Int. J. Aquat. Biol. (2020) 8(1): 56-65

ISSN: 2322-5270; P-ISSN: 2383-0956

Journal homepage: www.ij-aquaticbiology.com

Original Article
Growth and mortality parameters of Caspian kutum, Rutilus kutum, in southern Caspian

Sea

Reza Shahifar1, Rahman Patimar*1, Hasan Fazli2, Hadi Raeisi1, Mohammad Gholizadeh1, Hojjatallah Jafaryan1

1Department of Fisheries, Faculty of Agriculture and Natural Resources, Gonbad Kavoos University, Gonbad Kavoos, Iran.

2Caspian Sea Ecology Research Center, Sari, Iran.

Article history:
Received 7 July 2019

Accepted 11 August 2019

Available online 2 5 February 2020

Keywords:
Kutum

L-infinity

Total mortality

Natural mortality

Caspian Sea

Abstract: The Caspian Kutum, Rutilus frisii, is one of the endemic and most important commercial
cyprinid species in the southern Caspian Sea. A detailed study on growth and mortality parameters

of this species was conducted based on 700 samples collected from commercial catches of beach

seining in Guilan and Mazanderan provinces during fishing season 2017-2018. Females were

dominated in both studied populations. Size frequency distributions showed significant variation

among same sexes and between different sexes. The different WLRs were observed, positive

allometric in Mazanderan, and negative allometric in Guilan. There were significant differences in

growth parameters between sexes, females were of much greater asymptotic length than males, while

the male fish had a higher growth rate and attained a smaller theoretical L∞ size than females. The
theoretical maximum length (L∞) was larger than the maximum one recorded during sampling. Based
on the Bhattacharya method, the Caspian kutum from Guilan fishing grounds was more diverse, and

included nine cohorts, while the population from Mazanderan Province showed only six cohorts. The

linearized catch curve based on age composition data showed that total mortality rates (Z) are 1.32
year-1 and 0.63 year-1 for males and females of Guilan, respectively, that of males in Mazanderan is

1.04 year-1 and of females 0.86 year-1. The natural mortality rates (M) were 0.48 year-1 for males and
0.26 year-1 females in Guilan, and was found to be as 0.26 year-1 and 0.45 year-1 for males and females

of Caspian kutum caught in Mazanderan. The exploitation ratio (E) was found to be higher than 0.5
for both sexes from Guilan, and to be lower than the expected optimum level of exploitation in

Caspian kutum males and females caught in Mazanderan.

Introduction

The ichthyofauna of the southern Caspian Sea basin

includes 119 species belong to 63 genera (Esmaeili et

al., 2014, 2018). Caspian kutum, Rutilus kutum, is
endemic to this basin occurring mainly in the southern

coast of the Caspian Sea with a distribution extending

from the Terek River in the north to the Gorgan Bay

in the southeastern region. This species is among of

the most important commercial species for fisheries

and stock enhancement programs in the Iranian

Caspian coast (Kiabi et al., 1999; Farhang and

Eagderi, 2019). Despite intensive investigations on

Caspian kutum in the context of aquaculture and

ecotoxicology, few publications are available on its

growth parameters (Abdolmaleki et al., 2007; Hoseini

*Correspondence: Rahman Patimar DOI: https://doi.org/10.22034/ijab.v8i1.677

E-mail: rpatimar@yahoo.com

et al., 2010; Aghili and Mohamadi, 2011; Fazli et al.,

2013). However, no paper has so far considered the

variation in growth of different populations of this

species on a relatively broad regional scale.

Fish growth is an indeterminate plastic process that

can change considerably among populations of a

species in its distribution areas. Furthermore, growth

parameters are not only required input to several stock

assessment methods, but also allow tests of life-

history hypothesis (Stergiou, 2000). Therefore,

verification of local values and growth variability

within species (considered as inter-population

variations) has great importance in fisheries ecology

and fish population dynamics. Hence, this study aimed

to investigate in greater detail growth aspects of

Int. J. Aquat. Biol. (2020) 8(1): 56-65

Caspian kutum from two main fishery areas, to

increase our knowledge on the variability of its growth

parameters. The results could be used for better and

effective management practices of exploiting the

species stock in this basin.

