Author(s): Amy Teffer & Scott Hinch
Published by: Conservation Physiology (March 7, 2017)
Capture severity, infectious disease processes and sex influence post-release mortality of sockeye salmon bycatch
Adult salmon that are released after being captured are stressed and injured. Injured fish are an easy target for bacteria, viruses and other pathogens that can kill fish before they spawn, especially in warm water. Our research examines how gillnet entanglement duration and water temperature affect disease development and survival. We captured adult sockeye salmon during their spawning migration and brought them to the lab where we entangled them in a gillnet for either 20 seconds or 20 minutes and then air-exposed them for one minute. They were monitored for two months with another group of fish that were not net- or air-exposed. Water temperature was changed daily to match that experienced by these fish during upriver migration. We took blood and tissue samples at the start of the experiment and as fish died to measure how multiple infections and host responses changed over time. We did this by looking at genes expressed by both the fish and the infectious agents that they carried, which painted a picture of infectious disease development.
Overall, fish that were not entangled survived the longest, generally more than 20 days, which marks the start of the spawning period. Fish exposed to 20-minute entanglement survived the fewest days, averaging only 9 days for females and 16 days for males. Survival was not much better for 20-second entangled fish, which lived an average of 17 days if female and 24 days if male. Most mortality happened when temperatures were highest and few entangled fish survived to the spawning period. How much (and how long) fish struggled in the net and whether an anti-viral immune response was present during capture influenced how long fish lived after entanglement. Fish that died within two weeks had several heavy infections and were in poor health with low immune activity compared to survivors. This is some of the first evidence for pathogen development in released catch and supports keeping entanglement durations short and limiting fishing when rivers are warm. This is the first of a series of studies exploring the interacting effects of warming rivers and gillnet entanglement on disease development, migration success and survival of released salmon.