54.1 16.7 4.8 70.6 32.1 Polychaetes 19.1 5.7 20.8 6.5 22.9 17.1 Crabs 35.7 12.0 64.5 22.4 26.7 8.0 31.3 9.3 35.3 12.1 5.7 1.5 Shrimps 14.3 3.6 41.9 14.8 11.8 4.4 28.6 20.6 Sand/detritus 4.8 3.0 2.1 1.0 Hard tissues/parts 19.4 3.9 2.2 0.9 6.3 1.9 Unidentifiable/digested 18.1 2.1 6.5 1.0 15.6 2.3 14.6 1.9 25.7 11.4 Vacuity index (VI) 9.76 21.21 15.63 22.58 33.33 23.56 No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 89 average similarity) to each other. The low levels of simi- larity in prey consumed by fish were also seen among the E. coioides, A. africanus from the two estuaries and P. ar- genteus drawn from Wami estuary (40% average similarity, Figure 2). The results of PERMANOVA test paired with that of dendrogram showed significant differences among species (Pseudo-F=21.0; p=0.001; Table 2) as well as the interactions between main factors, species, and estuaries (Pseudo-F=21.0; p=0.001; Table 2). Even the pairwise PERMANOVA tests indicated spatial intra-specific diet variations of each of the three pairs of predatory fish species caught from Wami and Pangani estuaries (t≥1.88; p≤0.05; Table 3). Dietary niche breadth and overlaps of predatory species The values of index of dietary niche breadth (Ba) of the predatory fish species varied from 0.29 to 0.7 (Figure 3). Despite the dietary volume of E. coioides varied be- tween estuaries (Table 2), the species presented much higher values of Ba as compared to other species (Figure 3). Arius africanus from Wami showed moderate niche breadth, whereas specimens from Pangani estuary showed lower values of Ba (Figure 3). A different situation was noticed for the comparable pair of P. argenteus from Wami and S. sihama from Pangani where the former species showed lower, and the latter species showed moderate di- etary niche breadths (Figure 3). Furthermore, the analysis of dietary overlaps based on Pianka’s index (O) revealed much high values (O>0.6) for the following pairs: A. africanus of Wami and A. africanus of Pangani, E. coioides of Wami and E. coioides of Pangani, as well as P. argenteus of Wami and E. coioides of Pangani, (Table 4) which is an indication of low trophic flexibility of these species in two estuaries. However, the lowest index of overlap (O=0.2) was presented by the pair of P. argenteus of Wami versus S. Sihama of Pangani (Table 4). Variations in stable isotopes of predatory fish and their representative prey fish It was revealed that the δ13C and δ15N ratios were sig- nificantly different among species and between estuaries (PERMANOVA, Pseudo-F=27.3, p=0.001; Table 5 and Figure 4). Moreover, there were significant interactions Figure 3. Index of dietary niche breadth (Ba) of predatory fish species, Arius africanus, Epinephelus coioides, Pomadasys ar- genteus and Sillago sihama collected from Wami and Pangani estuaries. The dotted line indicates the classes of the Ba, 0 – 0.39 = low Ba; 0.4 – 0.6 = moderate Ba and 0.61 – 1 = high Ba. Table 2. Two-way PERMANOVA of diet composition in predatory fish species of Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama from Wami and Pangani estuaries. Sq.root CoV = square root component of variation. Source of variation df MS Pseudo-F p(perm) Sq. root CoV Estuary 1 10257 4.0 0.002 11.43 Fish species 3 53316 21.0 0.001 33.23 Estuary x Fish species 1 7927.5 3.1 0.012 23.51 Residual 213 2538 50.38 Table 3. Pair-wise PERMANOVA of spatial variations in diet composition of Arius africanus, Epinephelus coioides, Pomadasys ar- genteus and Sillago sihama from Wami and Pangani estuaries. Estuary comparisons of fish species Denominator df t p A. africanus (W) and A. africanus (P) 88 1.88 0.02 E. coioides (W) and E. coioides (P) 46 2.03 0.01 P. argenteus (W) and S. sihama (P) 88 4.17 0.001 No n- co mm er cia l u se on ly T.J. Lyasenga et al.90 between species and estuaries (PERMANOVA, Pseudo- F=22.6, p=0.001; Table 5). At spatial scale, the individual predatory fish species exhibited a clear distinction of iso- topic values between estuaries (Figure 4 and Table 6). As such, the mean isotope ratio of A. africanus (-17.3±1.2‰; 13.3±0.3‰) and E. coioides (-18.6±0.7‰; 13.6±0.5‰) of Wami estuary were significantly lower in terms of δ13C but higher in terms of δ15N ratios when compared to that of Pangani estuary (-16.3±2.1; 11.1±1.8‰; -18.0±0.7‰, 12.8±0.6‰ (Figure 4 and Table 6; Pair-wise PER- Table 4. Niche overlaps between pairs of predatory fish species analyzed from Wami (W) and Pangani (P) estuaries. Species A. africanus (W) E. coioides (W) P. argenteus (W) A. africanus (P) E. coioides (P) S. sihama (P) A. africanus (W) 0.42 0.33 0.94 0.42 0.58 