A review of bycatch and discard issues in Indian Ocean tuna fisheries

Ardill D, Itano D, Gillett R (2011) A review of bycatch and discard issues in Indian Ocean tuna fisheries. In: 7th Session of the Working Party on Environment and Bycatch. IOTC-2012-WPEB08-INF20, Maldives, p 44

Public awareness and concern over the environmental impact of food production and security is rising rapidly. Whether real or perceived, scientifically justified or completely false, these perceptions can shape fisheries by influencing marketing, demand and product flow. In the fisheries sector, impacts can include overexploitation of both target and non-target stocks, damage caused to the environment by lost or discarded fishing gear, “ghost fishing” and pollution caused by discards, as well as the “carbon footprint” of fishing and baiting operations. The most recent estimates of non-target, associated and dependent species (NTAD) taken by global fisheries is of 7.3 million tonnes annually,
63% of which results from trawl fisheries with only 5% of the total from all tuna fisheries combined.
There is general agreement that this level of waste is unacceptable. Furthermore, although retained non-target catch may be recorded and reported to flag state authorities and Regional Fisheries
Management Organizations (RFMOs), no track is usually kept of discards of dead organisms, whether or not of target-species, resulting in wastage and distortions of data sets used for stock-assessments. A clear distinction should therefore be made between bycatch and discards.
This study, based on official statistics and published material, concentrated on pole-and-line, purse seine and longline tuna fisheries of the Indian Ocean, which, although representing less than half the region’s tuna landings, are the only sectors having sufficient statistical data and governance to permit analysis and the application of mitigation measures. It should be noted that, while often having significant non-target catch, artisanal fisheries rarely discard and fully utilize their retained catch.

...By far the largest incidence of bycatch and of discards in the Indian Ocean tuna fisheries studied here comes from longline fisheries. Bait used in the fishery, which can be considered a “discard”, amounts to half the total catch. Bycatch consists of 87 species or species groups, including sharks, seabirds and turtles, many of which are listed by the International Union for the Conservation of Nature (IUCN) as being threatened or endangered. Mitigation measures appear to have reduced seabird mortality in temperate waters, but in those fleets not targeting sharks, the actual shark catch was probably up to three times that reported. The Portuguese fleet that utilizes wire leaders records high mortality levels of blue, mako, silky, whitetip and thresher sharks while 75% of blue sharks and most rays appears to survive. The France-Réunion fleet which uses monofilament nylon leaders registered 80% reduction in the number of sharks caught. Mitigation measures could thus include mandating the use of nylon leaders in those fleets not targeting sharks. Other mitigation measures suggested include full catch retention, which would improve food security and nutrition in coastal communities where bycatch is landed and mandatory monitoring of tuna vessels via observer programs or remote sensing devices.


This study concludes that the Indian Ocean tuna fisheries discussed in this paper have a very low level of bycatch, particularly in comparison with other gear types fisheries. The level of discarding also appears to be negligible, other than that of sharks in some longline fleets.