Materials and Methods

The samples were collected from commercial catches

in fishing grounds of Guilan and Mazanderan

provinces during 2017-2018 fishing season i.e. fall

and winter. However, the size range did not include

individuals smaller than the size at first capture

because they were not selected by the commercial

fishing gears. The fishing operations in these areas are

conducted using huge beach seine with mesh sizes of

26–32 mm. Then, the collected specimens were

transported to the laboratory, where their fork length

(FL; ±0.1 mm) and total weight (W; ±0.1 g) were

recorded. All specimens were dissected to determine

their sex by direct observation of the gonads. In

addition, 5-10 scales of each individual were removed

from the area just above the lateral line and mounted

between two glass slides for age estimation. Two

readers interpreted the growth marks on scales without

prior knowledge of length, weight or sex of the fish,

its date of capture or the previous reading to avoid

reading bias. Only coincident readings were accepted.

To describe size structures of the populations

studied, histograms of size-frequency distributions of

the Caspian kutums were made, using 4 cm FL

intervals. Two-sample Kolmogorov–Smirnov (K–S)

tests (α=0.05) were used to compare size frequency

distributions between sexes (Sokal and Rohlf, 1981).

Estimation of the Weight and length relationship was

made by adjustment of an exponential curve to the

data (Ricker, 1975): W = aLb, where W is the total
weight (g), L the fork length (cm), a the intercept
(initial growth coefficient or condition factor) and b
the slope (allometric exponent). The relationship

coefficients were calculated using non-linear least

squares estimation. Student’s t-test was applied to

determine the significance of differences between the

isometric growth and the estimated b-value of the
equation. Additionally, an analysis of covariance was

used to compare FL–W relationships between sexes

(Keivany et al., 2016).

Estimates of theoretical growth in length were

obtained by fitting the von Bertalanffy growth

function (VGBF) to the mean length at age data

separately for males and females by non-linear

regression: Lt= L∞[1-e-k(t-t0)] using the method of
Gulland-Holt (Everhart and Youngs, 1975), where Lt
is the length at age t, L∞ is the asymptotic length, k is
the growth coefficient, and t0 is the hypothetical time
when the fish total length is zero. The overall growth

performance index (Øʹ) was calculated based on the

growth parameter estimates by the equation of Munro

and Pauly (1983): Øʹ= log10k+ 2 log10 L∞. The index

was used to compare growth parameters obtained in

this study with those reported by other authors. In

addition, the weight-based of VBGF calculated as

Wt=W∞[1-e-k(t-t0)]b, where Wt is the weight of the fish
in g at age t, W∞ is the asymptotic weight of the fish
in g and b is the constant in the length–weight
relationship (Ricker, 1975; Sparre and Venema,

1992).

Monthly length-frequency data was analysed using

the package FISAT model progression analysis, and

Bhattacharya’s (1967) method subroutine (Sparre et

al., 1989; Gayanilo et al., 1994) for to identify the

modes in the polymodal length-frequency

distributions of Caspian kutum cohorts. The total

mortality rate (Z) for each sex of population was
estimated using Powell Weatherall plot, where the

regression equation has the form ln(N) = a + bt′, where
N is the number of fish in cohorts by means of
successive growth curves, t′ is the relative age of the
fish in that cohort, and b with the sign changed
provides an estimate of Z (Powell, 1979; Wetherall et
al., 1987). To obtain an independent estimate of

natural mortality rate (M), the Pauly’s empirical
equation (1980) log(M) = -0.0066-0.279 log(L∞) +
0.6543 log(K) + 0.4634 log(T) was employed. Here,
T is mean annual habitat temperature in southern
Caspian Sea and L∞ is expressed in cm.

Results

A total of 700 Caspian kutum were collected from

Shahifar et al./ Growth and mortality parameters of Caspian kutum

commercial catches in the Guilan (n=310) and

Mazanderan (n=390) provinces. The number of

females sampled was statistically more than the

number of males sampled (χ2=46.27, P<0.05). In both
sampling areas, the sex ratio was almost the same

(1:1.7) in favoure of females (Guilan: χ2=19.6,

Mazanderan: χ2=26.7, P<0.05). Size frequency
distributions showed significant variation among

same sexes and between different sexes (K–S test,

P<0.05) (Fig. 1).
The length–weight relationships (WLRs) of males

and females in both populations are given in Figure 2.

