Stock Assessment and Management of hilsa shad tenualosa ilisha in the meghna river of Southeastern Bangladesh considering the emerging Climate change

Abstract

The national fish of Bangladesh, Hilsa shad, Tenualosa ilisha (Hamilton, 1822) is a marine, brackish and freshwaters fish belonging the family Clupeidae (Riede, 2004). It is locally known as ilish or ilsha. The overall objective of this study is to assess the stock of T. ilisha from the Meghna River, Southeastern Bangladesh. A total of 1433 individuals were collected for the period of July 2018 to December 2020 where its body weight (BW) and total length (TL) were measured with 0.01 g and 0.01 cm accuracy for each specimen. The current study was on morphometric relationships through length-weight and length-length relationships, meristic features, condition factors (Fulton’s, KF; allometric, KA; Relative, KR) and form factor of T. ilisha. Physiological status was determined using the equation provided as: ā = W/Lb. If ā was close to the a value (a, LWR parameter) indicated the fish was in ideal condition, whereas ā > a pointed to fatty fish and ā < a to lean fish. The prey-predator status estimated through relative weight. Growth parameters were estimated through the von Bertalanffy equation. Data of length–frequency were analyzed with excel based program and FAO-ICLARM Stock Assessment Tool. The TL ranged from 15.3-57.8 cm while the BW was 37.17–2250 g. The overall KF was 0.7191-1.7098, KA was 0.0014-0.0151 and KR was 0.65-1.66 for T. ilisha in the Meghna River. The maximum KR was found in July while the minimum was in January. The KF was strongly correlated with TL and BW while KR was with BW in the Meghna River. KA has no significant relation neither TL nor BW. The maximum fatty fish was found in the months of December (44%) while the minimum was in October. The highest percentage of lean fish was found in the month February (41%) and the lowest was in September (22%). The WR indicated that the habitat was in stable circumstance for T. ilisha. The TL50 (the length at which 50% of specimens become matured) was calculated by using four models ((i) TL vs. Gonadosomatic index (GSI), Modified gonadosomatic index (MGSI) and Dobriyal index (DI) model, (ii) logistic model (iii) maximum length based model (Lmax) and from the following three models, mean Lm was 26.10 cm. Based on monthly variation in GSI, MGSI and DI value, T. ilisha spawn all over the year with two peaks in October and April in the Meghna River. Macroscopic observations of gonads showed five maturity stages. Based on macroscopic features, Hilsa spawn throughout the year but highest percentages in October. The total fecundity (FT) ranged from 65999 to 1575850. Fecundity was found to be highly correlated with TL and BW. Suitable temperature considered as 25-26◦c for the spawning. Furthermore, mean air temperature is increasing by 0.029 °C y-1, and rainfall is decreasing by 2.96 mm y-1. The growth parameters through von Bertalanffy model of T. ilisha was as Lt = 57.62 (1-exp(-1.13(t- 0.009))). The estimated L∞ = 57.62 cm, K = 1.13 y-1, W∞ = 2024.6 g, t0 = 0.009, tmax = 4.17 year. The overall growth-performance index (Ø') was 3.38. In addition, the age of T. ilisha was estimated as 3.10 years. Fishing mortality (F = 0.97 year-1) rate was lower than natural mortality (M =1.57year-1). Further, estimated exploitation rate (E=0.63) was higher than the maximum permissible exploitation rate (Emax = 0.47). Consequently, overfishing is the greatest pivotal threat for T. ilisha stock in the context of Meghna River, southeastern Bangladesh. The MSY was calculated at 286,327 MT. Scientists and conservationists will be able to establish early management methods and policies based on the findings of this study of T. ilisha to ensure long-term survival in the Meghna River and its adjacent ecosystems.---