Fraser River sockeye salmon: data synthesis and cumulative impacts

Research program presentations (continued)PredatorsMarine mammal predationAndrew Trites, UBCThe presentation covers which marine mammals eat sockeye, marine mammal population levelsand how much they eat. The assessment was based on peer-reviewed literature, governmentreports and unpublished data. Key metrics include numbers of marine mammals, populationtrends, diet composition and estimated consumption.Changes over time are linked to the history of hunts, culls, protection, recovery, stabilizationand/or increases (e.g. Stellar sea lions). There are temporal patterns, but no obvious connectionto sockeye abundance. Data is too sparse to say whether there is a difference in distribution thatmay relate to sockeye. Marine mammals move around – they are not sedentary. On whether anyone life stage appears to be impacted, there is no evidence of significant predation on smolts;predation is limited to adults and sockeye does not appear to be an important part of their diets.Predation by marine mammals has undoubtedly helped to shape the life history and physiologyof sockeye, including potential traits such as run timing, density of schools and swimmingperformance, all factors which would enhance their ability to escape being eaten.Evidence for stressor: Many species of marine mammals have been observed eating sockeye –people fishing notice the mammals taking sockeye off their lines. Many populations have alsorecovered to historic highs. And the Strait of Georgia has the highest density of harbor seals inthe world; and also because highest density of harbour seals in the world (are they eating sockeyeor something else?).However, sockeye is not a preferred prey species among those marine mammals that eat salmon.The seal population in Georgia Strait stabilized in the 1990s, before Fraser sockeye declined.Predation by marine mammals should also be equal across all Fraser River salmon stocks, andshould not just be affecting Fraser stocks since most marine mammals are not concentrated in theStrait of Georgia.The evidence supports some but not all of the conclusions in the PSC report. It is agreed thatpopulations of several species are back up, including Stellar sea lions, Pacific white-sideddolphins, harbor seals and humpback whales. However the evidence disagrees that total foodconsumption by marine mammals is large enough to affect sockeye populations. This would onlybe true if they only ate sockeye but the evidence suggests that sockeye account for less than 1%of their diet, not over 20% of the summer and fall diet of Stellar sea lions, as the PSC reportsuggested.Information gaps include outdated diet data for harbour seals, limited diet data for porpoises anddolphins, data for other seasons besides the summer months and more DNA studies to determinethe species of salmon consumed (DNA has been used successfully in the last 5 year).Salmon-eating marine mammals in BC:Stellar sea lions: Populations declined from the late 1950s due to culls, then saw a rapid risefrom the late 1990s and are still growing. Populations are still at all-time lows in Alaska.54