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New study explores post- release mortality in billfish

By Staff | Mar 15, 2018

Obtaining reliable estimates of post-release mortality in billfishes has only become possible in the last 25 years or so, largely due to the development of electronic tagging equipment (acoustic and satellite tags) that allows more effective tracking of tagged fish.

In addition, while there have been a number of well-conducted studies that have investigated billfish post-release survival using this technology, the high cost of electronic tags has precluded large numbers of billfish from being tagged in any single study, which subsequently limits the amount of statistical inference that can be derived from each study.

However, a recent study by Dr. Michael Musyl and colleagues published in the Canadian Journal of Fisheries and Aquatic Sciences compiled the results of 49 independent studies into a single meta-analysis to estimate post-release survival in six species of billfish. This study included billfish that were caught on both recreational and commercial fishing gear (longline).

The authors’ meta-analysis reported 41 instances of post-release mortality, with 90 percent of mortalities occurring within ten days of release. In addition, mortalities were tallied for individual species, and no statistical differences were reported among studies for a given species, in mortality between species, and between different gear types. Taken collectively, post-release survival was calculated among all species at roughly 86 percent. Results for individual species are as follows:

Blue Marlin (12 studies): Seven mortalities out of 142 tags, or roughly 90 percent survival.

Black Marlin (three studies): Three mortalities out of 26 tags, or roughly 86 percent survival.

Striped Marlin (seven studies): 14 mortalities out of 108 tags, or roughly 85.5 percent survival.

Sailfish (seven studies): Seven mortalities out of 85 tags, or roughly 90 percent survival.

White Marlin (six studies): Nine mortalities out of 94 tags, or roughly 89 percent survival.

Spearfish (one study): One mortality out of two tags, or roughly 50 percent survival.

While the authors could not detect any significant differences in overall post-release survival between gear types, they point out that this may be an artifact of unrepresentative studies and low statistical power in the analysis. That is, there may very well be significant effects in gear type, handling practices, etc.

This is evident in independent studies that have specifically compared the effectiveness of circle hooks to J hooks and have reported significantly higher survival with circle hooks. Given the relatively rare occurrence of post-release mortality in billfish, the authors point out that catch and release fishing is a viable fisheries management option.