Demographics of great sturgeon (Huso huso) in Iranian waters of the Caspian Sea (2008–2010)

The great sturgeon Huso huso is one of the most important sturgeon species in the world, occurring in the Caspian Sea, the Black Sea and the Sea of Azov (Ta’ati et al. 2011). It is the main species of sturgeons bred in Iran (Kazemi et al. 2014) and many other countries for meat and caviar production (Williot et al. 2000; Boscari et al. 2017). However, the combination of the species’ life cycle characteristics (long-lived, late-maturing and anadromous fish) with environmental degradation of water basins, water-level fluctuations, water pollution, damming of their spawning rivers, overfishing (including extensive poaching), poor fisheries management, intensive and selective illegal and unreported fishing, failure to protect their nursery grounds, and lack of regulation during the past 50 years have resulted in a drastic reduction in catches (Karayev 2006; Graham & Murphy 2007; Tavakoli & Khoshghalb 2018; Mirrasooli et al. 2019), thereby negatively impacting the fishing industry of this species from fishermen to consumers.

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 H. huso stock and determines total allowable catch limits based on these estimates (Ustaoglu & Okumus 2004; Mirrasooli et al. 2019). This method has been used to adjust the total allowable catches (TACs). However, more long-term data on population parameters along with analysis of behavior, age, growth and mortality of H. huso as well as their comparison with recent data are required. The objectives of this study were to estimate the age, to study the somatic growth, to determine the age structure and to estimate the mortality rate of great sturgeons collected between 2008 and 2010.

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 FL of males was 194.17 ± 8.10 cm (mean ± SE) with a range of 84 to 255 cm, while the mean FL of females was 224.17 ± 7.06 with a range of 84.0 to 305 cm (Table 1). The mean FL of males was not significantly different from that of females (Mann–Whitney U test, n = 105, p = 0.08). The mean total weight was 69.77 ± 6.72 for males with a range of 4 to 141 g, while the mean W of females was 111.26 ± 10.70 g with a range of 4 to 287 g. The mean W was significantly different between the two sexes (Mann–Whitney U test, n = 105, p = 0.03).

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, p < 0.05). The condition factor (CF), expressing feeding activity during the year, was significantly different between the two sexes (p < 0.001).

Figure 2

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, p = 0.18) in the age–frequency distributions for males and females. A higher proportion of females than males was found in the 18- and 19-year-old classes. It was determined that 73% and 71% of the fish caught were less than 25 years old for females and males, respectively, and the proportion of fish under 8 years old was low (Fig. 3). The age range for the fish caught was 5 to 30 years and the age classes of 17 to 25 years had the highest proportion, with 70% and 83% for females and males, respectively.

Figure 3

The von Bertalanffy growth parameters were determined for female (L_∞ = 275.784 cm; K = 0.082 per year; t₀ = −0.809 year) and male (L_∞ = 265.255 cm; K = 0.062 per year; t₀ = −2.208 year) great sturgeon (Fig. 4). The growth of females in relation to age was significantly higher (F = 16.67, df 3, 50, p < 0.001) compared to that of males (Fig. 4). Males were characterized by higher mortality rates Z (0.64) than females (0.46) and lower survival rates (53%) compared to females (63%), with annual mortality of 37%, which for females was 47%.

Figure 4

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 (Megalops atlanticus) (Cyr 1991) and white sturgeon (Acipenser transmontanus) (Rien & Beamesderfer 1994), which are also long-lived species. It was also found that the use of sectioned spines of pectoral fins as a non-lethal method of age determination is useful and precise, particularly for endangered species, such as great sturgeon, or when sacrifice of samples is undesirable (Glass et al. 2011). The APE was low (0.95%) and there were no differences in the profiles of pectoral fin spines and their images. The APE below 5% was consistent with the interpretation of age (Morison et al. 1998). Only the oldest fish showed multiple vascular channels and compressed or abnormal bands in the margin of the anterior fin spine, which made fin cross sections.

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. Pleuronectes platessa L. (Rijnsdorp, 1989), Gadus morhua (Olsen et al. 2005) and Micropterus dolomieu (Dunlop et al. 2005). The mortality coefficient of female great sturgeon in 1990–1994 (Taghavi-Motlagh 2001) and 1990–2011 (Fazli et al. 2020) was lower than that obtained in this study for females and males. This indicates that intensive commercial fishing may be a causative factor in the sharp decline of the great sturgeon stock, the increase in the annual mortality rate and the decrease in the survival rate.

Figure 5

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.