The WLRs for females and males were significantly

different (P<0.05). Therefore, the female and male
data cannot be pooled. Different types of growth were

observed: positive allometric in males and females

from Mazanderan, and negative allometric in both

sexes of Guilan (t-test, P<0.05).
Table 1 summarizes The von Bertalanffy growth

parameters and mortality rates estimated from our data

set. There were significant differences in growth

parameters between sexes, females were of much

greater asymptotic length than males, while the male

fish had a higher growth rate and attained a smaller

theoretical L∞ size than females. The theoretical
maximum length (L∞) was larger than the maximum
one recorded during sampling and no samples reached

or were longer than the theoretical value of calculated

for each sex. The plots of von Bertalanffy growth

functions for males and females fitted to the observed

length and weight-at-age data of each sex and year

separately show that the growth patterns of females

Figure 1. Relative proportion (%) of Caspian kutum males and females from southern Caspian Sea (2017-2018).

Table 1. Growth parameters of the Caspian kutum from southern Caspian Sea (2017-2018).

Location Sex Year W∞(gr) L∞(FL,cm) k(year-1) t0(year) Z M

Guilan Male 2017-2018 2053.41 60.74 0.20 -0.68 1.32 0.48

Guilan Female 2017-2018 4356.91 69.12 0.14 -0.99 0.63 0.26

Mazanderan Male 2017-2018 1610.10 52.77 0.36 -0.39 1.04 0.26

Mazanderan Female 2017-2018 2396.10 69.12 0.14 -0.99 0.86 0.45

Int. J. Aquat. Biol. (2020) 8(1): 56-65

and males are similar in younger ages, thereafter;

females grew to be larger than males (Figs. 3, 4).

The Bhattacharya method showed different cohorts

in commercial catch of Caspian kutum in the southern

area (Fig. 5). The Caspian kutum from Guilan fishing

grounds was more diverse, and included nine cohorts,

while the population from Mazanderan Province

showed only six cohorts. The linearized catch curve

Figure 2. Fork length–weight relationship for combined sexes, as well as male and female Caspian kutum, southern Caspian Sea (2017-2018).

Figure 3. Length-age based von Bertalanffy growth model for the Caspian kutum sampled in southern Caspian Sea (2017-2018).

Shahifar et al./ Growth and mortality parameters of Caspian kutum

based on age composition data, corresponding to the

slope of the descending limb of the catch curve, is

presented in Figure 6. Estimated instantaneous total

mortality rates (Z) were 1.32 year-1 and 0.63 year-1 for
males and females of Guilan, respectively, that of

males in Mazanderan was 1.04 year-1 and of females

0.86 year-1. Instantaneous natural mortality rates (M)
using the equation of Pauly (1980) was 0.48 year-1 for

males and 0.26 year-1 females in Guilan. The M
parameter was found to be as 0.54 year-1 and 0.45

year-1 for males and females caught in fishing ground

of Mazanderan. The exploitation ratio (E) was found

Figure 4. Weight-age based von Bertalanffy growth model for the Caspian kutum sampled in southern Caspian Sea (2017-2018).

Figure 5. The sequences of operation in the use of the Bhattacharya method of separating a length–frequency distribution into normal components

for Caspian kutum from southern Caspian Sea (2017-2018).

Int. J. Aquat. Biol. (2020) 8(1): 56-65

to be higher than 0.5 for both sexes of Caspian kutum

from Guilan, and to be lower than the expected

optimum level of exploitation (E=0.50) in Caspian
kutum males and females caught in Mazanderan,

indicating that Caspian kutum suffered great fishing

pressure in Guilan sea areas, and fishing pressures on

the Caspian kutum in Mazanderan sea areas have been

relatively low.

Discussions

As a comprehensive study on growth parameters of

Caspian kutum in the southern Caspian Sea can still

be considered as limited to date, the present study

provides a more detailed insight into the information

on growth of this species. Estimates of demographic

growth parameters are important in a broader context

as they provide a fundamental description of the life

history characteristics. The determination of age and

growth is of great importance to both fisheries biology

and management as it forms the basic knowledge

required for the estimation of mortality, recruitment

and yield (Mehanna, 1996).

