Juvenile Sockeye Research Articles

Infectious haematopoietic necrosis virus genogroup‐specific virulence mechanisms in sockeye salmon, Oncorhynchus nerka (Walbaum), from Redfish Lake, Idaho

Characterization of infectious haematopoietic necrosis virus (IHNV) field isolates from North America has established three main genogroups (U, M and L) that differ in host-specific virulence. In sockeye salmon, Oncorhynchus nerka, the U genogroup is highly virulent, whereas the M genogroup is nearly non-pathogenic. In this study, we sought to characterize the virus-host dynamics that contribute to genogroup-specific virulence in a captive stock of sockeye salmon from Redfish Lake in Idaho. Juvenile sockeye salmon were challenged by immersion and injection with either a representative U or M viral strain and sampled periodically until 14 days post-infection (p.i.). Fish challenged with each strain had positive viral titre by day 3, regardless of challenge route, but the fish exposed to the M genogroup virus had significantly lower virus titres than fish exposed to the U genogroup virus. Gene expression analysis by quantitative reverse transcriptase PCR was used to simultaneously assess viral load and host interferon (IFN) response in the anterior kidney. Viral load was significantly higher in the U-challenged fish relative to M-challenged fish. Both viruses induced expression of the IFN-stimulated genes (ISGs), but expression was usually significantly lower in the M-challenged group, particularly at later time points (7 and 14 days p.i.). However, ISG expression was comparable with 3 days post-immersion challenge despite a significant difference in viral load. Our data indicated that the M genogroup virus entered the host, replicated and spread in the sockeye salmon tissues, but to a lesser extent than the U genogroup. Both virus types induced a host IFN response, but the high virulence strain (U) continued to replicate in the presence of this response, whereas the low virulence strain (M) was cleared below detectable levels. We hypothesize that high virulence is associated with early in vivo replication allowing the virus to achieve a threshold level, which the host innate immune system cannot control.

Climate and intraspecific competition control the growth and life history of juvenile sockeye salmon (Oncorhynchus nerka) in Iliamna Lake, Alaska

Recent climate changes have had marked effects on the ice-free season and thermal conditions in many high-latitude lakes, but their ecological effects combine with density-dependent processes to affect fish growth and life history. To better understand the relative roles of climate and intraspecific density, we applied Gaussian state–space models to long-term data (1962–2006) on growth of juvenile sockeye salmon ( Oncorhynchus nerka ) in Iliamna Lake, Alaska, USA. Both temperature and density influenced fry size at the end of their first growing season, but the positive effect of temperature exceeded the negative effects of density. Fry growth was affected by the magnitude of their own cohort more strongly than by the previous brood (i.e., yearlings). In contrast, density was more important than temperature in Lake Aleknagik, also in Bristol Bay, over the same period of record, probably because Iliamna Lake is cooler and has generally lower densities of juvenile sockeye salmon than Lake Aleknagik. In both lakes, the size of the fish at the end of the first growing season affects smolt size and age at seaward migration, hence survival at sea and age at maturity, so the relative effects of climate and density depend on the ecological context.

Global climate change and potential effects on Pacific salmonids in freshwater ecosystems of southeast Alaska

General circulation models predict increases in air temperatures from 1°C to 5°C as atmospheric CO2 continues to rise during the next 100 years. Thermal regimes in freshwater ecosystems will change as air temperatures increase regionally. As air temperatures increase, the distribution and intensity of precipitation will change which will in turn alter freshwater hydrology. Low elevation floodplains and wetlands will flood as continental ice sheets melt, increasing sea-levels. Although anadromous salmonids exist over a wide range of climatic conditions along the Pacific coast, individual stocks have adapted life history strategies—time of emergence, run timing, and residence time in freshwater—that are often unique to regions and watersheds. The response of anadromous salmonids will differ among species depending on their life cycle in freshwater. For pink and chum salmon that migrate to the ocean shortly after they emerge from the gravel, higher temperatures during spawning and incubation may result in earlier entry into the ocean when food resources are low. Shifts in thermal regimes in lakes will change trophic conditions that will affect juvenile sockeye salmon growth and survival. Decreased summer stream flows and higher water temperatures will affect growth and survival of juvenile coho salmon. Rising sea-levels will inundate low elevation spawning areas for pink salmon and floodplain rearing habitats for juvenile coho salmon. Rapid changes in climatic conditions may not extirpate anadromous salmonids in the region, but they will impose greater stress on many stocks that are adapted to present climatic conditions. Survival of sustainable populations will depend on the existing genetic diversity within and among stocks, conservative harvest management, and habitat conservation.

