Seasonal use of nearshore intertidal habitats by juvenile Pacific salmon on the delta-front of the Fraser River estuary, British Columbia
Date
1984
Authors
Macdonald, Allan Lindsay
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Abstract
Seasonal occurrence, relative abundance, and distribution of juvenile salmon populations on the delta-front of the Fraser River (outer) estuary were examined for the periods June - August 1981 and April - August 1982. In 1981, the latter part of the chinook (Oncorhynchus tshawytscha) migration was observed. In 1982, juvenile pink (O. gorbuscha), chum (O. keta), and chinook were abundant in nearshore intertidal zones between Apr i 1 and August. The absence of the juvenile coho (O. kisutch) and sockeye (O. nerka) populations (except for incidental catches), despite substantial adult escapements to the Fraser River, indicates the nearshore region of the outer estuary was not utilized as nursery habitats by these species. Pink, chum, and chinook juveniles were distributed between intertidal zones independent of fish size, and seasonal salinity and temperature patterns. Significant seasonal increases in average length occurred for pink and chum (1982) and for chinook ( 1981 and 1982), and were the result of growth on the delta - front of the Fraser estuary as indicated by comparisons with previously studied life history patterns, migration timings, seaward migration routes, and size distributions of juveniles in the Fraser River and Strait of Georgia. Residency periods, for at least part of the juvenile populations in 1982, were estimated to be 8 to 15 days for pinks, 15 to 25 days for chum, and 60 to 75 days for chinook.
Stomach analyses of chinook revealed that epibenthic, pelagic and surface drift invertebrates, and larval and juvenile Pacific herring (Clupea harengus) were fed upon. Dietary differences between intertidal zones were relatively minor except for herring, which were taken more frequently in the offshore zone.
Herring were the most important prey as indicated by an index which incorporates frequency of occurrence, numerical abundance, and volume of organisms in the stomach. Most members of a resident herring population appeared to outgrow their vulnerability to chinook predation as the season progressed, which may have produced the observed temporal variations in the kinds of prey taken. Results of this study showed nearshore intertidal zones of the outer Fraser estuary comprise important rearing habitats for salmon juveniles, and thus expand the known limits of estuarine nursery areas which previously included primarily the marshes of the inner Fraser estuary.