The great sturgeon
This fish is common in commercial catches from the Iranian coastal waters of the Caspian Sea, accounting for less than 40% of the total sturgeon catch (WCMC 2010). Sturgeons tend to have a vulnerable and long life cycle, maturing at an advanced age (10 to 18 years) and spawning every 3 or 5 years with a generation duration of 20–25 years (Onara et al. 2014). Males spawn more intensively in a shorter time than females and reach sexual maturity 2–3 years before females (Holčík 1989). Males are caught more frequently in the inshore and estuarine waters because they spend more time reproducing in these habitats than females.
At present, there are many problems that affect the sustainable management of sturgeon populations from the Iranian coastal waters of the Caspian Sea. Therefore, different models of population dynamics are required to reflect the status of sturgeon populations, the consequences of overexploitation and aspects related to environmental pressure (Karayev 2006; Bakhshalizadeh et al. 2020). New approaches have emerged that combine aspects of ecosystem analysis and cognitive theory, such as those developed by Karayev (2006). The beluga fishery in the Caspian Sea is maintained through a stock enhancement program (Ustaoglu & Okumus 2004). In other areas (Danube River), beluga populations were found to continue to decline, showing long-term changes in their structure and size (Rosten 2012; Sandu et al. 2013). The Islamic Republic of Iran uses the catch per unit effort (CPUE) to asses the
The great sturgeon,
After all samples were sacrificed, gutted weight (W) in kg and fork length (FL) in cm were measured for each fish. The sex and the development stage were determined by the examination of gonads. Right pectoral fin spines were removed for the purpose of age analysis (Rien & Beamesderfer 1994).
Preparation and interpretation of fin spine sections were carried out according to the procedure used for sturgeons by Bakhshalizadeh et al. (2011). Images were used to estimate ring counts in the end spine sections. The age of all samples (194) was estimated by direct reading and estimates from the images. The variability of within-reader age estimates was calculated using the absolute percentage error (APE) between ring counts according to Beamish & Fournier (1981). To determine differences and precision of the estimated age, we used the T test and the coefficient of variation (CV), respectively, as indices of precision (Chang 1982). Differences in frequencies in the age distribution were determined by the Chi-square test of independence.
The length–weight relationship was analyzed by the function
The following formula was used (Gulland 1983) to estimate the annual instantaneous total mortality coefficient (
Statistical analyses and plotting of the data were performed using the STATISTICA (Version 5.5), Sigma plot (Version 2000) and Excel (Version 2016) software packages.
A total of 194 individuals were sampled, including 111 males and 83 females. The sex ratio of 1:0.75 was significantly different from the 1:1 ratio (χ2 = 1.5, n = 105). The mean
Descriptive length and weight composition of male and female great sturgeon from the Iranian waters of the Caspian Sea in 2008–2010
Size | Sex | Number | Mean | Standard error of the mean | Minimum | Maximum |
---|---|---|---|---|---|---|
Fork Length (cm) | Female | 33 | 214.17 | 7.06 | 84 | 305 |
Male | 24 | 194.17 | 8.10 | 84 | 255 | |
Weight (kg) | Female | 33 | 111.26 | 10.70 | 4 | 287 |
Male | 24 | 69.77 | 6.72 | 4 | 141 |
The relationship between length and weight was expressed for each sex using a power regression (Fig. 2). The data revealed that only females were characterized by positive allometric growth. The effect of sex on the length–weight relationships was significant (ANCOVA, n = 105,
To compare the age of the great sturgeon, the results obtained by direct reading and imaging were compared, finding no significant differences between the two techniques and the APE was 0.95%. During the study period, a mean of 21 and 20 years was determined with an interval of 5 to 30 years and 5 to 25 years for females and males, respectively (Fig. 3). There were no significant differences (X2 = 1.445, df 2,
The von Bertalanffy growth parameters were determined for female (
