Estimates of humpback (Megaptera novaeanglia), minke (Balaenoptera acutorostrata) and killer (Orcinus orca) whale fishing gear interactions in Norwegian fisheries suggest low anthropogenic mortality
Arne Bj?rge, Andr? Moan, Kathrine A. Ryeng, ?ystein R. Wiig
Electronic fish logs from reference vessels and fishery inspector logbooks were used to estimate fishing gear interaction rates for humpback (Megaptera novaeangliae), minke (Balaenoptera acutorostrata) and killer (Orcinus orca) whales in Norwegian pelagic fisheries from 2010 to 2020. Estimated rates were applied to fisheries data to estimate fleet-wide non-observed totals. Reference fleet data indicated that there were two minke whale catches, one taken in a trawl and one on a longline. Data from both sources indicated that there were no cetacean entanglements in Danish seines. The fishery inspector logbooks indicated that entrapments of humpbacks and killer whales occurred infrequently in purse seine fisheries, but with a large peak in 2017. Estimates showed that in an 11-year period, a total of 77 humpback whales (95% confidence interval CI 43 – 177) and 121 killer whales (95% CI 75 – 232) were entrapped in purse seines, with an estimated mortality of 5% (CV 0.69, 0.0% – 11.8%) and 6% (CV 0.48, 95% CI 0.3% – 11.9%), respectively. Estimates also showed that in a 10-year period, 12 and 45 minke whales (95% CI 0 – 36, 0 – 139) were bycaught on longlines and trawl, respectively. The average yearly mortality rates over the study period were thus approximately 0.67 killer whales, 0.35 humpback whales and 5.7 minke whales per year. Given the Potential Biological Removal sustainability limits of 98 humpbacks, 161 killer whales and 1,498 minke whales per year, it may be concluded that the average yearly mortality incurred to these whale populations in Norwegian waters by Norwegian pelagic fisheries does not constitute a significant risk to either of these species. The 2017 peak was most likely caused by the dynamic Norwegian Spring Spawning herring (Clupea harengus) stock overwintering in narrow fjords, causing a substantial spatial and temporal overlap between whales feeding and fishermen operating purse seines.