FISHERY BULLETIN: VOL, 77. NO, 3 



channel. Sampling along the east channel and 

 Holden Creek on 22 April showed that the marked 

 fish occurred all along the east channel and Hold- 

 en Creek but were most concentrated at Station 31 

 and for several hundred yards farther upstream. 

 Sampling in subsequent weeks indicated that 

 marked fish remained concentrated at Stations 30 

 and 31 as long as chinook salmon were abundant 

 there. With the shift in population center to Sta- 

 tions 28 and 29, the marked fish also shifted 

 downstream, but remained most abundant at Sta- 

 tion 29. A few marked fish were also captured in 

 samples taken across the delta front and across the 

 landward margin of the mud flat at high tide, but 

 none were captured in stream channels in the 

 center of the mud flat at low tide. In spite of their 

 twice daily migration up and down the delta with 

 the tide, therefore, marked fish remained concen- 

 trated in the area of marking, so that population 

 estimates from the recaptures referred only to the 

 east channel and Holden Creek, and underesti- 

 mated the population in this region as well. 



The instantaneous rate of disappearance of 

 marks from the sampling area after the April 

 marking was 0.117, or about 11%/day, and after 

 the May marking the rate was 0.137, or about 

 13%/day. These disappearance rates were used to 

 correct the summed release of marks each day to 

 an estimate of the marks remaining in the area 

 and to calculate estimates of the marked popula- 

 tion on all sampling dates following the last re- 

 lease of marks (Table 6). 



Estimates of population size throughout the 

 first period of marking were consistent, ranging 

 only 14,675-17,133, and estimates for 25 April and 



3 May were also similar (Table 6). On May 9, the 

 population estimate dropped to 5,708 and re- 

 mained at this level or lower throughout May. The 

 first population estimate from right pelvic clips 

 was on 17 May. Estimates based on this mark 

 ranged 4,629-9,544 between 17 and 19 May and 

 remained at this level throughout May. Estimates 

 for the first 2 wk of June from recaptures of right 

 pelvic clips were 2,352 and 1,204, respectively. 



Estimates from right pelvic recaptures in May 

 were twice as great, or greater, than estimates 

 from left pelvic recaptures. Possibly this differ- 

 ence occurred because fishing during 17-19 May 

 was concentrated where fry marked with left pel- 

 vic clips were most abundant, so that recaptures of 

 this mark were high. 



The estimates indicated that the population in 

 the east channel and Holden Creek was 12,000- 

 19,000 throughout April and early May and that 

 the population declined to 5,000-10,000 in the lat- 

 ter half of May and declined further to about 2,000 

 in early June. These changes are consistent with 

 changes in beach seine catches. 



During the first week of May sampling was per- 

 formed across the landward edge of the mud fiat at 

 high tide (13 sets) in the east arm and Holden 

 Creek (8 sets), across the delta front at low tide (8 

 sets), and in the stream channels crossing the 

 center of the mud fiat (7 sets). Although this sam- 

 pling was not at random with respect to either the 

 distribution of chinook salmon or marks, it does 

 permit a population estimate based upon sam- 

 pling areas outside the east channel and Holden 

 Creek. A total of 406 chinook salmon were cap- 

 tured, of which 12 were recaptures. The average 



Table 6.— Release and recovery of fin-clipped chinook fry, estimates of marks available and population estimates for the Nanaimo 

 River estuary in 1977. LV = left pelvic clip; RV = right [>elvic clip. Population estimates are the product of total catch and estimated 

 marks present divided by marks recaptured. 



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