FISHERY BULLETIN: VOL. 70, NO. 1 



of their minute scales by abrasion against net- 

 ting in the main cod end of the trawl. 



An Isaacs-Kidd midwater trawl was towed 

 from the Decibar to samjile the juvenile sockeye 

 at depths greater than 25 m and to evaluate the 

 fishing capabilities of the large midwater trawl 

 towed at the same depths. The mouth opening 

 of this trawl was 1.9 m^ and the net was con- 

 structed of 6.3 mm stretched mesh knotless 

 netting. 



FISHING STATIONS 



Juvenile sockeye were sampled with trawls 

 taken at intervals of about 3 weeks at 6 different 

 stations (Figure 1). Most of the tows were of 

 30 min duration but some tows wei-e shorter, 

 when the echo sounder traces indicated that 

 young sockeye were especially abundant between 

 12 and 14 m at night. 



ANALYSES OF SAMPLES 



The length of all fish was measured to the near- 

 est millimeter from the snout to the end of the 

 central rays of the caudal fin. This measurement 

 is referred to as the fork length. Lengths of 

 smaller fish were measured in a graduate tray 

 under a binocular microscope; calipers were 

 used for larger individuals. 



All fish were weighed by fork length groups 

 using a center-loading milligram balance 

 (KERN Model No. T1226-1) . Weights recorded 

 are from "blot-dried" specimens. Moisture was 

 blotted from the exterior of the fish, and gentle 

 pressure was applied to the buccal cavity and 

 branchial chamber to remove moisture from 

 these spaces. Age was determined from scales 

 using y 254 projections of thermoplastic impres- 

 sions. 



Stomach analyses for food were done on fish 

 selected to represent proportionally as complete 

 a size range as possible. The food weight was 

 measured by subtracting weight of stomach shell 

 from weight of stomach plus food. The number 

 of all species of food organisms were counted 

 according to size and state of condition of each 

 stomach examined. 



RESULTS 



FOOD OF UNDERYEARLING 

 (AGE 0) SOCKEYE 



During the latter part of March and up to 

 mid-April, 1970, pre-mature fry" (24 to 28 mm 

 fork length) with a small portion of the yolk 

 sac remaining were caught at night at a depth 

 of 14 m in midlake positions off the 3 major 

 spawning areas. A few fry (28-30 mm) with 

 empty stomachs were caught during the day at 

 depths between 35 and 100 m in late March, and 

 the first actively feeding fry (28 to 33 mm) were 

 caught at depths between 12 and 55 m at night 

 during the latter part of April. The number 

 of fry caught at midwater depths increased in 

 May at Stations 3, 4, 5, and 6 and reached a 

 maximum in June at all stations. Fry continued 

 to be caught in July and were still being caught 

 in trawl nets at night in late August and early 

 September. The fry and larger underyearling 

 sockeye ate the same food organisms throughout 

 the year, but the larger juvenile fish had more 

 food in their stomachs. 



Figure 2 shows the number and weight of all 

 species of food organisms per underyearling 

 sockeye (Age 0) from August, 1969, when in- 

 lake sampling began to April, 1970, when about 

 85 '^r of the fish migrating were yearling smolts.' 

 The percentage of the total number of the six 

 major food categories from all the fish sampled 

 for stomach contents through the same period is 

 shown in Figure 3. A list of the different gen- 

 era of food organisms found in the stomachs 

 of juvenile sockeye from 1969 to 1971 is given 

 in Table 1; the smallest is listed at the top of 

 the column and the largest at the bottom. 



Epischura was the predominant form (60%) 

 in the stomachs in August, 1969 but was almost 

 replaced by /fo/o?9edmm (60-80%) from Septem- 

 ber to December (Figure 3). The incidence of 



^ "Embryo" is defined as a larva minus its yolk-sac. 

 An "alevin" is a larva of an age following hatching but 

 prior to yolk absorption. Following this stage the fish 

 becomes a "fry" (cf. Bams, 1969). 



' In 1969 ca. 86% of migrant smolts were yearling, 

 10% were 2 year old, and 4% were 3 year old. In 1970 

 ca. 85% of smolts were yearling and 15% were 2 year old. 



40 



