Faced with sudden environmental changes, animals must either adapt to novel environments or go extinct. Thus, study of the mechanisms underlying rapid adaptation is crucial not only for the understanding of natural evolutionary processes but also for the understanding of human-induced evolutionary change, which is an increasingly important problem [1-8]. In the present study, we demonstrate that the frequency of completely plated threespine stickleback fish (Gasterosteus aculeatus) has increased in an urban freshwater lake (Lake Washington, Seattle, Washington) within the last 40 years. This is a dramatic example of "reverse evolution,"[9] because the general evolutionary trajectory is toward armor-plate reduction in freshwater sticklebacks [10]. On the basis of our genetic studies and simulations, we propose that the most likely cause of reverse evolution is increased selection for the completely plated morph, which we suggest could result from higher levels of trout predation after a sudden increase in water transparency during the early 1970s. Rapid evolution was facilitated by the existence of standing allelic variation in Ectodysplasin (Eda), the gene that underlies the major plate-morph locus [11]. The Lake Washington stickleback thus provides a novel example of reverse evolution, which is probably caused by a change in allele frequency at the major plate locus in response to a changing predation regime.

We compared temporal consumption rates by sockeye salmon Oncorhynchus nerka fry with food supply to evaluate how different survival and dispersal rates or additional hatchery production affected the winter carrying capacity of Lake Washington, Washington. Peak immigration of sockeye salmon fry into southern Lake Washington precedes the spring zooplankton bloom by 2–3 months. Zooplankton density, fish diet, and growth were sampled during winter and spring 2001, when a record 52.4 million fry entered the lake. Supplementary information on the distribution and dispersal of fry was collected in 2002 and 2003. We used bioenergetics modeling to estimate the temporal–spatial consumption by fry during early lake rearing. Cyclops bicuspidatus were dominant in the diet and zooplankton assemblage but declined from more than 30/L in late February and early March to fewer than 5/L during mid‐March and early April. Fry ate significantly larger Cyclops than the average size in the lake; approximately 83% of the food items in the stomachs were larger than 0.8 mm, which was the minimum food item size. Hydroacoustics and midwater trawling surveys during 2002 and 2003 suggested that fry dispersed quickly over the southern half of the lake and used depths of 0–30 m. Under the most realistic scenarios for fry dispersal, feeding, and survival, total consumption of all prey by sockeye salmon fry represented 5% of the average monthly biomass of Cyclops during March and early April, when their availability was lowest. Potential bottlenecks in the availability of Cyclops only developed in simulations when fry were restricted to feeding in 0–10‐m depths in the southernmost region of the lake. Despite the seemingly adequate food supply for sockeye salmon fry, other planktivores also consume significant fractions of the exploitable prey biomass, and the interannual availability of exploitable zooplankton varies considerably during winter and early spring. Either of these factors could impinge on the localized food supply for sockeye salmon and other species during some years and should be considered in the adaptive management of any enhancement program.

Stable isotopes of nitrogen and carbon and stomach content analysis were used to determine the trophic position and relative importance of benthic and pelagic pathways for different life stages and species of the major fishes and invertebrate prey in Lake Washington. Significant coupling of the benthic and pelagic pathways was evident in this complex food web across seasons and fish ontogenies. Among apex predators, cutthroat trout Oncorhynchus clarkii and northern pikeminnow Ptychocheilus oregonensis shifted ontogenetically from benthic omnivory to pelagic piscivory, whereas yellow perch Perca flavescens shifted from pelagic zooplanktivory to benthic piscivory. Apex predators continued to rely on benthic prey seasonally, particularly in winter and spring. Benthic pathways were less important to the current diets of apex predators than they were during the recovery from eutrophication in the 1970s. Surprisingly, the d 15 N values for copepods during winter and for zooplanktivorous longfin smelt Spirinchus thaleichthys and threespine sticklebacks Gasterosteus aculeatus were similar to those for top piscivores, whereas the significantly lower values for zooplanktivorous juvenile sockeye salmon O. nerka were more similar to expectations. Nitrogen and carbon isotope ratios of pelagic planktivores and invertebrates also varied seasonally. Mixing model results showed that stable isotopes and stomach contents were comparable for determining ontogenetic trends, but stable isotopes established these trends with many fewer samples and less variability and accurately portrayed ontogenetic trends when few stomach samples were available. However, stomach content analysis was critical in delineating seasonal trends in diets and for identifying specific prey species.