Biological and ecological traits of non-targeted species caught by the tuna purse seine fishery in the western Indian Ocean

The tuna purse seine fishery of the western Indian Ocean is estimated to have relatively low bycatch rates, i.e. about 3.4 % of the total catch in recent years. Yet, with the implementation of the yellowfin tuna quota (IOTC Res. 17/01) in January 2017 and discard ban policy (IOTC Res. 17/04) since January 2018, removal of non-targeted species is expected to increase due to the change in strategy now mostly oriented to fishing on Fish Aggregating Devices and potential increased mortality of bycatch. Purse seine observer programs provide information for monitoring the magnitude and composition of incidental catches. Nevertheless, little quantitative information exists on the biology and ecology of non-targeted species, particularly in the western Indian Ocean. Thus it is difficult to assess their removal effect on the role and function of the pelagic ecosystem. Within this context, three objectives have been defined for this paper. First the main biological and ecological traits for purse seine bycatch species were reviewed from the literature. Secondly, a new biological sampling launched in Seychelles in 2017 was presented. Finally, length-weight relationships for 15 non-target species were updated for the western Indian Ocean using morphometric data collected on board and at port.