ACTH does not mediate divergent stress responsiveness in rainbow trout

Two lines of rainbow trout selected for high (HR) and low (LR) responsiveness to a standardised confinement stressor displayed a sustained divergence in plasma cortisol levels during a 3 h period of confinement (max.: HR: 167 ± 13 ng ml-1; LR: 103 ± 8 ng ml-1; P < 0.001). However, no significant difference in plasma ACTH levels was evident (max: HR: 153 ± 9 pg ml-1; LR: 142 ± 7 pg ml-1). Dexamethasone (DEX) was administered to HR and LR fish to block endogenous adrenocorticotropin (ACTH) release. Administration of a weight-adjusted dose of ACTH to the DEX-blocked fish elevated plasma cortisol levels to a significantly greater extent in HR (233 ± 24 ng ml-1) than LR (122 ± 14 ng ml-1) fish (P < 0.001). Plasma cortisol levels in DEX-blocked HR and LR fish after sham injection were low but also significantly different (HR: 6.7 ± 1 ng ml-1; LR: 2.2 ± 0.2 ng ml-1; P < 0.001). These results indicate that modulation of cortisol responsiveness to stressors in HR and LR fish resides, at least in part, downstream of the hypothalamic-pituitary axis

Brain monoaminergic activity in rainbow trout selected for high and low stress responsiveness

This paper investigates whether two lines of rainbow trout displaying genetically determined variation in stress responsiveness and behavior also show differences in brain monoaminergic activity. In several brain regions, strains of rainbow trout selected for consistently high or low post-stress cortisol levels displayed differences in tissue concentrations of monoamines and/or monoamine metabolites, or in metabolite/monoamine ratios. High-responsive trout reacted to stress by an increase in the concentrations of both serotonin (brain stem), dopamine (brain stem), and norepinephrine (optic tectum, telencephalon), whereas low-responsive fish did not. Brain stem and optic tectum concentrations of monoamine metabolites were also elevated after stress in high responders, but not in low-responsive fish. The simultaneous increase in the concentration of monoamines and their metabolites suggests that both synthesis and metabolism of these transmitters were elevated after stress in high-responsive trout. A divergent pattern was seen in the hypothalamus, where low-responsive fish displayed elevated levels of 5-hydroxyindoleacetic acid (a serotonin metabolite) and 3-methoxy-4-hydroxyphenylglycol (a norepinephrine metabolite). In the telencephalon, both populations had elevated concentrations of these metabolites after stress. These results clearly suggest that selection for stress responsiveness in rainbow trout is also associated with changes in the function of brain monoaminergic systems. The possible functional significance of these observations is discussed with respect to the physiological and behavioral profile of these strains of fish. Literature is reviewed showing that several factors affecting brain monoaminergic activity might be altered by selection for stress responsiveness, or alternatively be under direct influence of circulating glucocorticoids

Differences in behaviour between rainbow trout selected for high- and low-stress responsiveness

Two F1 lines of rainbow trout divergent for plasma cortisol responsiveness were generated by individual selection for post stress cortisol values within the F0 generation. Adult females of the F1 generation were transferred to rearing in social isolation in observation tanks. After 6 days locomotor activity in high- and low-responding (HR, LR) individuals was quantified as time spent moving during a 20 min observation period. Behavioural observations were repeated the next day with a smaller conspecific intruder present in each observation tank. Differential hypothalamus-pituitary-interrenal axis activity in the two lines was subsequently confirmed by a standardised confinement stress test, which resulted in significantly higher plasma cortisol concentrations in HR than LR fish. HR fish displayed higher levels of locomotor activity than LR fish in the presence of an intruder, but not when in isolation. Aggressive behaviour towards the intruder was not seen, suggesting either state-dependent lack of territorial aggression, or chronic stress in the experimental fish. A significantly higher incidence of feed intake was seen in LR trout when held in observation tanks (40% vs 0 % of the fish took food when in isolation), suggesting that these fish acclimated more successfully to the experimental conditions than HR fish did. These results suggest that selection for stress responsiveness in salmonid fish leads to behavioural alterations, which are of potential importance to the performance of these fish in aquaculture rearing operations

Aspects of the ecology of killer whale (<i>Orcinus orca Linn.</i>) groups in the near-shore waters of Sub-Antarctic Macquarie Island

Occurrences of killer whales (Orcinus orca) in the waters surrounding Sub-Antarctic Macquarie Island have been recorded since the 1820s; however, their presence only became the focus of scientific research in the mid-1990s. The analyses of sightings data collected from the island between 1986 and 2015 are presented herein. The study provides evidence of a relationship between killer whale sighting probability and seasonal prey availability. Killer whales were present at the island year-round with a distinct seasonal peak in November–December, and coincident with a peak in occurrence of southern elephant seals (Mirounga leonina) due to breeding season activity, particularly the dispersal of weaned pups. Supporting this association and killer whales’ top-down influence on the survival of juvenile and adult southern elephant seals, pinnipeds accounted for 79% of prey identified, with weaned southern elephant seal pups contributing over a quarter of feeding events observed in the near-shore environment. Fur seals and penguins were also identified as prey. Killer whale groups had a median group size of three individuals, and groups of three to five individuals were most often observed feeding/milling in near-shore waters. The largest range in group sizes were observed during their peak occurrence in early summer, particularly in the number of sub-adult and female whales per group. Adult males made up 75% of single occurrences, and singletons were most often observed travelling. Overall, the ecology of killer whales at Macquarie Island was similar to that of killer whales studied at other Sub-Antarctic locations, with comparable seasonality, behaviour, diet, and group structure. Much remains to be learnt regarding the seasonal movements of whales and their diet at other times of year, their relationship to killer whales sighted in coastal Australian, New Zealand and Antarctic ecosystems, and impact on diet from commercial fisheries operations and fluctuating prey populations.</p