Growth Rate Potential of Juvenile Sockeye Salmon in Warmer and Cooler Years on the Eastern Bering Sea Shelf

A spatially explicit bioenergetics model was used to predict juvenile sockeye salmonOncorhynchus nerkagrowth rate potential (GRP) on the eastern Bering Sea shelf during years with cooler and warmer spring sea surface temperatures (SSTs). Annual averages of juvenile sockeye salmon GRP were generally lower among years with cooler SSTs and generally higher in offshore than nearshore regions of the eastern Bering Sea shelf during years with warmer SSTs. Juvenile sockeye salmon distribution was significantly (P<.05) related to GRP and their prey densities were positively related to spring SST (P<.05). Juvenile sockeye salmon GRP was more sensitive to changes in prey density and observed SSTs during years when spring SSTs were warmer (2002, 2003, and 2005). Our results suggest that the pelagic productivity on the eastern Bering Sea shelf was higher during years with warmer spring SSTs and highlight the importance of bottom-up control on the eastern Bering Sea ecosystem.

Long‐term changes in rearing habitat and downstream movement by juvenile sockeye salmon (Oncorhynchus nerka) in an interconnected Alaska lake system

Abstract – In some populations the phenomenon of partial migration develops where some individuals stay in a given habitat rather than move with the migratory component. Depending on the selective pressures, the individuals that stay may be larger, smaller or similar in size to those that move. Freshwater movements of juvenile sockeye salmon (Oncorhynchus nerka Walbaum) fry vary among and within populations, and can be complex, especially in interconnected lake systems. We examined variation of movement patterns by a sockeye salmon population in an interconnected lake system during a period of rapid natural habitat change and found that fry migrating downstream were shorter, had lower body condition, and were more likely ill and moribund compared with fish remaining in the lake. However, otolith microstructure measurements indicated that emigrants did not grow significantly slower than residents prior to downstream movement. We show that patterns (i.e., demography of migrants, timing of movement) of downstream movement have changed since the 1970s, corresponding to changes in rearing habitat. Our findings parallel the results with other salmonid species and are generally consistent with the paradigm that density‐dependent interactions from declining habitat availability or quality result in the downstream movement of competitively inferior individuals, although the mechanisms governing downstream migration are unclear in this system.

Effects of marine-derived nutrients on population dynamics of sockeye salmon (Oncorhynchus nerka)

The effects of marine-derived nutrients (MDN) on the productivity of sockeye salmon ( Oncorhynchus nerka ) stocks in Alaska, USA, were examined through nitrogen stable isotope analysis of smolts and mathematical models of the sockeye stock–recruit relationship. Smolt δ15N was used to infer the degree to which smolts depend on MDN for their growth. We found that characteristics of sockeye nursery lakes and watersheds significantly affected the availability of MDN to juvenile sockeye. The magnitude of escapement and water residence time were the most important factors affecting the MDN availability to juvenile salmon. Analysis of stock–recruit models indicated that regional environmental fluctuations had a large effect on stock productivities. However, stock–recruitment data showed little evidence that increasing MDN input to nursery lakes increased stock productivities. Stock–recruitment data may be poorly suited to detection of the influence of MDN because of the multitude of factors that influence juvenile survival in the first several years of their life.

Ontogenetic Diet Shifts of Prickly Sculpin in the Lake Washington Basin, Washington

AbstractWe 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.

Juvenile sockeye salmon distribution, size, condition and diet during years with warm and cool spring sea temperatures along the eastern Bering Sea shelf

Interannual variations in distribution, size, indices of feeding and condition of juvenile Bristol Bay sockeye salmon Oncorhynchus nerka collected in August to September (2000–2003) during Bering–Aleutian Salmon International Surveys were examined to test possible mechanisms influencing their early marine growth and survival. Juvenile sockeye salmon were mainly distributed within the southern region of the eastern Bering Sea, south of 57°0′ N during 2000 and 2001 and farther offshore, south of 58°0′ N during 2002 and 2003. In general, juvenile sockeye salmon were significantly larger (P < 0·05) and had significantly higher indices of condition (P < 0·05) during 2002 and 2003 than during 2000 and 2001. The feeding index was generally higher for age 1.0 year sockeye salmon than age 2.0 year during all years. Among‐year comparisons suggested that Pacific sand lance Ammodytes hexapterus were important components of the juvenile sockeye salmon diet during 2000 and 2001 (20 to 50% of the mean wet mass) and age 0 year walleye pollock Theragra chalcogramma were important components during 2002 and 2003 (50 to 60% of the mean wet mass). Warmer sea temperatures during spring and summer of 2002 and 2003 probably increased productivity on the eastern Bering Sea shelf, enhancing juvenile sockeye salmon growth.

