The paper analyzes the daily catch, fishing effort and fish size data of the
purse seine fleet fishing in the western Indian Ocean in February 2005, when
a major concentration of tuna occurred and was heavily exploited by this
surface fishery. This tuna concentration event occurred over a period of
just 12 days, in an area of about 3500 square nautical miles located to the
west of the Seychelles. This small stratum produced a total catch of 22 000 t, corresponding to 6.5% of the total fishing mortality of all adult
yellowfin (Thunnus albacares) in the entire Indian Ocean in 2005. Sets were made mainly on
free schools and the catch mainly composed of large yellowfin tuna. The
average CPUE and the average catch per set were very large, 65 t and 85 t
per fishing day, respectively. This “event” took place in a precise area
where a high concentration of chlorophyll had been localized 18 days before.
The subsequent concentration of tuna schools probably arose due to the high
densities of their prey feeding on this large phytoplankton biomass. This
phytoplankton bloom was observed at the edge of an anticyclonic eddy, but
its origin and its high density cannot be fully explained by available
environmental data. The adult yellowfin were probably at a reproductive
stage and actively feeding on the local food chain generated by the
phytoplankton bloom. Such an event is extreme, but typical of tuna purse
seine fisheries where fleets often search for such tuna patches. These
events play an important role in tuna fisheries due to the increase in
fishing effort and fishing efficiency of purses seiners, both of which
increase their impact on the resource. The fine scale study of such events
and their improved integration into tuna stock assessments is recommended.