Int. J. Aquat. Biol. (2023) 11(1): 20-29 ISSN: 2322-5270; P-ISSN: 2383-0956 Journal homepage: www.ij-aquaticbiology.com © 2023 Iranian Society of Ichthyology Original Article Population dynamics of two sympatric native and exotic cichlids in a tropical microtidal estuary, India Kuttanelloor Roshni1, Chelapurath Radhakrishnan Renjithkumar1,2 1Department of Aquatic Environment Management, Kerala University of Fisheries and Ocean Studies, Kochi 682506, India. 2St Albert’s College (Autonomous), Ernakulam, Kerala, India. s Article history: Received 14 June 2022 Accepted 24 December 2022 Available online 2 5 February 2023 Keywords: Alien species Growth Mortality Overfishing Vembanad Lake Abstract: Population dynamics of two sympatric cichlids, the native Etroplus suratensis and the alien Oreochromis mossambicus, were determined from the Vembanad Lake, the largest estuarine system of southern peninsular India. Based on annual length-frequency data, the results revealed that E. suratensis has a higher asymptotic length (L∞) (302 mm vs. 262mm), but a lower growth constant (K) (0.68 year-1 vs. 1.1 year-1) than O. mossambicus, suggesting the faster growth rate of the alien species. Natural mortality (M) rates recorded for E. suratensis (0.69 year-1) indicate lower levels of predation and other natural stressors in the lake. The exploitation ratio (E) was estimated to be 0.83 for E. suratensis and 0.78 for O. mossambicus, both values greater than the predicted optimum and suggestive of overexploitation. Our study provides the first information on the demography of wild E. suratensis, which would help inform future management strategies in the Vembanad Lake. Introduction Biological invasion is the second-most important threat to global biodiversity after habitat loss (Welcomme, 1988; Courtenay and Robins, 1989). The invasion and establishment of non-native species can impair ecosystem functioning through adverse impacts at the individual, population and community levels of native species, eventually leading to their population decline or extinction (Nyman, 1991; Sato et al., 2010). Ecological risks caused by alien fish species are difficult to be estimated in the initial stages, though the impacts are far-reaching and irreversible (Stapp and Hayward, 2002). Despite the high rate of global biological invasion, the ecological, economic and social impacts of alien species are poorly known to effectively address and manage the issue (Gu et al., 2015; Xia et al., 2019). Mozambique tilapia, Oreochromis mossambicus (Peters, 1852) is a benthopelagic, eurytopic and salt- Correspondence: Kuttanelloor Roshni DOI: https://doi.org/10.22034/ijab.v11i1.1621 E-mail: roshni.phd@gmail.com DOR: https://dorl.net/dor/20.1001.1.23830956.2023.11.1.3.3 tolerant mouth-brooding cichlid inhabiting slow- flowing rivers and streams, as well as the lakes and lagoons of Eastern Africa (Skelton, 1993). Due to its perceived utility as a candidate aquaculture species, it is one of the world's most translocated and widely distributed alien fish species (Canonico et al., 2005). The species survives in many unsuitable environments due to its huge ecological plasticity characterized by adaptable life-history traits, trophic flexibility, tolerance to extreme and unfavorable environmental conditions, immense breeding potential, sound parental care, high growth rate, and aggressive and territorial behaviour (Pérez et al., 2006; Maddern et al., 2007). Considering its ecological impacts, O. mossambicus has been listed as one of the 100 most invasive species in the world (Global Invasive Species Database, 2019) and is believed to create the most adverse ecological effects (Lowe et al., 2000). The Pearlspot, Etroplus suratensis (Bloch, 1790), 21 Int. J. Aquat. Biol. (2023) 11(1): 20-29 is a medium-sized euryhaline substrate-spawning fish inhabiting estuaries, coastal lagoons, and natural and man-made freshwater habitats of peninsular India and Sri Lanka (Munro, 1955; Baensch and Riehl, 1985). This species is a major contributor to inland fisheries in Peninsular India (Padmakumar et al., 2012) and Sri Lanka (Jones et al., 2018), supporting the livelihoods