Changes in Algal Species Composition Affect Juvenile Sockeye Salmon Production at Woss Lake, British Columbia: A Lake Fertilization and Food Web Analysis

AbstractIn recent decades, the abundance of sockeye salmon Oncorhynchus nerka in the Nimpkish River watershed (Vancouver Island) has declined by more than 80%. To improve sockeye salmon abundance, one of three Nimpkish nursery lakes (Woss Lake, 13.6 km2) was fertilized; a second (Vernon Lake, 8.4 km2) was maintained as an unmanipulated reference. For 3 years (2000–2002), we assessed changes in phytoplankton, zooplankton, and juvenile sockeye salmon and then used food web analysis to model the fate of fertilizer phosphorus (FP) as it moved up the food web to juvenile sockeye salmon. As the experiment progressed, we found two distinctly different results. During the first 2 years (2000 and 2001), fertilization was associated with higher concentrations of P, chlorophyll, and phytoplankton. However, because most of the algae were inedible diatoms (principally Rhizosolenia eriensis), only 8% of the algal standing stock was available to zooplankton and less than 0.1% of the FP was incorporated into sockeye salmon biomass via limnetic pathways. The result was that juvenile sockeye salmon growth rates in the treated and control lakes were not significantly different. During the third year (2002), a bloom of the diatom Leptocylindrus increased Woss Lake edible algal biomass by a factor of 7; zooplankton production tripled, and juvenile sockeye salmon production increased by 19%. Consequently, Woss Lake juvenile sockeye salmon grew twice as fast as they did in untreated Vernon Lake, and juvenile sockeye salmon fall weights were 50% higher than those recorded before the lake was fertilized. We conclude that for north Pacific coastal lakes, bottom‐up effects resulting from changes in phytoplankton species composition can profoundly alter rates of juvenile sockeye salmon production. Regulation of algal species composition through manipulations of fertilizer composition and application timing might be used to reduce blooms of nuisance algae and increase age‐0 sockeye salmon yield.

Long‐Term Population Dynamics of the Endangered Snake River Sockeye Salmon: Evidence of Past Influences on Stock Decline and Impediments to Recovery

AbstractDeclines in populations of Pacific salmon Oncorhynchus spp. have been most pronounced in the southern extent of their range, and numerous anthropogenic stressors and natural drivers have been identified as potential causes. Using a paleolimnological approach, we have reconstructed the natural variability in the population dynamics of endangered Snake River sockeye salmon O. nerka over approximately the past 1,370 years. The rearing habitat ecology of their main production system, Redfish Lake, Idaho was also assessed over the past 500 years. Siliceous algal remains (diatoms and chrysophytes), stable nitrogen isotope δ15N, cladoceran zooplankton subfossils, and other proxy indicators archived in dated lake sediment cores were employed in this analysis. The inferred natural salmon production was much higher than that recorded in fisheries escapement data. Unprecedented declines in Snake River sockeye salmon production and changes in rearing system ecology were inferred over the past 150 years. The early, negative influences of the Columbia River commercial salmon harvest (in the 1860s) and sustained interruption of migration access (1910‐1934) from hydroelectric damming were evident. Enhanced zooplanktivory, associated with nonnative stocking and possible expansion of kokanee (lacustrine sockeye salmon) beginning in the 1920s and 1930s, appear to have altered the forage base for juvenile sockeye salmon, probably contributing to further declines. Lake nutrient enrichment since the 1950s was inferred, which changed nursery lake nutrient and trophic dynamics. This study highlights both site‐specific and regional influences on declines in past Snake River sockeye salmon population abundance and changes in rearing system ecology for these fish over the past 150 years. Such a long‐term ecological perspective is important to the continued conservation and management of this endangered species and has broader applications for other sockeye salmon runs at risk throughout their range.

