Gregory T. Ruggerone

Consumption of Migrating Juvenile Salmonids by Gulls Foraging below a Columbia River Dam

Abstract Consumption of migrating juvenile Pacific salmon Oncorhynchus spp. and steelhead Salmo gairdneri by gulls was estimated below the turbine area of Wanapum Dam on the Columbia River in 1982. Foraging success of the gulls, chiefly ring-billed gulls Larus delawarensis, averaged 65% during bright light conditions and 51% during the evening. The number of salmonids consumed by gulls ranged from 50 to 562 fish/h. Multiple-regression analysis indicates that the number of salmonids consumed by gulls is significantly affected by the passage rate of fish through the turbines and spillgates, and by light intensity. The number of salmonids consumed by gulls foraging below the turbines during 25 d of peak salmonid migration was approximately 111,750 to 119,250 fish or 2% of the estimated spring migration. Although some salmonids consumed by gulls had been killed when passing through the turbines, the mortality of salmonids caused by gulls is potentially important. Inexpensive measures could be implemented to d...

Factors affecting marine growth of Bristol bay sockeye salmon

Abstract The growth of Bristol Bay sockeye salmon in freshwater and in the last few months at sea is dependent on their abundance (a negative correlate) and the prevailing water temperature (a positive correlate). In 1990 and 1991, the returning sockeye salmon to Bristol Bay were unusually small relative to their abundances and temperatures and this raised a concern for limited ocean carrying capacity because other stocks of salmon were also very numerous in those years. However, the inclusion of other stocks of sockeye salmon and the abundant runs of Asian chum salmon in a multiple regression analysis did not provide a better predictor of the body size of Bristol Bay sockeye salmon. Growth increments of Nushagak Bay sockeye salmon in their first and second years at sea (from scale measurements) were correlated with temperatures but not with the abundances since 1975. The growth increment in the third year of sea life was uncorrelated with temperature and abundance but was correlated with final adult length. This suggests that concerns for carrying capacity limitations should be placed on the migratory routes of returning adults when the fish are probably most concentrated and their growth most limited by their food supply.

Predation on Sockeye Salmon Fry by Juvenile Coho Salmon in the Chignik Lakes, Alaska: Implications for Salmon Management

Abstract -The consumption of recently emerged sockeye salmon Oncorhynchus nerka by juvenile coho salmon O. kisutch in Chignik Lake, Alaska, was estimated for 15 May–5 August, 1985–1987. Estimated daily consumption of sockeye salmon fry by individual coho salmon in Chignik Lake, based on a stomach evacuation method, increased from about 2.0 fry in late May to 3.3 fry in June, then it declined in July and early August to 1.1 fry. Average consumption during each year, based on a bioenergetic approach, was within 14% (range, 15–20%) of the stomach evacuation method estimates, whereas bioenergetic estimates on a given day generally were within 30% of the stomach evacuation method estimates. Few coho salmon were captured in nearby Black Lake, where consumption of sockeye salmon fry per juvenile coho salmon was low. Estimates of sockeye salmon fry consumed by coho salmon in Chignik Lake, based on two independent estimates of juvenile coho salmon abundance, were 68 million, 24 million, and 78 million, which repre...

Adaptive strategies and life history characteristics in a warming climate: Salmon in the Arctic?

In the warming Arctic, aquatic habitats are in flux and salmon are exploring their options. Adult Pacific salmon, including sockeye (Oncorhynchus nerka), coho (O. kisutch), Chinook (O. tshawytscha), pink (O. gorbuscha) and chum (O. keta) have been captured throughout the Arctic. Pink and chum salmon are the most common species found in the Arctic today. These species are less dependent on freshwater habitats as juveniles and grow quickly in marine habitats. Putative spawning populations are rare in the North American Arctic and limited to pink salmon in drainages north of Point Hope, Alaska, chum salmon spawning rivers draining to the northwestern Beaufort Sea, and small populations of chum and pink salmon in Canada’s Mackenzie River. Pacific salmon have colonized several large river basins draining to the Kara, Laptev and East Siberian seas in the Russian Arctic. These populations probably developed from hatchery supplementation efforts in the 1960’s. Hundreds of populations of Arctic Atlantic salmon (Salmo salar) are found in Russia, Norway and Finland. Atlantic salmon have extended their range eastward as far as the Kara Sea in central Russian. A small native population of Atlantic salmon is found in Canada’s Ungava Bay. The northern tip of Quebec seems to be an Atlantic salmon migration barrier for other North American stocks. Compatibility between life history requirements and ecological conditions are prerequisite for salmon colonizing Arctic habitats. Broad-scale predictive models of climate change in the Arctic give little information about feedback processes contributing to local conditions, especially in freshwater systems. This paper reviews the recent history of salmon in the Arctic and explores various patterns of climate change that may influence range expansions and future sustainability of salmon in Arctic habitats. A summary of the research needs that will allow informed expectation of further Arctic colonization by salmon is given.

