Salmonids often display a series of ontogenetic shifts in habitat, and these may also be associated with changes in diet. For example, adfluvial populations rear in streams for several years and then migrate to lakes. The patterns of habitat use, trophic ecology, and movements of such populations are commonly studied during the riverine stages. The lacustrine period is typically less well known, but salmonids may play an important ecological role as lake piscivores. In Lake Washington, Seattle, Washington, cutthroat trout Oncorhynchus clarki are a top native piscivore and may affect the dynamics of the fish and zooplankton upon which they prey. Our objective was to study the growth, diet, and size distribution of cutthroat trout in littoral and limnetic habitats of Lake Washington, with emphasis on consumption of two of the lake's dominant pelagic planktivores: juvenile sockeye salmon O. nerka and longfin smelt Spirinchus thaleichthys. Cutthroat trout entered the lake at approximately age 2. As they grew larger, the cutthroat trout became increasingly piscivorous and tended to occupy the limnetic zone after they reached about 250 mm fork length (FL). Specifically, percentages (by wet weight) of fish in the diet of cutthroat trout increased from 22.5% for cutthroat trout smaller than 200 mm FL to over 95% for cutthroat trout larger than 400 mm FL. Fish made up a higher percentage of cutthroat trout diets in fall and winter in both the limnetic and littoral zones, and a greater proportion of fish was consumed in the limnetic zone than in the littoral zone. Variation in diet was observed among years (1995–2000), apparently reflecting the relative abundance of longfin smelt. The role of cutthroat trout both as the object of recreational fisheries and as a predator on sockeye salmon (valued in commercial and recreational fisheries) complicates management of this large, urban lake.

In summer and fall 1989, six rainbow trout (Oncorhynchus mykiss) were tracked in Lake Washington with ultrasonic transmitters for a total of 349 h to determine their movements in relation to the distribution of possible prey species. The trout moved primarily in the nearshore area at an average velocity of 12.4 cm/s (--0.25-0.3 body lengths/s). Five of the six fish made one more rapid (>1 body lengths/s) excursion across the lake, then continued moving in the nearshore zone. The trout were generally inactive, staying close (<50 m) to shore at night, and became more active near dawn; however, the highest average velocities were at dusk. They spent over 90% of their time in the top 3 m of the water column and 10% in brief (2 mm), shallow (mean 6.6 m) dives. Dives occurred most frequently at dawn and during the day (0.8/h), less often near dusk (0.5/h), and seldom at night (0.1/h). The depth distribution and movement patterns suggest that the trout were feeding on Daphnia pulicaria during the day, in both nearshore and offshore areas, supplementing this diet with nearshore fishes such as prickly sculpins (Cottus asper). Predation on pelagic planktivores (longfin smelt, Spirinchus thaleichthyes, and juvenile sockeye salmon, Oncorhynchus nerka) was unlikely because the trout were primarily found nearshore and near the surface, whereas the planktivores are primarily offshore and closer to the bottom. Ré sumé : Au cours de l'é téet de l'automne de 1989, six Truites arc-en-ciel (Oncorhynchus mykiss) du lac Washington ont é tésuivies au moyen d'é metteurs d'ultra-sons pendant un total de 349 h, ce qui a nous permis d'é tudier la relation entre leurs ddplacements et la repartition des espèces proies disponibles. Les truites se dé placaient principalement dans les eaux du Bord a une vitesse moyenne de 12,4 cm/s (-0,25-0,3 longueurs totales/s). Cinq des six truites ont entrepris une excursion plus rapide (>1 iongueur totaie/s) d'une rive a i'autre du lac, puis ont repris leurs dé piacements près des rives. La nuit, les truites é taient gé né ralement inactives, restaient près du bord (<50 m) et elles reprenaient leurs activité s a l'aurore, mais leur vitesse moyenne maximale de nage a é téenregistré e au cré puscule. Les truites passaient plus de 90% de leur temps dans les 3 m supé rieurs de la colonne d'eau et 10% de leur temps a effectuer de courtes (2 mm) plongé es peu profondes (moyenne 6,6 m). Les plongé es avaient lieu surtout a l'aurore et durant le jour (0,8/h), moms souvent au cré puscule (0,5/h) et rarement la nuit (0,1/h). Leur repartition dans la colonne d'eau et leurs dé placements semblent indiquer que les truites se nourrissaient de Daphnia pulicaria durant le jour, aussi bien au bord que dans les zones situé es plus au large, et qu'elles consommaient des poissons littoraux, tels le Chabot piquant (Cottus asper), comme complements de nourriture. La predation d'espèces pé lagiques planctonophages (l'Eperlan d'hiver, Spirinchus thaleichthyes, et les stades juveniles du Saumon rouge, Oncorhynchus nerka) semble pe...

We studied the ontogenetic diet shifts of prickly sculpin Cottus asper (the largest North American freshwater cottid) in the Lake Washington basin from a variety of habitat types, including fluvial and lacustrine. In all habitats, prickly sculpin progressively shifted to larger prey, such as fish and crayfish (Decapoda), as they increased in size. In offshore areas of Lake Washington, amphipods were the dominant prey by weight consumed by prickly sculpin of 75-124 mm total length (TL). Although generally uncommon numerically in the diet, fish (primarily small cottids) made up a large percentage of the diet by weight for prickly sculpin larger than 125 mm TL. In the lower end of the Cedar River, juvenile sockeye salmon Oncorhynchus nerka (23-30 mm TL) were most commonly observed in the diets of 50-99-mm prickly sculpin, while larger prickly sculpin tended to consume larger fish, including adult longfin smelt Spirinchus thaleichthys, lampreys Lampetra spp. (ammocoetes and adults), and small cottids. For each habitat type, diet overlap tended to decrease as size-classes became more dissimilar. Overall, the size of fish eaten was strongly related to prickly sculpin size, but the type of fish eaten influenced the relationship between prey size and predator size. The higher percentage contribution of fish to prickly sculpin diets seen here relative to other studies can be explained by the fact that (1) we collected many large prickly sculpin and sampled a wide variety of habitat types and (2) potential prey fish in the Lake Washington system are abundant and diverse. Our results, in combination with other research on the Lake Washington ecosystem, suggest that because of their size, abundance, wide range in habitat use, and breadth of diet, prickly sculpin are an especially important species in the food web of this system. Prickly sculpin are directly linked to the pelagic food web as both predators and prey, and they play a key role as benthic predators.