Subordinate Fish Research Articles

Social stress reduces forebrain cell proliferation in rainbow trout ( Oncorhynchus mykiss)

Compared to mammals, teleost fish have a very high rate of adult brain cell proliferation. Still little is known about how this process is regulated in comparative models, and what its functional implications are. We investigated the effect of stressful social interaction on brain cell proliferation in size matched rainbow trout pairs after the formation of stable social hierarchies. After 4 days of interaction, socially subordinate fish displayed common signs of chronic stress including reduced feeding behaviour, elevated plasma cortisol levels, and up-regulated brain stem 5-HT activity. The number of newborn cells in the telencephalon was quantified using immunohistochemistry for the exogenously administered S-phase marker BrdU. Subordinate fish had 40% fewer BrdU-positive telencephalic cells compared to isolated controls, while dominant individuals showed a non-significant tendency towards reduced cell proliferation. Cell proliferation in subordinate animals correlated negatively with aggression received immediately after hierarchy formation, indicating that the level of cell division suppression is related to the severity of the social stressor. These findings are comparable to findings in mammalian models of psychosocial stress, indicating that the suppressive effect of social stress on brain cell proliferation is conserved, and thus likely confers adaptive benefits throughout the vertebrate subphylum.

Dominant convict cichlids (Amatitlania nigrofasciata) grow faster than subordinates when fed an equal ration

AbstractPrevious studies indicate that dominant fish grow faster than subordinate fish when fed equal rations. It is unclear, however, whether this growth differential is caused by intrinsic differences related to their propensity to become dominant, or by the extrinsic effect of the social stress experienced by subordinates. We first tested whether dominant convict cichlids (Amatitlania nigrofasciata) grew faster than subordinates when fed an equal amount of food. Second, we tested whether the growth advantage of dominants occurred when only visual interactions were allowed between pairs of fish. Third, we randomly assigned social status to the fish to rule out the possibility that intrinsic differences between fish were responsible for both the establishment of dominance and the growth differences. In three separate experiments, dominant fish grew faster than size-matched subordinate convict cichlids, but the growth advantage of dominants was higher when there were direct interactions between fish compared to only visual interactions. Our results provide strong support for the hypothesis that the slower growth rate of subordinate fish was due to the physiological costs of stress.

Higher levels of aggression are observed in socially dominant toadfish treated with the selective serotonin reuptake inhibitor, fluoxetine

The following study set out to test the hypothesis that acute treatment with the selective serotonin reuptake inhibitor, fluoxetine, would result in a rise in circulating 5-HT levels and consequently a decrease in territorial aggression in the Gulf toadfish, Opsanus beta. Size-matched pairs of toadfish were implanted intraperitoneally with the same dose of fluoxetine (0, 10 or 25 μg g − 1 ). After a social interaction between a pair of fish, circulating levels of serotonin (5-HT; 5-hydroxytryptamine) and cortisol were measured and relative mRNA expression of the 5-HT 1A receptor in the toadfish brain was determined using quantitative (real-time) PCR (qPCR). Behavioral endpoints such as the number of aggressive acts and swimming activity were also quantified so that dominant and subordinate fish could be identified. Fluoxetine treatment resulted in an increase in circulating levels of 5-HT, regardless of social status. Circulating cortisol concentrations were unaffected by fluoxetine, but were significantly higher in subordinate individuals when compared to dominant fish. Toadfish brain 5-HT 1A receptor mRNA expression was not affected by treatment or social status. Lastly and contrary to our predictions, fluoxetine treatment resulted in an increase in the number of aggressive acts made by dominant individuals, with no differences in the level of aggression or swimming activity of subordinate fish. This study is the first to describe elevated aggression in a teleost fish with elevated circulating levels of 5-HT.

