Revisiting the Vulnerability of Undersized Bigeye (Thunnus obesus) and Yellowfin (T. albacares) Tuna Caught by Purse Seine Fisheries while Associating with Surface Waters and Floating Objects

Scutt Phillips, J., G. M. Pilling, B. Leroy, K. Evans, T. Usu, C. H. Lam, K. M. Schaefer, & S. Nicol. (2017). Revisiting the Vulnerability of Undersized Bigeye (Thunnus obesus) and Yellowfin (T. albacares) Tuna Caught by Purse-seine Fisheries while Associating with Surface Waters and Floating Objects. PLOS ONE. In press.

• Bottom line: Most visits to surface waters of the western and central Pacific Ocean by medium-sized juvenile yellowfin and bigeye tuna are brief, according to a 2017 study. The study suggests that current strategies for preventing excessive catch of these juveniles may not be sufficient. It further suggests that any successful strategy will likely involve limiting fishing that uses fish aggregating devices (FADs)—artificial objects widely used to assist in catching tuna.
• Background: Tuna have a tendency to associate with logs and other natural floating objects, for reasons that are not entirely clear. In the Pacific, fishing vessels often take advantage of this tendency by using FADs. But these artificial floating objects also attract juvenile bigeye and yellowfin tuna. As a result, they get caught as “bycatch” in purse seine nets, ending up in cans of tuna along with their skipjack cousins. Catching too many juvenile fish can contribute to overfishing. Fisheries managers in the Pacific and elsewhere have enacted seasonal bans on FAD fishing in hopes of mitigating such overfishing.
• Methods: To explore the FAD-related behavior of bigeye and yellowfin tuna, an international research team, led by Dr. Joe Scutt Phillips at the University of New South Wales in Australia, assessed the duration of surface associations in these species. The study looked at data from 50 bigeye and 35 yellowfin tuna fitted with tags that record swimming depth, external water temperature, and ambient light level. The researchers used these data to estimate the duration of every extended visit to the surface. The data did not allow them to link individual visits to the presence or absence of FADs. However, they did estimate the frequency of surface-association in regions of differing FAD density.
• Findings:
  • Episodes of surface association lasted for a median of two days in both bigeye and yellowfin tuna. This is a relatively brief period, although much longer visits were observed.
  • Total time spent in surface association was also relatively short—17 percent for bigeye and 23 percent for yellowfin.
  • Areas with higher FAD density did not show any increase in surface association.
  • These results suggest that the main effect of FADs on bigeye and yellowfin tuna is to make aggregations containing them easier to locate and catch. There was no evidence for widespread, long-term FAD associations by juvenile yellowfin or bigeye.
  • In that case, the authors argue, the only way to reduce bycatch of such fish would be to reduce fishing effort on FADs.
  • Seasonal bans on FAD fishing would have limited effect unless they also reduced total FAD effort or occurred during times of particularly high bycatch. Other studies corroborate this, showing that some closures do not reduce catch or effort, but simply shift effort on FADs from one place or time to another.

The full publication is available here:

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179045.