Statistical and Monte Carlo analysis of the Hawaii deep-set longline fishery with emphasis on take and mortality of Oceanic Whitetip Shark

Originally published in 2021 - NOAA PIFSC data report: DR-21-006.

The study developed a process model from pelagic observer data to describe the take and mortality of oceanic whitetip shark (OCS) in the Hawaii deep-set longline fishery. The process model considered the: 1) probability of interaction (Catch model #1); 2) probability of branchline bite-off (Catch model #2); 3) probability of mortality at retrieval (Fate model #1); 4) probability of mortality due to handling between retrieval and release (Fate model #2); and 5) probability of post-release mortality and mortality of biteoff (Fate model #3). Three scenarios were considered for the OCS process models: 1) the current fishery use of using wire leaders and leaving ~10 m of trailing gear on a released shark (Scenario 1-Status quo); 2) intended use of monofilament, removing all trailing gear (0 m) on a released shark (Scenario 2Monofilament leaders); and 3) intended use of monofilament, removing all trailing gear (0 m) on a released shark and gear modification by eliminating three hooks adjacent to longline floats (Scenario 3Monofilament leaders and gear modification). Monte Carlo simulations were conducted for each of the three scenarios. The annual anticipated take level (ATL) for OCS has a mean of 1,708. Mortality at longline retrieval averaged 19.2% (95% CI, 13.1%–27.3%). There was a positive benefit of a reduced catchability for OCS estimated from the voluntary transition from branchlines that have 1 m of wire leader at the terminal end to branchlines being entirely composed of monofilament. Median estimates of annual OCS catch were 1,708 for the status quo, 1,153 for monofilament leaders, and 678 with monofilament leaders and no shallow hooks deployed. Median estimates of annual mortality were 362 for the status quo, 255 with monofilament leaders, and 150 with monofilament leaders and no shallow hooks deployed. The transition from wire to monofilament leaders was estimated to have a 32% and 30% reduction in catch and mortality, respectively. The lowest OCS catch and mortality occurred with monofilament leaders and no shallow hooks deployed; however, a large revenue decrease occurs when no shallow hooks are used due to reduced catch of target and incidental species.