42 



Fishery Bulletin 91(1), 1993 



is well mixed and uniformly cold, with mean tempera- 

 ture below 5 C. With onset of the phytoplankton bloom, 

 the Bay stratifies, with rapid warming of the mixed 

 layer to over 10°C by June (Bruce et al. 1977, Ziemann 

 et al. 1991). Zooplankton are in low density until the 

 end of the phytoplankton bloom, and are comprised of 

 relatively large plankton such as overwintering 

 copepedids and some meroplankton such as barnacle 

 larvae (Wing & Reid 1972, Coyle & Paul 1990, Paul et 

 al. 1991). The initiation of the herbivorous copepod 

 maximum marks the start of a period with relatively 

 high densities of smaller zooplankton, especially cope- 

 pod nauplii in the size range (150-350(0.) utilized by 

 synchronous species such as walleye pollock and 

 flathead sole larvae (Fig. 12; data from Paul et al. 

 1991). It seems clear that fish larvae hatched prior to 

 the phytoplankton bloom must be adapted to a very 

 different set of conditions than those that occur syn- 

 chronously with the herbivorous copepod maximum. 



Larvae spawned in winter apparently employ vari- 

 ous foraging strategies while utilizing similar ener- 

 getic principles. Bailey (1982) concluded that Pacific 

 hake larvae use energy slowly and grow slowly while 

 passively hunting large prey. In Long Island Sound 

 NY, larvae of American sandlance Ammodytes 

 americanus hatched in winter are herbivores that sur- 



MARCH 



Figure 1 2 



Densities of copepod nauplii ( 150-350|j.) averaged over depths 

 of 5, 10, and 15 m in Auke Bay, Alaska during spring. 1986- 

 89 (data from Paul et al. 1991). 



vive nonfeeding in cold temperatures until the spring 

 phytoplankton bloom commences (Monteleone et al. 

 1987). In both of these strategies, larvae survive by 

 relying on their ability to withstand long periods of 

 low food availability, largely as a result of low meta- 

 bolic rates at cold temperatures. 



Sherman et al. (1984) identified the synchronous 

 strategy as a major adaptive tactic for many north- 

 west Atlantic Ocean fishes. In Auke Bay, several taxa 

 including osmerids, walleye pollock, and flathead sole 

 consistently appeared at about the time copepod abun- 

 dance was maximized. Although prey is relatively plen- 

 tiful during the herbivorous copepod maximum, the 

 numbers of predatory invertebrates are rising 

 (Smetacek et al. 1984, Coyle & Paul 1990); conse- 

 quently, mortality rates of fish eggs and larvae are 

 probably rising rapidly. Higher temperatures during 

 this period may also limit the time available for larvae 

 to encounter suitable conditions by causing high meta- 

 bolic rates and more rapid depletion of energy reserves. 

 These species are probably most sensitive to 

 interannual variation in the production cycle and may 

 demonstrate the type of recruitment fluctuation de- 

 scribed by the match-mismatch hypothesis (Cushing 

 1975) or the critical-period hypothesis (Hjort 1914). 



Patterns of abundance observed in Auke Bay could 

 result from hatching of larvae in the Bay or from ad- 

 vective events that carried larvae into the Bay from 

 other areas. Auke Bay has only one deep (>20m) en- 

 trance, just east of Coghlan Island (Fig. 1). Water in 

 the Bay is quite persistent, with an average replace- 

 ment time of the water mass once a month or longer 

 during the March-June period (Nebert 1990). In other 

 studies of growth, length-frequencies were determined 

 for walleye pollock and flathead sole larvae (Haldorson 

 et al. 1989, 1990). Cohorts of both species first ap- 

 peared in Auke Bay as small larvae comprising length- 

 frequency modes that could be followed for several 

 weeks. Over the 4 years of the study there were very 

 few cases in which length-frequency modes occurred 

 that could not be identified in preceding weeks. Conse- 

 quently, we conclude that most of the larvae sampled 

 in this study originated from hatching within or near 

 the Bay, with possible exception of osmerids. 



The osmerids in our study are most likely eulachon 

 Thaleichthys pacificus and capelin Mallotus villosus. 

 Eulachon is an anadromous species that spawns dem- 

 ersal, adhesive eggs in rivers. After hatching, the lar- 

 vae are carried into nearby marine waters. The most 

 likely source of eulachon larvae in Auke Bay is the 

 Mendenhall River, a glacier-fed stream about 2 km east 

 of Auke Bay. The fresh and turbid waters from the 

 Mendenhall River form a surface lens that projects 

 out into nearby Fritz Cove and often intrudes into the 

 eastern edge of Auke Bay through a narrow passage 



