Metabolic rate, territoriality and life-history strategies of juvenile Atlantic salmon (Salmo salar L.)

[摘要] The relationships between relative standard metabolic rate, aggression, territoriality, growth and subsequent life-history strategies were studied in juvenile Atlantic salmon. In order to do this a method of calculating mass-independent relative standard metabolic rates is presented. This procedure involved using individual deviations from allometric predictions of standard metabolic rate based on body size (termed residual standard metabolic rate). As in a previous study, it was found that salmon with higher relative standard metabolic rates were more likely to acquire dominance, in both pairs and groups. However, fish with higher standard metabolic rates appeared to have smaller metabolic scopes within which they had to carry out dominance-acquiring costly activities such as aggression, although fish with higher standard metabolic rates did indeed acquire dominance through greater aggression. Fish with higher standard metabolic rates, although having a higher cost of maintenance, were found to have a lower feeding motivation, possibly because they had a smaller metabolic scope and movements associated with foraging are themselves energetically costly. Therefore it appears that juvenile salmon with high standard metabolic rates and a limited metabolic scope opt to be more aggressive and thus acquire dominance and a feeding territory at the expense of higher foraging rates, since both behavioural strategies are energetically costly. It was also found that in an environment with little food, fish with high standard metabolic rates grew less well than predicted given their position in an artificial stream than conspecifics with lower costs of maintenance. This indicates a potential cost of a high standard metabolic rate. There is a temporal component to acquisition of territories since juvenile salmon emerge from gravel redds over several days. Through this 'prior residence' effect, fish introduced into a new environment first were more likely to acquire territories than later-arriving conspecifics. First-arriving fish, as a consequence of acquiring a feeding territory, grew faster and were more likely to smolt a year earlier than late-arriving juveniles. However, they did not appear to choose the most profitable territories, implying a time constraint to searching for the best sites. If a salmon takes too long to choose a territory, it risks the territories filling up with later-arriving fish and not acquiring one at all. Prior residence appears to be a powerful asymmetry when tested in both pairs and groups, intruders having to be relatively much larger to overcome it and acquire dominance. Relative standard metabolic rate did not predict dominance when prior residence was included as a competitive asymmetry. However, fish with higher standard metabolic rates were more likely to emerge first since they absorbed their yolk-sacs faster and so needed exogenous food sooner. Therefore, a high standard metabolic rate conferred an indirect benefit since it increased the likelihood of a fish being a prior resident. Differences in aggression arising from differences in relative standard metabolic rate were also apparent in a hatchery situation. A group consisting entirely of salmon with high standard metabolic rates showed more aggression than a group of salmon with low standard metabolic rates. However, mean growth did not improve as a consequence of lower aggression rates, although the distribution of individual growth rates was more even in the group of fish with low standard metabolic rates. This may be a consequence of fewer fish in that group behaving despotically and monopolizing available food. As reported in earlier studies, differences in standard metabolic rate between the Upper and Lower Modal Groups of juvenile salmon became apparent during their first winter and spring. However, Upper Modal Group fish had higher weight-specific standard metabolic rates in December, earlier than previously documented, and higher mass-independent metabolic rates in May, prior to smoltification. This is suggested to be a pre-adaptation to the high metabolic demands the smolts will face when they migrate to sea. Individual residual standard metabolic rates varied more in the Upper Modal Group than the Lower Modal Group over winter and spring, possibly because respiratory enzymes in the Upper Modal Group were more seasonally adapted to the changing water temperatures of the period. They may have therefore worked inefficiently at the temperature at which metabolic rate was measured (since it remained constant while the ambient water temperature changed over time), being most inefficient when the difference between sampling and ambient water temperatures was greatest. However, individual residual standard metabolic rates remained broadly invariant throughout the period, demonstrating that individual standard metabolic rate is a relatively stable minimum to aerobic metabolic activity.

[发布机构] University:University of Glasgow

[关键词] Aquatic sciences, Physiology