Prey selection and efficiency of naïve and experienced juvenile sockeye salmon

Prey selection and growth efficiency of juvenile sockeye salmon Oncorhynchus nerka switched between live prey and pelleted diets were investigated. First feeding sockeye salmon fry were placed into one of three dietary treatments for 7 months prior to assessing potential differences with a growth and a behavioural assay. Dietary treatments were (1) adult Artemia franciscana for 1 month, followed by pelleted feed for an additional 6 months (Art− BD), (2) pelleted feed from first feeding for 7 months (BD) and (3) adult A. franciscana for 1 month, and a combination of pelleted feed and live adult A. franciscana for 6 months (Art+ BD). Equal numbers from each treatment group were then tagged, pooled into replicate ‘common garden’ tanks and fed novel live prey items (Daphnia sp. and mosquito Culex pipiens larvae) for an additional 3 weeks. No significant differences in the growth efficiency of sockeye salmon were found during the 3 week feeding trial on the novel prey items. Additional sockeye salmon from each dietary treatment were used in a behavioural assay to determine if the treatments had an impact on foraging efficiency (prey selection or time to capture prey). No significant differences in prey selection were found among treatment groups in time to capture pellets, A. franciscana or mosquito larvae. Also, no significant differences were found within treatment groups in time to capture different food sources. No substantive benefits in foraging efficiency of sockeye salmon associated with prior exposure to live prey were demonstrated. This suggests that altering existing hatchery practices for juvenile sockeye salmon by offering live food prior to release is unlikely to influence post‐hatchery feeding behaviour or increase post‐release survival.

A proposed life history strategy for the salmon louse, Lepeophtheirus salmonis in the subarctic Pacific

The sea louse, Lepeophtheirus salmonis, is commonly found on Pacific salmon that are rearing in the central North Pacific Ocean and adjacent seas (subarctic Pacific). Large numbers of sea lice have also been observed on all species of adult Pacific salmon when they return to coastal marine areas in the summer during their spawning migration. We propose that the transport of sea lice into coastal areas is a strategy employed by L. salmonis to improve their productivity by improving the transmission potential of the infectious stage when host densities are decreased in the open ocean and increased in the coastal areas. Juveniles of the species of Pacific salmon inhabit the same areas at the same time as the returning adult salmon and, according to the proposed strategy, will become infected from sea lice on the adult Pacific salmon. Juvenile pink, chum and sockeye salmon will carry these sea lice into the open ocean when they migrate away from the coastal areas later in the year. The offspring of these sea lice on pink, chum and sockeye would infect Pacific salmon on the high seas and thus maintain high abundances of sea lice. Juvenile coho and chinook salmon that remain in the coastal areas would serve as hosts for the sea lice over the winter. The sea lice on these resident coho and chinook salmon would infect juvenile Pacific salmon that enter the ocean in the early spring. The strategy would result in the infection of juvenile Pacific salmon throughout their range in all coastal areas.

Effects of surgically implanted acoustic transmitters >2% of body mass on the swimming performance, survival and growth of juvenile sockeye and Chinook salmon

The influence of surgical implantation of an acoustic transmitter on the swimming performance, growth and survival of juvenile sockeye salmon Oncorhynchus nerka and Chinook salmon Oncorhynchus tshawytscha was examined. The transmitter had a mass of 0·7 g in air while sockeye salmon had a mass of 7·0–16·0 g and Chinook salmon had a mass of 6·7–23·1 g (a transmitter burden of 4·5–10·3% for sockeye salmon and 3·1–10·7% for Chinook salmon). Mean critical swimming speeds (Ucrit) for Chinook salmon ranged from 47·5 to 51·2 cm s−1 [4·34–4·69 body lengths (fork length, LF) s−1] and did not differ among tagged, untagged and sham‐tagged groups. Tagged sockeye salmon, however, did have lower Ucrit than control or sham fish. The mean Ucrit for tagged sockeye salmon was 46·1 cm s−1 (4·1 LF s−1), which was c. 5% less than the mean Ucrit for control and sham fish (both groups were 48·6 cm s−1 or 4·3 LF s−1). A laboratory evaluation determined that there was no difference in LF or mass among treatments (control, sham or tag) either at the start or at the end of the test period, suggesting that implantation did not negatively influence the growth of either species. None of the sockeye salmon held under laboratory conditions died from the influence of surgical implantation of transmitters. In contrast, this study found that the 21 day survival differed between tagged and control groups of Chinook salmon, although this result may have been confounded by the poor health of Chinook salmon treatment groups.

