HEALEY: UTILIZATION OF THE NANAIMO RIVER ESTUARY 



estimated marked population for the week was 

 655, giving a population estimate of 22,148 for the 

 whole estuary. The average population of the east 

 channel for the week was about 15,000, or about 

 68% of this estimate. Total estuary population 

 may. therefore, be about 32% greater than the esti- 

 mate for the east channel and Holden Creek. 



Comparing beach seine catches for 1975-77 with 

 the mark recapture estimates indicated that the 

 peak population on the estuary was on the order of 

 20,000-25,000 in 1976 and 1977 but was probably 

 closer to 40,000-50,000 in 1975. These estimates 

 are comparable with a single day's fry migration 

 in 1975 and 1976. However, the slow rate of disap- 

 pearance of marked fry from the east channel indi- 

 cated a relatively long residence of fry on the es- 

 tuary (about 60 days). An accumulation of fry on 

 the estuary during downstream migration would, 

 therefore, be expected. Treating each daily run of 

 fry as a single cohort arriving on the estuary, and 

 reducing that cohort by 11-12%/day (the rate of 

 disappearance of marked fry from the east chan- 

 nel), produced estimates for the estuary popula- 

 tion of around 100,000 in 1975 and 50,000 in 1976, 

 or about twice the estimate based on mark recap- 

 ture results for 1977. Estimates of downstream 

 run are for the release point of the marks, how- 

 ever, and significant mortality might occur bet- 

 ween the release point and the estuary (Hunter 

 1959). Alternatively, the rate of disappearance of 

 marked fry may underestimate the rate of disap- 

 pearance of recent downstream migrants. A dis- 

 appearance rate of 11-12%/day suggested an aver- 

 age residence time of about 60 days, whereas 

 growth rates suggested that most fry should spend 

 only 25 days in the estuary. 



If downstream migrants spend only 25 days in 

 the intertidal area, and their rate of disappear- 

 ance is constant during that' time, then peak es- 

 tuary populations are 40,000 in 1975 and 20,000 in 

 1976, comparable with the estimate based on 

 mark recaptures in 1977. The estimate of disap- 

 pearance rate from mark returns has rather wide 

 confidence limits, 25 days being within the range 

 of 95% probability in estimates of residence time. 

 The apparent discrepancy between mark recap- 

 ture estimates of estuary population size and 

 downstream run can be resolved by assuming re- 

 sidence of 25 days, therefore. The assumption of a 

 constant rate of disappearance of chinook salmon 

 from the estuary population, however, implies the 

 disappearance of many juveniles <70 mm FL. Al- 

 though high mortality of salmon fry is a common 



assumption, no predators or important diseases 

 were obviously present in the Nanaimo estuary to 

 justify the assumption of heavy losses of small fish. 

 The tentative agreement between the various es- 

 timates of population size may therefore be spuri- 

 ous, and these estimates should be regarded as 

 preliminary at best. 



By comparison with the Fraser and the Sixes 

 Rivers, chinook salmon were rare in the Nanaimo 

 River. Dunford (1975) reported maximum densi- 

 ties in excess of 2 fish/m^ in Fraser River marshes, 

 compared with average densities of about 0.1 

 fish/m^ in the east channel and Holden Creek. For 

 the Sixes River estuary, an area about twice as 

 large as the east channel and Holden Creek, 

 Reimers (1971) reported maximum population es- 

 timates of 100,000-150,000. However, Reimers' 

 estimates were made 5 days after the release of 

 marked fish into the estuary, and, assuming his 

 marked fish were disappearing at a rate similar to 

 those in the Nanaimo River, the population in the 

 Sixes River estuary may have been closer to half 

 the values he reported. Nevertheless, this still 

 represents a population significantly more dense 

 than that in the Nanaimo estuary. In terms of 

 suitable habitat, however, the Sixes River may not 

 be greatly different from the Nanaimo River, as it 

 is about twice as large as the east channel and 

 Holden Creek, and probably supported about 

 twice the population of chinook salmon. 



Population of Juvenile Chinook Salmon 

 Outsicie the Estuary 



Beach seine samples in areas other than the 

 intertidal area of the estuary produced few- 

 juvenile chinook salmon. In 1975, 19 sets made in 

 mid-May yielded only 3 juveniles, and in 1976, 61 

 sets made during April-June yielded only 26. 

 Twenty-four of these were captured in the lagoon 

 behind Duke Point (area 16), adjacent to the es- 

 tuary. Apparently onshore areas away from the 

 estuary were not used by chinook salmon fry, al- 

 though all the beaches sampled were used by pink 

 and chum salmon fry. 



Juvenile chinook salmon were captured in most 

 locations sampled by the two purse seines in 1975 

 and 1976. Not all chinook salmon captured were 

 young-of-the-year, however. Catches prior to May 

 were mainly yearlings. In late May and early June 

 there was a large influx of young-of-the-year and a 

 subsequent decline in the catch of yearlings. The 

 influx of young-of-the-year (Figure 6) coincided 



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