Insight into the fishery, biology and population dynamics of Auxis rochei (Risso, 1810) along the south-west coast of India

Auxis rochei is one of the coastal tunnies contributing substantially to the tuna landings in India, with an average (2014-2019) annual catch of 7210 t. Kerala stood first, with an annual average landing of 3176 t. The fishery comprised 14.5 - 39.6 cm fork length (FL) size groups with a mean length of 27.64 cm and a major mode of 26 cm FL. The Length-weight (L-W) relationship of pooled fishes was expressed as W= 0.000003 L3.22 (r2= 0.91). The size at first maturity was estimated at 25.8 cm FL when the fish was 1.5 years old. The number of eggs per spawning ranged from 84,562 to 1,33,698 and the ova diameter ranged between 0.1 and 0.6 mm, indicating asynchronous oocyte development with multiple spawning. Higher gonadosomatic index (GSI) was recorded during April, May and June and coincided with the minimum gastrosomatic index (GaSI). The recruitment pattern was bimodal, with varied intensity ranging from 0.79-17.89%. Growth was allometric and the von Bertalanffy growth equation derived was Lt=43.5(1-e-0.8(t+0.0212)). The growth performance index (ɸ) was 3.18 and the longevity of the fish was estimated at 3.7 years. One-year-old plus fishes sustained the fishery (22 - 32 cm FL). Total mortality (Z), natural mortalty (M) and fishing mortality (F)were estimated at 3.70, 1.40 and 2.30, respectively, with an exploitation rate (E) of 0.62 and the exploitation ratio (U) of 0.60. Beverton and Holt yield per recruit (Y/R) and Biomass per recruit analysis indicated that the Emax was at 0.87, much higher than the present level of exploitation, reflecting the sustainable fishery along the coast. Standing stock biomass (SSB) was estimated at 7215.7 t, and the spawning stock biomass formed 55.5% of the SSB. Thompson and Bell analysis indicated the MSY as 8125 t, at F-factor, 2.6. The biological reference points FCURR/FMSY (0.4) and BCURR/BMSY (1.5) were at safe levels, signifying the scope for fishery enhancement