See the paper here, or email me for a reprint
Anadromous migrations are one of the most recognizable characteristics of salmon. Young salmon head to sea for rearing where productivity and growth potential is higher than in freshwater environments. Semelparous salmon (e.g. Chinook, coho, sockeye, chum, pink) return to freshwater after a year for spawning and death after a year or more at sea. However, the pattern of migration and maturity is complicated in the iteroparous, facultitvely anadromous species (bull trout, cutthroat trout, Dolly Varden, brown trout, Arctic char, steelhead, etc.) that may vary widely in age at ocean migration and duration of time at sea, even within a population. In some cases individuals may remain in fresh water their entire lives, forgoing any ocean migrations.
Dolly Varden, Salvelinus malma, are a char species (closely related to Arctic char, Lake trout, brook trout and others) that is often facultatively anadromous where ocean access is available. Like many char species, they seem to have short ocean migrations only lasting the few summer months. In Chignik Lakes, Alaska, they are found in nearly every habitat, but migratory individuals primarily enter saltwater shortly after ice out (~ May), remaining there until August or September when the move back upstream for spawning or overwintering. Given this, in midsummer we would expect to find larger, older Dolly Varden in marine waters, and the smallest, youngest ones in fresh water. However, when we set about sampling Dolly Varden throughout the watershed, the very largest,oldest individuals were found in the Alec River, a headwater river about 25 km from marine waters. This begged the question: Why weren’t these fish at sea with the others?
We came up with several hypotheses about why we might find this pattern. First, Dolly Varden in Chignik may exhibit two distinctly different life histories, some individuals are migratory and others are resident. Like steelhead and rainbow trout, it seemed plausible that the Alec River Dollies were simply resident fish that never went to sea.
Second, it is possible that the large fish in the Alec River were either males or females remaining in fresh water while the other sex was migratory. In some Atlantic salmon populations nearly all of the migratory fish are female, while males remain in fresh water. This is because in females egg number is tied to body size, and they may have much more incentive to go to sea and get big.
Third, it could be that all fish are migratory, but not in every year. Therefore, the individuals we observe in the Alec River are simply those that skipped migrating for a season before resuming.
Fourth, fish are migratory at young ages, but cease migrating as they age.
To determine which hypothesis was supported, we needed a way to evaluate the migratory history of many individuals. For this we employed otolith microchemistry. Otoliths (ear stones) form a permanent record of the water chemistry that fish encounter. Marine chemistry differs from that of fresh water, and is detectable in the otoliths. We scanned the otoliths of fish from throughout the system to compare the migratory histories of individuals of many ages. What we found was that most fish in the system make at least one migration to sea. However, many of them make one or two consecutive migrations then effectively “retire,” remaining in fresh water for the duration of their lives. Fish begin retiring at about age 4, and by age 6 most fish are retired. Additionally, as we might expect, fish that fish that go to sea are larger for their age than those that remain in fresh water. But, that difference disappears by age 4, the age at which fish start retiring. What this means is that the benefits of marine migration are relatively weak in this system. We don’t know why this is exactly, but Chingik is a very productive system, and salmon subsidies are large for Dollies remaining in freshwater. Our previous work demonstrated that larger fish can survive and reproduce on a seasonal diet of salmon eggs alone.
So, it appears that Dolly Varden go to sea when they are too young (small) to survive on salmon subsidies, entering risky marine waters for a rich diet of sand lance, amphipods and crab larvae. Then, once they are large enough to survive on eggs, they retire in the relative safety of the Alec River and wait for the salmon eggs to come to them. It may be that fish from other habitats or tributaries with fewer salmon continue migrating, but we don’t have those data yet. To our knowledge, this is one of the first examples of a fish initiating migratory behavior, then later abandoning it for a sedentary life.