Growth Compensation in Juvenile Atlantic Salmon: Responses to Depressed Temperature and Food Availability

Abstract

This study examines behavioral and physiological responses of juvenile Atlantic salmon (Salmo salar) adopting alternative life history patterns following a period of reduced growth. We manipulated the growth rates of premigratory and nonmigratory salmon by either reducing food availability or maintaining water at low temperature (4–6°C). A third group of fish was kept at ambient temperatures (12–14°C) and fed ad libitum to provide a control. Fish in both experimental groups exhibited compensatory growth after the manipulation period, even though the manipulations had slowed growth rather than caused mass loss. The timing and duration of compensatory growth were affected by the nature of the constraint and the developmental pathway adopted. Compensatory responses were more persistent and stronger among premigratory fish than among nonmigratory. Fish kept at low temperature did not accelerate growth immediately after transfer to ambient temperatures, but they subsequently grew faster than controls for up to 215 d after the end of the manipulation period. This mitigated the effects of the period of low temperatures, although by the end of the experiment they were still smaller than the controls. Fish on reduced rations showed no such time lag, and they grew significantly faster than controls immediately upon regaining access to full rations. These fish attained the same body size as controls by the end of the experiment (day 215). The manipulations caused fish to reduce their growth in mass more than their rate of skeletal growth, but all fish achieved “normal” mass for their length (as compared to controls) within a week of transfer to full feeding or ambient temperature. The main mechanism underlying compensatory growth rates was apparently the increase of intake rates, although this was insufficient to explain the strong compensation shown by temperature-manipulated fish in the presence of larger (and thus competitively superior) individuals. Instead these fish enhanced their growth rate by apparently increasing the duration of the daily feeding period, and avoiding aggressive interactions. We interpret the observed compensation for periods of slowed growth as indicating that growth rate is normally submaximal and can be increased if the animal has fallen below its expected trajectory; thus premigratory fish may have shown a greater compensation because survival rates during migration are strongly size-dependent.