BAILEY ET AL.: ZOOPLANKTON ABUNDANCE AND FEEDING HABITS OF FRY 



samples and 66 of the 1966 samples were examined 

 to determine numbers of plankters from larger 

 subsamples (764 to '4 of the sample) taken with a 

 Gushing subsampler (Gushing 1961). A comparison 

 of the results of the two analyses indicated that 

 the data from 1-ml subsamples overestimated the 

 number of organisms by an average of 20"^^ (range 

 15 to 30%). Therefore, estimates of zooplankton 

 densities using the first method were reduced by 

 20%. Plankton samples contained protozoans 

 (mostly tintinnids) and phytoplankton, but these 

 were not included in the estimate of standing 

 stock of plankton because salmon fry consumed 

 only the larger zooplankters. Rotifers and copepod 

 nauplii were the smallest plankters included in the 

 counts. 



FEEDING IN TRAITORS RIVER 



Although most of the pink and chum salmon 

 excavated from redds in Traitors River contained 

 items such as sand or detritus in their digestive 

 tracts, only a few individuals contained food or- 

 ganisms. Ghironomids (dipterans) were the most 

 frequently observed food item. Seventy juvenile 

 pink salmon (fork length, 33 to 41 mm) were 

 collected from spawning gravels for analysis of 

 contents; only three contained food— a chironomid 

 pupa and some unidentifiable insect remains (Ta- 

 ble 1). Seventy juvenile chum salmon (fork length, 



Table 1. -Frequency of occurrence of items in digestive tracts of 

 70 pink and 70 chum salmon juveniles excavated from redds in 

 Traitors River in 1964-65. 



Table 2. -Frequency of occurrence of items in digestive tracts of 

 40 pink salmon fry and 40 chum salmon fry trapped in nets while 

 migrating down Traitors River, May 1964. 



Pink salmon 



Chum salmon 



33 to 41 mm) were collected from spawning 

 gravels; nine contained food. Ghum salmon had 

 eaten only chironomid larvae and pupae, 

 plecopteran nymphs, ephemeropteran nymphs, 

 and an arachnid (spider). One chum salmon (41 

 mm) contained the remains of 24 chironomid 

 pupae, 2 chironomid larvae, 3 ephemeropteran 

 nymphs, and 3 plecopteran nymphs. The other 

 eight chum salmon that contained food were 37 to 

 38 mm long and had eaten only one to three items 

 each. 



Although none of the 40 downstream-migrating 

 pink salmon fry (length, 32 to 37 mm) contained 

 food, 9 of the 40 chum salmon (length, 35 to 42 mm) 

 contained substantial numbers of chironomid 

 pupae and plecopteran nymphs (Table 2). The 

 average for those that contained food was 6.7 food 

 items (range 1 to 27 items). 



Fine sand (diameter, 0.05 to 0.90 mm) and plant 

 detritus were common items in the digestive 

 tracts of both the gravel-resident and the migrat- 

 ing pink and chum salmon (Tables 1, 2). The sand 

 and detritus were more common in fish taken from 

 the redds than in those captured in the down- 

 stream traps. 



FEEDING IN THE ESTUARY 



We studied four aspects of the feeding of pink 

 and chum salmon fry in the estuary: 1) stomach 

 contents, 2) feeding behavior in relation to water 

 currents, 3) effect of daylight on feeding, and 4) 

 time required for evacuation of stomach contents. 



Stomach Contents of Pink Salmon Fry 



In the springs of 1964, 1965, and 1966, a total of 

 140 pink salmon (length, 28 to 56 mm) were 

 collected from the estuary during daylight, and 30 

 (length, 31 to 58 mm) were collected at night (Ta- 

 ble 3). All of the stomachs from the fry collected in 

 daylight contained food. Gopepods (calanoids and 

 cyclopoids) occurred in 94% of the stomachs and 

 constituted 77% of the total volume of stomach 

 contents. Barnacle nauplii (cirripedes) and 

 cladocerans (Podon sp. and Evadne sp.) each oc- 

 curred in 56% of the stomachs and constituted 6% 

 of the total volume. The remaining 11% of the food 

 volume consisted of various other planktonic 

 forms and occasional epibenthic organisms. Most 

 of the food items were between 0.3 and 3.0 mm 

 long. The smallest item in pink salmon stomachs 

 was a disc-shaped diatom and the largest were fish 



849 



