Bycatch Of The World’s Largest Fish

Commercial fisheries pose an increasingly significant threat to non-target species of fish that are captured as bycatch, including several species of shark. While bycatch is by definition incidental, in many cases fishing vessels purposely look for large fish because smaller schools of target fish aggregate around them. This paper, published in Biological Conservation, investigates bycatch capture and mortality rates of the world’s largest fish: the whale shark, a species that often attracts schools of tuna and therefore becomes a target of fishing operations.

The authors analyzed data on fishing activity and whale shark capture in the Atlantic and Indian Oceans obtained from two sources. First, logbook declarations completed by skippers from 1980-2011, covering 100% of the activity of French and Spanish fleets. Second, recordings from scientific observer programs from 1995-2011, covering approximately 10% of fleet activity. By calculating shark sightings per unit of effort (i.e. number of sightings per fishing activity), the authors identified hotspots where interactions with sharks are more likely to occur, including the eastern part of New Guinea in the Atlantic Ocean and the Mozambique Channel in the Indian Ocean. They also found that sightings were concentrated during specific times of the year relative to seasonal weather patterns and migratory behavior.

Notably, the authors found that the overall rate of whale shark sightings and capture was very low, at just 1.12% and .88% of all fishing activity, respectively. Furthermore, according to scientific observer data, only two out of 145 whale shark who were captured died, calculated as an overall mortality rate of 1.38%. The authors attribute these low rates in part to regulations in the Indian Ocean prohibiting intentional setting of nets around whale sharks and specifying proper release protocols; no such regulations are in place in the Atlantic Ocean.

While the overall low mortality and capture rates may seem like good news for advocates, the authors call attention to two caveats: data from observer programs only covers 10% of activities, and there is currently no data available on the longer-term, post-release status of captured whale sharks. The authors note that scientific observer programs in Europe are set to expand their coverage of fleets by 100% in coming years. They also recommend that observers tag and monitor captured sharks, particularly those found in hotspots, to investigate post-capture survival rates and define more precise conservation management measures for whale sharks and other large fish and marine mammals that become bycatch.