Reintroduction of threespine stickleback into Cheney and Scout Lakes, Alaska

Background: Marine or anadromous threespine stickleback have colonized many northern Holarctic lakes after glacial recession, and their freshwater descendants have diverged in characteristic ways. Such divergence begins within a few generations, but previous studies have sampled only one generation or initiated sampling several generations after the populations were founded. Rotenone treatment of two Cook Inlet lakes to exterminate invasive northern pike also eliminated native threespine stickleback, creating an opportunity to introduce anadromous stickleback and observe their adaptation to freshwater conditions for multiple generations, beginning immediately after we founded the populations. Methods: In 2009 and 2011, we released about 3000 reproductively mature anadromous threespine stickleback into each lake. We sampled the source population and made annual samples from the two introduced populations. These samples have been preserved for future analysis. We estimated annual variation in relative abundance, made preliminary morphological observations, and assessed parasite diversity. Results: Anadromous stickleback released into the lakes produced abundant progeny, many of which survived and became reproductively mature the following spring. Both populations experienced demographic bottlenecks in 2013 and 2014 and began to recover in 2015. Preliminary observations indicate that stickleback in both lakes resemble anadromous stickleback, but by 2015 about 20% of the specimens in one population had a highly heritable, freshwater phenotype. One lake population had roughly twice as many parasite species and a much higher prevalence of a large metazoan parasite than either the anadromous ancestor or the other lake population. Our preliminary observations indicate that substantial evolution occurred during the first few generations, our existing samples can be used to study contemporary evolution, and sampling should be continued.