E. coioides (W) 0.34 0.25 0.97 0.56 P. argenteus (W) 0.37 0.84 0.21 A. africanus (P) 0.31 0.30 E. coioides (P) 0.49 S. sihama (P) Table 5. Two-way PERMANOVA results on isotopic variation of predatory fish species at species and estuary level for Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama from Wami and Pangani estuaries. Source df MS Pseudo-F P(perm) Fish species 3 20.85 27.234 0.001 Estuary 1 12.43 16.242 0.001 Fish species x estuary 3 17.30 22.606 0.001 Residual 95 0.766 Table 6. Pair-wise PERMANOVA results of intraspecific variations in stable isotopes composition of Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama between Wami and Pangani estuaries. Estuary comparisons of fish species Denominator df t p A. africanus (W) and A. africanus (P) 30 4.37 0.001 E. coioides (W) and E. coioides (P) 16 2.61 0.004 P. argenteus (W) and S. sihama (P) 23 8.58 0.001 V. buchanani (W) and V. buchanani (P) 26 1.03 0.363 Figure 4. Bi-plot of mean and standard error (SE) of carbon and nitrogen stable isotopes of Arius africanus, Epinephelus coioides, Pomadasys argenteus and Sillago sihama and their main prey fish Valamugil buchanani from Wami and Pangani river estuaries. No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 91 MANOVA, t=4.4, p≤0.004). Similarly, the stable isotope ratio of S. sihama based in Wami when compared with P. argenteus of Pangani, were significantly variable. This meant that the isotopic ratios of S. sihama of Pangani es- tuary exhibited the lowest δ13C (-20.1±1.7‰) and higher δ15N (12.8±0.5‰) values when compared to that of P. ar- genteus from Wami estuary (-15.5±0.2‰, 12.4±0.1‰; Figure 4). For the potential prey-fish, V. buchanani despite the δ13C and δ15N mean values being slightly higher in Wami compared to that of Pangani (Figure 4), the isotopic variations were statistically not significant (pairwise PER- MANOVA, t=1.03, p>0.05; Table 6). Isotopic trophic diversity, niche width and overlaps of fish populations The CR (that measures the trophic diversity) and NR (measuring trophic length and diversity) of A. africanus were shorter in the Wami estuary compared to that of the Pangani estuary (Table 7). The situation was also similar for other metrics of trophic niche width of A. africaus, such as the Bayesian estimate of trophic niche width (SEAb) and SEAc (Figure 5 and Table 7). Likewise, the same trend was also noticed for the SEAb and SEAc of the other three pairs of species of comparison from Wami and Pangani estuaries (Figure 5 and Table 7). The results of the intra- and interspecific comparisons of SEAb within and between the estuaries varied from one pair to another of predatory species. The SEAb of A. africanus from Wami was significantly smaller than that of Pangani es- tuary with no intraspecific overlap in between Bayesian estimate of 95% CI (Table 7). This was also in line with estimate of the probability of SEAb of less than 1% for the A. africanus based in Pangani to be smaller than that of Wami estuary (Table 7). The SEAb of E. coioides from both Wami and Pangani estuaries were statistically similar to each other due to large overlaps between 95% CI and 50% in magnitude of probability of the SEAb for the two pairing species (Table 7). Even CR and NR of E. coioides Table 7. Bayesian standard ellipse area widths (SEAb), magnitude of probability of SEAb for the Wami-based species to be smaller than Pangani based estuarine species; SEAc, CR and NR for A. africanus, E. coioides, S. sihama, P. argenteus and V. buchanani studied in Wami and Pangani estuaries. CI – credible interval of 95%. Species Wami Pangani Probability (%) of SEAb for species of Wami < Pangani for 95%CI %CI Lower Upper %CI Lower Upper A. africanus 99 0.59 2.31 99 1.99 8.04 95 0.68 1.92 95 2.36 6.67 0.0005 50 0.95 1.35 50 3.20 4.60 Mode 1.11 3.81 SEAc 1.26 3.69 CR 3.56 7.00 NR 0.92 3.44 E. coioides 99 0.37 3.09 99 0.44 2.86 95 0.47 2.27 95 0.56 2.20 50 0.78 1.29 50 0.89 1.38 Mode 1.00 1.09 SEAc 1.27 1.32 CR 1.86 1.51 NR 1.50 1.46 P. argenteus (Wami)/S. sihama (Pangani) 99 0.02 0.11 99 0.84 4.03 95 0.02 0.09 95 0.99 3.20 50 0.04 0.05 50 1.49 2.18 50 Mode 0.04 1.81 SEAc 0.05 1.98 CR 0.61 4.38 NR 0.26 1.52 V. buchanani 99 0.87 4.36 99 1.34 5.89 95 1.06 3.53 95 1.67 4.77 50 1.57 2.35 50 2.39 3.37 - Mode 1.91 2.85 SEAc 2.18 3.19 CR 2.4 2.98 NR 2.55 2.56 No n- co mm er cia l u se on ly T.J. Lyasenga et al.92 from these estuaries were relatively similar. The SEAb, and thus, trophic niche width, of S. sihama of Pangani was significantly larger as compared to that of A. argenteus of Wami due to zero overlap between the 95% CI (Table 7). Contrary