Visible Gill‐Net Injuries Predict Migration and Spawning Failure in Adult Sockeye Salmon

Abstract

AbstractFish that survive nontake fisheries interactions may subsequently die, a phenomenon that is generically termed “fisheries‐related incidental mortality” (FRIM). Gill nets, which typically asphyxiate fish and visibly damage their integument, inflict higher rates of FRIM than other commonly used gears. To better define FRIM associated with gill‐net encounters, an observational study coupled with biotelemetry measured migration survival and spawning success of a Sockeye Salmon Oncorhynchus nerka population during the final 45 km of their freshwater spawning migration (in 2014, 2015, and 2016). The daily prevalence of gill‐net injuries ranged from 0% to 80% of fish, resulting in an annual prevalence of 21–29% for females and 13–22% for males (over 3 years). Fish with visible gill‐net wounds had a 16% lower probability of completing their migration, and female fish with gill‐net wounds had an 18% lower probability of successfully spawning. As a result, the annual proportion of effective female spawners that died in the final 45 km of their migration due to gill‐net injuries was estimated to range from 3.8% to 9.9% (500 to 1,600 females). In addition, stray Sockeye Salmon from upriver populations commonly died at the tagging site, and visible gill‐net injuries were observed in half of those fish during a year with high mortality. If wild salmon populations continue to decline as climate change progresses, fisheries managers will be under greater pressure to minimize FRIM.