Robards et al ; Variation in fish communities of lower Cook Inlet, Alaska 



963 



have also described seasonal variation in shallow 

 water fish assemblages (e.g. Horn, 1980: Allen, 1982; 

 Nash and Gibson, 1982; Nash, 1988; Bennett, 1989). 

 Little is known, however, about annual, seasonal, and 

 daily variation in fish abundance in Alaska. Under- 

 standing the responses of fish populations at this 

 scale are critical for examining finer-scale processes 

 such as trophic interactions. Responses at these time 

 scales also put interdecadal changes into perspec- 

 tive, and it is changes at the decadal scale that are 

 rapidly becoming the focus of fishery oceanographers 

 (Francis etal, 1998). 



Our primary objectives were to assess variability 

 in abundance, diversity, and distribution of nearshore 

 fishes in three oceanographically distinct locations 

 within lower Cook Inlet and to compare these data 

 with information collected by Blackburn et al. ( 1980) 

 20 years ago. Until our study, Blackburn et al.'s 1976 

 sur\'eys provided the only comprehensive description 

 of nearshore fish communities in the region. Con- 

 current sampling of offshore species with midwater 

 trawls also allowed us to make comparisons between 

 shelf and nearshore fish communities. 



Methods 



Study sites 



Lower Cook Inlet (Fig. 1), in south-central Alaska, is 

 the largest embayment in the northern Gulf of 

 Alaska. The area supports several important seabird 

 colonies and numerous marine mammals, as well as 

 important commercial and recreational fisheries for 

 salmon iOncorhynchus spp. ) and Pacific halibut 

 (Hippoglossus stenolepis). We studied three lower 

 Cook Inlet areas that primarily support colonies of 

 fish-eating common murres ( Uria aalge ) and black- 

 legged kittiwakes (Rissa tj-idactyla): Gull Island, in 

 Kachemak Bay; Chisik Island, on the western side 

 of Cook Inlet; and the Barren Islands, near the en- 

 trance to Cook Inlet. Investigations of nearshore fish 

 communities were initiated around these colonies to 

 provide information that can be used to improve un- 

 derstanding of how forage fish abundance and dis- 

 tribution may influence seabird diets and productiv- 

 ity in these areas. 



Kachemak Bay (Fig. 1) lies along the southeast- 

 ern shore of Cook Inlet. The bay is 38 km wide at its 

 entrance and 62 km long. The upper 6 km consists of 

 mud flats that are exposed during low tide. Depths 

 are relatively shallow, ranging from about 35 to 90 m, 

 with some deeper areas (100 to 165 m) present off 

 Gull Island along the south-central side of the bay. 

 Water entering the bay originates from the Gulf of 



Figure 1 



Map of lower Cook Inlet showing the three study areas, prevailing 

 currents, bathymetry, and both beach seine and midwater trawl 

 locations. 



Alaska and is largely oceanic (Fig. 1; Burbank, 1977 ). 



Chisik Island, on the western side of Cook Inlet 

 ( Fig. 1 ), is located in the mouth of Tuxedni Bay, which 

 receives freshwater from glacier-fed rivers. Water 

 passing outside the island is also estuarine, because 

 it receives significant glacier-fed freshwater input 

 from large rivers at the head of Cook Inlet before 

 circulating down the western side of the inlet 

 (Burbank, 1977; Feely and Massoth, 1982). Near- 

 shore habitats around Chisik Island contain few 

 sandy substrates and consist primarily of glacial silt 

 and mud flats interspersed with rocky substrates 

 that are exposed at low tides. 



The Barren Islands, near the entrance to Cook 

 Inlet (Fig. 1), are situated in a transition zone be- 

 tween deep Gulf of Alaska waters and the shallow 

 Cook Inlet estuary. The Alaska Coastal Current en- 

 ters Cook Inlet north of the Barren Islands, leading 

 to intense upwelling of cold, nutrient-rich waters onto 

 the shallow southeastern Cook Inlet shelf (Burbank, 

 1977). Because of the upwelling and strong tidal ac- 

 tion (second highest tidal range in North America), 

 waters in this area are turbulent and well mixed. 



