Explore the vast range of titles available.

The underlying causes of mortality during critical life stages of fish are not well understood, nor is it clear if these causes are similar for naturally versus artificially propagated (i.e. hatchery) individuals. To assess the importance of selective mortality related to production type (hatchery vs. naturally produced) and size at and timing of marine entry, we compared attributes of juvenile Chinook salmon Oncorhynchus tshawytscha from the upper Columbia River summer- and fall-run genetic stock group captured in the Columbia River estuary with back-calculated attributes of survivors captured in marine waters. We used genetic stock identification, otolith chemistry and structure, and physical tags to determine stock of origin, size at and timing of marine entry, and production type. Fish emigrated from fresh water in May to September and the majority of fish collected in the estuary (87%) had arrived within 3 d of capture. In 1 of 2 yr, timing of marine entry for both production types differed between the estuary and ocean: the ocean catch included a greater proportion of juveniles that emigrated in late July than the estuary catch. There was no evidence of selective mortality of smaller juveniles during early marine residence in hatchery or natural juveniles, but the mean percentage (±SE) of hatchery fish in ocean collections was 16 ± 5.8% less than in the estuary, which could indicate reduced survival compared to naturally produced fish. Results from this study highlight the need to understand the effects of hatchery rearing and how hatchery propagation may influence survival during later critical life-history transitions.

Blooms of jellyfish around the world have been correlated with climatic variables related to environmental causes. Sizeable populations of large medusae, primarily Chrysaora fuscescens and Aequorea sp., appear annually in shelf waters of the Northeast Pacific Ocean. Previous research has shown that C. fuscescens is abundant seasonally in the inner shelf and exhibits high feeding rates on zooplankton. We examined medusae caught in surface trawls over an 8-year period (2000–2007) using (1) mesoscale surveys sampling 8–10 transects in May, June, and September, and (2) biweekly surveys along two transects from April to August, relating abundance to environmental parameters. C. fuscescens abundances generally peaked in late summer, whereas Aequorea sp. peaked in May or June. General additive models of the mesoscale data indicated that station catches for both species correlated with latitude, temperature, salinity, and distance from shore (and chlorophyll a for Aequorea sp.). Analysis of interannual variability revealed that highest catches of medusae correlated with cool spring–summer conditions, or negative anomalies of the Pacific Decadal Oscillation, and low winter–summer runoff from the Columbia River. Results confirmed our hypothesis of connections between jellyfish populations and regional climate conditions in a region known for strong physical forcing of ecosystem processes.

Lipids and fatty acids (FA) were investigated in 4 species of forage fish: northern anchovyEngraulis mordax, Pacific sardineSardinops sagax, Pacific herringClupea pallasi, and whitebait smeltAllosmerus elongatus, for their ability to serve as biological indicators of ocean conditions in the California Current large marine ecosystem (CCLME). Samples were collected during the oceanographically contrasting years of 2005 and 2006. Upwelling was severely curtailed in the spring and early summer of 2005, leading to delayed biological productivity, whereas upwelling was relatively normal in spring 2006. Principal components analysis described 78% of the variance within the lipid and FA dataset using the first 2 principal components. We found significant intra- and interspecific, interannual, and seasonal differences in lipid and FA profiles using univariate and permutation-based multivariate analysis of variance. Indicator species analysis showed distinct lipid and FA properties associated with each fish species. Using the ratio of docosahexaenoic acid (C22:6n-3) to eicosapentaeonic acid (C20:5n-3), we detected a transition from a diet composed primarily of dinoflagellate origin in early 2005 to a diet resulting from diatom-based productivity by late summer 2006. This shift was due to interannual differences in primary production, which was confirmed through phytoplankton sampling. Our study demonstrates that lipid and FA biomarkers in the forage fish community can provide information on ocean conditions and productivity that affect food web structure in the CCLME.