of thousands of subsistence fishers. Etroplus suratensis has high acceptance in domestic and international markets and has recently been considered an ideal aquaculture species (Padmakumar et al., 2009). Though assessed as ‘Least Concern’ on the IUCN Red List of Threatened Species™, localized population declines of this species have been recorded (Abraham et al., 2019). Vembanad Lake is the largest tropical estuarine system located on the southwest coast of India, harboring a rich ichthyodiversity (Maitra et al., 2018). Etroplus suratensis, one of the most abundant species in the lake ecosystem (Samuel, 1969), has shown drastic population reductions due to a synergy of anthropogenic stress in the habitat (Kurup et al., 1995; Padmakumar et al., 2012). The landing of the species decreased from 1252 tonnes in the 1960s (Samuel, 1969) to 458 t in the middle 1980s (Kurup et al., 1995) and 135.28 t in 2012-2013 (Roshni et al., 2017). The reported reduction in E. suratensis has been concomitant with an increase in catches of the exotic O. mossambicus (Kurup et al., 1993; Padmakumar et al., 2012). This co-existence of E. suratensis and O. mossambicus results in competition for food and nesting habitats since both species belong to the same family (Cichlidae) and have similar habitat preferences (benthopelagic), feeding behaviour (omnivores) and reproductive requirements (mouth and substrate brooders requiring sufficient space for spawning) (Ward and Samarakoon, 1981; Bindu and Padmakumar, 2008; Russel et al., 2012; Froese and Pauly, 2019) leading to the potential competitive displacement of E. suratensis (Odum, 1971). In this context, this work aims to estimate the population characteristics of native E. suratensis and alien O. mossambicus in Vembanad Lake to inform appropriate management plans. Materials and Methods The Vembanad Lake (9°28′, 10°10′N, 76°13′, 76°31′E) is a shallow, complex bar-built micro-tidal estuary covering an area of 241 km2 with a catchment area of 14500 km2, depth range of 1-12 m and total length of 80 km, stretching from Munambam in the north to Alappuzha in the south in the state of Kerala having open permanent connections with Arabian Sea (Martin et al., 2011). It is listed as a Ramsar site (No. 1214) by the Convention of Wetlands of UNESCO in 1981, considering its ecological significance and global conservation importance. It is one of the fast- degrading estuaries in India, and its fish fauna is under severe depletion pressure due to lake reclamation, bottom dredging, pollution, overexploitation of fishery resources and alien species introductions (Maitra et al., 2018). Samples of E. suratensis and O. mossambicus were collected from commercial catches at major fish landing centers along the Vembanad Lake monthly from June 2017 to May 2018. Fish were caught using gillnets of different mesh sizes ranging from 2-9 cm and cast nets with a mesh size of 1.5 cm. Data were collected randomly from well-mixed catches to avoid size-based bias in the population estimation. The total length of each individual fish was measured (LT, to the nearest 0.01 mm), and data were grouped into 10 mm intervals for further analysis. The growth and mortality parameters were estimated for the pooled population. Asymptotic length (L∞) and growth coefficient (K) were estimated using von Bertalanffy growth formula (VBGF) (Von Bertalanffy, 1938; Gayanilo et al., 2005) using ELEFAN-1 (Electronic Length Frequency Analysis) incorporated in FiSAT-II (FAO-ICLARM Stock Assessment Tools) (see http://www.fao.org/fishery/topic/16072/en#4) software (Gayanilo et al., 2005). Based on the L∞ and K values, growth performance index () and potential longevity (3/K) were estimated (Munro and 22 Roshni and Renjithkumar / Population dynamics of two sympatric native and exotic cichlids Pauly, 1983) using the formula:  = Log 10 (K) + 2 + Log 10 (L∞). Total mortality (Z) was estimated using the length- converted catch curve method (Pauly, 1984). Natural mortality (M) was calculated using the empirical formula of Pauly (1980): In M = 0.0152-0.279 In (L∞) + 0.6543 In (K)+0.4634 In (T), where L∞ is the asymptotic length in mm, K