Kuskokwim Bay chum salmon (Oncorhynchus keta) energy density, distribution, and stomach data, 2004

In the lab, all juvenile salmon identifications were confirmed and all chum salmon juveniles counted. Catches were quantified as catch per unit effort (CPUE) as the number of chum salmon caught per 30 minute trawl. Juvenile chum salmon from each station were sorted into 10 mm size classes: 30-39 mm, 40-49 mm, 50-59 mm, and ≥60 mm FL to stratify sampling by size. Up to ten individuals from each size class were selected from each haul for analyses of energy density and diet. Wet weight of each fish was measured to the nearest 0.001 g. Otoliths were removed and gill rakers on the first gill arch were counted to confirm species identification. The cardiac and pyloric section of the stomach were removed, and weighed to the nearest 1.0 mg with contents. All prey items were emptied from the stomach and fixed in 10% formalin for a minimum of 6 weeks, after which they were preserved in 70% ethanol until diets were analyzed. The empty stomach was weighed to the nearest 1.0 mg. Weight of prey contents were recorded as the difference between full and empty stomach weights in mg. Empty stomachs and viscera were returned to the body cavity. Feeding intensity was calculated as a continuous value of percent body weight (%BW) where total prey weight was divided by the wet fish weight minus prey weight and multiplied by 100 following Sturdevant et al. (2012). Prey items were identified to the lowest taxonomic level reasonable and counted for diet analysis. Wet weights (WW) for taxonomic groups were determined either from literature, from taking a WW average of ≥ 100 combined individuals, or were provided by C. Stark (University of Alaska Fairbanks, personal communication). Whole fish minus otoliths and stomach contents were placed in whirl-pak bags and stored in a freezer at -30 ºC for later processing. Prior to bomb calorimetry, frozen samples were placed in a freeze dryer (VirTis, Freezemobile 12) at -60 °C until weight stabilized, confirming minimal moisture content; dry mass was measured to the nearest 1.0 mg. Each sample was homogenized with a mortar and pestle, and a ≤ 150 mg pellet was pressed from a sub-sample and weighed immediately. For juvenile chum salmon < 150 mg DW, the whole fish was used in the pellet. A semi micro Parr 1425 calorimeter was used to measure energy density following the Parr manual (Parr Instrument Company 1994). Energy density from the calorimeter (cal g-1 dry mass) was converted to kJ and reported per unit dry mass (kJ g-1 dry mass).

Sample collections were planned on a station grid of 21 regularly spaced stations every 15 minutes of latitude and 7.5 minutes of longitude. Station locations were modified in the field when areas were inaccessible due to shallow depths or consistently poor weather; additional stations were added in areas that were more accessible. Twenty-two stations were sampled across the entire 2004 season, with variability in the stations sampled across cruises due to inclement weather conditions or shallow depths at low tides (four to fifteen stations were sampled per cruise). Sampling was conducted off the 13.0 m F/V Namorada across four weeks from 17 May to 11 June 2004. At each station, water temperature and salinity were collected with a SeaBird Electronics SBE-19 Seacat Conductivity-Salinity-Depth profiler.The timing of collection was informed by the results from a 2003 pilot field season consisting of three monthly cruise aboard the 11.0 m F/V Eileen O’Farrel (June 23-25, July 24-26, and August 26-30). The pilot field season revealed an earlier than anticipated juvenile chum salmon presence in Kuskokwim Bay with most juvenile chum salmon captured during the June cruise (n = 56) compared to three fish collected during the entire July cruise and a complete absence of juvenile chum salmon during the August cruise. Fish sampling was conducted during daylight hours with a modified Kvichak trawl (3.1 m height x 3.1 m width x 15.0 m length). The Kvichak trawl was modified by adding two doors to provide horizontal spread for the net in lieu of a second vessel. Floats at the headrope and weights at the footrope provided vertical spread and assured that the net fished at the surface. Given the shallow depths, the trawl often sampled the entire water column including epibenthic habitat as evidenced by flatfish bycatch. The trawl was fished at a target speed of 2 knots for 30 minutes at each station. After recovery of the trawl, all fish were anesthetized in MS-222 prior to handling, identified to species, and counted. All chum salmon juveniles were measured for fork length (FL) to the nearest 1.0 mm and frozen immediately at -30 °C.