Comparison of models for defining nearshore flatfish nursery areas in Alaskan waters

Because it is unrealistic to assess fishes in all locations, models are nee ded to characterize the nursery habitats of flatfish species. Descriptive h abitat 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 elasso don); age-0 Pacific halibut (Hippoglossus stenolepis); age-1 yellowfin sole (Pleuronectes asper); and age-0 rock sole (Pleuronectes bilineatus), In 19 95, collections were made in bays along the Alaska Peninsula (an area never before sampled for juvenile flatfishes) and were compared with the previou s models developed for Kodiak Island in 1991 and 1992. Very simple descript ive models for Kodiak Island (Norcross et al., 1995, Neth. J. Sea Res. 34: 161-175) accurately predicted the abundance along the Alaska Peninsula of f lathead sole (78%), Pacific halibut (96%), yellowfin sole (75%) and rock so le (99%) in specific depth ranges and on specific substrate types. More com plex 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 locati ons. Flathead sole were found at temperatures less than or equal to 8.9 deg rees C on mud and mixed mud substrates in similar proportions in 1995 (66%) as in 1991-92 (71%). Similarly, Pacific halibut were less than or equal to 7.9 km inside bays and at depths less than or equal to 40 m in 93% of the sites of this study, compared with 89% previously. Seventy-five per cent of yellowfin sole were at depths less than or equal to 28 m on mixed substrat es in both study periods. Rock sole were found on sand and muddy sand subst rates at temperatures above 8.7 degrees C in 52% of the sites in present an d 69% of the sites in previous studies. This field test demonstrated that b oth descriptive and CART models were very useful at identifying juvenile ha bitat parameters, but are insufficient for areas not previously sampled. Th e resource-selection models verified the importance of a subset of paramete rs 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.