is the growth constant in year–1 and T is the annual mean temperature (29◦C). The instantaneous fishing mortality rate (F) was computed as F = Z-M. The exploitation rate (E) was estimated as E = F/Z (Gulland, 1970). Emax (maximum yield per recruit) and E50 (exploitation that retains 50% of the biomass) were calculated from relative yield per recruit (Y/R) and relative biomass per recruit (B/R) analysis using the knife-edge selection method (Pauly and Soriano, 1986). The recruitment pattern was determined by reconstructing the recruitment pulses from a time series of length- frequency data (Gayanilo et al., 2005). Results A total of 745 specimens of E. suratensis (67-300 mm) and 865 specimens of O. mossambicus (62-260 mm) were analysed during the study period. Using ELEFAN-II, the growth parameters, i.e. asymptotic length (L∞), growth constant (K) and age at which length equals 0 (t0) were estimated as 302 mm, 0.68 year-1 and -0.01527 year for E. suratensis, and 262 mm, 1.1 year-1 and -0.0041 year for O. mossambicus, respectively (Fig. 1a, b). The growth performance index () was estimated as 4.87 and 4.79 for E. suratensis and O. mossambicus, respectively. Based on the growth equation, E. suratensis attained a length of 151 mm and 225 mm at the end of the first and second years of growth, respectively, while O. mossambicus reached 175 mm and 233 mm during the same period. Total (Z), natural (M) and fishing (F) mortality coefficients estimated for E. suratensis were 4.04 year-1, 0.69 year-1 and 3.35 year-1, and Z, M and F for O. mossambicus were estimated as 4.42 year-1, 0.99 year-1 and 3.44 year-1 (Fig. 2a, b). The exploitation rate (E) of E. suratensis was estimated as 0.83 and for O. mossambicus as 0.78. The exploitation rate at which marginal increase in relative yield per recruit Figure 1. (a and b). von Bertalanffy growth curve of Etroplus suratensis and Oreochromis mossambicus from Vembanad Lake, India. 23 Int. J. Aquat. Biol. (2023) 11(1): 20-29 becomes 1/10th of its value (E10), the exploitation rate at which the stock would be reduced to 50% of its unexploited biomass (E50), and exploitation rate at which yield per recruit reaches maximum (Emax) were estimated as 0.550, 0.368 and 0.675 for E. suratensis, and 0.600, 0.386 and 0.722 for O. mossambicus (Fig. 3a, b). The size at first capture for E. suratensis was estimated as L25 = 136.38 mm, L50 = 153.25 mm and L75 = 170.12 mm, and for O. mossambicu as L25 = 136.30 mm, L50 = 149.48 Figure 2. (a and b). Length-converted catch curve for Etroplus suratensis and Oreochromis mossambicus from Vembanad Lake, India. Figure 3. (a & b). Relative yield-per- recruit (Y/R) and relative biomass- per-recruit (B/R) analysis using knife-edge method for Etroplus suratensis and Oreochromis mossambicus from Vembanad Lake, India. Figure 4. (a & b). Recruitment pattern for Etroplus suratensis and Oreochromis mossambicus from Vembanad Lake, India. 24 Roshni and Renjithkumar / Population dynamics of two sympatric native and exotic cichlids mm and L75 = 162.12 mm. Recruitment of E. suratensis and O. mossambicus to the fishery was continuous and took place throughout the year, with around 64% of the recruitment occurring between July and September for both species (Figs. 4a, b). Discussion Though O. mossambicus attains a maximum length of 390 mm (Wohlfarth and Hulata, 1983), no fish in this size range were obtained in the present study. It has been suggested that O. mossambicus can alter the life history traits in favour of the surrounding environment; hence, a standard growth pattern with respect to a particular ecosystem or geographical area is difficult to fix (Pérez et al., 2006; Maddern et al., 2007). Menon (1999) estimated a maximum attainable length of 400 mm for E. suratensis; however, individuals encountered in the present study showed a significantly lower length range which may be due to the high levels of anthropogenic stress and their subsequent impacts limiting the growth of the species (Padmakumar et al., 2012). Bindu and Padmakumar (2014) reported a maximum length of 340 mm for E. suratensis from Vembanad Lake, comparable with the maximum estimated