to that, the substantial overlap between the 95% CI for the V. buchanani from both Wami and Pangani es- tuary (Table 7) was considered as an indication that the trophic niche width of the species in these habitats is somewhat similar. Furthermore, the SEAb between the following pairs of species were also significantly different when their 95% CIs were compared: both pairs of A. Africanus versus E. coioides from within Pangani and Wami estuaries, E. coioides versus P. argenteus of Wami as well as A. Africanus versus P. argenteus of Wami estu- ary (Table 7). Similar to A. africanus, the CR and NR of S. sihama (CR=4.4‰, NR=1.5‰) and V. buchanani (CR=3.0‰, NR=2.6‰) were consistently higher in Pan- gani estuary than those of their comparable species in the Wami estuary (P. argenteus CR=0.6‰, NR=0.3‰; and V. buchanani CR=2.4‰, NR=2.5‰; Figure 5 and Table 7). The SEAc showed substantial overlap between the standard ellipses for A. africanus and E. coioides from Wami estuary, and between all comparable species from Pangani estuary (Figure 5). This indicated that the degree of dietary resource sharing by predatory fish was higher in Pangani compared to that of Wami estuaries. Specifi- cally, the estimates of SEAc overlap analyses revealed that, in Wami estuary, the percentage of overlap between SEAc for A. africanus onto that of E, coioides, and vice versa, was 54%. No overlap of SEAc was found between A. africanus versus P. argenteus nor E. coioides versus P. argenteus from Wami estuary. Contrary to that, in Pangani estuary, the largest percentages of overlap were indicated by SEAc for the S. sihama onto that of A. africanus (62%) and for the E. coioides onto that of A. africanus (61%). The percentage of SEAc for the A. africanus versus E. coiodes was 21% and A. africanus versus S. sihama was 34%. More so, the percentage of E. coioides that over- lapped with S. sihama was 58%, while that of S. sihama onto the SEAc of E. coioides was 16%. The results of intra-specific overlap between estuaries indicated that the SEAc for the A. africanus of Pangani onto that of Wami was 11%, but the values of the overlap for the vice versa was 33%. For the E. coioides, the overlap percentages be- tween estuaries were 57% and 59%. Again, there was no ellipse overlap for the P. argenteus of Wami and S. sihama of Pangani. Trophic niche and redundancy at estuary level The SEAc and TA that infer the total trophic niche area of fish community were smaller in Wami (SEAc=5.4; TA=14.6) contrary to that of Pangani estuaries (SEAc=7.6; TA=25.1; Figure 6). The comparison test for the Bayesian estimate 95% CI of the two estuaries re- vealed that SEAb of Wami was slightly smaller than that of Pangani as they showed substantial overlap between the 95% CI (Wami estuary 3.90-6.94 of 95% CI; Pangani estuary 5.70-9.69 of 95% CI). However, the magnitude of probability for SEAb of Pangani to be smaller than that of Wami was impossible to occur (<1%). More so, the high level of overlap between SEAc of two estuaries was noticed whereby the SEAc of Wami was greatly enclosed within that of Pangani estuary (Figure 6). As well, the Layman community indices showed that the two estuaries differ in trophic resources and level of trophic diversifications of the same fish species investi- gated (Figure 7). The trophic length as indicated by the NR of Pangani estuary (NR=2.45) was marginally shorter when correlated with that of Wami (NR=2.91; Figure 7). The opposite situation was noticed for the indicator of Figure 5. The convex hull area (dotted lines) and standard el- lipse areas (SEA) measuring the trophic niche width of individ- ual predatory fish: 1.1 Arius africanus from Pangani; 2.5 A. africanus of Wami; 1.2 Epinephelus coioides of Pangani; 2.6 E. coioides of Wami; 1.3 Sillago sihama of Pangani; 2.7 Po- madasys argenteus of Wami and their prey fish; 1.4 = Valamugil buchanani from Pangani and 2.8 = V. buchanani of Wami and Pangani estuaries. No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 93 basal and average trophic diversity of fish as expressed by the CR and CD (Figure 7). The trophic redundancy (measured as M-NND) and its standard deviation (SDNND) differed substantially between the two estuaries with M-NND being smaller and SDNND larger in Wami than Pangani estuary (Figure 7). DISCUSSION The findings of this study indicate how anthropogenic activities in the upstream of the river catchments induce changes to the estuarine ecosystem’s functioning. The re- sults suggest that despite the Wami estuary being under conservation status, the high level of disturbances up- stream of the river