Dominance Hierarchies in Zebrafish (Danio rerio) and Their Relationship with Reproductive Success

The zebrafish has considerable potential for use as a model in the study of behavior in social systems, particularly dominance hierarchies, which are widespread in nature and can affect the lifelong success of individuals. There is, however, a paucity of information relating to the characterization of social groups and significance of dominance hierarchies in the zebrafish model. This study set out to bridge this knowledge gap and better characterize dominance and its implications for reproductive success in both male and female zebrafish in colonies comprising of two males and two females. Analyses of four aggressive behaviors (chase, bite, repel, spar) were conducted twice daily over a 5-day period, and fertilized eggs were collected for parentage analyses using DNA microsatellite markers. Dominant-subordinate relationships occurred both between males and between females, and in both sexes, dominance was associated with a greater body size and higher levels of aggression. During the spawning period, dominant females were, however, less aggressive toward their subordinates than dominant males to their subordinates. Aggressive behaviors employed for maintaining dominance did not differ between the sexes, but in females, in contrast with males, the level of aggression directed toward the subordinate fish increased over the study period. Overall, dominance resulted in a greater total reproductive success in males but not in females; however, dominant females sired more offspring with the dominant male. The findings illustrate that energy invested in dominance behavior appears beneficial for both sexes in zebrafish.

Factors influencing the establishment of dominance hierarchies of the grey triggerfish Balistes capriscus

Abstract Unlike other balistids, grey triggerfish Balistes capriscus occur in social groups in subtropical reef assemblages and have been noted to cooperate in capturing large crustacean prey. The objective of this study were to determine the structure of dominance hierarchies of these social groups and the factors that influence hierarchies of wild-caught grey triggerfish in a naturalistic setting. From observations of four groups of triggerfish (n = 19 fish) in both dyad and group (4 - 5 fish) settings, we provide a description of triggerfish behaviors and coloration patterns and an explanation of the social context in which suites of behaviors are used by dominant, middle-ranking, and subordinate fish. Sixteen behaviors and nine coloration patterns were noted for grey triggerfish. Grey triggerfish groups form linear hierarchies in both dyads and groups as measured by Landau's Index of Linearity (h = 1.0 for Groups 1, 3, and 4 and h = 0.95 for Group 2 in dyads; h = 1.0 for all groups in group settings). Dyadic hierarchies, however, were not necessarily good predictors of the hierarchies found in larger group settings, as they only predicted two of the four group hierarchies. Sex played no role in influencing status or behavior. Size had the greatest influence on dominance status, with larger fish being more dominant than smaller fish. An individual's dominance ranking influenced both body coloration and posture. These results suggest that color patterns and body postures may also be used by observers as an indicator of an individual's social status in groups.

Habitat structure directly affects aggression in convict cichlids Archocentrus nigrofasciatus

Abstract Aggressive behavior can be an important factor in determining how animals use and divide space and resources. Previous studies have shown that aggression in fishes can be influenced by a variety of factors, including water temperature and resource levels. In this study, we tested if the amount of habitat structure in the environment affected aggression levels in female convict cichlids Archocentrus nigrofasciatus. We performed a laboratory experiment in which we placed female convict cichlids into an aquarium with low or high amounts of habitat structure and monitored the dominant female's behavior toward the subordinate female. Aggressive behavior in convict cichlids primarily consists of chases and bites. We found that the total time the dominant female spent chasing the subordinate female was greater when there was a low amount of habitat structure as compared to when there was a high amount of habitat structure. We also found that both the average duration of a chasing bout and the number of bites directed at the subordinate fish increased when there was a low amount of structure, but the number of chases did not. These results indicate that increased habitat structural complexity decreases aggressive behavior in convict cichlids.

Agonistic encounters and cellular angst: social interactions induce heat shock proteins in juvenile salmonid fish

Juvenile salmonid fish readily form dominance hierarchies when faced with limited resources. While these social interactions may result in profound behavioural and physiological stress, it is unknown if this social stress is evident at the level of the cellular stress response--specifically, the induction of stress or heat shock proteins (Hsps). Thus, the goal of our study was to determine if Hsps are induced during hierarchy formation in juvenile rainbow trout (Oncorhynchus mykiss). To this end, we measured levels of three Hsps, Hsp70, Hsc (heat shock cognate)70 and Hsp90 in the white muscle, liver and brain of trout that had been interacting for 36 h, 72 h or 6 days. Our data indicate that Hsps are induced in both dominant and subordinate fish in a time- and tissue-specific manner. In further mechanistic experiments on fasted and cortisol-treated fish, we demonstrated that high plasma cortisol does not affect Hsp induction in trout white muscle or liver, but both conditions may be part of the mechanism for Hsp induction with social stress in the brain. We conclude that the behavioural and physiological stress experienced by juvenile rainbow trout in dominance hierarchies can be extended to the induction of Hsps.

