Pacific Ocean Perch - Diet and Condition

We examined juvenile Pacific ocean perch (POP) diet and zooplankton composition in two areas of the Aleutian Islands: a site north of Samalga Pass and a site south of Samalga Pass. Juvenile POP (<250 mm) were collected in August 2004 using a standard bottom trawl (above left), and zooplankton samples were collected with a 1-m ring net (505 um) (above right).

Size classes of juvenile Pacific ocean perch captured at the site
north of Samalga Island and south of Samalga Island.

Fish length frequencies indicated there were three size-classes of juveniles: small (<160 mm), medium (160-210 mm) and large (210-250 mm) (chart above). The abundance of juvenile POP was much lower at the north site than at the south site. Juveniles of all sizes were also in better condition at the north site than at the south site (chart below). Potential reasons for this difference in fish condition could be competition for food or habitat.

Comparison of condition of fish captured
at northern and southern Samalga Island sites.

Large copepods comprised the majority of juvenile POP diets in August for all size classes; however, larger prey items such as euphausiids were consumed by medium- and large-sized juveniles.

Diet composition in proportions by numbers and weights
of each type of prey for fish captured in the north and south.

Juvenile POP diets were very similar between the two study areas indicating diet differences were not responsible for differences in fish condition between the two sites (chart above).

Stomach fullness as a percent of body weight was also similar between sites, indicating that there was no difference in the amount of food fish were consuming between the two sites. Stomach fullness, however, can be misleading in the case of POP, because many fish tend to lose their stomach contents when they are caught in a trawl and it presents only a “snapshot” in time.

Composition of zooplankton in the water column
captured north and south of Samalga Island.

Temperature profiles for the water column in August for
northern and southern Samalga Island measured with a CTD.

Zooplankton samples were therefore examined to determine if there were differences in the composition or amount of prey available at the two sites. Zooplankton samples were dominated by large copepods, and there was no difference in the proportion of large copepods between the north and south sites (top right). The zooplankton biomass, however, was three times higher at the north site than at the south site, indicating there was more food available to juvenile POP at the north site.

Temperature data indicates the water column at the north site was mixed, whereas, the south site was stratified (bottom right). Other studies have shown the north area to be an area of upwelling and high zooplankton biomass.

Other differences between the two sites include the abundance of potential predators and differences in physical habitat. Pacific cod, arrowtooth flounder, and Pacific halibut are known potential predators of juvenile POP and were much more abundant at the south site than at the north site.

We hypothesize that juvenile POP condition varied between our study sites due to one or more factors: the abundance of food, predators, and/or habitat. Since juvenile POP consume pelagic prey, local- or basin-scale climate changes that affect water column structure and, in turn, pelagic productivity could affect juvenile POP survival to recruitment.

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