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

Stock dynamics of Auxis rochei

Abstract

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 mortality (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.

Keywords:
Biological reference point, Fecundity, Growth, GSI,
L-W relationship, Mean length, Mortality, SSB