The amazing, changing gut of the Dolly Varden
If you have journal access, see the paper here. Otherwise, read on about the study:
The Alaska Salmon Program has been working in the Chignik Lakes watershed in Southwestern Alaska, for over 50 years. Each summer a few graduate students and technicians make Chignik Lake their home for three months while they conduct research related to the fish and environment.In 2008 I started a new project that I hoped would explain some of the curious migratory behaviors of Dolly Varden (Salvelinus malma) in the Chignik system and elsewhere. Dolly Varden in Chignik are anadromous, meaning that they are born in freshwater and eventually move to the ocean for feeding and growth. Dolly Varden are thought to move to the ocean in their third or fourth year of life, and only remain there for the summer months, returning to freshwater to overwinter.
In the June of 2009, we were investigating one of the main sockeye salmon spawning tributaries in the Chignik watershed, the Alec River (pictured), when we noticed something odd. There were lots of relatively large Dolly Varden in the river, exactly when we expected them to be at sea. Why were these fish in the river when they could be feeding at sea? Before salmon arrive, in August, there is not much to eat in the river, especially for these bigger fish. Our assumption was that we would only find young Dolly Varden (~6″) that were feeding on insects prior to their migration to the ocean. Oddly, there did not appear to be any small fish there, only larger (16″-20″), presumably older fish.
In addition to being large and abundant, Chignik Dolly Varden in June were also really skinny (pictured). They looked like they were wasting away. A check of their stomach contents confirmed that they weren’t eating much. Fellow graduate student Jonny Armstrong has been thinking a lot about subsidies and movement of fish, so we teamed up to delve deeper into the Alec River Dolly Varden, and why they did not migrate when most other fish appeared to do so. We know that for many fish migrating to sea is a risky proposition, and fish only do it if the net benefits (e.g., size, fecundity, weighed against risk of mortality) of moving exceed those of staying. Maybe the Alec River fish had found a way to survive and reproduce year after year without going to sea? We did some bioenergetics calculations that just didn’t make sense. The only thing the fish appear to eat is salmon eggs, and they eat lots of them. During spawning Dolly Varden might have several hundred eggs in their gut at one time. The fish get obese during this time and gain a lot of weight. But, our calculations indicate that they would still run out of energy by the middle of the next summer; not surviving until the salmon return in August.
Plasticity in gut size allows many organisms to change their energetic costs, a fact that has been well documented for birds and snakes among other organisms. We went back to the Alec River to see if changing the size of the gut was allowing large bodied Dolly Varden to survive the lean time between egg pulses, while maximizing their egg intake in the short time while salmon spawn.
To see what we found, see these comparisons of two fish of the same length from June (Top, before eggs are available, 10 months after the last pulse of eggs), and late August (Bottom, after about 4 weeks of feeding on eggs:
The fall fish is fat, but not just because it has a stomach full of eggs. The organs inside have changed too:
A closeup of the gut (stomach, pyloric caeca, and intestine) show that the gut doesn’t just stretch out to accomodate eggs, the digestive machinery has gotten much larger (2-4x larger in mass).
So what does this mean for the fish? Once Chignik Dolly Varden get to be near their full adult size, it appears that they can use the flexible gut strategy to remain in freshwater, acquiring enough resources to reproduce and live until the next bout of eggs. It may be that other species have similar strategies to deal with pulsed resources, particularly eggs, in the same way. However, many watersheds do not have nearly the egg subsidy they once had because of a reduction in population size, or eggs ending up in hatcheries rather than streams.