The paper is open access from Ecology and Evolution, click here to download it
It’s hard to appreciate just how many small streams, many unnamed, exist along the Alaska Peninsula. Dolly Varden are known to be early colonizers of these streams following deglaciation. However, Dolly Varden life histories often seem paradoxically unsuited for very small coastal streams. Dolly Varden, like many other char, head to sea for the first time at a fairly large size compared to other salmon. This means that they might rear in freshwater for several years before achieving a large enough size to smolt. So, very small streams may become inhospitable in winter months as flows decrease or waters freeze. In streams that drain to lakes, young Dolly Varden fry can move into the lake, or through it to a larger stream. What do Dolly Varden do in the myriad small streams that drain to saltwater?
We hypothesized that Dolly Varden in small streams draining to saltwater might be adapted to smolt, or tolerate seawater, at the end of their first year of life, as streams dry or freeze in winter months. Although we did not have the capacity to directly measure this, we electrofished several small streams and found Dolly Varden fry using these habitats.
Two lines of evidence support our hypothesis. First, streams draining directly to saltwater were genetically distinct from those draining to lake or large river habitats. The level of distinction was surprising given that some lagoon streams are only separated from freshwater streams by 5 km or so. Second, we didn’t find the larger, older Dolly Varden that one might expect if they were rearing in these streams for multiple years.
More work is needed to know what exactly is driving the patterns that we have observed, but there is a strong implication of alternate life histories in lagoon streams have led to the observed genetic isolation. These processes may be prevalent outside of Chignik, as many, many, small streams dot the coastline throughout Southwestern Alaska.