Pleuronectes asper

Abstract: Juvenile yellowfin sole (Pleuronectes asper), rock sole (Pleuronectes bilineatus), and Pacific halibut (Hippoglossus stenolepis) had reduced growth following 30-90 days of exposure to sediments lad...

Abstract: Because it is unrealistic to assess fishes in all locations, models are needed to characterize the nursery habitats of flatfish species. Descriptive habitat models of species presence and categorical analysis regression tree (CART) models of species abundance have been developed in previous studies. Based on collections around Kodiak Island, Alaska in 1991 and 1992, these models have been developed for: age-0 flathead sole (Hippoglossoides elassodon); age-0 Pacific halibut (Hippoglossus stenolepis); age-1 yellowfin sole (Pleuronectes asper); and age-0 rock sole (Pleuronectes bilineatus). In 1995, collections were made in bays along the Alaska Peninsula (an area never before sampled for juvenile flatfishes) and were compared with the previous models developed for Kodiak Island in 1991 and 1992. Very simple descriptive models for Kodiak Island ( Norcross et al., 1995 , Neth. J. Sea Res. 34: 161–175) accurately predicted the abundance along the Alaska Peninsula of flathead sole (78%), Pacific halibut (96%), yellowfin sole (75%) and rock sole (99%) in specific depth ranges and on specific substrate types. More complex CART models of species abundance ( Norcross et al., 1997 , Fish. Bull. (US) 95: 504–520) were more precise but not as accurate as the descriptive models because some parameters were not always available at the test locations. Flathead sole were found at temperatures ≤ 8.9°C on mud and mixed mud substrates in similar proportions in 1995 (66%) as in 1991–92 (71%). Similarly, Pacific halibut were ≤ 7.9 km inside bays and at depths ≤ 40 m in 93% of the sites of this study, compared with 89% previously. Seventy-five per cent of yellowfin sole were at depths ≤ 28 m on mixed substrates in both study periods. Rock sole were found on sand and muddy sand substrates at temperatures above 8.7°C in 52% of the sites in present and 69% of the sites in previous studies. This field test demonstrated that both descriptive and CART models were very useful at identifying juvenile habitat parameters, but are insufficient for areas not previously sampled. The resource-selection models verified the importance of a subset of parameters used in earlier models and provided a statistical means (78–87% correct) for prediction of fish distribution in similar areas of the eastern North Pacific.

Abstract: -Juveniles offour species of pleuronectid flatfishes were abundant in bays and nearshore areas around Kodiak Island, Alaska, during August 1991 and 1992. The four most abundant species of juvenile (age-O or age-1) flatfishes were rock sole (Pleuronectes bilineatus). flathead sole (Hippoglossoides elassodonJ, Pacific halibut IHippoglossus stenolepisJ. and yellowfin sole (Pleuronectes asper). These species appeared to share nursery areas; however, physical characteristics ofthe nursery areas occupied by each species limited the amount of true overlap among species. Tree-based regression of catch-per-unit-of-effort data on physical parameters was used to refine conceptual models of species distribution, which were originally based only on 1991 data. Threshold values of the physical parameters were specified that best discriminated among stations with different abundances. Highest abundances of age-O rock sole were found on sand or muddy sand at temperatures greater than 8.7°C. as well as on other mixed sand stations less than 28 m deep. Ageoflathead sole were most abundant at temperatures less than 8.9°C and on mixed mud substrates. At warmer temperatures, abundances were high only if the depth was greater than 48 m, regardless ofsediment type. Age-O Pacific halibut were most abundant in depths less than 40 m at sites more than 2.9 km outside the mouths of bays. Inside bays, halibut were found in lower abundances in water over 9.0°C and on sediments containing both sand and mud. Age-1 yellowfin sole were always found in depths less than 28 m on mixed mud substrates. They were usually found within bays, with highest abundances at heads oflarge bays more than 32 km from the bay mouth. These four most abundant flatfishes therefore appeared to partition the available habitat in ways that minimized resource competition. Manuscript accepted 26 February 1997. Fishery Bulletin 95:504-520 (1997). Brenda L. Norcross* *