Mesoscale exploitation of a major tuna concentration in the Indian Ocean

Abstract

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.

Habitat characteristics of skipjack tuna (Katsuwonus pelamis) in the western North Pacific: a remote sensing perspective

“…This attracts nekton, with a net effect of aggregation of apex predators to forage on the lower trophic level organisms around the eddy edge (Ramos et al. , 1996; Fonteneau et al. , 2009).…”

Estimation of yellowfin tuna (Thunnus albacares) habitat in waters adjacent to Australia’s East Coast: making the most of commercial catch data

“…(2008) determined that relatively low concentrations of chlorophyll a were optimum for longline capture of YFT in the Northern Indian Ocean. Conversely, anomalously large purse seine catches in the equatorial Western Indian Ocean were linked to a high concentration phytoplankton bloom associated with a large anticyclonic eddy (Fonteneau et al. , 2008).…”

Biodiversity and environmental characteristics of the bycatch assemblages from the tropical tuna purse seine fisheries in the eastern Atlantic Ocean

“…With respect to the rest of variables, Fonteneau et al () found interesting relationships between chlrophyll‐a peak levels 18 days prior and free‐swimming tuna abundance in the Indian Ocean. However, and in accordance with the work of Lopez, Alvarez‐Berastegui, et al, , Lopez, Moreno, et al, , these relationships have not been found yet in the Atlantic Ocean for tuna and nontuna species.…”

“…Chlorophyll plays a key role in marine ecosystems as it is the source of the energy circulating through the trophic levels, and therefore, it can be seen as a proxy of prey enrichment (Marsac, ). In this study, chlorophyll taken 18 days before was selected as Fonteneau, Lucas, Tewkai, Delgado, and Demarcq () found interesting relationships between circumstantial Chl‐a levels 18 days before and free‐swimming tuna abundance in the Indian Ocean. Moreover, this variable has been also studied for the same reasons in the Atlantic Ocean to study relationships between environment characteristics and tuna distributions in DFAD sets (López, ).…”