increase vulnerability of the estuarine food web structure and ecosystems functioning. This is probably connected to a lower primary production, to changes in trophic positions of consumers, and thus to modifications occurring in the trophic niche of the preda- tory fish. As a result, the reduced trophic diversity, eco- logical redundancy, and complexity of the ecosystem are most likely experienced in Wami estuary than Pangani es- tuary. This was different from the expectation as the con- sidered highly disturbed Pangani estuary presented a relatively stable, resilient, and ecologically redundant ecosystem. Even though our findings are based on few compo- nents of the estuarine food webs, they include important compartments that greatly shape the complexity and thus determine the ecological redundancy of the estuarine ecosystems of concern. Furthermore, the inferences drawn from the present study are based on data collected in a spe- cific seasonal period, so that isotopic values gathered from estuarine fish with large body size (i.e., predatory fish) mirror the exploitation of natural resources during their growth period. Moreover, data collection that adheres to seasons may elucidate more on long-term changes in the use of dietary resources and other niche characteristics of estuarine fish assemblage. Moreover, our results explain how both predatory and prey fish respond to the dynamics of dietary resources, representing with a good approxima- tion the food web state in tropical estuaries differing in the degree of anthropogenic disturbances. Our findings showed that both stomach contents and stable isotopes can reveal significant differences in diet and isotopic values among fish species in different estu- aries. Matching of these findings confirm that both meth- ods can characterize the foraging flexibility of predatory fish species of marine origin in estuaries. Except for A. africanus, the results of the index of dietary niche breadth, Ba, have matched well with that of isotopic trophic niche metrics of the species between estuaries. As such, the ob- served consistent highest Ba of E. coioides in both estu- aries correlated with the similarity in their SEAb between Figure 6. Trophic niche width indicated by TA (dotted lines) and SEA (solid lines) for the pooled stable isotopes of carbon and nitrogen of fish from Wami and Pangani estuaries (n = 103). Cycle points represent individuals of all the species measured in each estuary and standard ellipse/circle represent the maxi- mum likelihood of isotopic niche of the species analyzed. Figure 7. The estuarine community-wide Layman metrics of Wami and Pangani estuaries that showed the (i) trophic length, or complexity, NR (dY_range), (ii) trophic diversity, or CR (dX_range), (iii) niche width, or convex hull area (TA), (iv) av- erage trophic diversity, or mean distance to the centroid (CD), (v) trophic redundancy, or the mean nearest neighbor distance (NND), and (vi) evenness of species packing, or extent of trophic diversification, that is, the standard deviation ) of NDD (SDNND). Black dots are the modes, boxes indicate the credible intervals at 50%, 75% and 95%. The numbers above each box are modes; the cross (red) is the values of the true population. No n- co mm er cia l u se on ly T.J. Lyasenga et al.94 estuaries. Similarly, the estimated moderate Ba of S. si- hama from Pangani and the lowest Ba of P. argenteus from Wami correlated with the larger SEAb of Pangani- based S. sihama versus the comparable smaller SEAb of P. argenteus of Wami estuary. However, the mismatch of trophic feeding niche of A. africanus by the two methods indicates that the trophic dynamics in highly fluctuating estuarine ecosystems requires robust methods that should complement the conventional stomach content analysis for drawing a sound and logical conclusion. The applica- tion of stable isotope in trophic ecology studies is widely accepted (e.g. Pasquaud et al., 2010; Olsen et al., 2011; Layman et al., 2012; Cummings et al., 2012) as it is among the powerful tools that ensure the determination of the integrated diet consumed and assimilated at a coarse taxonomic scale but over a long period (Michener and Lajtha, 2007; Selleslagh et al., 2015). Relative to that, the stomach content analysis determines the diet ingested instantly by the consumers at a finer taxonomic level (Ma- hesh et al., 2018). The marked variations in the dietary composition and trophic niche among the predatory fish species found in our study imply that they largely consume similar preys but in different proportions in the two estuaries. Differ- ences