The population and growth parameters of great sturgeon show a large variation due to intrinsic and extrinsic factors. The species is characterized by slow growth, larger size and late gonadal maturity. Growth and population data were obtained from samples collected during several annual cycles (2008–2010). The sampled fish represented the southern populations from the Caspian Sea. The maximum age determined for this species in 1940 ranged from 107 to 118 years (Babushkin & Borzenko 1952). It was later estimated at 50 to 63 years (Kosarev & Yablonskaya 1994; Fazli et al. 2020) but our study indicated that the maximum age is 25 to 30 years. The presence of females in older classes (Pirogoviskiy & Fadeyeva 1982) may represent a life strategy that gives them a longer life expectancy (Roff 1984). The precision of age estimation based on pectoral fin spines for great sturgeon is acceptable compared to tarpon (
The mean fork length of both sexes revealed a decrease in size compared to data for great sturgeon from 1990–1994 (Taghavi-Motlagh 2001) and from 1990–2011 (Fazli et al. 2020), and also the mean fork length of both females and males was lower than that reported by Ruban & Khodorevskaya (2011) for great sturgeon from the Volga. These differences may be related to the capture of predominantly mature migrating fish in this area, while the catches in the southern part contained both mature and immature fish (Taghavi-Motlagh 2001; Ghadirnejad et al. 2010).
The mean weight of the great sturgeon determined in this study was lower compared to that found in the periods from 1990 to 1994 (Taghavi-Motlagh 2001) and from 1990 to 2011 (Fazli et al. 2020), and with larger mature individuals which enter the Volga River (Ruban & Khodorevskaya 2011). In general, it was found that females had a greater fork length and body weight compared to males.
The length–weight relationship for the population of female great sturgeon from the Iranian coastal waters of the Caspian Sea (slope = 3.22) is similar to that for the populations of the sturgeon species that are distributed within the geographical range, which differs very little from 3.3 (Craig et al. 2005). This relationship shows that despite the conditions to which great sturgeon populations are exposed in the Iranian coastal waters of the Caspian Sea, the populations are relatively healthy (mean slope = 2.94, both sexes), with a better condition of females.
The length and weight-at-age relationships were asymptotic, with a rapid growth rate in the younger stage, decreasing at older stages. This growth behavior was determined in other sturgeon species from the same region (Brennan & Cailliet 1989). At the age of sexual maturity, the growth rate decreased and a physiological change from somatic growth to reproductive development was observed (Bruch & Binkowski 2002).
The growth parameters estimated in this study were within the range recorded for this species by Froese & Pauly (2019) and were lower than those obtained in 1990–1994 by Taghavi-Motlagh (2001). The length determined by age was generally greater for females than for males (Fig. 4). This may be due to the faster growth of females in a brood compared with that of the brood that underwent sexual maturation (Stamps 1993) and higher age at maturity of females (Scarnecchia et al. 2007). Estimates of the asymptotic length for females in 1990–1994 were greater than those for fish in 2008–2010, which indicates a reduction in size.
Estimates of the asymptotic length for females in 1990–1994 were much lower than for individuals in 2008–2010, and a decrease in size was observed (Fig. 5). Harvesting pressure could show slower growth and smaller size, which is in agreement with data reported for i.a.
Overfishing and several environmental factors, such as pollution and modification of hydrological conditions, may have contributed to severe depletion of the great sturgeon stock (Pourkazemi 2006). The current fishing pressure on the great sturgeon is high. Fish stock analysis by continuous sampling is necessary to assess the status of the stock of this species in order to obtain technical data that will serve in the state regulation of the fishery of this species in the Iranian coastal waters of the Caspian Sea.
The age range of the fish caught was 5 to 30 years, with the highest proportion of age classes being 17 to 25 years – 70% and 83% for females and males, respectively. The annual mortality rates were calculated at 47% for males and 37% for females. The data obtained in this study and their comparison with previous studies indicate that the great sturgeon stock is exploited in an unsustainable manner. Overfishing and several environmental disturbances, such as pollution and modification of hydrological conditions, may have led to the severe depletion of the great sturgeon stock.