Risk to the Indo-Pacific Ocean whale shark population from interactions with Pacific Ocean purse-seine fisheries

Whale sharks are globally distributed throughout tropical and sub-tropical seas. By occupying similar habitat to many tropical pelagic species, including tuna, they interact with purse-seine fisheries targeting these species. Due to their particular life-history, whale sharks are thought to be vulnerable to fishing mortality, but it is unclear whether present levels of fishing mortality pose a risk to whale shark populations.

Here, we examined observer data from the Western and Central Pacific Ocean tropical purse-seine fishery. First, we described whale shark interactions and trends within that fishery. We then attempted to standardise data from fishery interactions to investigate if fluctuations in oceanographic conditions (described by sea surface temperature and chlorophyll a) can explain temporal patterns in interactions.

Although changes in oceanic habitat variables could not account for the temporal shifts in interaction rates, we found that estimated environmental effects on spatial interaction rates produced estimates of spatial habitat suitability that appeared consistent with available information. We used this map of predicted habitat suitability to define the overlap between Pacific Ocean (Western and Central Pacific Fisheries Commission and Inter- American Tropical Tuna Commission) tuna fisheries and whale sharks, and to estimate total mortalities expected within these fisheries.

We also used life-history information and life-history theory to estimate risk for the Indo-Pacific Ocean whale shark population from Pacific Ocean purse-seine fisheries. To estimate the un-observable post-release mortality, we conducted a Delphi survey of experts, and summarised the information using a statistical model. The risk assessment model suggested that the risk from Pacific Ocean fisheries alone is moderate to low, but not insignificant given potential other sources of mortality and uncertainty. In accordance with suggestions from the experts in the Delphi survey, we suggest that a strict application of best-practice release protocols can significantly reduce post-release mortality and, therefore, risk for whale shark populations.