Fraser River sockeye salmon: data synthesis and cumulative impacts

Information gaps: The most important unknown is ecosystem monitoring for key predators.Information is poor for the open ocean and in freshwater. DFO’s attempts to implementintegrated management have been very limited and the recent ecosystem initiative was so poorlyfunded as to be meaningless. Key research needs include learning more about the abundance ofnon-commercial species and what’s happening in the open ocean.DiscussionPeterman: You have made an important point that sockeye could represent only 0.1% of the dietof dogfish and still suffer significant impact, given the abundance of the latter. We need to dothe rest of these calculations. We haven’t got the total salmon biomass that could be eaten bymarine mammals.Christensen: We can say let’s have an experimental fishery. Stomach samples reveal the averagediet. With DNA, you can identify prey that comprises only a very small proportion of thediet.Trites: It’s very simple to do the next step of putting a number on it. We need to do it for the fishas well because the fish eat far more. A big part of the diet of harbour seals was hake. Whatare those hake eating? So the seals could also be controlling other salmon predators, thushaving a positive as well as a negative impact. You can’t just put one number out withoutcontext – ecosystem models are important.Christensen: It’s very clear from this that we can’t point to one single species so we are probablylooking at cumulative effects.McKinnell: We also have to consider the behaviour of these species. Which predators have abehaviour that could put them in contact with high densities of outgoing sockeye smolts (e.g.which ones operate in the top 10 metres of water)?Christensen: This highlights the point in the quote about the salmon ocean refuge being 10miles offshore. About 15% of recent sockeye samples had daggertooth wounds.McKinnell: Regarding the diet of salmon sharks, it’s important to ensure that the target speciesfor the fishery where they are caught for stomach analysis was for salmon sharks. They willeat whatever else is in the net, so if they are caught in a salmon fishery, that will create bias.Routledge: Do Pacific mackerel only come north when the ocean is warmer?Christensen: That was not a major part of the argument. What is important is the big changeshappening in the Gulf of Alaska.Routledge: It has been hypothesized that mackerel are a major predator during El Nino events.Christensen: I don’t know.Martin: There has been a large change in the biomass of pilchards and mackerel over time.Christensen: There are now studies showing how much individual salmon sharks move aroundbut little is known about overall populations.Trites: Stellar sea lions are probably the ones that have been best studied, and pilchards show upin their diets when there are big changes.Dorner: The likely suspects are in the Gulf of Alaska, but Alaska stocks have not been affected.57