in the amounts of preys consumed may be linked to the prey abundance and catchability along with prey visibility by predatory species, plus the feeding strategies and energy requirements (Kulbicki et al., 2005; Kroetz et al., 2016). Both stomach contents and stable isotope-de- rived Layman metrics showed a considerable degree of trophic resource sharing and thus trophic niche overlaps among these predatory fish in estuaries. This finding agrees with those from the study of Matich et al. (2017). This possibly does not imply competition in trophic re- sources but rather the prey-catch strategies that define the prey types consumed. For instance, the higher trophic niche breadth and similarity in isotopic niche width of E. coioides in Wami and Pangani estuaries could be favored by its ambush strategy and structural complexity of the estuaries (Gibran, 2007). Complex habitats such as these mangrove-sheltered estuaries provide enhanced cover that reduces the possibilities of predators to be detected by the preys (Mwijage et al., 2018a). This situation is probably connected with the highest overlap of SEAc of the species between estuaries. Inter- and intra-species variations in isotopic values could mean considerable individual variability in the feed- ing strategies with individuals consuming prey of different trophic spectra and the use of basal nutritional sources they rely on. Differences in both dietary niche breadth and isotopic niche width at the estuary level suggest that fish species may have high trophic niche plasticity or flexibil- ity, which is indicated by Matich et al. (2017) as a mech- anism to respond to natural and human-induced environmental change. This is further indicated by the dis- similarity in magnitude of the isotopic metric-based over- lap of each possible pair of the predatory fish species in the two estuaries examined. The highest degree of overlap for the fish species of Pangani might be an indication of shifts in the trophic positions of organisms preyed upon by these species in two estuaries resulting to the observed differences in trophic length, diversity and trophic niches justified by NR, CR, CD and SEAc/SEAb. The shifts in trophic positions of consumers within comparable aquatic environment is mainly driven by differences in individual diet, trophic discrimination or enrichment factor (Villa- marín et al., 2018), as well as differences in the level of nutrients load fueling the base of the food web (Warry et al., 2016). Furthermore, our findings suggest a possibility of presence of ontogenetic shift in trophic positions (Park et al., 2018; Villamarín et al., 2018) for the comparable es- tuaries examined herein. Although the length size of the individuals was not considered during analysis, it might also influence the results of the differences in isotopic val- ues of the species between the estuaries. This is mainly linked with the size range (total length, TL) of A. africanus from Wami versus of Pangani and P. argenteus of Wami versus S. sihama of Pangani. The literatures em- phasize that resource utilization patterns of fish change markedly with ontogeny (Davis et al., 2012; Park et al., 2018; Villamarín et al., 2018). Specifically, with the exclusion of E. coioides, the larger CR of predatory fish populations of Pangani estuary might be linked with their prey to feed on the diet with varied nutritional sources. The largest SEAb of A. africanus population of Pangani could be probably asso- ciated with a high level of opportunistic or generalist feed- ing mechanism as a coping strategy resulting from exposing the estuary to more frequent human perturba- tions. The smallest SEAb of P. Argenteus of Wami and its δ13C mean values (-15.5±0.2‰) is an indication that the riverine or terrestrial derived dietary sources of which their δ13C ratio range from -23 to -30‰ (Bouillon et al., 2011) had little influence on the diet dynamics of this species. This is somewhat contributed by the sampling lo- cation of the samples. Pomadasys argenteus was caught in the estuarine plume away from the river mouth, and this environment might have been less influenced by the riverine dietary sources. The same situation further ex- plains the observed non-overlap of SEAc between P. Ar- genteus and any other pair of predatory fish studied. More so, the similarity in SEAb of E. coioides between estuaries suggests low trophic niche flexibility of the species. In view of that situation, the environmental filtering hypoth- esis plays a role in this assembly that was not similar due to divergence in the environmental conditions of the species