Why do some fish fight more than others?

Reversible changes in how readily animals fight can be explained in terms of adaptive responses to differences in the costs and benefits of fighting. In contrast, long-term differences in aggressiveness raise a number of questions, including why animals are consistent with respect to this trait, why aggressiveness is often linked to general risk taking, and why aggressive and nonaggressive animals often coexist within a population. In fish, different levels of aggressiveness bring several direct fitness-related consequences, such as when aggressive individuals monopolize a limited food supply and grow fast. They also bring indirect consequences, such as when aggressive fish are more susceptible to predation and when they require a larger respiratory surface to service a higher metabolic rate. Fitness consequences of aggressiveness are often context dependent, with aggressive fish tending to do well in simple, predictable conditions but not in complex, less predictable conditions. The diverse, context-dependent consequences of aggression mean that aggressive and nonaggressive fish flourish in different conditions and explain in general terms why these behavioral phenotypes often coexist. There are a number of candidate evolutionary frameworks for explaining why individual differences in aggressiveness are often, but not always, consistent over time and often, but not always, linked to differences in general risk taking

Quantitative analysis of the fine structure of the fish gill: environmental response and relation to welfare

Methods were developed to quantify variation in gill size and microstructure and applied to three fish species: brown trout, Arctic charr and common carp. Measurements of arch length, number and length of gill rakers, number and length of gill filaments and number, length and spacing of the lamellae were taken for each gill arch and combined by principal component analyses to give length-independent scores of gill size. Levels of fluctuating asymmetry in gill arch length were also examined. Buccal and gill cavity volumes were measured from silicon moulds. Standard histological methods were used to examine gill microstructure. Benthic-feeding charr from a sample collected in Loch Awe, Scotland had relatively larger heads and buccal cavities than did sympatric pelagic-feeding fish Allowing for body size, they also had a more extensive respiratory surface, perhaps reflecting exposure to poorly oxygenated water while feeding on the loch bottom and/or a more active life style. Levels of asymmetry in gill arch length were higher in the pelagic-feeding form, which grow faster than the benthic-feeding form (Chapter 2). Gill size and structure were compared in carp (Chapter 3) and trout (Chapter 4) classified by a standard test as having proactive, reactive or intermediate stress copping styles. Proactive carp and trout had more extensive respiratory surfaces and lower levels of hyperplasia than did reactive fish, intermediate fish lying in between. The opposite was the case for density of mucous cells, which was highest in reactive fish and lowest in proactive ones. These data suggest that maintaining a large respiratory surface may represent an unrecognised cost of a proactive coping style. Common carp were held in mixed groups of proactive and reactive fish in one of 6 combinations of temperature (20oC and 25oC) and dissolved oxygen (3-4, 5-6 and 7-8 mg O2 L-¹) for 10 weeks. At the higher temperature fish had relatively larger heads and longer secondary lamellae, but had fewer mucous cells and a lower percentage of hyperplasia. At the lowest oxygen levels fish had relatively larger heads and a higher degree of hyperplasia than those held in normoxic and hyperoxic conditions. These results suggest that, over weeks, carp are able to “remodel” their respiratory structures in response to their current oxygen requirements. Few clear differences in response were found between proactive and reactive fish (Chapter 5). In semi-extensively farmed carp sampled over their final production year. Shortterm, acute husbandry stressors (grading and crowding) produced striking changes in several potential welfare indicators, including reduced body condition, increased in plasma glucose, lactate and cortisol levels and higher level of body damage. Percentage hyperplasia and secondary lamella number and length also increased. Long-term acute stress (pre-harvest crowding in concrete tanks) was associated with increased levels of skin and fin damage and in hyperplasia and mucus cell number, reflecting high stress levels and/or poor water quality. Glucose, lactate and cortisol levels fell, suggesting either habituation to current conditions or differential mortality by physiological stress status (Chapter 6). The results of Chapters 2-6 are synthesised in a general discussion (Chapter 7) and considered in the context of the existing literature on trophic polymorphism, on stress coping strategies, on the effects of environmental conditions of the welfare of cultured fish and on how gill structure and microstructure relate to other indicators of welfare

Respiratory function in common carp with different stress coping styles: a hidden cost of personality traits?

The purpose of this study was to compare investment in structures for gas exchange in common carp, Cyprinus carpio, with different stress coping styles, which are known to differ in resting metabolic rate. Common carp were classified as proactive or reactive on the basis of rate of emergence from cover into a novel, potentially dangerous environment in three successive tests. The fish were then killed and their gill arches removed. Length-independent estimates of the size of the respiratory structures were derived by multivariate analysis of arch length and the number and length of gill filaments and lamellae for all gill arches. Overall gill area was also estimated. Filaments from the second gill arch were sampled for histological estimation of the extent to which the respiratory surface is covered by epithelial cells (hyperplasia). Proactive carp had longer gill filaments and more, longer lamellae, and consequently a larger gill surface area, than reactive carp. In contrast, the extent of hyperplasia was higher in reactive than proactive carp. Thus, compared to reactive fish, proactive carp had a larger respiratory surface, more of which was exposed to the surrounding water rather than being covered by epithelial cells. We suggest that the higher metabolic rate of proactive fish requires greater investment in, and greater exposure of, the respiratory structures. This is likely to make oxygen uptake more effective, but may also impose hidden costs of a proactive, aggressive lifestyle

Gill development in sympatric morphs of Arctic charr from Loch Awe, Scotland: a hidden physiological cost of macrobenthos feeding?

The development of the respiratory surfaces was compared in two sympatric, lacustrine morphs of Arctic charr. A macrobenthic invertebrate feeding specialist, that forages in the littoral benthic zone, had a gill cavity that was 54% larger in volume and had 31% greater respiratory surface area than that of a zooplankton feeding morph that forages in the pelagic zone. The large respiratory surface area in the benthic-feeding form was the result of longer gill arches, more and longer gill filaments and more numerous secondary lamellae. The difference in gill cavity volume and filament length appears to be the result of a larger head, but not body size, in the benthic-feeding form. This suggests that differences in these characteristics may have arisen as a by-product of the expression of larger head size commonly described in macrobenthos foraging specialist charr. The other differences, particularly the more numerous secondary lamellae and the length of the gill arches, were not the result of head size differences between morphs, and thus, these are most likely an adaptation to greater respiratory requirements. Benthic-feeding fish may have a greater respiratory capacity to allow them to forage in areas with lower levels of dissolved oxygen and/or engage in a more active lifestyle compared to the pelagic-feeding form. In any event, the strikingly larger respiratory surface is likely to impose an additional ionoregulatory stress on the benthic-feeding and thus may represent a hidden physiological cost of specialisation for foraging on benthic prey