Social relationship as inducer of immunological and stress responses in matrinxã ( Brycon amazonicus)

Matrinxã ( Brycon amazonicus) is a freshwater neotropical fish species with a social interaction and aggressive behavior, especially in crowded environments. This species' social structure is established by agonistic interactions, which increase significantly at the fifth hour, when compared to the first and second hours of confinement, when each dominant fish competes with other individual for a conditioned territorialism. This social relationship also induces a complex physiological response in the organism, which generates an acute stimulation from the stressor agent. A stress situation modulates the physiology of the subordinate fish, which undergo significant increases in cortisol, glucose, hematocrit, and hemoglobin, when compared to the control fish. The immune system also indicates a modulation caused by cortisol, which results in an increase in neutrophils and a significant decrease in thrombocytes in subordinate fish, in comparison with control fish; however, the dominant fish show a significant increase in monocytes and a decrease in lymphocyte levels, when compared control fish. The agonistic interactions in B. amazonicus during crowding are not favorable to the physiology and immune system of the fish, mainly subordinate fish, and should be avoided in rearing systems.

Establishment of dominance relationships in gilthead sea bream Sparus aurata juveniles during feeding: effects on feeding behaviour, feed utilization and fish health

In gilthead sea bream Sparus aurata held in groups of two, five or 10 fish, social hierarchies were observed. Subordinate S. aurata were characterized by elevation of basal levels of plasma cortisol, together with a reduced immunological potential. Subordinate fish also showed lower feed intake, feed utilization and lower growth. Fatty acid composition was also affected by social status, with a lower content of saturated acids, oleic and eicosapentaenoic, in muscle and liver of fish considered as subordinate. Results show that social hierarchy acts as a stressor in S.aurata.

Long-term growth and movement in relation to food supply and social status in a stream fish

Long-term patterns of residency, growth, and movement among drift-feeding fish in relation to their social status and macroinvertebrate drift were determined. Each month, over 21 months, we quantified macroinvertebrate drift, fish abundance, and biomass, recorded individual fish lengths, weights, and social rank, and monitored individual diel activity of fish residing in different pools. Despite marked variation in food supply between pools, average growth rates within each pool were not significantly different. However, size and growth of the largest (dominant) fish (i.e., ones that had priority access to preferred feeding positions and times) was positively related to food supply. In contrast, the distribution and growth of smaller subordinate fish were only weakly related to the distribution of resources. Over the 21 months of study, growth rates of the first-ranked fish were high and they rarely relocated to a new pool. Second- and third-ranked fish exhibited comparatively low growth rates and relocated to new pools more frequently than fish of any other ranks (higher or lower). Movement of these fish from their original pool of residency did not always result in increased growth rates. Overall, our study demonstrates that short-term behavioral responses to spatial and temporal resource availability and intraspecific competition (i.e., social status) interact to determine long-term patterns of distribution, movement, and growth.

Effect of noxious stimulation upon antipredator responses and dominance status in rainbow trout

A potentially painful experience may modify normal behavioural responses. To gauge the importance of pain relative to predation or social status, we presented competing stimuli, a predator cue or an unfamiliar social group, to two groups of noxiously treated rainbow trout, Oncorhynchus mykiss. In the predator cue experiment, fish were classified as bold or shy. Noxiously stimulated fish did not show antipredator responses, suggesting that pain is the imperative. In the social status experiment, noxiously stimulated fish held individually and undisturbed showed an increase in respiration rate and plasma cortisol. As a comparison, we used the dominant or subordinate fish in a group as the noxiously stimulated fish. After the noxious treatment, we returned this test fish to a familiar or unfamiliar social group. Neither dominants nor subordinates showed a negative change in physiology compared to their controls. However, in a familiar group the dominant was much less aggressive, suggesting a behavioural impairment in response to noxious stimulation. In an unfamiliar group, no reduction of aggression was seen, suggesting that maintaining dominance status took priority over showing signs of pain. These findings may reflect an ability to prioritize motivational drivers in fish, and as such provides evidence for central processing of pain rather than merely showing a nociceptive reflex.

