FISHKRY BULLETIN, VOL 77. NO. 3 



anchored in two narrow stream channels near the 

 mouth of the Nanaimo River (Figure 1). (These 

 traps were similar in design to those described by 

 Lister etal. 1969.) The mouth opening of each trap 

 was 30 cm wide by 60 cm deep. Four traps were set 

 side by side in one channel and three in the other. 

 Nylon netting of 5 cm mesh was run between the 

 traps and shore in an attempt to lead additional 

 fry into the traps. The traps were operated in 1975 

 and 1976 and were set and fished the same way 

 each year. In 1975 the traps were in place from 

 early March to late May, while in 1976 they were 

 in place from early April to late May. Although the 

 main river flow was down a third channel to the 

 west of the traps, a significant fraction of the 

 Chinook salmon run passed down the trapping 

 channels and, as will be shown later, the traps 

 captured about l.S'^r of the run. 



Figure l. — The Nanaimo River estuary. Vancouver Island, 

 showing the location of the fry traps (20) for juvenile chinook 

 salmon; the stations sampled weekly on the east arm of the river 

 and Holden Creek, (28), (29), (30), (31); the general location of 

 seine sets made to determine the distribution of chinook salmon 

 fry in the estuary, ^■, and the location of purse seine sets made 

 over the intertidal flats at high tide, 0. Small circles show the 

 location of pilings to which log rafts are moored Most raft stor- 

 age is on the west side. 



654 



Fry captured in the traps between 0800 h of 1 

 day and 0800 h of the next were counted as a single 

 day's catch. In 1975 the fry were held in live pens 

 in the river and marked a few hours after capture 

 by spraying with fluorescent grit (Healey et al.*^). 

 After they were sprayed, the fry were held a 

 further 24 h to recover and then were released in 

 the late evening into the river about 2.5 km up- 

 stream from the traps. Most mortality from mark- 

 ing occurred in 24 h and was normally <5% 

 (Healey etal.'). Each daily catch was examined for 

 marked fry, and total daily run was estimated by 

 mark recapture techniques (Ricker 1975; Healey 

 et al, see footnote 7). In 1976 the fry captured each 

 day were counted and released downstream from 

 the trapping site. 



By changing the color of marking grit several 

 times during the run I determined that, on aver- 

 age, 75'7r of recaptures from a single release were 

 made the night of release, a further 17% on the 

 next night, and the remaining8'7f over the next 14 

 nights. I assumed that these percentages repre- 

 sent the proportions of the marked fry which mi- 

 grate the night of release or delay migration one or 

 more days. Also, <100''^ of sprayed fry received a 

 mark. Samples of marked fry examined a few days 

 after spraying showed that usually 95*^^ or more of 

 the fry were marked. The total number of marked 

 fry migrating downstream each night was, there- 

 fore, estimated to be the number of fry released, 

 corrected for the proportion unmarked, minus the 

 number expected to delay migration, plus the 

 number expected to be migrating from previous 

 releases. Total daily run was estimated as the 

 product of daily catch and the estimate of marks 

 migrating divided by the number of recaptures. 

 Trap efficiency was the ratio of recaptures to esti- 

 mated marks migrating. 



During about half the trapping days in 1975 no 

 recaptures were made. On these days the run was 

 estimated as the trap catch divided by the overall 

 estimate of trapping efficiency for the year (total 

 recaptures/total marks migrating). Total run in 

 1976 was estimated from the overall estimate of 

 efficiencv for 1975. 



"Healey, M. C, F. P. Jordan, and R M Hungar 

 1976. Laboratory and field evaluating of fluorescent grit as a 

 marking material for juvenile salmonids. Fish. Res. Board 

 Can. Manuscr. Rep. 1392, 17 p. 



'Healey, M. C, R. V Schmidt, F P Jordan, and R. M Hun- 

 gar. 1977. Young salmon in the Nanaimo area 1975: I. Dis- 

 tribution and abundance. Fish. Res Board Can. Manuscr Rep 

 1369, 161 p. 



