Abundance, condition, and diet of juvenile Pacific ocean perch in the Aleutian Islands

Fishery Bulletin, July, 2009 by Jennifer L. Boldt, Christopher N. Rooper

Abstract--The relative value of pelagic habitat for three size classes of juvenile Pacific ocean perch (Sebastes alutus) was investigated by comparing their abundance and condition in two areas of the Aleutian Islands. Diet, zooplankton biomass, and water column temperatures were examined as potential factors affecting observed differences. Juvenile Pacific ocean perch abundance and condition, and zooplankton biomass varied significantly between areas, whereas juvenile Pacific ocean perch diet varied only by size class. Observed differences in fish condition may have been due to the quantity or quality of pelagic prey items consumed. For the delineation of essential demersal fish habitat, important ecological features of the pelagic habitat must therefore be considered.

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Studies defining essential fish habitat often focus on associations of marine fishes with seafloor characteristics (Neuman and Able, 1998; La Mesa et al., 2002). Although identifying these associations is necessary to define habitats used by fishes, it is also important to explore other factors that may affect fish survival. Pelagic production of food resources, such as zooplankton, may be a factor that affects the survival of demersal fish. Bottom-up control through variation in pelagic production, may in part explain the observations of large climate-change effects on demersal fishes, including rockfishes (Hollowed and Wooster, 1992; Hollowed et al., 2001).

Pacific ocean perch (Sebastes alutus; POP) are an important rockfish species that migrate vertically off the seafloor and consume pelagic prey (Brodeur, 2001), thus providing a direct linkage between pelagic zooplankton and demersal fish production. In general, juvenile POP (defined as individuals < 250 mm fork length [absolute value of FL] based on their size at first maturity) are associated with high-relief structured habitats, such as rocky outcrops, boulder fields, deep-water sponges, and corals (Rooper and Boldt 2005; Rooper et al., 2007). Little is known of the costs and benefits to juvenile POP of residing in different habitat types or the potential linkage between pelagic prey and demersal habitats.

One measure of the relative value of different habitat types to juvenile POP may be fish condition. Fish diet, habitat, and water temperature are integrated into overall fish condition, which may affect fish growth and subsequent survival (Paul, 1997; Boldt and Haldorson, 2004). The goal of this study was to examine the relative abundance and condition of three size classes of juvenile POP in two different areas of the Aleutian Islands. Fish occupying suboptimal habitats were expected to be in poor condition compared to those living in optimal habitats. Additionally, we explored potential causes for observed differences in fish condition by examining POP diet, prey availability, and water temperature observed in the different habitats.

Methods and materials

Study site

Fieldwork for this study was conducted from 28 May to 9 June 2004, and from 11 August to 23 August 2004, at two sites near Samalga Pass in the Aleutian Islands (Fig. 1). One site was located on the south (S) side and one on the north (N) side of Samalga Island (Fig. 1). Two stations were sampled at each of the two sites in August and two stations were sampled at the N site in June. All the stations were located at depths from 117 to 146 m in high relief habitat (boulder fields) inhabited by juvenile POP.

[FIGURE 1 OMITTED]

Juvenile POP abundance and size classes

Juvenile POP (< 250 mm FL) were collected during daylight hours with a standard NMFS poly Nor'eastern bottom trawl. A net (with average net width of 16.5 m) was towed at a target speed of 3 knots over distances of 0.4-1.0 km. The catch per unit of effort (CPUE in number of juvenile POP per hectare) at each site was calculated by using the area swept. Juvenile POP were measured for fork length and frozen for laboratory analyses. Temperature was measured twice daily at each site with a SeaBird CTD (conductivity, temperature, depth recorder, Sea-Bird Electronics, Inc., Bellevue, WA) to obtain temperature-depth profiles.

In the laboratory, juvenile POP were thawed, measured, weighed, and dissected. Fish stomachs were weighed full and empty, and stomach contents were preserved in 10% buffered formalin for later diet analyses. Length frequencies of juvenile POP were plotted and natural breakpoints were used to divide the fish into three size categories (< 160 mm FL, 160-210 mm FL, and > 210 mm FL). Diet and fish condition were analyzed by these size categories and by site.

Juvenile POP condition

Fish condition was assessed by using log-transformed length-wet-weight regression residuals (indicator of somatic growth and hereafter referred to as wet-weight residuals; n=226) and log-transformed length-dry-weight regression residuals (indicator of energy storage and hereafter referred to as dry-weight residuals; n=226). Also, the energy content of dried fish (n=226) was determined with a Parr 1425 Semimicro bomb calorimeter (Parr Instrument Company, Moline, ILL If available, at least 10 fish of each size category per site were dried in a 60[degrees]F drying oven or in a freeze dryer until a stable weight was reached. To test for differences in dry weights between the drying methods, thirty randomly chosen fish were individually ground and divided in half; one half of each fish was dried in the drying oven and the other half in the freeze dryer. Also, twenty two randomly selected fish were homogenized and divided in half; one half was freeze-dried, the other half was oven-dried, and the energy content of both halves was determined with the carorimeter to determine if there were differences in energetic content attributable to the two drying methods.