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AFSC/ABL: Gulf of Alaska Diel Trawl Survey, 2005-2006
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Diel epipelagic sampling for juvenile Pacific salmon (Oncorhynchus spp.), rockfish
(Sebastes spp.), sablefish (Anoplopoma fimbria), and associated species was conducted
in order to identify factors that may affect year-class success of these commercially
important species. Sampling occurred in offshore marine habitats of the coastal northeast
Pacific Ocean from 10-20 August 2005 and was conducted with a surface trawl fishing
the upper 20 m of the water column along transects up to78 km offshore near 58 N.
Three habitats were sampled along each transect over a 24-hr period: the continental
shelf (<200 m depth), the continental slope (400-750 m depth), and the abyss (>2,000
m depth). A total of 38,747 fish and squid representing 24 species were sampled in
56 trawl hauls. Of the targeted juvenile fish species, a total of 587 salmon, 11 rockfish,
and 70 sablefish were captured. Sampling during day (1500-1900) and night (2200-0200)
periods indicated that biomass of fish and squid was 2-4 times higher at night at
(each?)all habitat types pooled across transects. No distinct patterns between day
or night occurrence were noted for juvenile pink salmon (O. gorbuscha), chum salmon
(O. keta), sockeye salmon (O. nerka), or coho salmon (O. kisutch), however, juvenile
Chinook salmon (O. tshawytscha) were encountered only at night. Catches of juvenile
rockfish and juvenile sablefish were quite low in this study, and larger sample sizes
of these fish are needed to adequately determine their diel distribution. Diel differences
were apparent with forage species such as Pacific herring (Clupea pallasi), capelin
(Mallotus villosus), and eulachon (Thaleichthys pacificus) that were almost exclusively
sampled at night. The offshore distribution patterns of target species were distinctly
different, with the most common occurrences of juvenile salmon over continental shelf
habitats, juvenile sablefish over continental shelf and slope habitats, and juvenile
rockfish over slope and abyss habitats. Pacific herring, capelin, eulachon, and Pacific
sardines (Sardinops sagax) were found over continental shelf habitats, whereas small
squid and myctophids occurred primarily at slope and abyssal habitats. The greatest
overall catch biomass was of gelatinous species (jellyfish), which was consistently
higher than that of all fish and squid combined, usually by an order of magnitude.
Individual fish or squid species with highest average weight per haul were pomfret
(Brama japonica), adult coho salmon, Humboldt squid (Dosidicus gigas), and blue sharks
(Prionace glauca). The occurrence of the latter two warm-water species and Pacific
sardines were of interest because this study occurred during an anomalously warm year
and the capture of Pacific sardines and Humboldt squid represent northern range extensions
for these species. Stomach content analysis of potential predator species of the target
species showed that only adult coho salmon were predating on juvenile salmon and sablefish,
and only pomfret were predating on juvenile rockfish. Further sampling of the target
species is needed in these habitats during more normal environmental conditions to
validate these observations.
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