Population structure and demographic history for year cohort dynamics of landlocked ayu Plecoglossus altivelis altivelis in dam reservoir of Japan.

Ayu Plecoglossus altivelis altivelis is a valuable osmeroid species for inland fishery in Japan. It is classified into two ecological forms of amphidromous migrating between rivers and sea and landlocked migrating between rivers and lakes or dam reservoirs. The number of dams and their reservoirs has remarkably increased in the twenty-first century under climate change, because of their respective roles in hydropower generation with negligible carbon emissions and in flood control. Dam reservoirs therefore become increasingly important as inland nursery grounds of ayu. In this study, we investigated the reproduction status of landlocked ayu migrating in the Haidzuka Dam reservoir and the Tabusa River in western Japan by molecular phylogenetic analysis based on population structure and demographic history for year cohort dynamics. A total of 849 individuals were collected monthly from October 2018 to September 2021 according to an annual life cycle of ayu. Nucleotide sequences of the partial mitochondrial DNA control region yielded 31 haplotypes, consisting of 4 shared haplotypes among the 2019, 2020 and 2021 cohorts and 27 unique haplotypes. The overall haplotype diversity and nucleotide diversity were calculated to be relatively low at 0.3503 ± 0.0206 and 0.0077 ± 0.0045, respectively, suggesting a founder event by dominant haplotypes. Star-shaped radiational haplotypes from dominant shared haplotypes on the median-joining network likely support a founder event. Although pairwise ФST values were determined to be very low among the year cohorts, only the 2019 cohort was found to have a significant difference from the 2020 and 2021 cohorts, for both of which Tajima's D values were also statistically significant. For the overall population, multimodal mismatch distribution and negative Tajima's D and Fu's Fs values in the neutrality test suggested population expansion or population subdivision. The native riverine population in the Tabusa River suffered habitat fragmentation and population bottleneck from dam construction, and therefore severe founder effect remained behind the artificially landlocked population with a low level of genetic diversity in the Haidzuka Dam reservoir.

Stock assessment of landlocked ayu Plecoglossus altivelis altivelis in Japan through length-based models.

Ayu Plecoglossus altivelis altivelis is a key commercially and culturally important freshwater osmeroid in Japan. Its native population is mostly an amphidromous form migrating between rivers and the sea, and not only native but also artificially landlocked forms are found in lakes and dam reservoirs. This study was undertaken to execute population feasibility and maximum sustainable yield (MSY) analysis of an artificially landlocked form of ayu during January 2018 to December 2020 in the Haidzuka reservoir and its connected Tabusa River, western Japan. FAO-ICLARM Stock Assessment Tools-II and empirical models were employed to estimate the growth function and population parameters. The estimated asymptotic length was 19.50 cm, and the growth coefficient was 0.73 year-1 with a growth performance index of 2.443. The length at first maturity and length at optimum yield were calculated as 10.77 cm and 12.63 cm, respectively, which were lower than the length at first capture (Lc = 13.15 cm), suggesting the mesh size of fishing gear favoring the sustainability of the reproductive potential of this population. The calculated total, natural, and fishing mortalities were 2.45, 1.19, and 1.26 year-1, respectively. The recruitment pattern was continuous round the year with two pulses where the peak was during July. The current level of exploitation (0.51) was slightly higher than the maximum exploitation rate (0.45), indicating a little overharvesting. The MSY of ayu in the Haidzuka reservoir was estimated to be 211 metric tons if the recommended Lc is maintained. Results of this study illustrate the first information on population characteristics of landlocked ayu and will help the development of suitable management policies for ayu fishery in lakes and dam reservoirs.

Estimation of life history parameters for river catfish Eutropiichthys vacha: insights from multi-models for sustainable management.

The river catfish, Eutropiichthys vacha is a vital protein source for rural communities and has high commercial value, but understanding its life history and management strategies reveals major inadequacies and ambiguities in the riverine ecosystems. Consequently, this study employs multi-models to analyze the life history parameters of E. vacha in the Ganges River (northwestern Bangladesh) from January to December, 2020. The total length (TL) and body weight (BW) of 362 individuals (male = 170, female = 192) were measured by a measuring board and a digital weighing balance, respectively. The overall sex ratio (male: female) was 1.0: 1.13 and did not oscillate statistically from the standard 1:1 ratio (p > 0.05). The TL varied from 6.7-19.2 cm for males and 6.3-19.0 cm for females. The length-frequency distributions (LFDs) revealed females outnumbered in 8.0-9.99 cm TL whereas males in 7.0-7.99 cm TL. The slope (b) of the length-weight relationship (TL vs. BW) for both sexes (b = 2.87) was substantially lower than isometry, specifying negative allometric growth pattern for E. vacha. Sex-specific relative (K R ) and Fulton's (K F ) condition analysis revealed better state of well-being of males than females. Only K F exhibited significant correlation with both BW and TL, hence making it ideal condition for predicting the fitness of E. vacha in this river. Moreover, the relative weight (W R ) suggests an imbalanced habitat for females with higher abundance of predators but suitable for males. The form factor (a 3.0 ) was 0.0062 and 0.0065, whereas the size at first maturity (L m ) and mean natural mortality (M W ) were 11.38 and 11.27 cm TL and 1.29 and 1.28 year-1 for the respective sexes. Besides, the calculated mean optimum catchable length (L opt ) was 13.58 and 13.09 cm TL for each sex. These findings will be crucial for further studies and to recommend appropriate strategy for the sustainable management of E. vacha in the Ganges River and adjacent watersheds.

Reproduction of Eutropiichthys vacha (Schilbeidae) in the Ganges River (NW Bangladesh) with special reference to potential influence of climate variability.

The Batchwa vacha, Eutropiichthys vacha is commercially important, supporting a viable small- and large-scale fishery throughout the Ganges River, NW Bangladesh. This study provides detail information on reproduction of E. vacha including size at sexual maturity, spawning and peak spawning season, and fecundity based on 734 female specimens through regular monthly sampling using cast net, gill net, and square lift net in the Ganges River during January to December 2016. Also, our study estimated the effects of climate change including temperature and rainfall on reproduction of E. vacha in the Ganges River. For each individual, lengths (total length, TL; standard length, SL) and body weight (BW) were measured with slide caliper and digital balance, respectively. Gonads (ovaries) were collected carefully by ventral dissection of each female specimen and weighed to the nearest 0.01 g accuracy. The gonadosomatic index (GSI % = (GW/BW) ×100), modified gonadosomatic index (MGSI % = (GW/BW - GW) × 100), and Dobriyal index (DI = [Formula: see text]) were calculated to estimate the size at sexual maturity (L50) and spawning season. Based on GSI, MGSI, and DI, the L50 was calculated as 12.5 cm TL for female. The TL50, the TL at which 50% of individuals become mature, was calculated by logistic equation as 12.7 cm. Also, on the basis of higher values of GSI, MGSI, and DI, spawning season was ranged from April to August, with the peak in June-July, signifying the peak spawning season for E. vacha in the Ganges River. The total fecundity (FT) ranged from 4800 to 77,976 (mean ± SD, 31384 ± 23,747) and was highly correlated with TL and BW. Water temperature during the spawning period ranged from 28 to 34 °C, with an average of 31 °C and there was significant correlation between temperature and GSI. Also, the spawning season coincides with the peak rainfall and there was significant correlation between rainfall and GSI. Additionally, analysis of long data series indicated that annual average air temperature is increasing by 0.0258 °C/year, while the annual average rainfall is decreasing by 3.107 mm/year. Finally, the findings of this study would be very effective to impose specific management for E. vacha in the Ganges River and surrounding ecosystems.