**5. Competion for food between brown trout and other sympatric fish species**

Trophic interactions between species are important factors structuring animal communities. Brown trout are top-consumers in freshwater habitats and play an important role as carriers of energy from lower to higher trophic levels (i.e. predators). Many freshwater fish species tend to occupy a specific type of habitat but there are lots of exceptions. For example, spatial niche overlap is considerable where Atlantic salmon and brown trout co-occur, although young Atlantic salmon tend to occupy faster flowing and shallower habitats [e.g. 74]. More‐ over, when both fish species co-occur, the habitat used by Atlantic salmon is restricted through interspecific competition by the more aggressive brown trout, indicating an interactive segregation between fish species [e.g. 41,74]. Indeed, it is well known that brown trout is a territorial drift feeder [3,81], and several authors have reported the behavioural dominance of trout over cyprinids in streams [82-84].

Firstly, the use of microhabitats is often different between species, due to segregation of microhabitats, an important factor in reducing the effects of competition for food [69,88,89]. For example, *Barbus bocagei* Steindachner, 1865 occupied deeper habitats and selected lower positions in the water column than *Pseudochondrostoma polylepis* (Steindachner, 1865), and *Squalius pyrenaicus* (Günther, 1868), *P. polylepis* occupied microhabitats with greater velocities than the other two species and *S. pyrenaicus* selected shallower habitats than the other two species [69]. In another study, S*. trutta* showed wider diversity in the habitat used for feeding than *Squalius carolitertii* (Doadrio, 1988), *Pseudochondrostoma duriense* (Coelho, 1985) and *B. bocagei* [30]. Hence, differences were found among species in their ability to feed at different depths of the water column [29,30] as shown in snorkelling studies into microhabitat use in

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**Figure 10.** Diet composition consumed by each fish species in the Tormes River (Central Spain) during summer.

Secondly, different species may specialise in different resources. For example, many cyprinid fish sympatric with trout feed on a significant amount of detritus and plant material not used by trout, leading to reduced inter-specific competition [29,30]. Moreover, resource partitioning may also occur at the level of prey size [29,30,90], although it is not clear whether this size selective strategy is adopted to reduce interspecific competition or it is the result of foraging

fish [69].

The competitive coexistence between species occupying similar niches may be facilitated by a generalisation of niche width as predicted by the optimal foraging theory (OFT), rather than the specialised niche width predicted by the classic niche theory as a response to interspecific competition [85]. However, studies on food partitioning in fish communities have obtained contradictory results. Whereas several authors have found differences in diet composition among sympatric fish species [e.g. 86,87], other researchers concluded that the same food resource can be shared by several species [29,30,85]. In these cases, the differences in behav‐ ioural feeding habits, handling efficiency and feeding habitat utilization are important adaptive features that may reduce the inter-specific competition in the fish community and permit the partitioning of food that allows coexistence [29,30]. Thus, sympatric fish species can adopt different strategies to overcome competion and food resource partitioning can occur at different levels.

Ontogenetic Dietary Shifts in a Predatory Freshwater Fish Species: The Brown Trout as an Example of a Dynamic... http://dx.doi.org/10.5772/54133 289

grams) and multiplied by 100) varies during ontogeny [78,79]. In salmonids the results are contradictory: in brook charr *Salvelinus fontinalis* (Mitchill, 1814) no differences have been found in the stomach fullness [80], whilst other researchers have demonstrated that the stomach fullness of brown trout varies among size classes [47]. Brown trout between the size of 40 mm and 320 mm fed more intensively, whilst the intensity declined above 320 mm length [47]. In the River Furelos (NW Spain), we have found that stomach fullness during the summer is different among age classes (Kruskal-Wallis test; *p* < 0.001), being higher in age-0+ (9% ± 0.64) than age-1+ (1.1% ± 0.14), age-2+ (1% ± 0.25) and age-3+ (1.1% ± 0.24) (all Mann–Whitney U test, *p* < 0.001) but no differences have been found between ages-1+, 2+ and 3+ (all Mann– Whitney U test, *p* > 0.05). Moreover, stomach fullness decreases with fish size (*r* = -0.72; *p* < 0.001) (unpublished data). Hence, stomach fullness can vary among age classes; however additional studies are needed in order to clarify whether stomach fullness varies during

**5. Competion for food between brown trout and other sympatric fish**

Trophic interactions between species are important factors structuring animal communities. Brown trout are top-consumers in freshwater habitats and play an important role as carriers of energy from lower to higher trophic levels (i.e. predators). Many freshwater fish species tend to occupy a specific type of habitat but there are lots of exceptions. For example, spatial niche overlap is considerable where Atlantic salmon and brown trout co-occur, although young Atlantic salmon tend to occupy faster flowing and shallower habitats [e.g. 74]. More‐ over, when both fish species co-occur, the habitat used by Atlantic salmon is restricted through interspecific competition by the more aggressive brown trout, indicating an interactive segregation between fish species [e.g. 41,74]. Indeed, it is well known that brown trout is a territorial drift feeder [3,81], and several authors have reported the behavioural dominance of

