Salmon, caribou, and a possible dissertation topic
Last night after I went to bed, I started thinking about salmon. Before I went to bed, I was thinking about the population dynamics of caribou herds and the recent sequencing of mammoth DNA. These two thoughts led me start asking myself some questions about salmon for which I don't think we have answers yet (if we do, they are probably buried in some obscure journal of quaternary studies). I think these questions would make a great dissertation topic for the right biology student. Here's how it came to me.
In the Northwest, salmon are the most important fish that there is. The British and Icelanders might think that cod is important, Southerners think bass and catfish matter, and PZ Myers will tell you more about zebra fish than you ever thought possible, but we Northwesterners know that they have all misplaced their fishy affections. Salmon is the most important fish that there is. Commercial salmon fisheries are one of the biggest industries in any territory bordering the North Pacific. Tourists pay thousands to pilots and guides who will take them to streams where they can stand six feet from some other tourist trying to catch an eighty-pound king salmon. Salmon are the main food supply for all the bears in Pacific-draining territories. The Tlingit and Haida knew the importance of the salmon and made it one of the most important totems of their people.
In school, our biology classes spent a great deal of time telling us about the life cycle of the salmon. Adult salmon swim upstream each summer to spawn in the gravel of shallow and clear mountain streams. They have sex once and die. The newly hatched little salmon swim back to the sea and spend a few years eating and growing, and when they are fully adult, they return to the stream of their birth to spawn, starting the cycle anew. Each salmon swims thousands of miles and somehow manages to navigate back to the exact place where they were born and only saw that one time. This feat of navigation is one of the great miracles of nature. Unfortunately, it's also partly baloney.
Consider: 20,000 years ago the Puget Sound region where I live was buried under a thousand feet of ice. The Cordilleran ice sheet spread out from the peaks of the coastal ranges and filled Puget Sound. It covered Seattle, Vancouver, Juneau, and Anchorage. It filled all of the valleys of the northwest coast and carved the fijords of British Columbia and Southeastern Alaska. It filled Prince William Sound and Cook Inlet. It covered the Alaska Peninsula out to the beginning of the Aleutian Islands and it reached out to cover the first few of those islands. But it didn't move north into Alaska. Most of the interior was a grassy steppe with mammoths and caribou and bears.
So much water was locked up in the Cordilleran ice sheet and its sister ice caps that the oceans were 600 feet lower. Great stretches of land were exposed. The North Sea was dry attaching Great Britain to Europe. The Falklands were indisputably part of Argentina. The islands of Indonesia were mountain peaks that surveyed a vast forested subcontinent. Alaska and Siberia were a single dry landmass that modern geologists call Beringia. Most of the rivers and streams between the Columbia and the Yukon rivers were buried under ice and could not have provided spawning ground for the salmon.
About twenty years ago, I saw a documentary on the population dynamics of caribou herds and their synchronization with the wolf population. The two populations rise and fall in a rather simple cycle. From a starting point with a low population of both, the caribou herd begins to grow. There are few competitors for the grazing land, so when the wolves are few the caribou begin a population explosion. However, when the numbers of caribou increase, that means more food for the wolves and they begin to have bigger litters and more frequent litters. The wolves embark on a population explosion of their own, lagging behind the caribou by a year or two. Eventually, the caribou population saturates its grazing land and has to face the rapidly growing wolf population. The growth of the caribou slows, stops, and then collapses. The wolf population, deprived of it's caribou smorgasbord, experiences the same stop and fall. Then the cycle starts over.
This is a very simplified version of the story. Wolves have other food sources, but prefer caribou when they can get it. People complicate matters considerably. Finally, there is not just one caribou population facing one wolf population. Across the Arctic North, there are a half dozen caribou herds. Each goes through the same cycle of population growth and collapse. But here's the kicker, they do it out of cycle with each other. When one herd reaches its maximum and competition for resources becomes rougher, some members will leave and join a neighboring smaller herd. Caribou are a migratory animal with set feeding and calving grounds, but they are able to overcome this and learn the ways of a new herd. This emigration gives valuable genetic diversity to the herds and often helps bring the declining herds back from the brink.
There are endless ecological points that I could make here, but let's get back to the salmon.
When the Cordilleran ice sheet melted 13,000 years ago, it exposed new valleys to the sea. Today these valleys are all stocked with salmon. How did they get there? Some salmon must have simply swam up the wrong streams, their famous navigation not being all we were taught. From the Yukon in the north and the Columbia in the south, salmon must have slowly stumbled onto new streams and colonized them. Was there a systematic overflow, like caribou at the top of their population cycle, or did they just get lost? Do salmon still wander into the wrong stream, or was this a temporary event limited to the period when the seas were rising and the coastlines changing?
That's question number one: how did the salmon colonize the northwest coast? Unless salmon have been observed colonizing new spaces, I'm not sure we can answer this one with anything other than just-so stories like the ones I just gave.
If you look at a textbook map of the last ice age, it will probably show the two North American ice sheets: the Laurentian, covering most of Canada, and the Cordilleran, which I described above. Your map will probably show the peak expansion of both when they touched and merged in Alberta. Your map probably will also show both coming right down to the oceans on the eastern and western margins of the continent. That detail is wrong.
The Cordilleran ice sheet probably met the sea in much the same way that the final remnants of it do today. That is, tongues of ice flow down valeys until they meet the sea, where they float and break off. Between these rivers of ice are un-glaciated ridges and bits of coastline that form islands in the sea of ice. Geoleogists are just beginning to map these islands. Many were large enough to hold entire small ecosystems that called refugia. One known refugia included Prince of Wales Island in Southeastern Alaska. The bears of some Southeastern islands were separated from their mainland brethren for long enough that they form a genetically distinct population. Did they have salmon to eat?
That's question number two: who were the salmon colonists of the northwest coast? Were they primarily from the Yukon and Columbia? Did they come more from one direction than from the other? Were there indigenous salmon in the coastal refugia to resist this invasion of yankee creek grabing aliens? Did they succeed? It seems to me that this is a doable dissertation or post-doc project. It requires mapping the DNA relationships between the local populations of one of the four species of salmon on the coast.
If you do this project, write me and let me know what you find out, because I really want to know how all of those salmon got where they are.