Social hierarchies, growth and brain serotonin metabolism in Atlantic salmon ( Salmo salar) kept under commercial rearing conditions

Monitoring social interactions between individuals in large, high-density groups poses several challenges. Here we demonstrate that relative concentrations of serotonin (5-Hydroxytryptamine, 5-HT) and its principal catabolite 5-Hydroxyindoleacetic acid (5-HIAA) in brain tissue of individual fish reflect social organisation in large groups of farmed Atlantic salmon. In the central nervous system of vertebrates, the monoamine neurotransmitter/neuromodulator 5-HT is critical for maintaining adaptive physiological, cognitive and emotional processes. In both teleost fish and mammals it has previously been shown that social interactions in small groups lead to elevated 5-HT release and/or metabolism in subordinate individuals, as indicated by the 5-HIAA/5-HT concentration ratio. In the current study, evidence is presented of non-linear dominance hierarchies in farmed Atlantic salmon ( Salmo salar) kept at high rearing densities. The physiological effect of these hierarchies was decreased when food resources were abundant, although some subordinate fish still showed altered brain serotonergic activity and failed to grow even feed was available in excess. The largest effect of decreased feed rations was seen in fish of intermediate size, where competition appeared to increase with reduced access to feed. The largest individuals in each rearing unit showed low 5-HIAA/5-HT ratios independent of feeding regime. A novel observation, with respect to previous studies, was that elevated brain 5-HIAA/5-HT ratios resulted from decreased 5-HT concentrations rather than elevated 5-HIAA in small fish. Thus, in light of the serotonin deficit hypothesis of depression, it cannot be excluded that social stress is important for animal welfare even in large, relatively homogenous groups of animals reared in captivity.

Effect of water renewal on dominance hierarchy of juvenile Nile tilapia

Nile tilapia social position ( Oreochromis niloticus) can be mediated by multiple channels, including chemical communication. Absence of chemical cues in the environment prevents hierarchical settlement among pairs, and enhances time spent in confrontations. The aim of this study was to test the effect of continuously renewed water flow on the establishment of hierarchical dominance in Nile tilapia juveniles. In this condition, a high frequency of attacks and disruption on hierarchical stability were expected because chemical cues for hierarchy maintenance could be washed out. After 3 days in isolation, the fish were paired by standard size but not by sex, and submitted to two conditions: continuously renewed water flow (RENEWED, n = 7) and non-renewed water flow (NONRENEWED, n = 8). The paired fish were placed in an aquarium (40 cm × 30 cm × 40 cm) for 3 h; four 10-min sessions were video-recorded: the first, immediately after the fish were paired and the others 1, 2, and 3 h after pairing. Hierarchy was identified by a dominance index (DI = given attacks/received + given attacks) for each fish. The hierarchical stability was achieved by analyzing the difference between dominant DI and subordinate DI (DI-D). Hierarchy was established in both groups after second session because the DI was significantly higher for one fish of the pair. The frequency of attacks of the dominant fish in RENEWED and NONRENEWED conditions was similar in all observation sessions. The attack frequency by subordinate fish was also similar during the first three sessions (2-h pairing). However, the frequency of attacks by subordinate fish in the RENEWED condition was higher than in the NONRENEWED situation at the fourth observation session (means ± S.E.: RENEWED = 2.83 ± 0.94 × 10 min −1 and NONRENEWED = 0.25 ± 0.16 × 10 min −1; Mann–Whitney, p = 0.04). At this point, a significant reduction of the DI-D was observed (means ± S.E.: RENEWED = 0.70 ± 0.11 and NONRENEWED = 1,00 ± 0.002; Mann–Whitney, p = 0.04). The changes in DI-D were related to more frequent attacks by the subordinated fish in renewed water flow. According to our results, the unsteady agonistic interaction under renewed water flow leads to social instability. Thus, continuous water renewing can wash out relevant chemical substances and therefore disturb the dominance recognition by subordinate fish.

Is diel activity determined by social rank in a drift-feeding stream fish dominance hierarchy?