length observed in the present study. There is no published information on the growth (K, L∞ and ) and mortality parameters (Z, M and F) of E. suratensis which makes comparing different geographic populations impossible. The estimated asymptotic length of E. suratensis is higher and the growth coefficient lower than that of O. mossambicus indicating that O. mossambicus grows much faster than E. suratensis facilitating its rapid colonization in the lake (Table 1). Slow-growing species are more vulnerable to collapse, especially when climatic variability and harvest dynamics contribute unfavorably to fish survival rates (Pinsky and Byler, 2015). The slow growth of E. suratensis could probably increase the depletion pressure of the species if extreme climatic variations and severe overexploitation continue in their natural habitat. The L∞ estimated in the present study is almost similar to that of O. mossambicus populations in African waters (Weyl and Hecht, 1998), while the species showed their highest growth rate in Vembanad lake than any other region, which suggests that the lake provides the best habitat requirement for O. mossambicus, facilitating its range expansion and population explosion. The growth performance index value () values observed for E. suratensis (4.87) and O. mossambicus (4.79) is high, reflecting excellent environmental conditions and high food availability in the habitat, favouring rapid growth of the species. The growth performance () values recorded for O. mossambicus was higher than those observed in previous studies (Moreau et al., 1986; de Silva et al., 1988; de Silva and Amarasinghe, 1989), which may be attributable to the ability of the species to utilize a wide range of food items (Maitipe and de Silva, 1984) in the Vembanad Lake. The higher natural mortality of O. mossambicus in the present study could be due to active fish predators in the lake (Gosavi et al., 2019), and the higher fishing mortality suggests high exploitation (Glamuzina et al., 2007; Panda et al., 2018). Overfishing is a major threat to inland waters (Allan et al., 2005). In the present study, a higher fishing mortality was observed for O. mossambicus compared to populations from Sri Lanka (Amarasinghe and De Silva, 1992; Amarasinghe, 2002; Amarasinghe et al., 2017). For an exploited fish stock of optimal utilization, the rate of fishing mortality (F) should be equal to the rate of natural mortality (M), giving an exploitation rate (E) of 0.5 (Gulland, 1970). The relative yield per recruit (Y/R) and biomass per recruit (B/R) determined based on knife-edge selection estimated the exploitation rate (E) of E. suratensis and O. mossambicus to be greater than 0.5. Also, the computed value of exploitation was greater than the predicted Emax, indicating that both the fishes are under excessive fishing pressure. Etroplus suratensis is exploited more intensively, particularly due to a targeted fishery due to their high economic value and market demand. Bindu and Padmakumar (2014) reported that 25 Int. J. Aquat. Biol. (2023) 11(1): 20-29 E. suratensis attains maturity at a length of 195 mm in males and 200 mm in females in Vembanad Lake. The length at first capture (Lc) of E. suratensis was estimated as 153.25 mm in the present study, indicating that the exploited stock mainly comprises immature individuals. The capture of smaller-sized individuals before reaching maturity can affect population stability and recruitment (Gwinn et al., 2015; van Overzee and Rijnsdorp, 2015). However, Allen et al. (2002) reported O. mossambicus reaches sexual maturity at a length of 150 mm, and the length at first capture (Lc) estimated for the species in the present study was 149.48 mm, almost reaching reproductive maturity. For E. suratensis, the overexploitation of immature fishes could be a major threat in the lake, affecting recruitment and leading to population decline. Meanwhile, the overexploitation of near-mature individuals of O. mossambicus is unlikely to threaten its population size due to the species continuous spawning, stable recruitment, and rapid growth. Recruitment of both E. suratensis and O. mossambicus