Monte Carlo simulation modelling of possible measures to reduce impacts of longlining on oceanic whitetip and silky sharks
The paper develops and applies a model for how oceanic whitetip shark and silky shark might interact with longline gear; using it to quantify potential sources of fishing-related mortality. It integrates available information of gear characteristics, spatial differences in the density of the two species, and the results of previous studies on catchability and survival. With this model we evaluate four simple potential management measures: (1) removal of shallow hooks; (2) removal of shark lines; (3) requirement for circle hooks; and (4) requirement for monofilament leaders. The analyses, and measures considered, represent an example of what can be undertaken with the simulation model developed and we anticipate examining other scenarios at the request of SC11 or CCMs. The key conclusions of the analyses are: 1. There are still critical gaps in our knowledge of longline gear configurations – with respect to the key variables important to sharks – for almost all Distant Water longline fleets due to the paucity or absence of observer data; 2. Collection of data on hook location (i.e., whether the hook was swallowed, or the shark was hooked in the lip), and how oceanic whitetip shark and silky shark are released (e.g., cut-off on the line in water or brought on-board and hook retrieved) will be critical for better understanding the potential fishing-related mortality and the effectiveness of the current Conservation and Management Measures (CMMs). We recommend that collection of these data by observers be considered; 3. There are few relevant scientific studies of likely rates of release mortality for oceanic whitetip shark and silky shark caught and released under commercial fishing conditions. This should be a priority for field studies, but the design of such studies should consider information under (2) above, particularly how sharks are released; and 4. Given available data and assumptions made in the analyses - the following results were obtained from application of the model through Monte Carlo simulation testing: • The initial interaction of silky shark and oceanic whitetip shark with longline gear can be reduced by both the banning of shark lines or the removal of “shallow-hooks”, which we defined as the three hooks closest to the start/end of the basket; • Banning shark lines has the potential to reduce fishing mortality by 14.7% and 23.3% for silky shark and oceanic whitetip shark respectively, and removing shallow hooks has the potential to reduce fishing mortality by 11.7% and 6.7% respectively; • Banning wire trace – while unlikely to influence initial interaction – lead to increased bite-offs which resulted in the greatest reductions in fishing mortality of the measures considered – 17.6% and 23.3% for silky shark and oceanic whitetip shark respectively; • Prohibiting both shark lines and wire trace is predicted to reduce mortality by 29.4% and 40% for silky shark and oceanic whitetip shark respectively. • The tendency for greater lip-hooking with circle hooks and therefore fewer bite-offs meant little predicted benefit from requiring circle hooks; and • Given the high levels of fishing mortality experienced by these two species, it is unlikely that the options under the shark CMM (2014-05) of either banning shark lines or wire traces will result in sufficient reductions in fishing mortality. Strengthening this measure may be necessary. We invite SC11 to consider the specific recommendations of this work and the preliminary results from the model. We particularly invite comments on the model assumptions, potential sources of information to better support the model, and potential mitigation measures that could be examined.