Reproductive potential and stock status of the anadromous shad, Tenualosa ilisha (Hamilton, 1822): Implications for managing its declining fisheries in northern Bay of Bengal bordering India

Abstract

The anadromous shad, Tenualosa ilisha is a fish of high economic importance in countries flanking the Bay of Bengal (BoB). Its fishery in the northern BoB bordering India and the associated Bhagirathi–Hooghly river system has been declining. Scientific explanations on the existing fishery management recommendations, especially the length at first maturity (Lm50), minimum legal size (MLS), minimum mesh size for fishing nets and stock status were limited, raising serious concerns on the sustainability of its fisheries. Through the examination of its reproductive cycle, using histological maturity identifiers, the Lm50 of males and females were redefined at 265 mm TL/ 1.0 year age and 305 mm TL/ 1.2 year age, respectively. The size-based fecundity, sex ratio and abundance revealed that females of 281–360 mm TL/ 1.0–1.6 year of age have maximum reproductive potential. In the catch, 45%–65% of males and females were below Lm50. The temporal progression of mean length in the catch (MLC) and female length at maturity showed declining trend. Application of length-based indicators (LBI) and stock assessment models showed the fishery in near full exploitation, less optimal and undergoing multi-generation, indiscriminate, size-selective overfishing. The spawning stock biomass (SSB) has declined to 26.8% of the virgin SSB, while the yearly spawning potential ratio (SPR) ranged from 0.04 to 0.29, which is lower than the SPR threshold (0.3) suggested for sustaining a fishery. The species is both growth and recruitment overfished. Though the mean annual catch is marginally lower than the estimated maximum sustainable yield (MSY) of 32,953 t, any increase in fishing effort would further impact the SSB. Based on the results, a MLS of 360 mm TL, minimum mesh size of 110 mm for fishing gear and a precautionary annual catch limit of 30,000 t suggested for management, and the potential impact of implementing these on fishers’ catches discussed.