was continuous throughout the year, with a major pulse during July- September producing 64.30 and 64.24% recruitment, respectively. Spawning and recruitment of the two fish species occurring in the same month could lead to severe inter-specific competition for available food resources and space due to overcrowding of fingerlings. Our study indicates the presence of a well- established population of O. mossambicus in Vembanad Lake, contributing significantly to the total annual fishery. This will lead to the rapid occupation of a vacant spatial and trophic niche by the species resulting in the competitive displacement of many native fishes, especially those sharing similar ecological resources like E. suratensis and Pseudetroplus maculatus (Raghavan et al., 2008). Conclusion Currently, E. suratensis in Vembanad Lake is experiencing severe fishing pressure, as evident from the high rate of exploitation, including a greater proportion of juveniles in the catch. This is in addition to threats from alien species such as Atractosteus spatula, Clarias gariepinus, Pangasianodon hypophthalmus, Piaractus brachypomus and Pterygoplichthys multiradiatus (Krishnakumar et al., 2011; Roshni et al., 2014; Bijukumar et al., 2019). There is an urgent need to reduce the fishing effort and the mesh sizes of fishing gears to avoid the future collapse of this fish species. Also, a more detailed investigation is recommended to understand the life history traits and threats to Species L∞ K  Z M F E E50 Lc Location Source O. mossambicus 448 0.52 2.8 1.39 1 0.394 0.282 - 18.8 Kadulla Reservoir, Sri Lanka Amarasinghe and De Silva (1992) O. mossambicus 460 0.45 2.77 3.03 0.955 2.076 0.685 - 19.6 Minneriya Reservoir,Sri Lanka Amarasinghe and De Silva (1992) O. mossambicus 266. 0.23 - - - - - - - Chicamba lake, Mozambique Weyl and Hecht (1998) O. mossambicus 378 0.51 2.69 2.83 1.1 1.73 0.61 - 15.9 Tabbowa Reservoir, Sri Lanka Amarasinghe (2002) O. mossambicus 314 0.84 - - - - - - - Salton Sea, California, USA Caskey et al. (2007) O. mossambicus 402 0.42 2.64 2.15 0.93 1.22 0.57 0.75 22.4 Minneriya Reservoir,Sri Lanka Amarasinghe et al. (2017) O. mossambicus 424 0.31 2.55 2.34 0.75 1.59 0.68 0.75 22.5 Udawalawe Reservoir, Sri Lanka Amarasinghe et al. (2017) O. mossambicus 432 0.29 2.54 1.96 0.69 1.27 0.68 0.75 23.7 Victoria Reservoir, Sri Lanka Amarasinghe et al. (2017) O. mossambicus 262 1.1 4.79 4.42 0.99 3.44 0.78 0.368 149.5 Vembanad lake, India Present study E. suratensis 302 0.68 4.87 4.04 0.69 3.35 0.83 0.386 153.3 Vembanad lake, India Present study L∞, asymptotic length (mm); K, growth coefficient (year-1); , growth performance index; Z, total mortality (year-1); M, natural mortality (year- 1); F, fishing mortality (year-1); E, current exploitation rate, E50, exploitation rate where stock is reduced to half its virgin biomass; Lc, length at first capture (mm). Table 3. Growth, mortality and exploitation parameters of Oreochromis mossambicus and Etroplus suratensis from various geographical regions. 26 Roshni and Renjithkumar / Population dynamics of two sympatric native and exotic cichlids E. suratensis for implementing proper conservation plans. Reward-based encouragement to the local fishers for collecting large quantities of O. mossambicus, detailed scientific research on the impacts on native species, and the development of novel eradication measures are also required for managing the fast-growing population of O. mossambicus. Acknowledgments The authors are thankful to the Head, Department of Aquatic Environment Management, KUFOS, Kochi, India, for providing the necessary facilities for the research. We express our sincere gratitude to Rajeev Raghavan, Assistant Professor, KUFOS, Kochi, India, for his constructive comments on the draft manuscript. The first author thanks the Kerala State Council for Science Technology and Environment (KSCSTE), India, for the financial support to carry out the research. References Abraham R., de Alwis Goonatilake S., Fernado M, Kotagama O. (2019). Etroplus suratensis. In: The IUCN Red List of Threatened Species 2019. e.T172368A60612143. (International Union for Conservation of