No abstract

The patterns of homing and straying of spring chinook salmon (Oncorhynchus tshawytscha) from the Cowlitz River, Washington, were analyzed, based on coded wire tag recoveries. Out of an estimated escapement of 41 085 chinook salmon, 98.6% returned to Cowlitz River and the rest were recovered in other rivers. Almost all strays were within the Columbia River system, and most were found in the Lewis and Kalama rivers, upstream of Cowlitz River. Straying was positively correlated with age at return and negatively correlated with number of returning salmon.

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.

Fisheries oceanography is the study of ecological relationships between fishes and the dynamics of their marine environments and aims to characterize the physical, chemical, and biological factors that affect the recruitment and abundance of harvested species. A recent push within the fisheries management community is toward ecosystem-based management. Here, we show how physical and biological oceanography data can be used to generate indicators of ocean conditions in an ecosystem context, and how these indicators relate to the recruitment of salmonids, sablefish, sardines, and rockfish in the California Current.

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.