There is a great deal that is not known about what Alaska’s salmon eat when they are out surviving and fattening in the ocean. It would be very expensive to find out, but with today’s tools it would be possible, I think. What I bring to the table here from the Alaska Marine Science Symposium is fragmentary and inferential, but concerning. This is unpublished research that won’t make it into the peer-reviewed literature for some months or even years.
Two Bering Sea posters drew my attention in this regard, along with Russ Hopcroft’s biological sampling along the Seward Line in the Gulf of Alaska. UAF graduate student Tessa Minicucci’s poster “Competition among Western Alaska Chum Salmon and Asian Pink and Chum Salmon in the Bering Sea?” took an award for the way it begins to confirm a “density-dependent effects” hypothesis that there might be too many fish for the preferred prey. Basically, she is showing that the range of pink salmon from Russia and Japan greatly overlaps the range of the chums in the Bering/Aleutians in the spring and the fall. And when diet is measured (by the NOAA lab in Juneau,) there is a big difference in what they find in chums between odd and even years.
For those unfamiliar with this subject, I’ll add here that there is a well-known odd/even-year signal in pink salmon abundance. There are far more out there in odd years than even ones.
Minicucci shows the chums eating higher nutrition prey in even years, when there are fewer pinks. In odd years, they eat fewer euphausids, amphipods and calanoid copepods, and more jellyfish, pteropods and baby fish. In other words they may be losing out to the pinks for the higher nutrition prey.
Another poster, from Sonia Batten of the Sir Alister Hardy Foundation for Ocean Science, which has been monitoring Atlantic plankton since 1931, dealt with Pacific plankton sampled in the same area of the Aleutians along the great circle shipping route by a plankton net attached to a cargo vessel since the year 2000. Guess what? They found an odd/even abundance signal for calanoid copepods and the opposite signal for the diatoms the copepods like to eat, which “suggests that grazing pressure on phytoplankton by copepods us determined by pink salmon predation on the copepods, i.e. top down control.” Sonia didn’t get any awards for that one, but I saw several fish biologists scratching their heads in front of that poster.
Oh those pink salmon! Everybody knows they are big eaters and a few, a very few, have dared over the years to speculate that all those hatcheries set up in Southeast Alaska and Prince William Sound might be putting out so many of those eaters that somebody else would have to pay the price. But nothing is simple in this business. The pinks we’re talking about in these two studies come from Russia and Japan, at least for the most part. But poor runs of undersize chums to Arctic, Yukon and Kuskokwim rivers continue to be a reality. “At sea” conditions are blamed, and that’s convenient, because “at sea” is also still kind of a black box, though, as we are seeing, less of one than it used to be.
And all this is happening as the ecosystem goes through changes that are increasingly off the scale of what scientists are used to dealing with. There has been quite a bit of speculation that salmon from AYK would increasingly pasture in the colder waters of the more northern part of the Bering Sea, but that cold was not holding so well last month, though may be doing better now.
And out along the Seward Line, extending all along the shelf, and down to deeper waters, the plankton changed with “the blob.” Hopcroft showed the big calanoid copepods giving way to larger abundances of smaller species from waters to the south. As noted earlier, the big copepods, along with euphausids, are the better quality prey for salmon.
But it’s never simple. Hopcroft and other scientists say phenology needs to be understood here. Phenology is the timing of biological onsets and changes. The birds, for instance, need to arrive in the arctic at the time when their food appears, so they can gain the nutrition to reproduce, and then again the energy to migrate. In the case of calanoid copepods, they are adapted to the seasonal cold. They rise to feed, store enough fat to support a winter in the deeper cold waters, and go back down in the fall and become dormant. So they only have this desirable fat content in the spring and fall. Salmon seeking food in later spring might actually be better off with other species of copepods at that time.
And one more uncertainty here. Plankton nets were not really designed for what we now know is out there. Even today they are not very good at trapping euphausids, so that’s a particularly fuzzy part of the food chain picture. They are picked up in stomach contents, though, and they are more than half the diet of chum salmon. In odd years, by Minicucci’s data, that drops by about 50 percent.