Nature and Natural Resources). Retrieved from https://www.iucnredlist.org/species /172368/60612143 (Accessed on 10 June 2022). Allen G.R., Midgley S.H., Allen M. (2002). Field guide to the freshwater fishes of Australia, Western Australian Museum: Perth, Western Australia. Allan J.D., Abell R., Hogan Z., Revenga C., Taylor B.W., Welcomme R.L., Winemiller K. (2005).Overfishing of inland waters. Bio Science 55(12): 1041-1051. Amarasinghe U.S. (2002). The fishery and population dynamics of Oreochromis mossambicus and Oreochromis niloticus (Osteichthyes, Cichlidae) in a shallow irrigation reservoir in Sri Lanka. Asian Fisheries Science, 15(1): 7-20. Amarasinghe U.S., De Silva S.S. (1992). Population dynamics of Oreochromis mossambicus and Oreochromis niloticus (Cichlidae) in two reservoirs of Sri Lanka. Asian Fisheries Science, 5: 37-61. Amarasinghe US., Jayasinghe R.P.P.K., Moreau J. (2017). Length-based stock assessment of Oreochromis mossambicus and Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae) in multi- mesh gillnet fisheries in reservoirs of Sri Lanka. Acta Ichthyologica Piscatoria, 47(3): 265-277. Baensch H.A., Riehl R. (1985). Aquarien atlas. Band 2. Megus, Velag fur Natur-und Heimtierkunde GmbH: Melle, Germany. Bijukumar A., Raj S., Arjun A., Katwate U., Raghavan R. (2019). Jurassic invaders: flood-associated occurrence of arapaima and alligator gar in the rivers of Kerala. Current Science, 116: 1629-1630. Bindu L., Padmakumar K.G. (2008). Food of the pearlspot Etroplus suratensis (Bloch) in the Vembanad Lake, Kerala. Journal of Marine Biological Association of India, 50(2): 156-160. Bindu L., Padmakumar K.G. (2014). Reproductive biology of Etroplus suratensis (Bloch) from the Vembanad wetland system, Kerala. Indian Journal of Geo Marine Science, 43: 646-654. Canonico G.C., Arthington A., McCrary J.K., Thieme M.L. (2005). The effects of introduced tilapias on native biodiversity. Aquatic Conservation: Marine and Fresh Water, 15(5): 463-483. Caskey L.M., Riedel R.R., Costa-Pierce B.A., Butler J, Hurlbert S.H. (2007). Population dynamics, growth, and distribution of tilapia (Oreochromis mossambicus) in the Salton Sea, 1999-2002, with notes on orange mouth corvina (Cynoscion xanthulus) and bairdiella (Bairdiella icistia). Hydrobiologia, 576: 185-203. Courtenay W.R Jr., Robins C.R. (1989). Fish introductions: good management, mismanagement or no management? Reviews in Aquatic Sciences, 1(1): 159-172. de Silva S.S., Amarasinghe U.S. (1989). Stunting in Oreochromis mossambicus (Peters) (Pisces, Cichlidae): An evaluation of past and recent data from Sri Lankan reservoir populations. Journal of Applied Ichthyology, 5(4): 203-210. de Silva S.S., Moreau J., Senaratne K.A.D. (1988). Growth of Oreochromis mossambicus (Pisces: Cichlidae) as evidence of its adaptability to Sri Lankan Reservoir. Asian Fisheries Science, 1: 147-156 Froese R., Pauly D. (2019). FishBase World Wide Web electronic publication. Retrieved from www.fishbase. org (Accessed on 12 July 2022). Gayanilo F.C., Sparre P., Pauly D. (2005). FAO- ICLARM Stock Assessment Tools II (FiSAT II). 27 Int. J. Aquat. Biol. (2023) 11(1): 20-29 Revised version User Guide FAO Computerised information Series (Fisheries). Vol 8 Rome, Italy. Glamuzina B., Dulčić J., Conides A., Bartulović V., Matić-Skoko S., Papaconstantinou C. (2007). Some biological parameters of the thin-lipped mullet Liza ramanda (Pisces, Mugilidae) in the Neretva River delta (Eastern Adriatic, Croatian coast). Vie et milieu- Life Environment, 57(3): 7-13. Global Invasive Species Database. (2019). Species profile: Oreochromis mossambicus (IUCN Invasive Species Specialist Group). Available at http://www. iucngisd.org/gisd/speciesname/Oreochromis+mossa mbicus (Accessed on 8 August 2021). Gosavi S.M., Kharat S.S., Kumkar P., Tapkir S.D. (2019). Assessing the sustainability of lepidophagous catfish, Pachypterus khavalchor (Kulkarni, 1952) from a tropical river Pachaganga, Maharashtra, India. Journal of Basic and Applied Zoology, 80(9): 1-10. Gu D., Ma G., Zhu Y., Xu M., Luo D., Li Y., Wei H., Mu X., Luo J., Hu Y. (2015). The impacts of invasive Nile tilapia (Oreochromis niloticus) on