This study described the phenology and patterns of variability of forage fish and mesozooplankton populations near the Columbia River plume. Our objective was to identify the timing of the seasonal appearance of forage fish and to characterize the temporal patterns of abundance in relation to ocean conditions including zooplankton availability. Observations were collected at 2 stations in 2008 and 2009 using 200 kHz bio-acoustic moorings, and with bi-weekly net sampling surveys conducted at nearby stations to measure total fish density and relative species composition. Acoustic time series results revealed that the seasonal timing of acoustic schools, representing the forage fishes northern anchovyEngraulis mordax, whitebait smeltAllosmerus elongatus, and Pacific sardineSardinops sagax, occurred abruptly, with specific dates of appearance identified at each station in mid-May in both years. Both 2008 and 2009 represented very similar years for the timing and strength of wind-driven coastal upwelling. The timing of forage fish appearances in these years was linked with ocean temperature and salinity data collected at a nearby oceanographic buoy but was poorly correlated with mesozooplankton abundance, which was highly variable and fluctuating over a period of hours to days. Fluctuations in zooplankton and forage fish populations drive many trophic interactions, including juvenile salmon, seabirds, and large predators (e.g. adult salmon) that rely on the timing and abundances of these populations and have significant implications for ecosystem based management.

Concern about bycatch of marine mammals by fishery research gear has led to the use of mammal excluder devices (MEDs) in some surveys. We measured the effect of a rigid‐grate MED on trawl fishing effectiveness for juvenile Pacific salmon Oncorhynchus spp. and other upper‐pelagic species in the northern California Current. Catches of 11 taxa were compared among sets of a pelagic rope trawl without an MED and with an MED in two orientations: angled upward or angled downward. We estimated differences in overall catch ratios and size selectivity for each MED orientation compared to the unmodified net. The MED orientation had a species‐specific influence on both catch ratio and size selectivity, with median ratios ranging from 0.0 to 1.8 for the upward orientation and from 0.4 to 2.7 for the downward orientation. Video observations helped to explain performance differences and may help to improve MED design. Use of an MED in trawl surveys may substantially increase or decrease the catch of various taxa, contributing further uncertainty to the analysis of survey time series for either abundance or catch composition.

Dramatic changes in the physical and biological conditions off Washington and Oregon, USA, have occurred since 1998, including extreme El Niño (warm) and La Niña (cool) years, high and low Columbia River flow years, a major intrusion of subarctic water, and a low oxygen event on the shelf. The occurrence of contrasting environmental conditions provided an excellent opportunity to examine pelagic nekton distributions and their abiotic and biotic associations. Pelagic surface trawl surveys conducted during June and September from 1998 to 2002 off northern Washington to central Oregon revealed a nekton community dominated by Pacific herring (33.5% of total catch), Pacific sardines (29.9%), and northern anchovy (12.3%). Between 1998 and 2002, species composition shifted from a community dominated by southern species (mackerels and hake) to one dominated by northern species (squid, smelts, and salmon), but the transition was gradual, and small pelagic species (sardines, herring, and anchovy) showed no consistent trends in abundance over time. Species diversity/evenness was highest in September 2002. Cluster analysis identified 7 species and 6 station clusters. Subyearling Chinook salmon, market squid, Pacific sardine, yearling coho salmon, and Pacific saury were the strongest indicator species for 5 of these cluster groups. Cluster group distributions differed between both inshore/offshore and north/south. A 3-dimensional ordination explained 55% of the total variance with bottom depth, distance from shore, and sea-surface temperature correlated with the first axis, latitude with the second axis, and surface salinity, surface density and stability with the third axis. Our results suggest that the habitats occupied by pelagic nekton species expand and contract in relation to the dynamic nature of the California Current and are affected by changing ocean conditions at both seasonal and interannual periodicities.