Virulence Comparisons of Infectious Hematopoietic Necrosis Virus U and M Genogroups in Sockeye Salmon and Rainbow Trout.

Infectious hematopoietic necrosis virus (IHNV) is an aquatic rhabdovirus that infects salmonids in the Pacific Northwest of the United States, Europe, and Asia. Isolates of IHNV have been phylogenetically classified into three major viral genogroups, designated U, M, and L. To characterize virulence of IHNV in the context of these three viral genogroups, seven strains of IHNV (three U genogroup strains, three M strains, and one L strain) were compared for their pathogenicity in juvenile sockeye salmon Oncorhynchus nerka, kokanee (lacustrine sockeye salmon), and rainbow trout O. mykiss. Fish were waterborne-exposed to the different viral strains, and virulence was assessed by comparing mortality curves and final cumulative percent mortality (CPM) in both species of fish at 10°C and 15°C. In sockeye salmon and kokanee, the U genogroup virus types were extremely virulent, causing average CPMs of 69-100%, while the M genogroup virus types caused very little or no mortality (CPM = 0-4%). The endangered Redfish Lake sockeye salmon stock exhibited extreme differences in susceptibility to the U and M genogroups. Conversely, in two stocks of rainbow trout, the M genogroup virus types were more virulent, inducing average CPMs of 25-85%, while the U genogroup viruses caused lower mortality (CPM = 5-41%). In both fish species, the single L genogroup strain caused low to intermediate mortality (CPM = 13-53%). Viral glycoprotein sequence comparisons of the seven challenge strains revealed three amino acid sites (247, 256, and 270) that consistently differed between the U and M genogroups, possibly contributing to pathogenicity differences.

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

AbstractStable 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 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.

Swimming performance and morphology of juvenile sockeye salmon, Oncorhynchus nerka: comparison of inlet and outlet fry populations

We raised two populations of sockeye salmon fry from fertilized eggs in the laboratory and tested the hypothesis that outlet fry popula- tions, fish which must migrate upstream to reach rearing lakes after yolk-sac absorption, have better swimming ability and morphological char- acteristics conducive to enhanced swimming per- formance than inlet fry populations, fish which migrate downstream to rearing lakes. Despite being of identical age, fry from the outlet popu- lation were larger (approx. 6.7% longer, ~ 5m m on average) and more laterally compressed than inlet fry at the time of our initial experiments. Using an open-top box flume, we found that the burst-swimming performance (in cm s -1 ) of the outlet population was 31% better. We found no differences between populations in prolonged- swimming performance. We were unable to find any direct relationships between measures of swimming performance and size or shape vari- ables, suggesting that the larger, more robust morphology of outlet fry was not responsible for the superior burst ability. Recent biochemical studies indicate outlet fry may be metabolically better provisioned for burst swimming than inlet fry. It is possible that the morphological differ- ences between the populations of fry reflect adaptations needed by adults during their migra- tion and spawning.

Environmentally controlled Daphnia spring increase with implications for sockeye salmon fry in Lake Washington, USA

In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka) strongly prefer Daphnia over other prey, uniformly to Daphnia when the threshold abundance of 0.4 Daphnia L-1 is achieved. Using long-term Lake Washington data (1978-2001) and fry trap data (1992-2001) from a major tributary, we examined the following: (i) factors that predict Daphnia pulicaria and Daphnia thorata increase to this threshold switching abundance, (ii) trends in Daphnia dynamics that may affect sockeye foraging and (iii) temporal correspondence of Daphnia increase and fry arrival. The winter abundance of D. pulicaria, in combination with basic parameters of spring conditions, was an important predictor of the date of D. pulicaria spring increase, indicating greater reliance on pelagic population dynamics (versus diapause hatch) than D. thorata exhibited. In addition, D. pulicaria was a more consistent prey than D. thorata, the latter exhibiting larger population fluctuations. Thus, recently increasing D. thorata prominence could decrease diet consistency for sockeye fry. Additionally, the timing of sockeye arrival to Lake Washington and Daphnia's increase to the threshold has become less concordant, so that fry in recent years have had to rely upon less profitable prey for longer periods. Long-term trends and species-specific differences in Daphnia phenology may affect fry through altering diet composition, with additional implications for other zooplankton withstanding greater predation pressure in Daphnia's absence. Recent decades of warming in Lake Washington are consistent with the warming of lakes worldwide, and complex phenological responses such as those reported here may be common as the climate continues to change. (Less)