the fisheries in the main rivers of Guangdong Province, China. Biochemical Systematics and Ecology, 59: 1-7. Gulland J.A. (1970). The fish resources of the ocean, FAO Fisheries Technical Paper, No. 97 Food and Agriculture Organization of the United Nations, Rome, Italy. Gwinn D.C., Allen M.S., Johnston F.D., Brown P., Todd C.R., Arlinghaus R. (2015). Rethinking length-based fisheries regulations: The value of protecting old and large fish with harvest slots. Fish and Fisheries, 16(2): 259-281. Jones B.L., Unsworth R.K.F., Udagedara S., Cullen- Unsworth L.C. (2018). Conservation Concerns of Small-Scale Fisheries: By-Catch Impacts of a Shrimp and Finfish Fishery in a Sri Lankan Lagoon. Frontier in Marine Science, 5: 52. Krishnakumar K., Ali A., Pereira B., Raghavan R. (2011). Unregulated aquaculture and invasive alien species: a case study of the African Catfish, Clarias gariepinus in Vembanad Lake (Ramsar wetland), Kerala, India. Journal of Threatened Taxa, 3(5): 1737-1744. Kurup B.M., Sebastian M.J., Sankaran T.M., Rabindranath P. (1993). Exploited Fishery Resources of the Vembanad Lake. Indian Journal of Fisheries, 40: 199-206 Kurup B.M., Sebastian M.J., Sankaran T.M., Rabindranath P. (1995). Exploited fishery resources of the Vembanad Lake. Estimates of marketable surplus of production. Journal of Marine Biological Association of India, 37(1&2): 1-10. Lowe S., Browne M., Boudjelas S., De Poorter M. (2000). 100 of the World’s worst invasive alien species - a selection from the global invasive species database, invasive species specialist group (ISSG) a specialist group of the Species Survival Commission (SSC) of the World Conservation Union (IUCN), New Zealand. Maddern M.G., Morgan D.L., Gill H.S. (2007). Distribution, diet and potential ecological impacts of the introduced Mozambique mouth brooder Oreochromis mossambicus Peters (Pisces: Cichlidae) in Western Australia. Journal of Royal Society of West Australia, 90(4): 203-214. Maitipe P., de Silva S.S. (1984). Switches between zoophagy, phytophagy and detritivory of Sarotherodon mossambicus (Peters) adult populations in twelve man-made Sri Lankan lakes. Journal of Fish Biology, 26(1): 49-61. Maitra S., Harikrishnan M., Shibu A.V., Sureshkumar S., Ranjeet K., Nandan S.B. (2018). Studies on temporal variations of exploited fishery resources and their trophic levels in a tropical estuary. Regional Studies in Marine Science, 22: 61-69. Martin G.D., Nisha P.A., Balachandran K.K., Madhu N.V., Nair M., Shaiju P., Joseph T., Srinivas K., Gupta G.V.M. (2011). Eutrophication induced changes in benthic community structure of a flow-restricted tropical estuary (Cochin backwaters), India. Environment Monitoring and Assessment 176(1): 427-438. Menon A.G.K. (1999). Check list - freshwater fishes of India. Records of the Zoological Survey of India, Miscellaneous Publication, Occasional Paper No. 175, Kolkata, India. 175 p. Moreau J., Bambino C., Pauly D. (1986). Indices of overall fish growth performance of 100 tilapia (Cichlidae) populations, In: L. Maclean, L.B. Dizon, L.V. Hosillos (Eds.), Proceedings of the First Asian Fisheries Forum, Asian Fisheries Society: Manila, Philippines. pp: 201-206. Munro I.S.R. (1955). The marine and freshwater fishes of Ceylon, Department of External Affairs: Canberra. 351 p. Munro J.L., Pauly D. (1983). A simple method for comparing the growth of fishes and invertebrates. ICLARM Fish byte, 1(1): 5-6. 28 Roshni and Renjithkumar / Population dynamics of two sympatric native and exotic cichlids Nyman L. (1991). Conservation of freshwater fish. Protection of biodiversity and genetic variability in aquatic ecosystem, Fisheries Development Series 56, Swedmar and WWF: Sweden. 38 p. Odum E.P. (1971). Fundamentals of Ecology, Saunders: Philadelphia. 