Risk to the Indo-Pacific Ocean whale shark population from interactions with Pacific Ocean purse-seine fisheries
Note: also published as IOTC-2018-WPEB14-INF06
This WCPFC SC14 paper includes the original report with a covering note, which summarises the findings of the study and, subsequently, invites SC14 to consider the following actions:
• Accept the results of the quantitative risk assessment of the impacts of Pacific Ocean purse seine fishing on Indo-Pacific whale sharks;
• Conclude that the available data indicate there is a moderate to low probability that the Indo-Pacific whale shark is at risk from Pacific purse seine fisheries (probabilities of generally <20% that current risk levels exceed a range of life history-based notional reference points); and
• Recommend that the WCPFC initiate concerted efforts to identify and promote best practice safe release methods for whale sharks and quantify post-release mortality rates under a variety of release scenarios.
Summary of findings:
• A nominal trend of high interactions in 2006-2008, followed by lower rates thereafter (Figures 6 and 7), was not altered by standardization and is consistent with trends found in the Eastern Pacific Ocean by Román et al. (2018)4 (p. 35). These decreasing annual trends in interactions do not appear to result from management measures as prohibitions on intentional setting of purse seines on whale sharks were adopted by the PNA in 2010, by the WCPFC in 2012 and by the IATTC in 2015. Furthermore, the trends may have been influenced by low WCPO observer coverage rates prior to 2010 (Table 1).
• Given the consistency in annual interaction trends over a broad area of the Pacific, it is possible that these trends relate to basin-wide oceanographic/ environmental conditions which mediate the overlap of whale sharks and the purse seine fishery (p. 35).
• Strong correlations were found between environmental variables and whale shark interaction rates for most set types except free school sets which show the highest interaction rates (Figure 7). One potential explanation for the lack of consistent correlations with free school sets is that whale sharks’ habitat preferences relate to fronts and clines that are not well-resolved in the aggregated oceanographic data used in the predictive modelling and which are more important in determining the locations of free school sets (p. 35).
• The spatially predictive model was able predict ‘hotspots’ for whale sharks which are generally in line with known areas of occurrence (p. 35). However, environmental predictors used in the model did not explain temporal shifts in interaction rates (Figure 13 and p. 3).
• In recent years, the number of interactions recorded as resulting in an immediate whale shark mortality was less than 1 in 1000 sets (p. 21). However, the probability of post-release mortality, which was estimated at ~10% (with a significant tail extending to higher value; Figure 14) based on an expert survey, was the greatest source of uncertainty in the assessment (p. 37). Understanding and reducing post-release mortality is recommended as one of most effective approaches to maintaining acceptable risk levels (pp. 37-38).
• For all scenarios the risk ranged from near 0% to as high as 54% of the most precautionary notional reference point (which is defined as “MSM” or maximum sustainable fishing mortality which is equivalent to half of the maximum population growth rate (rmax)) (p. 28). As the risk of exceeding any one of the three notional limit reference points is generally less than 20% since 2009, the risk from Pacific Ocean fisheries alone is considered moderate to low. The total risk to the Indo-Pacific whale shark population may however be higher if there are differential impacts to more vulnerable population segments within the Pacific and/or higher fishing mortalities outside of the region (e.g. the Indian Ocean) (p. 37).