University of Tasmania
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Social behaviour, aggression and the indirect effects of competition and predation in the fish species Galaxias maculatus and Gambusia holbrooki

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posted on 2023-05-28, 12:39 authored by Higgins, KL
This study investigated interactions between native Galaxias maculatus and invasive Gambusia holbrooki, from the freshwater and tidally influenced wetlands of the kanamaluka/Tamar River system in cool temperate northern Tasmania. Galaxias maculatus is a valued species for human consumption and as a member of the ecosystem, and Gambusia is the target of an eradication program since its illegal introduction into the area in 1992, due to its detrimental impacts on the native habitat and the native fish and frog fauna. In addition to manual removal methods, the approach being developed (Trojan Y) aims to eradicate the population through the release of genetically manipulated Trojan Y fish which, through breeding with wild fish, would bias the sex ratio of the population to males. To maximise efficacy, we need to understand the behaviour of released fish and how they interact with wild fish. This study examines sublethal effects of predation and competition, including changes in habitat use, social interaction, and behaviour, with implications for interrupted feeding, potentially decreasing tolerance and raising vulnerability to other predators. Cover was chosen for the main habitat variable as it is important in the field and often used in habitat restoration. As these effects can have as strong an impact on the affected species' fitness as lethal effects, this study will inform G. maculatus management and G. holbrooki eradication, including providing a baseline against which the behaviour of sex-reversed Trojan Y fish can be compared. To examine how sexual dimorphism affects G. holbrooki behaviour, all-male, all-female, and mixed sex groups of fish were tested in experimentally manipulated tanks for changes in aggression, schooling, and position in the water column. Male G. holbrooki were more aggressive, but there was more display behaviour in all-female groups. Fish were mostly solitary, which was more marked in males. Males used surface water less than females, even when in single sex groups. The effect of cover on social, aggressive, and protective behaviours was tested separately for each species. Galaxias maculatus had low rates of conflict, usually formed schools, and used cover regularly, which allowed them to increase swimming activity (swimming in and out of cover), presumably due to the protection it provided reducing the need to use relative immobility as a defence. Conversely, G. holbrooki rarely used cover, and both sexes were mostly solitary with high aggression towards other fish, with higher rates in males. The role of different levels of cover and G. holbrooki sex ratios were tested with the two species combined, focussing on interspecific aggression, avoidance, and co-existence. Gambusia holbrooki positioned themselves higher in the water column than G. maculatus when both species were present, similar to when each species was in mono-specific groups. Galaxias maculatus used cover less than without G. holbrooki, but still increased activity when cover was present. Aggression by G. holbrooki was lower than when in mono-specific groups, but the rate of aggression was not reduced by cover. The presence of G. holbrooki, especially females, greatly reduced G. maculatus schooling, while Galaxias aggression, both towards conspecifics and G. holbrooki, increased in the presence of G. holbrooki males. Social, aggressive, and defensive behaviours were examined for adult and immature G. maculatus to test if these behaviours changed with size in G. maculatus and whether there was greatervulnerability in smaller fish. It was found that behaviours were generally similar with size, with the only major difference being that large fish swam in small groups more often than small fish. Although interspecific aggression was much rarer than intraspecific aggression, G. holbrooki primarily affected G. maculatus behaviour by reducing schooling and cover use. In addition, G. maculatus increased intraspecific aggression when G. holbrooki was present, whereas G. holbrooki decreased their intraspecific aggression in the presence of G. maculatus. This research provides the most detailed baseline of behaviour in G. maculatus and G. holbrooki in for both monospecific groups and interspecific interactions; and adds to the understanding of the behaviour of these species in temperate conditions. This can be used to evaluate the behaviour of sex-reversed (Trojan Y) fish. Behavioural changes play a major role in G. maculatus impacts, suggesting that habitat restoration alone may not be sufficient to mitigate G. holbrooki impacts on G. maculatus. The observed higher intra- and lower inter-specific aggression in male G. holbrooki suggests that a male-biased ratio, a potential outcome of a proposed control program (Trojan Y), may be positive for native fish populations.


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