The effect of visual isolation on territory size and population density of juvenile rainbow trout (Oncorhynchus mykiss)

Abstract

Visibility is thought to affect the territory size of visually oriented animals but there have been few experimental tests of the hypothesis. We re-examined the relationship between visibility and territory size in juvenile salmonids to test the hypothesis that increasing habitat heterogeneity results in a reduction in territory size and consequently in higher population densities. Equal densities of young-of-the-year rainbow trout (Oncorhynchus mykiss) were stocked in two experimental treatments with low visibility and a control treatment with high visibility. Visibility was decreased by placing large stones or plywood dividers onto the substrate of experimental stream channels. As predicted, the size of individual territories decreased with decreasing visibility of the habitat. However, the treatments did not differ significantly in population density or growth rate of the fish. While this study confirms the inverse relationship between habitat visibility and territory size, the decrease in territory size did not produce an increase in population density of juvenile salmonids.Résumé : La visibilité affecte, croit-on, la taille du territoire chez les animaux qui s'orientent par la vue, bien qu'il y ait peu de vérifications expérimentales de cette hypothèse. Nous avons examiné de nouveau la relation qui existe entre la visibilité et la taille du territoire chez des jeunes salmonidés dans le but d'éprouver l'hypothèse selon laquelle une hétérogénéité accrue de l'habitat réduit la taille du territoire et, par conséquent, permet des densités de population plus élevées. Nous avons établi des densités égales de jeunes truites arc-en-ciel (Oncorhynchus mykiss) de l'année dans deux sites expérimentaux à visibilité réduite et un site témoin à visibilité élevée. De grosses pierres ou des panneaux de contre-plaqué placés sur le substrat des ruisseaux artificiels ont servi à diminuer la visibilité. Tel que prédit, la taille des territoires individuels diminue lorsque la visibilité dans l'habitat est réduite. Cependant, il n'en résulte pas de différences dans la densité de population, ni dans le taux de croissance des poissons. Bien que notre étude confirme l'existence d'une relation inverse entre la visibilité dans l'habitat et la taille du territoire, la réduction de la taille du territoire n'entraîne pas d'augmentation de la densité de population chez les jeunes salmonidés.[Traduit par la Rédaction] Imre et al. 309

Environmental enrichment reduces aggression of pearl cichlid, Geophagus brasiliensis, during resident-intruder interactions

“…Territorial enrichment with physical barriers such as kelp models and pebbles can cause fish to have smaller individual territories with less visibility of surrounding habitat (Imre et al, 2002;Höjesjö et al, 2004). We suggest that reduced visibility of territory could increase energetic costs for resident animals when they attack an intruder and, thus, fish in this situation would either decrease aggressive interactions or choose co-habitation as their best strategy.…”

“…We added no objects into the non-enriched tanks. These experimental conditions (enriched vs. bare tanks) were based on Imre et al (2002) and Salvanes et al (2007). Water temperature, aeration and photoperiod of experimental aquaria were similar to stoke-tank holding conditions.…”

“…Nijman & Heuts (2000) found the effect in the cichlid, Haplochromis burtoni, however without showing an increase in aggressive interaction frequency. In contrast, environmental enrichment actually decreases aggression in rainbow trout (Imre et al, 2002) and brown trout (Höjesjö et al, 2004), although these studies did not perform tests within an owner-intruder paradigm. Accordingly, in rainbow trout greater environmental complexity leads to smaller individual territories, while in brown trout the subordinate fish have more protected area in an enriched environment.…”

Habitat Complexity, Brain, and Behavior

“…For us to move beyond 'just-so' stories speculating on the connection among environmental forces, brain and behavior, hypotheses on these connections must be specifically tested [Gould and Lewontin, 1979]. Recent papers on fishes have demonstrated experimentally that habitat complexity causes greater species diversity and richness [e.g., Lingo et al, 2006] and that limiting the visual field influences home range size, mating and feeding, showing the influence of visual stimuli on how an animal uses its space [Imre et al, 2002;Rilov et al, 2007]. Most significantly, Sol et al [2007] tested the hypothesis that big brains confer an evolutionary advantage, an assumption not tested by previous researchers.…”

What tunes aggression in fish during fighting in environments that vary in enrichment?

“…Even then, our conclusion regarding the EE and aggression remains. Moreover, during our manuscript preparation and publication of our article (published in Neotropical Ichthyology Apr-Jun/ 2010 but online since Feb/2010), other reports were published that showed that EE reduces aggression or the territory size defended aggressively (leading to increased population density -cohabitation) in salmonids (Imre et al 2002;Höjesjö et al, 2004), and these reports were included in our article (Kadry & Baker, 2010). Heuts & Nijman (2011), however, arbitrarily restricted their analysis to cichlids and ignored those articles about salmonids and the article about convict cichlids (Archocentrus nigrofasciatus) by Barley & Coleman (2010).…”