BARRACLOUGH and ROBINSON: LAKE FERTILIZATION. III. 



1400 1600 1800 2000 2200 2400 0200 0400 0600 OBOO lOOO '2°° _, 



(500 1700 SOO 2100 2300 OiOO 0300 0500 0700 0900 "OO 'JTO 



Figure 10. — (a) Depth of the densest portion of the 

 layer of juvenile sockeye salmon at different times of the 

 day and night from June 17 to June 18, 1970 is indicated 

 by a broken line. A secondary layer is shown at 105 m 

 for a 2-hr period. The depth of each tow with a 

 midwater trawl is shown relative to the depth of the fish, 

 (b) Number of all food organisms of underyearling 

 sockeye. (c) Food species of underyearling sockeye 

 salmon expressed as a percentage of the total number 

 of organisms. 



A second feeding period was noted at the time 

 of the diel migration downward. Stomach sam- 

 ples from juvenile sockeye collected during this 

 period contained many fresh Daphnia and Cy- 

 clops in tows 8 to 10. Only 2^/r of the zoo- 

 plankton in the stomachs of sockeye caught at 

 midday were in a fresh condition (Table 2), 

 which indicates a marked reduction in feeding 

 activity. 



GROWTH OF UNDERYEARLING 

 SOCKEYE 



The average size of underyearling sockeye 

 (Age 0) in 1969 and 1970 in Great Central Lake 

 is shown in Figures 11 and 12. A total of 1,760 

 underyearling sockeye were caught in 1969 and 

 20,783 fish in 1970 from all six stations. A com- 

 plete record of all data on which this analysis 

 is based has been reported by Barraclough and 



70 



60 



E 



t 50 



,o 40 



30 



Or 1969 Underyeorlings 

 a -. 1970 Underyeorlings 



^^IwARlflPRlMflYljUNljUL I AUG I SEP'OCTInOV I DECl JAN I FEBI MAR 



Figure 11. — Average length of underyearling sockeye 

 salmon in each month, 1969 and 1970. 



2 50 



- 2 00 



E 



1 50 



100 



050 



o = 1969 Underyeorlings 

 O = 1970 Underyeorlings 



D — a-o-<S^ 



marIapr'may'jun IjulIaug'sepIoctInovIoecI janI feb'mar 



Figure 12. — Average weight of underyearling sockeye 

 salmon in each month, 1969 and 1970. 



Robinson (1971).' Although Httle data on the 

 size of in-lake juveniles prior to August, 1969 

 are available, it is evident that a marked initial 

 increase in length and weight of underyearlings 

 occurred from June to July, 1970 at a time when 

 sustained additions of nutrients were made to 

 the lake. Growth continued steadily during the 

 rest of 1970 and through winter months in 1970- 

 1971. 



"■ Barraclough, W. E., and D. G. Robinson. 1971. 

 Length, weight, age and food of juvenile sockeye salmon 

 {Oncorhynchns nerka) from Great Central Lake, British 

 Columbia, May 1969 to February 1971. Fish. Res. Board 

 Can., Manuscr. Rep. No. 1128, 268 p. 



45 



