Differences in reaction of bigeye tuna (Thunnus obesus) and skipjack tuna (Katsuwonus pelamis) to intermittent light

Abstract

The use of fish aggregating devices (FADs) in tuna purse seine fisheries are considered to affect the stock status of bigeye tuna (Thunnus obesus) in the Pacific, Indian and Atlantic Oceans as juvenile bigeye tunas tend to aggregate around the FADs and are caught along with skipjack tuna (Katsuwonus pelamis), the target species. International efforts to recover the stock status of bigeye tuna to sustainable level are under way. Regional fisheries management organizations have recommended measures to control fishing mortality of bigeye tuna by the FAD sets. In contrast, the skipjack tuna stock, the main target of the fishery, is considered healthy. As the two species coexist in a limited space around FADs, bigeye tuna is usually caught with skipjack tuna. To minimize the bigeye tuna bycatch at FADs while maintaining the catch of skipjack tuna, it is necessary to determine species-specific stimuli that can spatially separate the two species. We investigated the feasibility of using intermittent light as one such stimulus. The reactions of the two species to intermittent light of various frequencies (0.3, 0.5, 1.0, 1.7 and 5.0 Hz) and to combined intermittent (0.5, 1.0 and 5.0 Hz) and continuous light were tested. Bigeye and skipjack tuna samples were introduced alive into a 20-m-long cage and exposed to light stimulus. Behavior of fish in the cage was observed using two counter-oriented echo sounders. An underwater camera was used to monitor fish behavior and species identification. Skipjack tuna tended to move away from high-frequency intermittent light (5.0 and 1.7 Hz). In contrast, bigeye tuna showed less avoidance or even attraction to the same stimuli. Consequently, high-frequency intermittent light could be used to separate the two species.