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Fisheries Behavioral Ecology Program - Newport Laboratory

Temporal and Ontogenetic Shifts in Habitat Use by Juvenile Pacific Cod
(Gadus macrocephalus)

examining cod in a flume tank
Figure 11.  The behavior of juvenile age-0 and
age-1 cod was examined in the 10-m large flume tank at the Fisheries Behavioral Ecology Programís laboratory at the Hatfield Marine Science Center in Newport, OR.  A horizontal gradient of light was set-up overhead to examine how fish respond to changing light conditions as a function of depth and time of day.

Our understanding of the distribution of Pacific cod (Gadus macrocephalus) throughout their entire early life history is incomplete. Pacific cod spawn demersal eggs in winter-spring, and larvae rise to the upper reaches of the water column where they are thought to be transported shoreward. Pelagic juveniles settle in coastal regions of Alaska in July, where they are known to prefer structural habitats such as Laminaria spp. and seagrass beds through August. Because the distribution of older juvenile cod is relatively unknown, including their residency in coastal waters during fall and during the subsequent year as 1-year-olds, we examined habitat use by age-0 and age-1 juvenile Pacific cod in coastal regions near Kodiak Island, Alaska, over daily, seasonal, and annual scales. We used a combination of field gear, such as seine nets and baited cameras, and laboratory techniques (Fig. 11) to follow the distribution and behavior of juvenile cod from the region. Available data indicate that the 2006 year class was an exceptionally strong recruitment event in nursery areas around Kodiak Island, allowing for a unique opportunity to follow the temporal and ontogenetic changes in habitat use by this abundant juvenile Pacific cod cohort.

Figure 12, click to enlarge
Figure 12.  Click image to enlarge.

Figure 13, click to enlarge
Figure 13.  Click image to enlarge.

Figure 14, click to enlarge
Figure 14.  Click image to enlarge.

Based on Atlantic studies, we predicted there could be behavioral and survival consequences to newly settling juvenile cod if the 2006 cohort remained resident in the nearshore as age-1 fish in 2007. We also examined the abundance, distribution, and diet of juvenile saffron cod (Eleginus gracilis), another highly abundant gadid in the region, as a potential competing species of Pacific cod in the nearshore. Catch data indicated highly variable recruitment to nursery areas and a strong separation of depth distribution by age and species.

Age-0 Pacific cod and saffron cod were most abundant in the nearshore (< 3 m) whereas age-1 Pacific cod were typically found in depths ranging from 9.0 to 13.5 m (Fig. 12). In comparison, age-1 saffron cod were found in depths less than or equal to 4.5 m and sometimes overlapped with age-0 Pacific cod during the day. At night, age-1 Pacific cod moved into shallow regions and co-occurred with age-0 cod to a greater extent (Fig. 13). Laboratory light-gradient experiments indicated that age-0 Pacific cod tolerated intense lighting typical of shallow water regions whereas larger age-1 Pacific cod strongly avoid bright light given the choice (Fig. 14). However, while diet data indicate age-1 fish of both species are moderately piscivorous (3% saffron cod; 16% Pacific cod), we found no direct evidence of predation on smaller conspecific cod, possibly due to the low densities of age-0 cod in the year of the diet study.

Together, these results suggest coastal regions continue to serve as nursery areas beyond the first year of development for juvenile gadids, and that small-scale temporal and depth partitioning in these regions may be a mechanism by which varying cod species and age groups co-occur. Our current focus is trying to understand the degree to which the survival and distribution of coastal juvenile cod influence neighboring and offshore cod population dynamics.

By Benjamin J. Laurel

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