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A morphological and biochemical comparison of articially and naturally-reared salmonids Ludwig, Bryan William


Wild salmonids are usually superior to hatchery salmonids in marine survival. In this study, comparisons were made between wild and artificially-reared coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdneri) to identify morphological and biochemical differences and the causes of these differences. Coho salmon were obtained from three types of rearing facilities: a hatchery production-channel, a semi-natural rearing-channel (pool-riffle sequence with insect drift) both at the Qualicum river, and Burrows ponds at the Capilano river. Coho were fed an Oregon Moist Pellet diet. Steelhead trout, fed a Silver Cup diet, were reared in lake net-pens (O'Connor lake). Wild fish were captured from the same watersheds in which the cultured fish were reared. For Qualicum coho, comparisons were made at various periods in the life history. Coho smolts, reared in the production-channel (11.8 cm, 17.5 g) and Burrows ponds (12.4 cm, 18.4 g) were longer and heavier than wild coho from the Qualicum river (10.1 cm, 11.6 g) and Capilano river (11.1 cm, 14.6 g). Rearing-channel smolts (10.0 cm, 11.8 g) were similar in size to wild coho from the Qualicum river. Condition factors were higher in wild coho. Production-channel and Burrows pond smolts were lower in moisture (76.3%, 77.3% respectively) but higher in lipid (4.4%, 3.8%) than their wild counterparts (79.1%, 79.7% moisture; 3.0%, 2.0% lipid). There were few differences in ash or protein (% wet tissue). Wild coho were higher in ash and protein and lower in lipid (% dry tissue) but there were few differences when the data were expressed as a percent of the lipid-free dry tissue. Therefore, differences in proximate composition were confined to moisture and lipid. Phospholipid content was similar in wild and cultured coho averaging 12.9 mg/g wet tissue. In coho, the greatest differences in fatty acid composition were in neutral lipid (NL). Production-channel and rearing-channel smolts were higher in monounsaturates (58.2, 55.5% respectively) and ω6 fatty acids (9.0, 7.5%) but were lower in ω3 fatty acids (12.7, 14.9%) than the wild coho (43.9% monounsaturates, 5.7% ω6 and 28.4% co3 fatty acids) . Differences in polar lipid (PL) fatty acids were slight. In wild coho, the NL content of 18:2 ω6 (3.8%) was 0.6 times that of the production and rearing-channel coho while the 22:6 ω3 content (14.8%) was 2.2 times that of the production and rearing-channel coho. In steelhead, pen-reared and wild smolts did not differ in length (18.5 cm), weight (62.4 g), condition factor (4.62), moisture (76.4%), ash (2.6%), protein (16.8%) and phospholipid (9.2 mg/g wet tissue). The values given are averages for pen-reared and wild smolts. Pen-reared smolts (2.9%) were significantly lower in lipid than the wild steelhead smolts (4.2%). Fatty acid content of pen-reared and wild steelhead differed in both neutral and polar lipid. Pen-reared smolts had an ω6 content of 29.2% NL and 10.0% PL and an ω3 content of 13.2% NL and 51.2% PL. The wild smolts were lower in ω6 fatty acids (4.9% NL, 3.5% PL) but were higher in ω3 fatty acids (39.6% NL, 55.2% PL). The content of 22:6 ω3 (21.3%) in neutral lipid of wild smolts was 2.7 times larger than that of the pen-reared smolts. The high lipid content in cultured coho was attributed to their high ration level and the higher lipid content in Oregon Moist Pellet (18.3% dry matter) than in aquatic insects (12.4% dry tissue) (Phillips et al. 1954). The higher lipid content in wild smolts may be a result of their older age at smolting (2-3 years), although a definitive explanation is not yet available. Differences in fatty acid composition were due to the diet. Oregon Moist Pellet was much higher in monounsaturates (58.9%) than aquatic insects (Plecoptera, Trichoptera, Ephemeroptera, Chironomidae) (40.7%). Silver Cup was much higher in ω6 fatty acids (19.3%) than the aquatic insects (6.8%).

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