Groups of 0+ Atlantic salmon (Salmo salar) smolts were transferred to duplicate seawater tanks, and subjected to five different ration levels, 0% (starved), 25%, 50%, 75% or 100% (full fed). Waste feed was collected after each meal. After six weeks all groups were re-fed in excess. During the trial period body weight and length increased significantly in the 50, 75 and 100% groups, while no significant changes in body weight were observed in the 0% and 25% groups. A significant decrease in SGR was observed in the 0 and 25% groups during the first month in sea water. After re-feeding, SGR increased in all groups. All groups, except the previously starved group, showed peak SGR between weeks 6-8 and 8-12. Food restriction at 0% and 25% of full ration for a period of six weeks resulted in significant osmotic disturbances. After six weeks in sea water, plasma Cl⁻ levels were higher in the 0% group than in the other groups. Branchial Na⁺,K⁺-ATPase activity increased in all groups following exposure to seawater. Re-feeding caused a transient increase in branchial Na⁺,K⁺-ATPase activity after two weeks in the previously starved group, with a concurrent reduction in plasma Cl⁻ levels. Previous exposure to different ration levels significantly influenced growth rate and mean body size. Compensatory growth and partial size compensation was seen in the 0, 25 and 50% feed deprivation groups, whereas full size compensation was found in the 75% group.

Wild Atlantic salmon smolts were captured during spring out-migration in the Northwest Miramichi River, New Brunswick, Canada, and placed on an isotopically distinct hatchery diet to determine the relative contributions of growth and metabolic turnover to isotopic change. As expected for an ectothermic species, growth explained a large amount of isotopic variation in changing stable carbon ratios of muscle tissue (average r ²= 0.46) for the 3 years of study. Turnover rates of muscle carbon in all 3 years in growing fish (0.24–0.66 month⁻¹) were higher than previously reported values for other ectothermic species, but there was little evidence for isotopic change in non-growers (average r ²= 0.041, p > 0.1). It is unlikely that non-growers had consumed any of the hatchery diet over a 2-month period, thus preventing them from acquiring the new carbon isotopic signature. This period of food deprivation resulted in nitrogen-15 enrichment in liver relative to muscle (p= 0.003). It is advised that future isotope studies of metabolic turnover rates in ectotherms be conducted on slow-growing animals over a long time period. This would serve to avoid the obscuring effects of growth on isotopic change, and provide stronger comparisons to endothermic tissue turnover rates.

We assessed the food resource partitioning of three fish species (Arctic charr, Atlantic salmon and alpine bullhead) living in sympatry in a subarctic river. Fish were sampled monthly during the ice-free season (May–October), and dietary overlap among the species was calculated according to Schoener’s index. In October, the diet overlap among all three species was high (>70%). In contrast, large to modest food resource partitioning occurred among Arctic charr and the other two species from May to September (27–59% overlap), whereas there was a distinct diet overlap between Atlantic salmon and alpine bullhead in May, August and September (>64%), but not in July (53%). Surface prey (terrestrial and emerged aquatic insects), probably caught at the surface, were important for Arctic charr in August and September (24.9 and 46.6%, respectively), whereas the other fish species mainly fed on Apatania stigmatella, Mystrophora intermedia and Ephemerella aurivilli. Alpine bullhead seemed to feed close to the bottom, Atlantic salmon used both the bottom and water of various depths, whereas Arctic charr showed the greatest capacity to forage at the water surface. This vertical segregation may be important for fish assemblages in subarctic rivers, allowing food resource partitioning and coexistence of sympatric species.