602 p. Padmakumar K.G., Bindu L., Manu P.S. (2009). Captive breeding and seed production of Etroplus suratensis in controlled systems. Asian Fisheries Science, 22(1): 51-60. Padmakumar K.G., Bindhu B., Manu P.S. (2012). Etroplus suratensis (Bloch), the state fish of Kerala. Journal of Bioscience, 37(1): 925-931. Panda D., Mohanty S.K., Pattnaik A.K., Das S., Karna S.K. (2018). Growth, mortality and stock status of mullets (Mugilidae) in Chilika Lake, India. Lakes & Reservoirs: Research & Management, 23(1): 4-16. Pauly D. (1980). On the interrelationships between natural mortality, growth parameters and mean environmental temperature in 175 fish stocks. ICES Journal of Marine Science, 39(20): 175-192. Pauly D. (1984). Fish population dynamics in tropical water, a manual for use with programmable calculators, ICLARM Studies and Reviews, International Centre for Aquatic Living Resources Management: Manila, Philippines. Pauly D., Soriano M.L. (1986). Some practical extensions to Beverton and Holt’s relative yield-per-recruit model. In: Maclean J.L., Dizon L.B., Hosillos L.V. (Eds.), The First Asian Fisheries Forum, Asian Fisheries Society: Manila, Philippines. pp: 491-496. Pérez J.E., Alfonsi C., Nirchio M., Barrios J. (2006). The inbreeding paradox in invasive species. Interciencia, 31(7): 544-546. Pinsky M.L, Byler D. (2015). Fishing, fast growth, and climate variability increase the risk of collapse. Proceedings of the Royal Society B- Biological Science, 282(1813): 20151053. Raghavan R., Prasad G., Ali A., Pereira B. (2008). Exotic fishes in a global biodiversity hotspot-a case study from River Chalakudy, part of Western Ghats, Kerala, India. Biological Invasion, 10(1): 37-40. Roshni K., Renjithkumar C.R., Kurup B.M. (2014). Record of a newly introduced fish, red-bellied pacu Piaractus brachypomus (Cuvier, 1818) (Characiformes, Serrasalmidae), in a tropical wetland system, India. Journal of Applied Ichthyology, 30(5): 1037-1038. Roshni K., Renjithkumar C.R., Kurup B.M. (2017). The down turn of state fish, Etroplus surstensis in Vembanad lake- A Ramsar site In Proceeding- Perspectives on Biodiversity in India. Volume III, In: P.N. Krishnan, S. Jayakumar, A. Bijukumar, C.K. Peethambaran, K.G. Ajithkumar, E.K. Eswaran, C. Sureshkumar, P.K. Priya (Eds.), CISSA, Thiruvanthapuram, India. pp: 352-354. Russell D.J., Thuesen P.A., Thomson F.E. (2012). A review of the biology, ecology, distribution and control of Mozambique tilapia, Oreochromis mossambicus (Peters 1852) (Pisces: Cichlidae) with particular emphasis on invasive Australian populations. Review in Fish Biology and Fisheries, 22(3): 533-554. Sato M., Kawaguchi Y., Nakajima J., Mukai T., Shimatani Y., Onikura N. (2010). A review of the research on introduced freshwater fishes: New perspectives, the need for research and management implications. Landscape Ecology and Engineering, 6(1): 99-108. Skelton P.H. (1993). A complete guide to the freshwater fishes of southern Africa, Southern Book Publishers: Halfway House, South Africa. 388 p. Samuel C.T. (1969). Problems and prospects of the estuarine fisheries of Kerala, First All India Symposium on Estuarine Biology, Tambaram, Madras. Stapp P., Hayward G.D. (2002). Effects of an introduced piscivore on native trout: insights from a demographic model. Biological Invasions, 4(3): 299-316. Van Overzee H.M.J., Rijnsdorp A.D. (2015). Effects of fishing during the spawning period: Implications for sustainable management. Review in Fish Biology and Fisheries, 25(1): 65-83. Von Bertalanffy L. (1938). A quantitative theory of organic growth. Human Biology, 10(2):181-213. Ward J.A, Samarakoon J.I. (1981). Reproductive tactics of the Asian cichlids of the genus Etroplus in Sri Lanka. Environment Biology of Fishes, 6(1): 95-103. Welcomme R.L. (1988). International introductions of inland aquatic species. FAO Fisheries Technical., Rome, Italy. 294 p. Weyl O.L.F., Hecht T. (1998). The biology of Tilapia rendalli and Oreochromis mossambicus (Pisces: Cichlidae) in a subtropical lake in Mozambique. South African Journal of Zoology, 33(3): 178-188. Wohlfarth G.W., Hulata, G. (1983). Applied genetics of 29 Int. J. Aquat. Biol. (2023) 11(1): 20-29 tilapias. ICLARM Studies and Reviews. 2nd Edn, International Centre for Living Aquatic Resource Management: Manila, Philippines. 26 p. Xia Y., Zhao W., Xie Y., Xue H., Li J., Li Y., Chen W., Huang Y., Li X. (2019). Ecological and economic impacts of exotic fish species on fisheries in the Pearl River basin. Management of Biological Invasion, 10(1): 127-138.