Rearing conditions to develop seawater tolerance in underyearling sockeye salmon smolt

In order to confirm the conditions required to produce underyearling smolts, juvenile sockeye salmon Oncorhynchus nerka were reared under different photoperiod and temperature conditions. The level of smoltification was assessed by plasma sodium concentration 24 h after transfer to sea water (plasma-Na), gill Na+,K+-ATPase activity (gill-ATPase), and plasma thyroxine (plasma-T4) and growth hormone (plasma-GH) concentrations. Mean fork length of all groups reached more than approximately 8.5 cm at the end of the experiment. The fish that experienced an alteration in photoperiod from short day (8∶16 h light-dark cycle) to long day (16∶8 h light-dark cycle) showed significant elevation in gill-ATPase from 9 to 16 μmol inorganic phosphate per milligram protein per hour (μmol Pi/mg pro/h) and reduction in plasma-Na from 194 to 154 mM. This group displayed distinct pigmentation on their fin margins. However, in groups reared under constant short day and constant long day, changes in gill-ATPase and plasma-Na occurred vaguely at each different time and pigmentation on fin margins were not observed. The observed photoperiod-induced changes were greater at 7°C than at 11°C. Further, the alteration of the photoperiod induced two-threefold increases in plasma-GH and plasma-T4. These results suggest that lengthening of photoperiod from short to long daylight at 7°C causes smoltification accompanied by active secretion of growth hormone and thyroxine in underyearling sockeye salmon when the fish are larger than 8.5 cm in fork length.

Temporal dynamics in foraging behavior of a pelagic predator

A paradox exists between common observations of highly complex food webs in nature and prevailing ecological theory suggesting that complex food webs are unstable. Qualities of food webs, such as dynamic trophic links, that were not previously considered in theoretical models have been suggested to stabilize complex food webs and resolve this paradox. Flexible foraging behavior can result in dynamic trophic linkages between a predator and its prey over short time scales. Using 4 years of data on diets of juvenile sockeye salmon (Oncorhynchus nerka) and the zooplankton community from Lake Washington (Seattle, Washington, USA), we determined that juvenile sockeye show dynamic foraging behavior that depends solely on the density of its preferred prey, Daphnia. Juvenile sockeye consumed only Daphnia when their density was greater than 0.4·L–1. Long-term patterns in Daphnia densities demonstrated that for the last 28 years, trophic links between sockeye and the less preferred copepods are eliminated for approximately 6 months of each year when Daphnia is above this switching density threshold. Our data provide a convincing empirical example of the activation and inactivation of trophic links occurring on short time scales as a result of dynamic foraging and help to explain the commonness of high-complexity food webs.

Estimation of Stock Composition and Individual Identification of Sockeye Salmon on a Pacific Rim Basis Using Microsatellite and Major Histocompatibility Complex Variation

AbstractThe variation at 14 microsatellite loci and one major histocompatibility complex (MHC) locus was surveyed for over 48,000 sockeye salmon Oncorhynchus nerka sampled from 299 localities ranging from the Columbia River to Japan. For the microsatellite loci, the number of alleles observed at a locus was related to the power of the locus in providing accurate estimates of stock composition of single‐population mixtures. In an analysis of single‐population mixtures where the Pacific Rim baseline was used for estimation of stock identification, 80% accuracy for the average population was achieved by employing approximately 80 alleles in the analysis. Increasing the accuracy of estimated stock compositions to 90% for the average population required approximately 400 microsatellite alleles. When all loci were used to estimate stock compositions, estimates were above 80% for all sampling sites or populations, above 90% for the lake of origin, and generally above 95% for the region of origin. Analysis of known‐origin samples indicated that accurate lake or regional estimates of stock composition were obtained. The accuracy of identification of individual fish to the correct lake of origin was above 90%, regardless of whether the lakes were geographically widespread or within a single watershed. The estimated stock compositions of mixed‐fishery samples from the western Bering Sea, from the continental shelf near Kodiak Island in the Gulf of Alaska, from Southeast Alaska, and from Johnstone Strait in southern British Columbia were markedly different among samples. These stock compositions reflected geographical variation in fishery locations and variation in the migration pathways of either juvenile or maturing sockeye salmon. Variation of DNA enabled us to estimate accurately the origin of individual fish and the composition of mixed‐stock samples from any location in the Pacific Rim distribution of sockeye salmon.