Growth hormone increases growth and dominance of wild juvenile Atlantic salmon without affecting space use

Abstract

Growth hormone (GH) was applied to Atlantic salmon Salmo salar parr (the pre-migratory freshwater life stage) to manipulate growth potential experimentally and to elucidate the effects on dominance status, actual growth, exploratory activity and home range. Experiments were conducted using seven groups of eight parr from May to September of two successive years. The fish were tagged with passive integrated transponders (PIT tags), tested for dominance, and then held in an enclosed section of a natural stream which was fitted with an array of PIT tag detectors to record space use at a definition of c. 2 m. Relationships between dominance rank, space use and growth were established over 2 weeks. The four lowest ranking fish in each group were then given a slow-release GH implant while the other fish received a placebo. The GH stimulated increase in fork length (LF) and mass and decrease in condition factor due to the relatively greater increase in LF. There was, however, an interaction between GH-stimulated increase in growth and season, with the hormone having an effect only during the early part of the summer. Regardless of treatment, fish that moved most around their home range grew fastest. Increased growth in GH-treated fish was associated with an increase in growth per unit movement, not increased total movement. This suggested that GH-treated fish increased their rate of short-distance (<2 m) foraging movements. Overall, space use, measured in terms of home range size and time allocation throughout the range, did not vary consistently in response to application of GH. There was a strong correlation between the weighted centre of the home range (a measure of position within the enclosure) before and after treatment, irrespective of whether fish were given GH or a placebo. The study shows that when density is low relative to carrying capacity, GH stimulates increased dominance and growth in a near-natural environment without having measurable effects on space use at a definition of c. 2 m. The results are interpreted as suggesting that high dominance status gives no significant growth advantage in a highly competitive situation, but increases foraging rate when food is abundant. Increased foraging appears to result from local changes in time budgeting rather than variations in the extent of home range and larger-scale movements within it. Thus, in areas with declining wild Atlantic salmon populations where the habitat is unsaturated and food is abundant, introduced domestic Atlantic salmon may be competitively superior.