Evaluation of post-release mortality for porbeagle and shortfin mako sharks from the Canadian pelagic longline fishery
The majority of shark catches from the Canadian pelagic longline fleet are discarded alive at sea, making post-release mortality (PRM) estimates critical to understanding total fishing mortality. We found little evidence that at-vessel mortality has changed from 2001-2018, suggesting that capture characteristics are similar and previous satellite tagging can be combined with more recent tagging to describe PRM. Estimated rates were 14% for porbeagle (6% for healthy and 40% for injured) and 28% for shortfin mako (27% for healthy and 33% for injured), which is approximately ½ of the previous estimate for porbeagle and essentially the same for shortfin mako. We propose that this difference for porbeagle is related to handling characteristics during tagging, which switched from bringing animals on board to tagging in the water. This conclusion is supported by an analysis of recovery times for surviving animals where median recovery time was 1 day (shortfin mako) or 1.5 days (porbeagle) longer when the shark was tagged onboard as compared to in the water.