habitats (Pereira et al., 2017). No n- co mm er cia l u se on ly Estuarine fish trophic dynamics and anthropogenic disturbances 95 Furthermore, the higher values of δ15N and NR of Wami-associated species may be an indication of the predatory fish to occupy higher trophic position usually connected to the nature of the diet assimilated by individ- uals, the trophic enrichment factor and high levels of nu- trients load (Villamarín et al., 2018), as opposed to Pangani estuary. The effects of high values of δ15N at low trophic level are transmitted upwards higher up to top predators of estuarine food web, thus affecting the overall trophic organization (Warry et al., 2016). Parallel to this, the substantial low δ13C ratio could be contributed by the Wami estuarine food web to be fed by riverine primary food sources and high inorganic nitrogen load that elevate δ15N values in secondary production (Michener and La- jtha, 2007; Woodland and Secor, 2011; Warry et al., 2016). In the Pangani estuary, water abstractions upstream of the river due to multi-reservoirs for hydropower gen- eration and irrigation is linked with trophic interactions downstream the estuarine and coastal food webs (Mwi- jage et al., 2018b). These human activities contributed to modify the nutrients’ biogeochemistry in the estuary and increased retention time of sediment, organic particles and accompanied nutrients in the reservoirs, upstream of Pan- gani river (Selemani et al., 2017). Eventually, Pangani es- tuary experience lower δ13C values of terrestrial derived basal food sources (Mwijage et al., 2018a) and regulated nutrients load including nitrogen compounds. At the estuarine level, the differences in mean distance to the centroid (CD) and SEAc of the Wami and Pangani fish species may be linked to the variations in estuarine primary productivity. The high loads of suspended sedi- ments in the Wami estuary as reported by Kiwango et al. (2015) contribute to reducing trophic diversity and abun- dance resulting from the low rate of aquatic primary pro- duction (Selemani et al., 2017). Light penetration is constrained by the suspended particles that, consequently, limit photosynthetic activities. In that way, trophic web connectance, or the number of trophic links to the primary producers, tend to be small. This also may be a factor con- tributing to the low level of trophic overlap of Wami and consequently, low trophic redundancy (de Carvalho et al., 2017; Lira et al., 2018). The overall effect is to make food web components vulnerable to secondary extinctions as a result of the reduced complexity of the ecosystem (Lira et al., 2018). This supports our findings but not what was expected of Wami to exhibit higher trophic redundancy than Pangani. As such, Wami estuary presented a large SD-NND which suggests a reduced trophic redundancy. This is in contrast to the Pangani estuary that according to Hellar-Kihampa et al. (2013), have relatively low sed- iment loads as large volumes are retained in the dams lo- cated upstream of the estuary. This concurred with the findings of Abrantes et al. (2014) who reported that the high sediment loads affect aquatic primary productivity and hence small SEAc of fish populations and communi- ties in the Betsiboka and Tana estuaries. The predatory fish assemblages in the Wami estuary are probably less opportunistic in feeding that implies a low level of resilience upon disturbances. Selleslagh and Amara (2014) also showed that fish in the Canche Estuary showing very low anthropogenic pressure, exhibited a specialist feeding strategy. The resilience of the estuarine ecosystem is contributed by among others, trophic com- plexity or multiplicity of trophic linkages along the trophic web and species diversity (Lira et al., 2018). In that sense, the present study agreed with the findings of Pasquaud et al. (2010) and Donázar-Aramendía et al. (2019) who showed that high degrees of anthropogenic pressures modify the structural components and ecologi- cal roles of the species in estuarine ecosystems. Structural modifications in Wami estuary are certainly influenced by the high turbidity that, apart from affecting primary production, hampers species of higher-order con- sumers or predatory fish because of lower performances in prey hunting. The reduction of predatory fish species from a community lead to the dominance of its prey at the lower trophic level and subsequent secondary extinctions of other carnivores (Sanders et al., 2018). This also leads to a reduction in trophic redundancy and consequently complexity and stability of the estuarine food