During the summer of 1994, 1996, and 1997, we conducted hydrographic and plankton surveys of the upper 70 m in the nearshore, the continental shelf, and off-shelf oceanic waters off Oregon, USA. Copepod densities and biomass were estimated along 4 transects from each cruise. The on-shelf copepod biomass was 2.8 times greater than off-shelf biomass. Nonmetric multidimensional scaling identified an on-shelf and an off-shelf copepod community. The change in community composition usually occurred at or slightly offshore of the continental shelf break, defined here as 180 m water depth. Indicator-species analysis identified the subarctic neritic species,Calanus marshallae,Pseudocalanus mimus, andAcartia longiremis, as good indicators of continental shelf waters. ‘Warm water’ speciesMesocalanus tenuicornis,Calocalanus styliremis,Clausocalanusspp., andCtenocalanus vanuswere indicators of off-shelf waters. The copepod communities off the coast of Oregon during the summer upwelling season reflect the origins of the dominating currents within each habitat. The coastal copepod community is subarctic neritic in origin, consistent with southward coastal flows. The offshore copepod community is a mixture of species with origins in the Transition Zone as well as species that are typical of the coastal region of the California Current off central and southern California. We speculate that the central and southern California Current species are present in offshore waters of Oregon in summer because they are transported north with the Davidson Current in winter but, with the initiation of the upwelling season in spring, they are transported into offshore waters where they establish viable populations.

Well-defined fronts develop at the seaward edge of riverine plumes where suspended materials and planktonic organisms are concentrated by convergent water flows. Riverine plume fronts have been hypothesized to be favorable fish habitats because they can lead to localized prey aggregations. We examined the spatial distribution of juvenile Pacific salmonidsOncorhynchusspp. in and around plankton-rich frontal regions of the Columbia River plume to test the hypothesis that juvenile salmonids aggregate at riverine plume fronts to feed. Juvenile salmonids tended to be abundant in the frontal and plume regions compared to the more marine shelf waters, but this pattern differed among species and was not consistent across the 2 study years. Stomach fullness tended to be higher in the more marine shelf waters than either the front or plume areas, which does not support the hypothesis that salmonids consistently ingest more prey at frontal regions. Many prey organisms were disproportionately abundant at these fronts, but salmon stomach-content analysis did not reveal higher stomach contents at fronts or identify prey groups indicative of feeding in the frontal areas. Although our results indicate that the Columbia River plume influences the distributions of juvenile salmon, our observations do not support the hypothesis that juvenile salmonids congregate to feed at fronts at the leading edge of the Columbia River plume. The short persistence time of these fronts may prevent juvenile salmon from exploiting these food-rich, but ephemeral, features.

Salmonid run sizes are strongly affected by their early marine stage. Fully understanding the life history of salmonids means understanding how they interact with their marine environment and with other fishes. Changes in the biological and physical environment off the Columbia River region affects the distribution and abundance of predatory fishes and their feeding, forage fishes, and juvenile salmonid marine survival. From 1998--2004, forage fish and predatory fish distribution and abundance off the Columbia River was quantified by surface trawling at night during spring/summer. The effect of predation on salmonids was measured by stomach analysis of predatory fishes. During the study period (1998--2004), forage fishes increased in abundance by orders of magnitude and were strongly related to the abundance of cold-water copepods the previous year. Higher forage fish populations were also linked to cooler ocean conditions and perhaps fewer predatory Pacific hake (Merluccius productus). Most forage fishes were distributed nearshore while predators had a more offshore distribution. Pacific hake was most abundant in 1998, 2003, and 2004; warm ocean years. Jack mackerel (Trachurus symmetricus ) was most abundant during 1999--2002; relatively cool ocean years. Deep (50-m) ocean temperatures and the date of the spring transition, when nearshore currents switched from northward to southward, were good predictors of Pacific hake abundance in the study area. Forage fish or salmonid occurrence in a haul was negatively related to the occurrence of predators. Pacific hake and jack mackerel ate primarily euphausiids and small fishes. Salmonids were rarely eaten by either predator. However, because the Pacific hake population can be very large, hake predation can be a significant source of juvenile salmon mortality off the Columbia River during some years. A trophic model showed that marine mortality of Columbia River juvenile salmonids may be related to the abundance of Pacific hake and forage fishes. A multiple regression using the predictions from the trophic model of annual numbers of juvenile salmonids eaten by hake and Columbia River spring flows as independent variables, accounted for much of the variation observed in the annual marine survival of Columbia River coho (Oncorhynchus kisutch) and Chinook salmon (O. tshawytscha), the dependent variable. Future research should identify the physical and biological forces that alter the feeding habits, migration and movements of Pacific hake and jack mackerel off the Northwest, and how Columbia River flows affect trophic interactions.