332 



Fishery Bulletin 98(2) 



Bay, pink salmon samples for diet analysis were 

 taken from low-gradient beaches where there is a 

 high production of epibenthic harpacticoid copepods 

 (Landingham, 1982; Cordell, 1986). However, even 

 at this habitat type, pelagic zooplankton was an 

 important dietary component. Similarly, Sturdevant 

 et al. (1996) found zooplankton to be the dominant 

 dietary component of pink salmon juveniles from all 

 nearshore habitat types sampled in Prince William 

 Sound, Alaska. 



From a bioenergetics standpoint, prey density 

 and water temperature are critical factors affecting 

 fish growth by influencing consumption rate, meta- 

 bolic rate, and gastric evacuation rate (Willette, 

 1996). Temperature was significantly correlated with 

 observed growth of juvenile pink salmon, but indi- 

 ces of prey abundance were not consistently corre- 

 lated with growth and were even inversely related 



7 ^ 



5 - 



£ 

 If) 



/ 



Brood years | 

 1985, 1986, and 1988 



/ 



I 



All years 



65 



Growth rate (% bwd) 



Figure 9 



Interannual .survival depicted as the regression of survival ladjusted for catch 

 rate) versus gi-ovvth rate of tagged Auke Creek pink salmon. When all four 

 brood years are included in the regression, a poor relationship results (;-=(). 02. 

 P>().;i97l. However, when brood year 1987 is excluded from the regression, the 

 relationship between survival and gi'owth rate improves i/--=().88, /'=(). 001 i. 

 This relationship is al.so vci'v good when ba.sed on data from just brood year 

 1987ir-=0.9.'i./'=0.001l. 



in the case of harpacticoid copepods. This lack of 

 correlation may be due to the typically high and 

 variable productivity of zooplankton and littoral epi- 

 benthos found in subarctic ecosystems such as Auke 

 Bay (Cordell, 1986; Coyle and Paul, 1990). Assum- 

 ing there is some threshold level of prey density 

 required to sustain maximum growth of juveniles, 

 then the pronounced spikes in biomass of prey popu- 

 lations associated with the spring bloom would mean 

 that much of the variability in prey occurs above 

 the density threshold, thereby masking a relation- 

 ship between prey biomass and juvenile pink salmon 

 growth. Any such relationship should occur primar- 

 ily before or after the prey population peaks. Anal- 

 ysis of residuals between expected growth of pink 

 salmon and observed initial growth indicated that 

 food abundance did limit growth at low water tem- 

 peratures, (i.e. growth rates observed in the early 

 period of nearshore residency were 

 possibly constrained by food avail- 

 ability as well as lower temperatures 

 (Fig. 7). 



The emigration of Auke Creek pink 

 salmon extended from late March to 

 mid-May; most of the fish emigrated 

 within 2-3 weeks in mid- to late 

 April. Holtby et al. (1989) propo.sed 

 that the synchronous timing of emi- 

 gration of chum salmon may be an 

 adaptive feature based upon advanta- 

 geous growth conditions in the estu- 

 arine environments, with synchrony 

 acting as a mechanism to saturate 

 predators and enhance siu'vival. By 

 this premise, it would be beneficial 

 for ail Atike Creek juveniles to emi- 

 grate together, later in the spring, 

 within a narrow time window, so as to 

 ensure optimum growth conditions. 

 Each year, however, a portion of the 

 Auke Creek pink salmon juveniles 

 emigrate in early spring and they 

 consistently encounter poor growth 

 conditions. Water temperatures are 

 usually 4 C, and zooplankton abun- 

 dance is very low. These juveniles 

 take longer to reach a particular 

 size than do later emigrants and 

 generally reside longer in estuarine 

 nursery areas than the later, faster- 

 growing emigrants (Table 1). Even 

 though predator populations are rel- 

 atively low in early spring, the early 

 emigrants are exposed to predation 

 over a longer period thtui are later 