The competitive coexistence between species occupying similar niches may be facilitated by a generalisation of niche width as predicted by the optimal foraging theory (OFT), rather than the specialised niche width predicted by the classic niche theory as a response to interspecific competition [85]. However, studies on food partitioning in fish communities have obtained contradictory results. Whereas several authors have found differences in diet composition among sympatric fish species [e.g. 86,87], other researchers concluded that the same food resource can be shared by several species [29,30,85]. In these cases, the differences in behav‐ ioural feeding habits, handling efficiency and feeding habitat utilization are important adaptive features that may reduce the inter-specific competition in the fish community and permit the partitioning of food that allows coexistence [29,30]. Thus, sympatric fish species can adopt different strategies to overcome competion and food resource partitioning can occur at

ontogeny in brown trout.

288 New Advances and Contributions to Fish Biology

trout over cyprinids in streams [82-84].

**species**

different levels.

**Figure 10.** Diet composition consumed by each fish species in the Tormes River (Central Spain) during summer.

Firstly, the use of microhabitats is often different between species, due to segregation of microhabitats, an important factor in reducing the effects of competition for food [69,88,89]. For example, *Barbus bocagei* Steindachner, 1865 occupied deeper habitats and selected lower positions in the water column than *Pseudochondrostoma polylepis* (Steindachner, 1865), and *Squalius pyrenaicus* (Günther, 1868), *P. polylepis* occupied microhabitats with greater velocities than the other two species and *S. pyrenaicus* selected shallower habitats than the other two species [69]. In another study, S*. trutta* showed wider diversity in the habitat used for feeding than *Squalius carolitertii* (Doadrio, 1988), *Pseudochondrostoma duriense* (Coelho, 1985) and *B. bocagei* [30]. Hence, differences were found among species in their ability to feed at different depths of the water column [29,30] as shown in snorkelling studies into microhabitat use in fish [69].

Secondly, different species may specialise in different resources. For example, many cyprinid fish sympatric with trout feed on a significant amount of detritus and plant material not used by trout, leading to reduced inter-specific competition [29,30]. Moreover, resource partitioning may also occur at the level of prey size [29,30,90], although it is not clear whether this size selective strategy is adopted to reduce interspecific competition or it is the result of foraging behaviour and/or morphological constraints such as gape size [29,91]. Also, terrestrial prey are present primarily on the stream surface and although tend to be absent from the diets of benthic feeders such as *B. bocagei* (Figure 10), terrestrial inputs may constitute an important food resource for freshwater fish species and especially for brown trout. Thus, the utilization of allochthonous food resources such as terrestrial invertebrates by fishes may reduce competi‐ tion facilitating the partitioning of resources [30].

\*Address all correspondence to: javier.sanchez@usc.es

tiago de Compostela, Spain

ty of A Coruña, Spain

vedra, Spain

**References**

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2 Station of Hydrobiology "Encoro do Con", Castroagudín s/n, Vilagarcía de Arousa, Ponte‐

3 Department of Animal Biology, Vegetal Biology and Ecology, Faculty of Science, Universi‐

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Thirdly, diel segregation is possible among fish species, and this may also lead to reduced interspecific competition between fish [29,92,93]. According to macroinvertebrate trait analyses, sticklebacks (*Gasterosteus aculeatu*s Linnaeus, 1758) and *P. duriense* show a slight preference for prey that drift during the day, whilst age-0 *S. trutta* seem to prefer to feed at dusk, whereas *Achondrostoma arcasii* (Steindachner, 1866) differs from the other three species due to its preference to feed on prey on organisms with weak or no tendency to drift [29]. However, the "diel drift behaviour" of macroinvertebrate prey of brown trout and three sympatric cyprinids is similar [30]. Hence, the differences in the diel feeding behaviour among sympatric fish species might only be adopted in highly competitive communities, where food is a more limiting resource.

## **6. Conclusion**

To summarize, the present study supports the hypothesis differences in the feeding habits and habitat utilization of different age classes of trout could reduce competition for food, by allowing food resource partitioning. Hence, age-related diet shifts occur at five different levels: (1) diet composition changes with fish age; (2) prey selection varies with fish age, probably due to prey-size selection which is in turn dependent on the size-frequency distribution of the available prey; (3) mean prey size increases with fish size and age; (4) habitat utilization for feeding may be different among age classes; (5) niche breadth tends to increase with age and fish size. Finally, also the stomach fullness can vary among age classes. However, additional studies are needed in order to clarify whether stomach fullness varies during the ontogeny in brown trout.