The size frequency used in the present study, on

which growth and mortality estimates depended, were

not based on a scientific sampling schedule but rather

on random subsamples taken from commercial

catches. Fishing gear selectivity is an important

consideration in studies determining demographic

characteristics of fish populations. Here, the nets used

in the beach‐seine fishery were selective for the

Caspian kutum. Furthermore, the catching this species

in the study area was seasonal, which was not caught

year-round (Hasanpour et al., 2016). Therefore, this

study lacks data some lengths and ages. Irrespective of

this selectivity, this study was able to develop growth

protocols, provide representative population

parameters and mortality rates of this important

exploited species. In this study, specimens were

successfully aged using scales, shown to be

problematic for some Caspian cyprinids in previous

studies. In general, scales were hard to read due to the

existence of numerous annuli. This is relatively often

an important source of error in ageing Cyprinids. It is

unlikely that our scale ages represent the true age of

the Caspian kutum, despite the fairly precise age

estimates achieved by multiple readers using scales.

The two data sets used to assess the Caspian kutum

populations’ growth variability in the present study

were collected approximately within the same time

period, and we assume that the pool specimens’ data

Figure 6. Linearized catch curve of the Caspian kutum and the regression equation obtained- southern Caspian Sea (2017-2018).

Shahifar et al./ Growth and mortality parameters of Caspian kutum

represents the average stock composition. Although it

is difficult to compare size and length data among the

various studies because of seasonal differences and

methods of collection, length frequency analysis

applied to the Caspian kutum from the study areas

revealed that the stocks of the species consist of

different sizes groups. This might be explained by the

fact probably stemming from fishery selections on the

populations.

Length-weight relationships provided a good fit to

data for the Caspian kutum, while there was a great

deal of individual variability for the populations. This

variability could be attributed to greater differences in

size selectivity and/or condition factors among

individuals of this species. The length–weight

relationship equations had b-values significantly
differ from “3” reflecting an allometric mode of

growth for the Caspian kutum. The allometry

coefficient of the length-weight relationship shows

positive allometric growth for the population from

Mazanderan (>3) and negative mode for the Guilan

(<3) populations. However, the b-values lie still
between the expected range of 2.5–3.5 (Bagenal and

Tesch, 1978). The estimated exponent value for

female population from Guilan and male population

from Mazanderan shows that the fish body is close to

the isometric growth model, i.e. body weight increases

approximately three times faster than body length.

According to Froese (2006), if b=3, then small

specimens in the sample under consideration have the

same form and condition as the large specimens.

Patimar et al. (2009) noted that a variation in b-values
among populations can be affected by the geographic

location as well as the environmental conditions. The

difference in the exponents of Caspian kutum could be

attributed to the different sampling areas as well as the

differences in number of specimens and length ranges

of the populations belong to distinct regions (Patimar

et al., 2009).

The present b-value of Caspian kutum is more or
less than those reported by previous studies;

Abdolmaleki et al. (2007) gave b=3.04, while in
Aghili and Mohammadi (2011), 2.96 in the

southeastern Caspian Sea and Fazli et al. (2012)

reported 3.12 for this species, which all differ from the

present ones. These differences in b-values could
probably be explained by the catches being through

several different seasons or they could be attributed to

different size distribution, as well as to differences in

age composition in different years.

Estimates of von Bertalanffy (VBGF) growth

parameters indicated that females reach a larger

asymptotic size than males. The larger asymptotic

length for females could be attributed to their

relatively longer longevity and greater absolute

growth rate. The longer lifespan and larger size of

females could be considered a life history strategy for

supporting increasing egg production (Roff, 1983).

The values of L∞ which are lower than the maximum
FL should be accepted considering that no male and

female longer than these asymptotic sizes were

caught. The estimated VBGF parameters show

significant differences between sexes and areas as

well, which can be attributed to inter- and intra-

population variation of Caspian kutum. Growth

parameters (L∞, K and t0) are the basic input data into
various models used for managing and assessment the

