The Causes, Resolution and Consequences of Contests for Space in Atlantic Salmon (Salmo salar L.)

O'Connor, Kirstine I (1999) The Causes, Resolution and Consequences of Contests for Space in Atlantic Salmon (Salmo salar L.). PhD thesis, University of Glasgow.

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The objectives of this thesis were firstly, to examine fighting behaviour in juvenile Atlantic salmon (Salmo salar) and to relate this to the natural situation under which these contests will occur; and secondly, to examine the factors that determine dominance and their consequences. Analysis of the behaviour of contesting dyads of fish revealed a tendency for subordinate fish to darken in body and sclera colour during aggressive encounters with a dominant conspecific. Since this darkening accompanies a reduction in the aggressive behaviour of the dominant fish it is hypothesised that darkening signals submission to an opponent. This hypothesis is compatible with both the Sequential and Cumulative Assessment Models. These models predict that in deciding what strategy to employ at any time during a contest an individual should consider all the information it has obtained up to that point, including the cumulative damage it has incurred during the fight. Both of these models suggest that combatants will alter their behaviour as a fight progresses and end the contest once one combatant acknowledges its opponent's superiority. By darkening in colour a subordinate fish is recognising its opponent's superiority and avoiding a protracted fight. Since fighting is costly, natural selection will favour any mechanism such as submission signalling that leads to shorter and hence less costly fights. When fights between familiar opponents were compared to those between unfamiliar opponents, there was a trend for subordinate fish to darken at lower fight intensities when fighting familiar opponents. It is suggested that, in this context, darkening is used as a signal of submission when an individual faces an opponent it knows to be of superior resource holding potential. In this way potentially costly escalated fights are avoided and a subordinate fish may avert a costly battle and conserve energy for a fight against a stranger where the outcome is less predictable. The influences of asymmetries in standard metabolic rate (SMR), date of first feeding and prior residence upon dominance, territory acquisition and growth were examined. Contrary to previous work there did not appear to be a correlation between SMR and dominance or growth. However, when salmon fry were separated into Early First Feeders (EFF), Late First Feeders (LFF) and Intermediates (according to the amount of yolk remaining in their yolk sacs and thus their anticipated date of switching to exogenous food) and placed into a stream tank simultaneously there was a clear effect of first feeding date. EFF tended to have more stable territories and LFF tended to dart higher into the water column to intercept prey items. These results suggest that EFF have an intrinsic advantage over LFF, which means that they are more likely to establish and maintain feeding stations. To examine the influence of prior residence on behaviour and growth performance, the residence of groups of salmon fry was manipulated so that one group arrived at a new habitat three days ahead of a second group. Although there did not appear to be any difference in the dominance rank of the two groups of fish, the first fish to arrive in the habitat fed at a higher rate and subsequently grew faster. These results suggested that the advantage of early emergence is not entirely attributable to intrinsic differences and the advantage is partly mediated through a 'prior residence' effect. Although this study did not find any advantages of high SMR it is hypothesised that SMR, date of first feeding and prior residence are so interwoven that to analyse their individual effects masks the true extent of their influence upon dominance and consequently growth. Since fish with a high SMR would tend to exhaust their yolk sacs sooner they will have an earlier first feeding date than fish with low SMRs. In addition, their early switch to exogenous feeding means that they emerge from the redd sooner than low SMR fish and thus gain a prior residence advantage. In this way the indirect advantages of a high SMR may be stronger than its direct influence when it is examined out of the context of a natural stream environment. As the stream environment is extremely heterogeneous and food supply unpredictable, the stability of SMR under conditions of food deprivation were examined. Fish reduced their SMR after a period of food deprivation. However, the reduction was not uniform, so that the ranking of SMR, and by inference the dominance hierarchy, changed. This suggests that fish differ in their ability to 'down regulate' their SMR and may subsequently vary in their ability to withstand periods of low food availability. This is particularly interesting as it suggests that the flexibility of SMR rather than its magnitude may be a more realistic predictor of dominance in salmon parr. While this thesis focuses on the short-term consequences of dominance (e.g. initial growth rate and territory acquisition), an individual's position in the hierarchy may have long term repercussions through its effect on growth rate.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Neil Metcalfe
Keywords: Zoology, Evolution & development, Aquatic sciences
Date of Award: 1999
Depositing User: Enlighten Team
Unique ID: glathesis:1999-76354
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 15:22
Last Modified: 19 Nov 2019 15:22

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