EVALUATION OF INVENTORY PROCEDURES FOR HATCHERY FISH .2. VARIATION INSPECIFIC GRAVITIES OF PACIFIC SALMONIDS DURING REARING

Four salmonid species were examined for variations in specific gravity over time and between species. Possible causes of the variation were also examined. Monthly estimates of specific gravity ranged from 1.00 to 1.04. Specific gravity decreased over a 1-year rearing cycle for co ho salmon (Oncorhynchus kisutch), spring chinook salmon (O. tshawytsch a), winter steelhead (O. mykiss), and rainbow trout (nonanadromous O. mykiss). Although specific gravity decreased for all species, the patt erns were different. Fish length did not appear to affect specific gra vity. Lipid content had a slight effect on specific gravity, but only in fish fed at high feeding rates (only coho salmon and rainbow trout were tested). Deflation of the swim bladder significantly increased sp ecific gravity in the species tested. Thus, the swim bladder and facto rs that affect it seem to be the main causes of variations in specific gravity. Fish inventories are often made by water displacement estima tes based on a fish specific gravity of 1.02. This assumed specific gr avity is used for all species at all times during their rearing cycle. An inventory based on a specific gravity of 1.02 when the actual valu e was 1.00 would overestimate the number of fish by approximately 2%.