A flotsam-associated fishery in the Indian Ocean, and its potential impact on pelagic bycatch species

In the deep-sea fishing boats operated from the fishery harbours of southern Sri Lanka, in addition to gillnets and longlines, encircling nets are taken onboard to operate whenever the fishing boat encounters a floating object (flotsam) in the high seas. These flotsam-associated fisheries target shade-loving fish species such as Decapterus russelli (Rüppell) and Elagatis bipinnulata (Quoy & Gaimard), while juvenile tuna species are also caught as bycatch. The present study was an attempt to investigate the status of the fisheries of D. russelli and E bipinnulata using length-based stock assessment and to elucidate the impact of these fishing strategies on three species of small tuna and three species of large tuna. The analysis revealed that D. russelli stock was optimally exploited, while E. bipinnulata showed sub-optimal exploitation, indicating that both stocks remain at safe levels. Length-based indicators (LBIs) showed that the proportion of fish exceeding the mean length at first capture was very low or zero for both small and large pelagic tuna species, while the proportion of immature individuals in the landings was high, suggesting potential adverse impacts of ring netting on these stocks. Although there are claims that catching juvenile tuna around floating objects does not necessarily lead to overfishing, the impact of ring netting on large pelagic tuna fish stocks in the Indian Ocean is a matter of concern, particularly given the substantial increase in ring net effort reported in this study. Therefore, the expansion of this fishery underscores the urgent need for regulatory measures to prevent the risk of future overfishing of tuna stocks.