Gastric evacuation of single and multiple meals by piscivorous coho salmon, Oncorhynchus kisutch

SynopsisGastric evacuation of multiple meals of recently emerged sockeye salmon, Oncorhynchus nerka, consumed by juvenile coho salmon, O. kisutch, were compared with that expected from a single meal evacuation rate model developed for the estimation of food consumption by coho in the field. Significant interaction occurred between meals consumed two hours apart (p<0.001). Evacuation of the first of two meals (29.9% of initial weight remaining) was significantly faster (p<0.05) than that of a single meal (36.1% of initial weight remaining) after four hours, whereas evacuation of the second meal (72.2% of initial weight remaining) was significantly slower (p<0.001) than that of single meals (50.6% of initial weight remaining) after two hours. The total weight of the multiple meal remaining after four (0.191 g), six (0.138 g) and eight (0.070 g) hours averaged within 6% of that predicted by the single meal evacuation model (0.186 g, 0.115 g and 0.073 g, respectively). These data suggest that the stomach evacuation model based on single meals is adequate for estimating the evacuation of prey consumed by continuously feeding coho salmon.

Evidence for competition at sea between Norton Sound chum salmon and Asian hatchery chum salmon

Increasing production of hatchery salmon over the past four decades has led to concerns about possible density-dependent effects on wild Pacific salmon populations in the North Pacific Ocean. The concern arises because salmon from distant regions overlap in the ocean, and wild salmon populations having low productivity may compete for food with abundant hatchery populations. We tested the hypothesis that adult length-at-age, age-at-maturation, productivity, and abundance of a Norton Sound, Alaska, chum salmon population were influenced by Asian hatchery chum salmon, which have become exceptionally abundant and surpassed the abundance of wild chum salmon in the North Pacific beginning in the early 1980s. We found that smaller adult length-at-age, delayed age-at-maturation, and reduced productivity and abundance of the Norton Sound salmon population were associated with greater production of Asian hatchery chum salmon since 1965. Modeling of the density-dependent relationship, while controlling for other influential variables, indicated that an increase in adult hatchery chum salmon abundance from 10 million to 80 million adult fish led to a 72% reduction in the abundance of the wild chum salmon population. These findings indicate that competition with hatchery chum salmon contributed to the low productivity and abundance of Norton Sound chum salmon, which includes several stocks that are classified as Stocks of Concern by the State of Alaska. This study provides new evidence indicating that large-scale hatchery production may influence body size, age-at-maturation, productivity and abundance of a distant wild salmon population.

Size Selectivity of Predation by Brown Bears Depends on the Density of Their Sockeye Salmon Prey

Can variation in prey density drive changes in the intensity or direction of selective predation in natural systems? Despite ample evidence of density-dependent selection, the influence of prey density on predatory selection patterns has seldom been investigated empirically. We used 20 years of field data on brown bears (Ursus arctos) foraging on sockeye salmon (Oncorhynchus nerka) in Alaska, to test the hypothesis that salmon density affects the strength of size-selective predation. Measurements from 41,240 individual salmon were used to calculate variance-standardized selection differentials describing the direction and magnitude of selection. Across the time series, the intensity of predatory selection was inversely correlated with salmon density; greater selection for smaller salmon occurred at low salmon densities as bears’ tendency to kill larger-than-average salmon was magnified. This novel connection between density dependence and selective predation runs contrary to some aspects of optimal foraging theory and differs from many observations of density-dependent selection because (1) the direction of selection remains constant while its magnitude changes as a function of density and (2) stronger selection is observed at low abundance. These findings indicate that sockeye salmon may be subject to fishery-induced size selection from both direct mechanisms and latent effects of altered predatory selection patterns on the spawning grounds, resulting from reduced salmon abundance.

Trends and Factors Influencing the Length, Compensatory Growth, and Size-Selective Mortality of Juvenile Bristol Bay, Alaska, Sockeye Salmon at Sea

Abstract The productivity of Bristol Bay, Alaska, Sockeye Salmon Oncorhynchus nerka increased during the mid-1970s. This increase is believed to be partially due to an increase in early marine growth associated with the 1976–1977 cool-to-warm shift in summer sea surface temperature (SST). The body size of juvenile salmon during their first year at sea is believed to regulate their ability to survive over winter. The back-calculated smolt length, first-year ocean growth, and total juvenile length of Sockeye Salmon from five Bristol Bay river systems (Egegik, Kvichak, Naknek, Ugashik, and Wood) and two smolt ages were used to examine trends and factors influencing total juvenile length, compensatory growth, and size-selective mortality in the first year in the ocean from 1962 to 2007. Juvenile length increased in relation to summer sea temperature, the 1977–2001 and 2002–2007 warm temperature regimes, smolt length, and compensatory growth. Compensatory growth—an inverse relationship between first-year ocean growth and smolt size—increased over time as well as after the 1976–1977 climate regime shift, was more common in age-1.0 fish than in age-2.0 juveniles, and was important in determining the length of juvenile Sockeye Salmon from the Wood River (the shorter fish among rivers and smolt ages). The coefficient of variation in length did not change with SST, suggesting that size-selective mortality occurred prior to the end of the first year at sea for all 10 fish groups. The predictor variables that were significant in the models varied among river systems and smolt ages. This study demonstrated that the frequency of compensatory growth and the total lengths of juvenile Sockeye Salmon during their first year at sea increased with summer SST (range, 7.5–10.5°C) in the eastern Bering Sea, a possible mechanism for the increased productivity of Bristol Bay Sockeye Salmon associated with warmer sea temperatures.