Indicators of depredation impacting Reunion Island pelagic longline fishery
Depredation is defined as the damage or removal of fish from fishing gear by predators. Depredation raises concerns about the conservation of marine protected species involved, fisheries yield and profitability, as well as stock assessment of target species. There is an obvious lack of knowledge about depredation impacting pelagic longline fisheries, especially in the southwest Indian Ocean. Thus, there is a real need for the development of accurate indicators to assess its impact in a given fishery. In Reunion Island, pelagic longliners targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from short-finned pilot whale (Globicephala macrorhynchus), false killer whale (Pseudorca crassidens) and various pelagic sharks. Catch and depredation data collected during self-reporting, commercial and experimental cruises between 2007 and 2015 were used to calculate depredation indicators such as the depredation occurrence (Interaction Rate), the proportion of fish depredated among the overall catch (Gross Depredation Rate), the average proportion of fish depredated per depredated set (Damage Rate) and the number of fish depredated per 1000hooks (Depredation Per Unit Efiort). Here we show that shark depredation impacted more fishing sets (IRs=31%) than toothed whale depredation (IRc=14%), but when depredation occurred, toothed whale depredation impact was higher: the number of fish damaged per 1000 hooks and the average proportion of fish damaged per set were greater for toothed whale depredated sets (DPUEc*=2.7 fish and DRc*=16.8%) than for shark depredated ones (DPUEs*=1.2 fish and DRs*=6.4%). Since 2011, when the pelagic fleet concentrated its fishing effort around the Reunion EEZ and the east coast of Madagascar, the gross depredation rate increased and ranged from 4 to 6.3%. These are minimum depredation estimates, since several uncertainties could not be taken into account: total depredation leaving no trace on the hook, bait depredation by small delphinids indirectly leading to catch loss, toothed whale presence scaring fish away or additional running costs when leaving a fishing area to avoid predators. Thus, combined with marginal profits, increased running costs and low fish prices, toothed whale depredation is likely to induce disastrous effects on Reunion pelagic longline fishery. Therefore, direct and indirect costs linked to depredation are being assessed in an on-going study.