As fish are poikilothermic i.e. their body temperature is not constant and hence is influenced by outside temperature, they have to adapt to any change in environmental conditions, within their range of tolerance, by behaving so as to minimize the impact of environmental conditions on their activity. Fish movements can be seen simplistically as the tool to achieve the best equilibrium possible between the physiology (energy budget) and the environmental conditions. Internal factors include genetic and ontogenetic factors, i.e. “the factors related to the genetic code of an individual as well as to its development and growth (life-cycle)” (Campbell, 1993). They are also linked to the physiology of an individual, for example, energy expenditure. Anderson (2002) described fish behaviour as a reaction to agents such as prey, predators and habitat features that affect fish fitness.
Every agent and/ or reaction is analysed by the fish in terms of energy costs and benefits.
Fish need to adapt their behaviour in order to minimize energy loss. This behaviour is also known as the optimal foraging theory where a fish, at every given time, acts in order to maximise the energy trade off towards benefits. In winter, specific choice of habitats and the behavioural patterns adopted by brown trout have been suggested to be governed by the need to minimize energy expenditure, i.e. selection of positions in habitats with low velocities and suitable cover and physico-chemical attributes but where energy depletion is minimized (Cunjak, 1996). Internal factors explain the various strategies used by different species to use their habitat. For example, stream fishes use different strategies for over wintering, depending on the species and life-stages. Among salmonids, behavioural movements and habitat use vary between year-classes (Elliot, 1986). Among non-salmonid species, Fox (1978) determined that ontogenic factors were responsible for the switching from larval stages to sedentary, territorial behaviour in bullhead and the resulting choice of habitat where the dominant substrate was of coarse type. Legalle et al. (2005) observed that bullhead switched habitat according to their age and body size. This conclusion confirmed that fish habitat occupancy depends on the species and size of individuals
(Heggenes, 1996). Indeed, in Newfoundland Rivers, both habitat use and habitat preference differed between young-of-the-year and parr Atlantic salmon (DeGraff and Bain, 1986).
However, even within the same species and same population, individual variations in habitat use occur, due to an individual own physiological state or energy budget.
Greenberg and Giller (2000) observed substantial individual variation in brown trout habitat use on a daily basis with some individuals using the same habitat all day while others switched habitat between day and night.
Internal factors, as described above, play an important role in fish behaviour and constitute the basis for fish adaptation to environmental conditions. However, their influence on the behaviour displayed by fish is also triggered by interactions with external, environment-related factors.
2.5.3.2 External biotic factors
Biotic factors include intra- and inter-specific competition for shared resources such as preys, habitat and refuges, as well as predator-prey interactions. These different types of biotic interactions and their importance for fish habitat use are discussed in further details in the following sections.
2.5.3.2.i Intra-specific competition
Intra-specific competition is linked directly to the density of individuals of a same species in a particular area of the stream for example. (Downhower et al., 1990). In theory, density has an impact on fish distribution and behaviour because as it increases, so does the competition for resources (food, habitat, refuges, cover, etc.). Elliot (1986) concluded that the spatial distribution of brown trout in a Lake District stream in the U.K. was density-dependent and that the behavioural movements of the different life-stages was also a result of intra-specific and life-stage specific competition. On the other hand, in a study on bullhead, Utzinger et al. (1998) found there was no significant correlation between population density and fish movements. These observations show that density alone does not appear to be responsible for intra-specific competition. Resource shortage, whether
they are food resources, mating partners or suitable habitats can be responsible for intra- specific competition. Elliot (1986) concluded that population density was the chief factor to affect between-year-class variation in spatial distribution for brown trout of similar age.
This pattern might result from territoriality and hierarchy, which are key characteristics of trout populations. It thus appears that some species and some life-stages are more sensitive to intra-specific competition than others. Brown trout 0+ density was found in some streams to be regulated by intracohort competition (Cattanéo et al., 2002). Hierarchy and territoriality also play a role in the way fish use available habitat. A study of red spotted masu salmon (Oncorhyncus masu ishikawai) in a Japanese mountain stream revealed the existence of size structured dominance hierarchy with the most dominant fish having access to areas of pools allowing them to get primary access to drifting preys (Nakano, 1995).
2.5.3.2.ii Inter-specific competition
This density-dependent factor that occurs when several species have the same diet or the same habitat requirements and that the density of individuals is too high for the available food or habitat resources (Campbell, 1993). Competition between fish species can result in niche segregation for species living in sympatry, e.g. Atlantic salmon (Salmo salar) and brown trout. Indeed, brown trout favoured the more slow flowing habitat types while Atlantic salmon preferred more fast flowing habitat. Salmon parr would use a wider range and, in general, deeper (mean=82 cm) habitat, than trout did (mean= 70cm) as well as faster flowing areas. In the absence of brown trout, Atlantic salmon widen their use of depths, but where other pool-dwelling fish species are abundant, the density of salmon in deep-slow water is reduced (Heggenes et al., 1996; Heggenes and Dokk, 2001). Some species can also live in allopatry at a basin scale, i.e. the different species occur in different parts of the catchment with little or no overlap between them. An example of this behaviour could be observed in a stream basin in Utah where cutthroat trout (Oncorhynchus clarki utah) dominated reaches at higher altitude while brown trout was the most dominant in lower altitude reaches (de la Hoz Franco and Budy, 2004).
2.5.3.2.iii Predation
Predator-prey interactions play an important role in the regulation of fish populations. Fish are both predators and prey in river and their movements will occur according to their status: predators will use habitats where they can find appropriate prey and prey will tend to move to refuge habitats. Predator-prey interactions often explain the diel patterns of movements observed in streams. Most fish tend to feed at night, first, to increase their chances to catch prey, and secondly to have less risk of being spotted by predators.
Bullhead and salmonids are mutual predators: bullhead is known to influence salmonid distribution though predation of the salmonid eggs in locations where there are high densities of adult bullhead (Carter et al., 2004). Bullhead adopt a cryptic behaviour during the day, hiding in refuges, as this species is very vulnerable to predation (Tomlinson &
Perrow, 2003) by carnivorous fish such as brown trout, pike (Esox lucius) and chub (Leuciscus cephalus), and piscivorous birds like the grey heron (Ardea cinerea) and kingfisher (Alcedo atthis), as well as the introduced North American signal crayfish (Pacifastacus leniusculus), the latter predating both on eggs and adults.