After each trawl haul, the fish were anesthetized with tricaine methanesulfonate (MS-222), identified, enumerated, measured, labeled, bagged, and frozen. After the catch was sorted,fish and squid were measured to the nearest mm fork length (FL) or mantle length with a Limnotera FMB IV electronic measuring board (Chaput et al. 1992). Usually all fish and squid were measured, but very large catches were subsampled due to processing time constraints. Up to 50 juvenile salmon of each species were bagged individually; the remainder were bagged in bulk. All fish were frozen immediately after measurement. During times of extended processing, fish were chilled with ice packs to minimize tissue decomposition and gastricactivity. All chinook and coho salmon were examined for missing adipose fins indicating the possible presence of implanted CWTs; those with adipose fins intact were again screened through a detector in the laboratory. The snouts of these fish were dissected later in the laboratory to recover CWTs, which were then decoded and verified to determine origin.
Frozen individual juvenile salmon were weighed in the laboratory to the nearest gram (g).Mean lengths, weights, and Fulton condition factors (g/FL^3 * 10^5; Cone 1989) were computed for each species by habitat and sampling interval. To identify stock of origin of juvenile chum,sockeye, coho, and chinook salmon, sagittal otoliths were extracted from the crania and preserved in 95% ethyl alcohol. Laboratory processing of otoliths for thermal marks was contracted to DIPAC. Otoliths were prepared for microscopic examination of potential thermal marks by mounting them on slides and grinding them down to the primordia (Secor et al. 1992).Ambiguous otolith thermal marks were verified by personnel at the Alaska Department of Fish and Game otolith laboratory. Stock composition and growth trajectories of thermally marked fish were then determined for each month and habitat.
Whole body energy content (WBEC) was determined by bomb calorimetry in the laboratory for juvenile coho salmon caught in the strait habitat. After removing the stomach contents, individual juvenile coho were dried at 55 Celsius to a constant weight in an oven. Fish were homogenized in a micro-mill to yield a uniform powder, from which 0.50 +- 0.02mg pellet subsamples were formed. Pellets were combusted in a Parr micro-bomb calorimeter following standard methods (Parr Instrument Co. 1994), and WBEC values were expressed in energy units of calories/g dry weight.
Potential predators of juvenile salmon from each haul were identified, measured, and weighed onboard the vessel. Their stomachs were then excised, weighed, and classified by percent fullness. Stomach contents were removed, empty stomachs weighed, and total content weight determined by subtraction. General prey composition was determined by estimating contribution of taxa to the nearest 10% of total volume. The wet-weight contribution of each prey taxon to the diets was then calculated by multiplying its percent volume by the total content weight. Fish prey was identified to species, if possible, and lengths were estimated. The incidence and rate of predation on juvenile salmon was computed for each potential predator species. Overall diets were summarized by percent weight of major prey taxa and the frequency of feeding fish.
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