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Prey selection and growth efficiency of juvenile sockeye salmon Oncorhynchus nerka switched between live prey and pelleted diets were investigated. First feeding sockeye salmon fry were placed into one of three dietary treatments for 7 months prior to assessing potential differences with a growth and a behavioural assay. Dietary treatments were (1) adult Artemia franciscana for 1 month, followed by pelleted feed for an additional 6 months (Art− BD), (2) pelleted feed from first feeding for 7 months (BD) and (3) adult A. franciscana for 1 month, and a combination of pelleted feed and live adult A. franciscana for 6 months (Art+ BD). Equal numbers from each treatment group were then tagged, pooled into replicate ‘common garden’ tanks and fed novel live prey items (Daphnia sp. and mosquito Culex pipiens larvae) for an additional 3 weeks. No significant differences in the growth efficiency of sockeye salmon were found during the 3 week feeding trial on the novel prey items. Additional sockeye salmon from each dietary treatment were used in a behavioural assay to determine if the treatments had an impact on foraging efficiency (prey selection or time to capture prey). No significant differences in prey selection were found among treatment groups in time to capture pellets, A. franciscana or mosquito larvae. Also, no significant differences were found within treatment groups in time to capture different food sources. No substantive benefits in foraging efficiency of sockeye salmon associated with prior exposure to live prey were demonstrated. This suggests that altering existing hatchery practices for juvenile sockeye salmon by offering live food prior to release is unlikely to influence post-hatchery feeding behaviour or increase post-release survival.

The potential to use adult Artemia to deliver erythromycin to first-feeding sockeye salmon, Oncorhynchus nerka (Walbaum), was investigated in three trials. In the first trial, first-feeding sockeye were fed live erythromycin enriched adult Artemia or pellets containing equal amounts of erythromycin for 35 days. At the end of the trial, tissue erythromycin concentration of the fish fed the live Artemia was significantly greater (P < 0.05, 25.52 +/- 1.29 microg mL(-1); mean +/- SEM), than the tissue concentration of the fish fed the pellets (0.72 +/- 0.01 microg mL(-1)). In the second trial, first-feeding sockeye were fed either live or freeze-dried bioencapsulated erythromycin (adult Artemia) or pellets containing erythromycin daily for 21 days. Mean daily erythromycin concentration in fish fed the freeze-dried Artemia, live Artemia, or pellets did not differ significantly. In the third trial, apparent erythromycin digestibility was determined. Significantly more (P < 0.05) erythromycin was retained by juvenile sockeye fed freeze-dried bioencapsulated erythromycin (98.3 +/- 1.0%) compared with medicated pellets (89.2 +/- 1.7%). Uptake of bioencapsulated erythromycin from adult Artemia (live or freeze-dried) appears to be greater than uptake from pellets. Freeze-dried and live Artemia were equally effective at delivery suggesting enriched freeze-dried adult Artemia could be produced into a highly palatable, consistent, off-the-shelf product.

Live adult brine shrimp, Artemia franciscana (Latreille), were enriched with erythromycin to determine if Artemia could accumulate therapeutic levels for subsequent feeding to young fish. Three trials were conducted to determine the erythromycin incorporation and survival rates of enriched Artemia when fed either liposomes containing erythromycin or various erythromycin suspensions. Erythromycin concentration in Artemia fed a liposome suspension was low (∼ 5 μg mL−1) relative to Artemia fed the direct suspension (> 100 μg mL−1) over the same time period. When enriched with suspensions up to 1 g erythromycin L−1 sea water for 14 h, Artemia survival was not significantly affected (P > 0.05) relative to controls. Using a suspension of 1 g L−1, tissue erythromycin concentrations of 109 ± 16 μg erythromycin mL−1 Artemia homogenate (mean ± SEM) were achieved after 12 h. Concentrations above 170 μg mL−1 were obtained using suspensions of 2–5 g L−1, but Artemia survival significantly (P