Growth rates of wild stream-dwelling atlantic salmon correlate with activity and sex but not dominance

Abstract

1. It is generally assumed that high social status confers benefits such as increased resource acquisition and growth rate, higher survival and/or increased reproductive output relative to subordinate individuals. 2. The hypothesis that dominant juvenile Atlantic salmon would have higher growth rates than subordinates in a flow-regulated natural stream was tested. Over 2 years, seven groups of eight size-matched wild fish were tagged with passive integrated transponders (PIT tags) and assessed for dominance using a serial removal method. Fish were then introduced together into an enclosed stretch of stream and allowed to forage freely for 14 days. Their use of space was recorded by an array of eight in-stream detectors. Using in-stream video recording, a correlation between dominance measured prior to and during the trials was confirmed. 3. Contrary to our hypothesis, there were no significant correlations between dominance status and specific growth rate. 4. Mean growth rate varied significantly between trials. In females, growth declined from May through to August. Maturing male fish exhibited a prepubertal growth spurt during June and July when their rates of growth significantly exceeded those of females. 5. There were significant positive relationships between rates of movement and growth of both Sexes. Each salmon generally used only a restricted portion of the stream but there were considerable overlaps between the home ranges of individuals. 6. Sampling over space and time on two occasions revealed complex variations in rankings of patches with respect to the abundance of drifting invertebrates of aquatic and terrestrial origin. 7. Our data are consistent with the hypothesis that a high degree of unpredictability in food supply results in correlation between growth and movement but not the ability to dominate high-quality feeding patches exclusively. Variation in growth between and within sexes might result from trade-offs between the costs (e.g. increased predation risk) and benefits (increased food acquisition rate) of sampling movements. The marked prepubertal growth spurt in maturing males is consistent with a strong advantage of large size of male parr during spawning.