Dominance hierarchies among pool-dwelling drift-feeding fish can determine spatial and temporal segregation of feeding territories. We tested the hypothesis that primarily nocturnal, dominant giant kokopu, Galaxias argenteus, force smaller, primarily diurnal, subordinate giant kokopu to feed by day. To test whether subordinate fish would become more nocturnal in the absence of dominant fish, we temporarily removed dominant fish from stream pools in which stable dominance hierarchies operated. Removal of the dominant fish resulted in a significant increase in the number of smaller giant kokopu active in stream pools at night; however, no change in activity was detected by day. The results indicate that large dominant fish control diel activity of subordinate fish, excluding them from stream pools at times (night) when invertebrate drift densities are at their highest.

The effects of hatchery domestication on competitive dominance of juvenile spring Chinook salmon (Oncorhynchus tshawytscha)

We tested the null hypotheses that competitive dominance among juvenile Chinook salmon (Oncorhynchus tshawytscha) in contest and scramble experiments would not be affected by domestication selection after one generation of state-of-the-art hatchery culture. Dyadic challenges of size-matched juvenile fish were conducted after a 6-day acclimation in 113.4 L aquaria. Differences in dominance and frequency of different types of agonistic interactions used were not significantly different in contest (n = 505) or scramble (n = 363) competition experiments (P > 0.05). However, wild origin fish were more aggressive than hatchery origin fish in both types of experiments (P ≤ 0.05). Furthermore, wild origin fish gained more weight than hatchery origin fish during contest experiments, and hatchery origin fish lost less weight than wild origin fish in scramble experiments (P ≤ 0.05). Dominant fish, regardless of origin, grew more than subordinate fish in both contest and scramble experiments (P ≤ 0.05). Our results indicate that aggression, and growth mediated by competition, can be affected by domestication after only one generation of state-of-the-art hatchery culture; however, impacts to competitive dominance appear to be small.

Stress and stress coping in fish, behavioural correlates and neuroendocrine mechanisms

The development of a dominance based social hierarchy is common among teleost fish and individual social history greatly affects physiology and behaviour of the fish. Long-term social subordination inhibits aggressive behaviour and predisposes the fish for losing subsequent fights for social dominance, whereas experience of being dominant, has the opposite effect, making the fish more likely to win future encounters. Subordinate individuals are exposed to chronic stress and show a chronic activation of the hypothalamic-pituitary-interrenal (HPI) axis and the brain serotonergic system. Elevated brain serotonergic activity is important for mediating the behavioural inhibition observed in subordinate fish. However, cortisol and neuropeptides involved in the control of the HPI axis also seem to play important roles in mediating behavioural effects of social experience. Moreover, in rainbow trout (Oncorhynchus mykiss), competitive ability is linked to suite of other behavioural traits and to the neuroendocrine stress response profile. Trout responding to stress with a low increase in plasma cortisol have an advantage in dyadic fights for social dominance with a conspecific responding to the same stressor with a larger increase in plasma cortisol. Moreover, rainbow trout selectively bred for low post-stress plasma cortisol (LR trout) have an advantage over trout selected for high post-stress cortisol (HR trout) in dyadic encounters. Thus, in addition to the epigenetic effects of social history there are also genetic factors predisposing individuals for dominant and subordinate social positions.

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of for- aging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor chang- ing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhyn- chus clarki clarki, when searching for food at the pool scale by artificially increasing prey avail- ability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was moni- tored from stream bank platforms under unma- nipulated control conditions and under seven experimental sets of conditions involving differ- ent combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10-0.25 m s -1 ) locations and within 1 m of structure cover, but would readily move to shal- lower ( < 30 cm) water away from cover if veloc- ities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water ( < 20 cm) if prey were delivered there. Responses of individual trout to manipu- lations indicated that water velocity was the main physical cue used by fish to decide where to for- age, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not ''perfect searchers'' that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish's normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about hab- itat conditions at the reach scale.

