David A. Beauchamp

Seasonal and Diel Distribution and Movement of Cutthroat Trout from Ultrasonic Telemetry

Abstract Our objective was to track the diel movements and distribution of large Bear Lake cutthroat trout Oncorhynchus clarki utah in relation to vertical environmental gradients and prey distribution in Strawberry Reservoir, Utah, during summer and autumn 1997. Implanted ultrasonic transmitters provided the depth and general location of the fish, while a differentially corrected Global Positioning System recorded boat positions that were then overlaid on a bathymetric map to determine temporal and spatial habitat use patterns. Tagged cutthroat trout used both nearshore and offshore habitats during both summer and autumn, but they were generally more pelagic during summer and used shallower nearshore zones more extensively in autumn. During early August, large cutthroat trout were restricted to the thermocline (metalimnion; 10-15 m deep) between a hypoxic hypolimnion and a warm epilimnion during all diel periods. When hypoxia affected the lower metalimnion in mid-August, the fish moved up into the upper ...

Ontogenetic Diet Shifts of Juvenile Chinook Salmon in Nearshore and Offshore Habitats of Puget Sound

Abstract Marine growth and survival of juvenile Chinook salmon Oncorhynchus tshawytscha depend in part on the quality and quantity of prey consumed during this potentially critical life stage; however, little is known about the early marine diet of these fish or factors that affect the diets variability. We examined the recent (2001–2007) dietary habits of Puget Sound, Washington, Chinook salmon (listed as threatened under the U.S. Endangered Species Act) during their first marine growing season (April–September). Juvenile Chinook salmon initially fed in nearshore marine habitats and then shifted to feed primarily offshore during July–September. Diet composition varied significantly among sampling regions (northern, central, and southern), habitats (nearshore, offshore), years, months, and fish size-classes. At nearshore sites, insects (all months) and gammarid amphipods (July) were dominant prey sources, whereas in offshore diets decapods (primarily crab larvae; July) and fish (September) were most impo...

Energy Density of Patagonian Aquatic Organisms and Empirical Predictions Based on Water Content

Abstract We measured the energy density for key species in the freshwater and marine ecosystems of Patagonia, providing the first database of such information for poorly known fauna, including fish, cephalopods, crustaceans, gastropods, annelids, and insects. We then considered an empirical model linking energy density (which is costly to estimate) to water content (which is easy to estimate) and compared the fit of the model with data from different taxonomic groups. Finally, we evaluated the predictive power of models with different levels of taxonomic aggregation to estimate energy density from water content. Fish (7,148-3,443 J/g of wet weight) had the highest energy density, followed by crustaceans (5,906-2,507 J/g), insects (5,794-1,334 J/g), mollusks (5,014-1,661 J/g), and annelids (4,542-1,954 J/g). The use of water content as a predictor of energy density provided greatly improved predictions as compared with use of the mean energy density value. As expected, the predictive power improved when mo...

Ontogenetic Trophic Interactions and Benthopelagic Coupling in Lake Washington: Evidence from Stable Isotopes and Diet Analysis

Abstract 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 δ15N values for copepods during winter and for zooplanktivorous longfin smelt Spi...

Early Marine Growth of Pink Salmon in Prince William Sound and the Coastal Gulf of Alaska During Years of Low and High Survival

Abstract Although early marine growth has repeatedly been correlated with overall survival in Pacific salmon Oncorhynchus spp., we currently lack a mechanistic understanding of smolt-to-adult survival. Smolt-to-adult survival of pink salmon O. gorbuscha returning to Prince William Sound was lower than average for juveniles that entered marine waters in 2001 and 2003 (3% in both years), and high for those that entered the ocean in 2002 (9%) and 2004 (8%). We used circulus patterns from scales to determine how the early marine growth of juvenile pink salmon differed (1) seasonally during May-October, the period hypothesized to be critical for survival; (2) between years of low and high survival; and (3) between hatchery and wild fish. Juvenile pink salmon exhibited larger average size, migrated onto the continental shelf and out of the sampling area more quickly, and survived better during 2002 and 2004 than during 2001 and 2003. Pink salmon were consistently larger throughout the summer and early fall duri...

