586 



Fishery Bulletin 92(3), 1994 



Discussion 



Distribution of pollock larvae in relation to 

 hydrography 



The vertical distribution of larval walleye pollock is 

 influenced by behavioral responses to gravity, light, 

 thermal stratification, turbidity, and turbulence (Olla 

 and Davis, 1990). Even yolk-sac larvae are capable 

 of oriented vertical movement. Olla and Davis ( 1990) 

 found that larvae moved away from 3°C water in a 

 vertical temperature gradient. Thus, temperature 

 gradients may be reflected in the vertical distribu- 

 tion of walleye pollock larvae. 



The results of this study show that the vertical 

 distribution of walleye pollock larvae differed be- 

 tween the inner and the outer basin of Resurrection 

 Bay in May 1989. Larvae at the outermost stations, 

 RES 4 and GAK 1, were distributed deeper in the 

 water column (Fig. 3). This is consistent with an 

 upward migration of young larvae after hatching. 

 Larvae are significantly younger at the outer sta- 

 tions and are distributed deeper in the water col- 

 umn and closer to the depth of hatching. Alterna- 

 tively, larvae may select a preferred temperature by 

 avoiding layers of cold water. Water temperatures 

 below 40 m were about 1°C warmer at GAK 1 than 

 at all stations inside the sill (Fig. 2). Temperatures 

 at RES 4, located between GAK 1 and the sill, were 

 intermediate. Cold water of less than 4°C below 40 

 m in the inner basin might prevent larvae from de- 

 scending in the water column, resulting in the ob- 

 served shallow distribution. 



The horizontal distribution of larvae is largely de- 

 termined by upper layer flow. Surface inflow of wa- 

 ter into Resurrection Bay has been observed in acous- 

 tic doppler current profiler (ADCP) transects across 

 the fjord, and average flow at 15 m depth at a moor- 

 ing location above the sill was up-fjord between June 

 and October 1989 (Weingartner 2 ). If the water in this 

 layer flowed up the fjord during April and May, it 

 would provide a mechanism for advection of larvae 

 into Resurrection Bay. Inflow of water at 15 m re- 

 quires a compensating outflow. If the upper layer flow 

 is divided in the horizontal plane with inflow on one 

 side of the fjord and outflow on the other side, larvae 

 may simply be transported through the fjord and 

 their residence time could be very short. Alterna- 

 tively, if surface inflow is compensated for by sub- 

 surface outflow or outflow in a shallow low-salinity 

 surface layer, larvae could accumulate inside the fjord 

 if they maintain their vertical position in the water 

 column. 



The available evidence suggests that the former 

 mechanism, i.e. two-way surface flow, dominates in 

 the outer fjord basin. The relatively high surface sa- 

 linity at GAK 1 suggests that the water in the outer 

 basin originates on the shelf. A salinity transect 

 across GAK 1 shows relatively low salinities at both 

 ends of the transect and higher salinities in the cen- 

 ter. This does not imply, but is consistent with, an 

 inflow of water along the east side of the outer fjord 

 basin and an outflow along the western shore. In- 

 flow of nearshore water along the eastern shore into 

 Resurrection Bay can be seen in satellite images of 

 the area (Royer 3 ) and there is evidence from ADCP 

 transects for a counterclockwise circulation in the 

 outer basin (Weingartner 4 ). Larvae that originate on 

 the shelf thus may be carried counterclockwise 

 through the outer basin. Larvae could be carried into 

 the inner fjord by intrusions of surface water across 

 the sill. We probably observed such an intrusion be- 

 tween 1 and 3 May 1989 (Milter, 1992). 



It has been demonstrated for several fjords in Nor- 

 way that water exchange processes can have a pro- 

 found influence on the community structure within 

 fjords (Lindahl and Perissinotto, 1987). Advective 

 processes can even be the major factor regulating zoo- 

 plankton biomass in a fjord (Lindahl and Hernroth, 

 1988). Advection of plankton into Resurrection Bay 

 from the shelf is evidenced by the fact that in addi- 

 tion to resident nearshore species like Pseudocalanus 

 spp., oceanic copepods iCalanus spp.) common in the 

 Alaska Coastal Current, are found in high concen- 

 trations inside the fjord (Smith et al., 1991). Larval 

 walleye pollock found inside Resurrection Bay could 

 similarly originate on the shelf and enter the fjord 

 as a result of advective processes. Plankton samples 

 collected in 1991 suggest that larvae entered the fjord 

 from outside (Muter, unpubl. data). However, acous- 

 tic surveys indicated the presence of adult walleye 

 pollock inside Resurrection Bay in the spring of 1983 

 and at least some spawning may occur inside the fjord 

 (Paul 5 ). 



Abundance 



Our results indicate that walleye pollock larvae were 

 abundant in Resurrection Bay and on the shelf out- 

 side Resurrection Bay, as represented by GAK 1. High 

 densities of larval pollock up to 55 larvaem ! were 



Weingartner. T. Institute of Marine Sciences, Univ. Alaska. 

 Fairbanks, AK 99775-1080. Unpubl. data. 1989. 



:1 Royer, T. Institute of Marine Sciences, Univ. Alaska, Fairbanks, 

 AK 99775-1080. Personal commun.. 1992 



4 Weingartner, T. Institute of Marine Sciences, Univ. Alaska, 

 Fairbanks, AK 99775-1080. Personal commun.. 1992. 



5 Paul, A. J. Seward Marine Center, Institute of Marine Sciences, 

 Box 730, Seward. AK. Personal commun.. 1992. 