Digestive capacity, growth and social stress in newly-metamorphosed Japanese flounder (Paralichthys olivaceus)

Hierarchies readily appear when rearing flounders through metamorphosis in space-limited conditions. In this experiment, subordinate fish were stressed, as suggested by their elevated cortisol level compared to dominant fish. Subordinate fish, although of smaller size than the dominant fish, showed no suppressed digestive capacity. By separating the two hierarchical groups into different tanks at low density, the cortisol level of the subordinate fish substantially decreased. The removal of the stressor also resulted in an increase in digestive function and improved coloration. Also, compensatory growth in length but not in weight was observed in the subordinate group, suggesting that the subordinate fish devoted their energy to increase their length. These results imply that the climax of flounder metamorphosis (i.e. the settlement stage) is a highly sensitive period, where social interactions may induce high levels of stress. However, Japanese flounder early juveniles prove to have a high recovery capacity from stress and show to allocate energy preferentially to growth in length. This seems to be an adaptation to diminish the probability of death by predation.

Growth Depression in Socially Subordinate Rainbow TroutOncorhynchus mykiss: More than a Fasting Effect

To assess the effects of subordinate social status on digestive function, metabolism, and enzyme activity in salmonid fish, juvenile rainbow trout Oncorhynchus mykiss were paired with size-matched conspecifics (<1.5% difference in fork length) for 5 d. Fish that were fasted for 5 d and fish sampled directly from the holding tank were used as control groups. Both subordinate and fasted fish experienced significant decreases in intestine mass (P = 0.043), and the gall bladder showed marked and significant changes in both size (P = 0.004) and appearance. These findings suggest that the negative effect of social subordination on digestive function reflects in large part a lack of feeding. Hepatic phosphoenolpyruvate carboxykinase activity was significantly higher in subordinate fish relative to dominants, whereas subordinate hepatic pyruvate kinase activity was significantly lower; activities of both enzymes were significantly correlated with plasma cortisol concentrations and behavior scores. Dominant-subordinate differences in the activities of these enzymes were eliminated by administration of the glucocorticoid receptor antagonist RU486, underlining a role for circulating cortisol in eliciting the differences. Significant increases relative to control fish were also detected in red and white muscles from subordinate fish in the activities of protein catabolic enzymes (aspartate aminotransferase, alanine aminotransferase, glutamate dehydrogenase). These differences occurred in the absence of any change in plasma free amino acid or ammonia concentrations, supporting an enhanced turnover of amino acids in muscle in subordinate fish. The results support the hypothesis that changes in metabolism, beyond those elicited by low food consumption, may be responsible at least in part for the low growth rates typical of subordinate fish and that these changes may be related specifically to circulating cortisol levels in subordinate fish.

The impact of social status on the erythrocyte β-adrenergic response in rainbow trout, Oncorhynchus mykiss

The objective of the present investigation was to determine whether chronic increases in circulating cortisol concentrations, resulting from the occupation of subordinate status in rainbow trout social hierarchies, resulted in an enhancement of the erythrocyte adrenergic response. Rainbow trout ( Oncorhynchus mykiss) were confined in fork length matched pairs for 6 h, 18 h, 48 h or 5–7 days, and social status was assigned through observations of behaviour. Erythrocyte adrenergic responsiveness, determined in vitro as changes in water content following incubation with the β-adrenoreceptor agonist isoproterenol, was significantly greater in subordinate than dominant fish at 48 h of social interactions but not after 5–7 days, nor when assessed as changes in extracellular pH (pHe). However, the activity of the Na +/H + exchanger (β-NHE), assessed in vitro as the pHe change following incubation with the permeable cyclic AMP analogue 8-bromo-cyclic AMP, was significantly lower in subordinate fish. The number of erythrocyte membrane-bound adrenergic receptors ( B max) was significantly higher in subordinate than dominant fish at 48 h, but had decreased by 5–7 days to a value that was not significantly different from that for dominant fish. The apparent dissociation constant ( K D) of these receptors was not significantly impacted by either social status or interaction time. Finally, the relative expressions of β-3 b adrenergic receptor (AR) and β-NHE mRNA were determined using real-time PCR and were found to be minimally affected by social rank. Relative to a control group, β-3 b AR mRNA was significantly up-regulated in both dominant and subordinate trout at all time periods, whereas the expression of β-NHE was in general significantly down-regulated. Unlike the situation in rainbow trout treated with exogenous cortisol, elevations in circulating cortisol resulting from low social status did not “pre-adapt” the erythrocyte adrenergic response, but rather may have served to offset the potentially adverse effects elicited by plasma catecholamines, which were elevated during social hierarchy formation.