Resource Assessment & Conservation Engineering (RACE) Division

Groundfish Assessment Program

Habitat Associations and Diet of Juvenile Pacific Cod

Pacific cod, Gadus macrocephalus, regularly rank second in catch and product value in the Alaska groundfish fishery, yet surprisingly little is known about the ecology of its juvenile stage. Although large-scale distributions have been described for adult Pacific cod, no investigations have focused on nursery areas and habitat associations during the juvenile stage. The Groundfish Assessment Program initiated a study to describe habitat associations of juvenile Pacific cod in Chiniak Bay, Alaska, located on the northeast side of Kodiak Island, where commercial fishing for Pacific cod is important for the local economy and where juvenile Pacific cod have been captured previously.

Figure 1.  Plotted generalized additive model (GAM) results for juvenile Pacific cod. Plots show the additive effect of each significant variable on the density of cod (GAM final model: R2 = 0.285, GCV = 0.617). Depth (P = 0.006) was non-linearly related to cod density such that abundance was highest at moderate depths (15-20 m). Percent cucumber mound cover (P = 0.013) and salinity (P = 0.033) were positively and linearly related to cod abundance. Dotted lines represent Bayesian 95% confidence intervals around the main effects, and vertical dashes along the x-axis show the distribution of points entering into the GAM model.

We sampled from 10 to 22 August 2002 at 66 stations. Depth, water temperature, salinity, sediment grain size, and the proportion of the seabed covered with emergent structure (i.e., tube-dwelling polychaetes, sea cucumber mounds, macroalgae) were measured prior to fishing with either a beach seine or small-meshed beam trawl. Generalized additive models (GAMs) were used to relate the abundance of juvenile Pacific cod to the following habitat variables: depth, temperature, salinity, sediment type, and percent cover of emergent structure.

The a priori hypothesis about juvenile Pacific cod habitat, based on ancillary data from studies in coastal Alaska and research on juvenile Atlantic cod, was that they congregated nearshore at shallow depths with eelgrass and macroalgae present. Instead, among a wide variety of nearshore habitats in Chiniak Bay, we found that depth (P = 0.006), percent cucumber mound cover (P = 0.013), and salinity (P = 0.033) were all significant covariates affecting juvenile Pacific cod density (GAM final model: R² = 0.285, GCV = 0.617), and these three covariates explained 35.5% of the deviance in the distribution of juvenile cod. Depth was nonlinearly related to cod density with abundance concentrated at moderate depths (15-20 m), declining between 20-25 m, and increasing at depths greater than 25 m (Fig. 1). Percent cucumber mound cover and salinity were positively and linearly related to cod abundance; however, due to the narrow range of salinity values among stations and the less significant P value, we considered the salinity result to be less robust than other significant variables effecting cod density.

No previous studies have documented fish utilizing sea cucumber mounds as habitat. Furthermore, eelgrass and macroalgae were inconsequential to cod distribution. Diets consisted mainly of small calanoid copepods, mysids, and gammarid amphipods and were significantly related to depth and percent mud. Cod predominately preyed on mysids in deeper water and larval crabs, larval barnacles, and small calanoid copepods in low-mud habitats.

Results from this study have described the habitat associations of juvenile Pacific cod during late summer, enabling further hypotheses about the function and quality of their habitat to be tested. Defining juvenile fish habitat is necessary in order to understand the causes of variability in growth, survival, and subsequent recruitment.

By Alisa Abookire
 

Annual Eastern Bering Sea Shelf Bottom Trawl Survey

The 30^th of May 2005 marked the start of the annual bottom trawl survey of the eastern Bering Sea (EBS) continental shelf in a long series dating back to 1971. The EBS shelf is a productive ocean region that contains some of the largest commercial fish and crab stocks in the world. Data and analyses resulting from this long time series are critical for assessing and managing commercial species such as red king crab, walleye pollock, Pacific cod, and yellowfin sole. The fishing vessels Arcturus and Aldebaran were chartered for a combined 130 sea days to sample 380 stations between the 20-m and 200-m isobaths and between the Alaska Peninsula and south of St. Lawrence Island.

In addition to the regular sampling, special studies are being conducted to investigate: 1) skate nursery areas; 2) groundfish feeding ecology; 3) in situ temperature and marbled eelpout distribution; 4) fur seal feeding strategies; 5) mercury level and bioenergetic content in forage fishes of the beluga whale; 6) in situ light levels and pollock distribution; 7) incidence of bitter crab disease and black mat syndrome; 8) molecular identification of Kamchatka and arrowtooth flounder, and 9) Steller sea lion prey DNA. Also completed prior to the start of the survey was a gear research experiment investigating the use of wire constrictors for limiting door spread of the EBS survey trawl. Various net mensuration data are currently being analyzed for determining how door constrictors affect trawl performance.

By Robert Lauth
 

Groundfish Systematics

James Orr and Duane Stevenson continue to work on the taxonomy and systematics of several families of fishes, most recently skates, snailfishes, and eelpouts. Their research on skates, in collaboration with Jerry Hoff and John McEachran (of Texas A&M), continues with the preparation of the description of a new species from the Aleutian Islands and a review of species related to the Alaska skate (Bathyraja parmifera), as well as a report of the deepwater species B. abyssicola and Amblyraja badia new to Alaska (in press with Northwestern Naturalist). Additional details of skate research may be found in "Recent Contributions to the Knowledge of the Skates of Alaska," by Stevenson and Orr published in the AFSC Quarterly Report for Jan-Mar 2005.

Research on snailfish systematics has continued with the submission of a manuscript by Orr and Morgan Busby on the taxonomy of the snailfish genus Allocareproctus. Four new species from the Aleutian Islands are described in the paper, based on collections of specimens from groundfish surveys conducted by RACE. In addition, a paper in press with Northwestern Naturalist by Orr, in collaboration with Beth Sinclair and Bill Walker of the National Marine Mammal Laboratory, reports on two deepwater species new to Alaska waters, the snailfish Paraliparis paucidens and the cuskeel Bassozetus zenkevitchi, based on midwater collections in the Bering Sea.

New species of eelpouts are being described under lead author Stevenson. Stevenson and Orr have submitted the description of a new species of Lycodes that is known only from the Islands of Four Mountains area in the Aleutian Islands. Stevenson is also collaborating with M. Eric Anderson (of the South African Institute of Aquatic Biodiversity) and Hisashi Imamura (of Hokkaido University, Japan) in producing a taxonomic revision of the eelpout genus Bothrocara. With Anderson, he has submitted the description of a new species of the genus from deep water in the Bering Sea.

By James Orr


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AFSC Quarterly Research Reports April-June 2005