A Flotsam-Associated Fishery in the Indian Ocean, and Potential Impact on Pelagic By-Catch Species

Abstract

In the deep-sea fishing boats predominantly operated from the fishery harbours of southern Sri Lanka, in addition to drift gillnets and longlines, an encircling net is taken onboard to operate whenever the fishing boat encounters a floating object (flotsam) in the high seas. These flotsam-associated ring net fisheries target shade-loving fish species such as Decapterus russelli and Elagatis bipinnulata. There are concerns among fisheries management authorities that juvenile tuna species are also caught in these ring nets. In the present study, an attempt was made to investigate the status of the fisheries of D. russelli and E bipinnulata using length-based stock assessment and elucidate how these fishing strategies impact on the stocks of three species of small tuna and three species of large tuna. It was revealed that D. russelli stock was optimally exploited and a sub-optimal level of exploitation of E. bipinnulata is evident from the analysis indicating the stock is still in the safe level. According to length-based indicators (LBIs), proportions of fish, bigger than mean length at first capture of both small and large pelagic tuna species were much lower or zero while those of immature fish in the landings were much higher indicating potential adverse impacts of ring netting on those fish stocks. As catching juvenile tuna around floating objects does not necessarily result in overfishing of stocks, the impact of ring netting on large pelagic tuna fish stocks in the Indian Ocean remains inconclusive.

Keywords: FADs, flotsam, Indian scad, LBI, rainbow runner, tropical tuna