webs. On the other hand, the large degree of overlaps in the isotopic niche of species from the Pangani estuary could not imply the existence of potential competitions of the trophic re- sources but rather the similarity in environmental condi- tions in microhabitats the fish species used for feeding. Greater diet overlaps increase trophic redundancy of the ecosystem (Sanders et al., 2018), a contention that seems to agree with our results which show that the Pangani es- tuary has greater trophic redundancy than that of Wami. CONCLUSIONS Overall, the community-wide metrics derived from sta- ble isotopes when complemented with dietary data eluci- date how predatory fish and their prey respond to human-induced changes in trophic niche width; this re- sponse determines cascading effects that impact the trophic redundancy and complexity of the estuarine ecosystems. Our findings indicate that anthropogenic activities in the upstream modified higher trophic position of fish con- sumers in Wami relative to that of Pangani estuaries. Thus, without management interventions, food web structure re- organization and disruption are likely to occur within the above-mentioned estuaries. Moreover, our findings should be taken with the caveat that not all components of the es- tuarine food webs were used to draw such inferences, but they could be used as indicators of showing that the two estuarine systems investigated are ecologically disturbed No n- co mm er cia l u se on ly T.J. Lyasenga et al.96 by anthropogenic activities. Therefore, management initia- tives should be strengthened for maintaining the structures and functioning of these fragile ecosystems. Corresponding author: Alistidia Paul Mwijage, Tanzania Fisheries Research Institute (TAFIRI) – Headquarters, P. O. Box 9750, Dar es Salaam, Tanzania. E-mail: alistidiamwijage@tafiri.go.tz; zawadi2007@gmail.com. Key words: diet; predatory fish; stable isotope; trophic niche; Lay- man community metrics; anthropogenic disturbances Funding: The University of Dar es Salaam through the CoAF- AQF19058 project number covered all costs from the study design through data collection and interpretation of results. Acknowledgements: The authors acknowledge the funder of this study, the University of Dar es Salaam that covered all costs from the study design through data collection and interpretation of the results. The authors also recognize and appreciate the technical support and research facilities provided by the Chemistry depart- ment of the College of Natural and Applied Sciences and School of Aquatic Sciences and Fisheries Technology of the University of Dar es Salaam. The support of Dr. Blandinga Lugendo (from Uni- versity of Dar es Salaam) who assisted the logistics of identifica- tion of a laboratory in United Kingdom to undertake the stable isotope analysis is also appreciated. The authors also acknowledge and appreciate the technical support and research facilities pro- vided OEA Labs limited in the United Kingdom. Authors’ contributions: All authors played different roles through- out the production of this manuscript. Project conceptualization was conducted by TJL, DAS, JAMM and LG. Data curation was conducted by PLM and DAS. Methodology and formal data analy- sis were conducted by PLM, APM, TJL and DAS. Manuscript writ- ing (original draft) was done by APM, TJL, DAS and PJM. Visualization and the first draft manuscript review were performed by DAS, JAMM, LG and DAS. Funding was collectively solicited by PLM, DAS, JAMM, LG and DAS. Conflict of interest: The authors declare no potential conflict of in- terest. Availability of data and materials: All data generated or analyzed during this study are included in this published article. Received: 18 July 2021. Accepted: 24 November 2021. This work is licensed under a Creative Commons Attribution Non- Commercial 4.0 License (CC BY-NC 4.0). ©Copyright: the Author(s), 2021 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2021; 12:9987 DOI: 10.4081/aiol.2021.9987 REFERENCES Abrantes KG, Barnett A, Bouillon S, 2014. Stable isotope-based community metrics as a tool to identify patterns in food web structure in East African estuaries. Funct. Ecol. 28:270-282. Abrantes KG, Barnett A, Marwick TR, Bouillon S, 2013. Im- portance of terrestrial subsidies for estuarine food webs in contrasting East African catchments. Ecosphere. 4:14. Anderson MJ, Gorley RN, Clarke KR, 2008. PERMANOVA+ for PRIMER: guide to software and statistical methods PRIMER-E. Anderson EP, McNally C, Kalangahe B, Gutmann Roberts C, Britton JR, 2018. 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