Quantifying Predation on Galaxiids and Other Native Organisms by Introduced Rainbow Trout in an Ultraoligotrophic Lake in Northern Patagonia, Argentina: A Bioenergetics Modeling Approach

Abstract Exotic rainbow trout Oncorhynchus mykiss support an economically valuable recreational fishery in Patagonia but also create concern for impacts on native organisms. These concerns are intensified by the possibility of hatchery release programs in this region. We estimated losses of different prey from predation by rainbow trout in Lake Moreno, Rio Negro Province, Argentina, using a bioenergetics model combined with input data from directed sampling on growth, seasonal diet, distribution, and thermal experience. The fish community was sampled seasonally using gill nets, hydroacoustics, and ichthyoplankton nets. Pelagic galaxiid larvae and benthic juvenile and adult small puyen Galaxias maculatus were the most important components of the diet. Bioenergetics simulations showed that over a 6-year life span in the lake (ages 1–7), rainbow trout attained a body mass of 2.3 kg and consumed 74.7 kg of food, of which 20% consisted of galaxiid larvae and 16% consisted of adult small puyen. Based on an esti...

Interannual Variability in Early Marine Growth, Size-Selective Mortality, and Marine Survival for Prince William Sound Pink Salmon

Abstract The main objective of this study was to use scale patterns to compare the early marine growth of the average pink salmon Oncorhynchus gorbuscha with that of fish from the same year-class that survived to adulthood to gain insight on critical periods for growth and survival. During 2001–2004, pink salmon that survived to adulthood were larger and grew faster than the average juvenile throughout the first growing season, indicating that larger, faster-growing juveniles experienced higher survival. Growth rate declined from mid–late June to early–mid-July for both juveniles at-large and fish that survived to adulthood. The adult survivors then grew at a faster rate than the average juvenile through September. Both the juvenile pink salmon population at-large and all cohorts that survived to adulthood grew at a faster rate during high-survival years than low-survival years from mid–late June to mid–late August. Greater variability in the growth trajectories of surviving adults was observed during high-survival years, potentially a result of diversified feeding or distribution strategies. This study supports findings that significant size-selective mortality of juvenile pink salmon occurs after the first growing season. Investigating the timing and magnitude of size-selective mortality on juvenile pink salmon during their first growing season is an initial step toward understanding the processes regulating growth and survival.

Seasonal Patterns of Predation on Juvenile Pacific Salmon by Anadromous Cutthroat Trout in Puget Sound

Abstract In the marine environment, Pacific salmon Oncorhynchus spp. suffer the greatest natural losses during early marine residence, and predation is hypothesized to be the key source of mortality during this life history stage. In the face of recent declines in Puget Sound salmon populations, our goal was to determine the extent of predation mortality on salmon during early marine life. In spring and summer of 2001-2003, we caught juvenile salmon and potential predators at nearshore areas in northern and southern regions of Puget Sound, Washington. We focused on the potential predation impact of coastal cutthroat trout O. clarkii clarkii, which were caught in low but consistent numbers in both regions and were the most abundant large-bodied potential predators of juvenile salmon in our catches. Cutthroat trout consumed a diverse and dynamic array of diet items and became increasingly piscivorous with increasing fork length above 140 mm. Cutthroat trout consumed a greater biomass of Pacific herring Clup...

Modeling Prey Consumption by Native and Nonnative Piscivorous Fishes: Implications for Competition and Impacts on Shared Prey in an Ultraoligotrophic Lake in Patagonia

Abstract We examined trophic interactions of the nonnative salmonids Rainbow Trout Oncorhynchus mykiss, Brown Trout Salmo trutta, and Brook Trout Salvelinus fontinalisand the main native predator Creole Perch Percichthys trucha in Lake Nahuel Huapi (Patagonia, Argentina) to determine the relative impact of each predator on their forage base and to evaluate the potential vulnerability of each predator to competitive impacts by the others. Using bioenergetics simulations, we demonstrated the overall importance of galaxiids and decapods to the energy budgets of nonnative salmonids and Creole Perch. Introduced salmonids, especially Rainbow Trout, exerted considerably heavier predatory demands on shared resources than did the native Creole Perch on both a per capita basis and in terms of relative population impacts. Rainbow Trout consumed higher quantities and a wider size range of Small Puyen (also known as Inanga) Galaxias maculatus than the other predators, including early pelagic life stages of that prey; ...

Extensive feeding on sockeye salmon Oncorhynchus nerka smolts by bull trout Salvelinus confluentus during initial outmigration into a small, unregulated and inland British Columbia river

Stomach contents were collected and analysed from 22 bull trout Salvelinus confluentus at the edge of the Chilko Lake and Chilko River in British Columbia, Canada, during spring outmigration of sockeye salmon Oncorhynchus nerka smolts. Twenty of the 22 (>90%) stomachs contained prey items, virtually all identifiable prey items were outmigrant O. nerka smolts and stomach contents represented a large portion (0·0-12·6%) of estimated S. confluentus mass. The results demonstrate nearly exclusive and intense feeding by S. confluentus on outmigrant smolts, and support recent telemetry observations of high disappearance rates of O. nerka smolts leaving large natural lake systems prior to entering high-order unregulated river systems.