nSHERY BULLETIN VOL, 77, NO 3 



across the landward margin of the intertidal area. 

 Apparently no chinook salmon, or very few, re- 

 mained over the intertidal flats at high tide The 

 redistribution of chinook salmon on each tidal cy- 

 cle, and their concentration in one of several low 

 tide refuges implied active habitat selection. Ac- 

 tive selection of habitats at low tide is further 

 indicated by the seaward movement of the center 

 of the population in the east channel and Holden 

 Creek as the season progressed. 



The habitats in which chinook salmon were cap- 

 tured ranged from a few centimeters to a meter or 

 more in water depth, on gravel, sandy, or muddy 

 substrates, with and without eelgrass, Zostera sp. 

 In the east river channel, concentrations of fry 

 were found mainly in pools and back eddies. There 

 were, however, no obvious qualitative differences 

 between preferred sites in Holden Creek where 

 chinook salmon were abundant and stream chan- 

 nels crossing the central and west sides of the 

 intertidal area where chinook salmon were scarce. 

 The upstream portions of the stream channels in 

 the central area of the delta, where they cut 

 through the marsh areas, were used as low tide 

 refuges in early spring. Where these stream chan- 

 nels cross the intertidal mud flat deep pools are 

 scarce and the water flow small. These features 

 may have made them unsuitable as refuges during 

 May. The absence of chinook salmon from the west 

 branch of the river could not be explained in this 

 way; however, disturbance of the estuary by log 

 rafting is greatest along the west branch and this 

 may have influenced chinook salmon distribution. 



Temperature and salinity in the east channel of 

 the river and Holden Creek indicated that the 

 chinook salmon were tolerating moderate 

 salinities and relatively high temperatures. Occa- 

 sional measurements of temperature and salinity 

 in other areas sampled at low and high tide were 

 comparable with those in the east channel at low 

 tide. Weisbart (1968) reported that juvenile 

 chinook salmon (parentage not identified) were 

 intolerant of direct transfer from freshwater to 

 31.8%o seawater, but that they had greater resist- 

 ance to seawater than either coho or sockeye 

 salmon, O. nerka. Mclnerney (1964) reported that 

 juvenile chinook salmon from the Samish hatch- 

 ery, Washington State, avoided all salinites above 

 0%o except for a brief preference for about 5%o 

 salinity in September tests. Presumably both tol- 

 erance and preference for salinity will vary among 

 stocks of salmon, and Nanaimo River chinook 

 salmon appear adapted to life in moderate salinity 



on the estuary. Temperatures experienced by the 

 chinook salmon at low tide were within their tol- 

 erance range but were generally above the 12°-13° 

 C reported to be their preferred temperature 

 (Brett 1952). 



Seasonal changes in the low tide distribution of 

 chinook salmon were not obviously correlated 

 with temperature and salinity in the east channel 

 and Holden Creek. Temperature at the upstream 

 stations often, though not always, exceeded that 

 at the downstream stations. Chinook salmon were 

 not captured at Stations 30 and 31 when tempera- 

 ture there exceeded 20° C. They were present at 

 Stations 28 and 29, however, when temperature 

 was 20"-21''C. Salinity wasonly slightly higher on 

 the average at the downstream stations, and often 

 the salinity at the upstream stations was the same 

 or slightly higher than downstream (Table 4). In- 

 creasing adaptation to salinity, therefore, ap- 

 peared not to be a factor in this seaward move- 

 ment. Possibly the disappearance of chinook 

 salmon from the shallow sampling stations in 

 Holden Creek as the season progressed was an 

 avoidance of the high temperatures that occurred 

 there on sunny days. 



The seasonal pattern of abundance of juvenile 

 chinook salmon in the Nanaimo estuary was the 

 same as that observed by Dunford (1975) in the 

 Fraser River, but different from that in the Sixes 

 River, Oreg. (Reimers 1971). In the Sixes River, 

 most chinook salmon apparently spent some 

 weeks in the river before moving into the estuary, 

 although some were considered to have moved di- 

 rectly to the estuary, and some even directly to the 

 sea. Reimers ( 1971 ) did not present information on 

 the temperature and salinity of the estuary habi- 

 tats he sampled. Dunford ( 1975) gave temperature 

 measurements for two habitat types in the Fraser 

 estuary, and these were lower than in similar 

 areas of the Nanaimo River. Chinook salmon dis- 

 appeared from Fraser River marsh habitats when 

 temperature reached about 15° C (Dunford 1975). 



Size and Growth of Chinook Salmon 

 in the Nanaimo Estuary 



Length and weight of chinook salmon captured 

 in the intertidal area of the estuary were only 

 slightly greater than those of downstream mi- 

 grants throughout the fry run. Toward the end of 

 the fry run, however, average length and weight of 

 chinook salmon captured in the estuary increased 

 rapidly and leveled off at around 70 mm fork 



662 