status of the exploited fish stocks, these parameters

facilitate the comparison between growth of

populations belonging to the same species at different

times and different localities. Comparison of VBGF

parameters from different populations in the southern

Caspian Sea, despite differences in methodology and

data quality, showed that the growth patterns were

different. In the previous studies, growth parameters

estimated for combined genders of Caspian kutum

were L∞=70.1 cm FL, k=0.38 per year, and t0=-1.56
years (Ghaninezhad et al., 2004), L∞=60.70 cm FL,
k=0.15 per year, and t0=-1.75 years (Abdolmaleki et
al., 2007), and L∞=58.00 cm FL, k=0.24 per year, and
t0=-0.16 years (Hosseini et al., 2010). The difference
is may be due to the difference in the ecological

parameters in different localities, or the maximum

observed lengths in the catch or the methods used in

calculations by different authors. We did not find any

studies describing weight-based growth parameters of

von Bertalanffy for the Caspian kutum. Because of the

lack of this kind of data, the parameter W∞ might not

Int. J. Aquat. Biol. (2020) 8(1): 56-65

be well-discussed in this study.

For fishes that grow according to the von

Bertalanffy function, the VBGF parameters cannot be

directly used to compare growth rates among

populations, there is practical problem that none of its

parameters have the dimensions of growth. The

growth performance index (Øʹ) emerges as an

alternative (Munro and Pauly, 1983; Pauly and

Munro, 1984). The relation between (ln)k and (ln)L∞
turns out to be a consequence of the so called Beverton

and Holt dimensionless life-history invariants

(Beverton and Holt, 1959; Beverton, 1992; Charnov,

1993), which allows the comparison of growth

performance as represented by the Øʹ index (Munro

and Pauly, 1983) of stocks of the same species. It also

represents a potential check for the accuracy of growth

parameter estimates. Comparison of Øʹ calculated

using reported VBGF parameters revealed that small

difference is in this index among populations, ranging

from 6.3 to 7.5. This confirms the accuracy of the

calculations, and different VBGF parameters seem to

indicate differences in dynamics status of the Caspian

kutum populations.

Higher total mortality (Z) and natural mortality (M)

for males suggest that the higher survival rate of

females may be due to mechanisms developed for

perpetuation of the species. Due to direct fishing

interest, Caspian kutum is an important species in

terms of fisheries management in southern Caspian

Sea. Therefore, large difference between the estimates

of mortality rates for different populations in the basin

indicates a different and variable natural and fishing

mortalities. The total and natural mortality rates of

Caspian kutum estimated in the present study were

larger than those estimated previously in the southern

Caspian Sea (Z=0.83 per year and M=0.31 per year for

sexes combined population) (Abdolmaleki et al.,

2007). This could potentially be due to a combination

of factors including (i) the previous estimates were

made potentially prior to any significant fishery-

associated impacts, and (ii) the effects of sustained

levels of harvesting of the species in our study area.

Nevertheless, the values of M seem reasonable,
considering the biological features of Caspian kutum.

The same species may have different natural mortality

rates in different areas (Sparre et al., 1989).

The estimate of the exploitation rate (E) indicates a

relatively low fishing pressure in Mazanderan fishing

areas; this should be treated with caution as mortality

rates were estimated from a single sampling season

and may be biased due to annual differences in year-

class strength. It is generally recommended that

assessment studies incorporate a range of mortality

estimation methods as a precautionary approach and

to improve certainty, particularly in data-poor

fisheries as studied here. In Guilan fishing grounds, at

the same time, the exploitation rate (E) of the Caspian

kutum exceeded an optimally exploited value

(Gulland, 1971), indicating that the stocks had been

overfishing. Therefore, an urgent fisheries

management response is required o prevent the

collapse of the Caspian kutum stocks in Guilan fishing

areas and to achieve sustainability. In addition, these

results could be underestimated because of the lack of

illegal catch information and other gear in current use,

which may include a significant portion of undersized

Caspian kutum.

In conclusion, the present study provides new

information about mortality and growth parameters of

Caspian kutum, needed for stock assessment and

management. However, further research is necessary

to elucidate if the relatively high degree of variability

found in some of growth parameters could represent

either adaptations to local selective pressures (both

fishery and environmental) or ecophenotypic

variations. The estimated Caspian kutum population

parameters indicated a trend toward its being a k‐
strategist species based on high maximum theoretical

length, low growth rate, long lifespan and low natural

mortality. This indicates that the regulation strategies

developed for the fishery management should be

based on the Caspian kutum’s different growth

patterns.

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