nSHERY BULLETIN VOL. 77. NO. 3 



with the decline in abundance of chinook salmon 

 in the intertidal area of the Nanaimo estuary. The 

 periodicity of catches in the estuary and adjacent 

 marine areas is indicative of a stage movement 

 away from the estuary and into deeper water by 

 young-of-the-year. Sampling by drum seine after 

 July 1976 indicated the persistence of moderate 

 numbers of juvenile chinook salmon in the 

 Nanaimo area until the end of October, after 

 which catches declined to the low levels observed 

 in spring (Figure 6). 



Catches of chinook salmon by the 92 m purse 

 seine in 1975 were mainly in area 10 (338 of 434 

 chinook salmon captured ), with smaller catches in 

 areas 6, 7, 8, and 11 and few elsewhere. Catches by 

 the 218 m drum seine in 1975 were also mainly in 

 area 10(101 of 205 captured), with the remaining 

 catch scattered throughout the sampling areas. 

 Chinook salmon were more scattered in 1976, area 

 10 yielding only 79 of 245 captured by drum seine 

 between April and July and areas 1.2, 5, and 6 also 

 providing good catches. Chinook salmon were of 

 similar abundance in drum seine catches between 



MARINE 



4-10 meter depth 



-°- ^'"""'^ 



MARINE 

 \ > 20 metef depth 



Figure 6.— Catch per set of juvenile chinook salmon by age and 

 life history type, by beach seine on the estuary, and by shallow 

 and deep purse seine in marine waters adjacent to the Nanaimo 

 River e.stuary. Data are averages for 1975-77, 



April and June 1975 and 1976 (CPUE 0.73 in 1975 

 and 0.83 in 1976) but were significantly less abun- 

 dant in July 1976 compared with 1975 (CPUE 3.30 

 in 1975 and 2.28 in 1976 x^ = 6.43, P<0.051. The 

 greater catch in July 1975 presumably reflected 

 the greater contribution of young-of-the-year from 

 the estuary in 1975. 



The presence of juvenile chinook salmon in the 

 Nanaimo area throughout the year in 1976 indi- 

 cates a local resident population that is 

 supplemented by young-of-the-year in June. The 

 appearance of juveniles in large numbers in area 

 10 coincident with their disappearance from the 

 intertidal area of the estuary indicates that these 

 fish were from the estuary population. The evi- 

 dence is not conclusive however, and e.xamination 

 of the catch at area lOin June and July 1977 for fin 

 clips from the estuary produced only 8 marked fish 

 out of 555 examined. This compares with approx- 

 imately 10%' of the estuary population marked in 

 April and May. Possible reasons for the low 

 number of marks in the catch at area 10 include 

 differential mortality of marks (the percentage of 

 mark returns in the estuary declined after each 

 marking), rapid dispersal of chinook salmon away 

 from the estuary, dilution of the fish of local origin 

 by fish from other systems, or dilution of the es- 

 tuary population by late migrants from the 

 Nanaimo River. In my view the most likely expla- 

 nations are rapid dispersal of juveniles from the 

 estuary population, and dilution of the estuary 

 population by late migrants from the Nanaimo 

 River. Chinook salmon reared in the intermediate 

 salinity of the estuary are probably already 

 adapted to seawater by the time they are ready to 

 leave the estuary while late migrants from the 

 river might be expected to stay close to the river 

 mouth for some time, adapting to salt water. Sam- 

 ples from area 10 may, therefore, contain a dispro- 

 portionate number of late migrants. 



An unknown proportion of the Nanaimo River 

 population probably disperses rather quickly 

 away from the Nanaimo area after leaving the 

 river. Some young-of-the-year, however, remain 

 in the Nanaimo area, at first concentrated rather 

 close to shore, but later moving to more offshore 

 sampling locations where they persist until at 

 least November (Figure 6 1. During the winter 

 these fish decline in numbers until by the follow- 

 ing spring there are only a few 1 + ocean fish in the 

 local area. Most of these disappear from the sur- 

 face waters in May coincident with a small influx 

 of yearling smolts from the Nanaimo River (Fig- 



666 



