ALBERT

Notes: This experimental study supports the hypothesis that the competitive advantage of large body size declines with increasing group size in rainbow trout Oncorhynchus mykiss, (Walbaum).

Notes: Both wild- and hatchery-reared brown trout Salmotrutta, 18 months of age and of the same genetic origin, responded with increased heart rates (tachycardia) to a simulated predator attack on 2 consecutive days. Brown trout reared in the hatchery showed a more rapidly induced tachycardia compared with wild-reared fish at day 1, but not day 2. During an undisturbed period several hours after attacks, hatchery-reared brown trout maintained higher heart rates compared to wild-reared fish on both days. Behavioural responses to the attack were very low for all fish, although hatchery-reared fish tended to be more active than wild fish after the attack day 2. The observed differences may have had a genetic background caused by different selection regimes in the hatchery- and wild-rearing environments, or could have been due to different phenotypic responses in the two environments.

Notes: 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.

Notes: Rainbow trout Oncorhynchus mykiss fry exposed to a paper mill effluent showed increased EROD (ethoxyresorufin-O-deethylase) activity and reduced territorial aggression in mirror image stimulation (MIS) tests. These results suggest that the MIS test could be a useful tool to detect sublethal behavioural effects of pollution in fish fry.

Notes: In salmonids, growth hormone (GH) stimulates growth, appetite and the ability to compete for food. This study tested the hypothesis that increased GH levels in GH-transgenic coho salmon Oncorhynchus kisutch (Walbaum) increase competitive ability through higher feeding motivation. The transgenic strain of salmon used contained a gene construct consisting of the sockeye metallothionein-B promoter fused to the type 1 growth gene coding region. The transgenic animals (mean size = 250 g) were F1 individuals. In six consecutive feeding trials, the intake of contested food pellets by size-matched pairs consisting of one control (1 year older non-transgenic coho salmon) and one GH-transgenic coho salmon was compared. Pellets were provided sequentially until neither fish took three consecutive pellets; the identity of the fish taking each pellet was noted. Calculated on the three first pellets offered at each feeding trial, the transgenic coho salmon consumed 2.5 times more contested pellets than the controls, supporting the hypothesis that GH transgenesis increases the ability to compete for food. Overall, the transgenic fish consumed 2.9 times more pellets that the non-transgenic controls, indicating a high feeding motivation of the transgenic fish throughout the feeding trials. It appears that GH transgenesis and GH treatments can induce similar changes in the feeding behaviour of salmonids. Depending on how transgenic and wild individuals differ in other fitness-related characters, escaped GH transgenic fish may compete successfully with native fish in the wild.

Notes: Foraging effort, swimming activity, vertical position and flight response were recorded in focal juvenile rainbow trout Oncorhynchus mykiss at three group sizes: without company, or in visual and chemical contact with either one or five companion fish at two levels of predation risk: high (simulated aerial predator attack) or low (no attack). The predator attack induced a pronounced flight reaction as well as a reduction in vertical position, feeding and swimming activity. The foraging effort of the focal fish increased with group size independent of the level of predation risk, which suggests that the group-mediated increase in foraging activity is caused by competition rather than by reduction in perceived risk. The flight response to the predator attack, however, was stronger when the focal fish had company, suggesting that individuals may benefit from copying the anti-predator response of other group members.

Notes: Hatchery-reared brown trout Salmo trutta stocked in a natural stream in addition to resident wild brown trout grew more slowly than those stocked with an experimentally reduced density of brown wild trout. In both cases, hatchery-reared brown trout grew more slowly than resident wild fish in control sections. Mortality and movements did not differ among the three categories of fish. The results showed that growth of stocked hatchery-reared brown trout parr was density-dependent, most likely as a consequence of increased competition. Thus, supplementary release of hatchery-reared fish did not necessarily increase biomass.

Notes: A comparison between a high-density (migratory) and a low-density (stream-resident) population of brown trout Salmo trutta, from the same stream, showed there was no support for the view that aggression was density-dependent.

Notes: The effects of ownership, energy status and relative size on the ability to defend a territory were studied in dyadic contests among wild brown trout fry. A logistic regression showed that the territory owner won 85% of the contests with opponents of equal size, and that a 30% size advantage was required to balance the advantage of ownership. Energy status did not affect the ability to win territorial contests.

Notes: Agonistic interactions were observed in dyadic contests between size-matched rainbow trout. The results suggest that male juveniles are more aggressive than females.