Hanging with the locals: A strategy to counteract Allee effects?

Shannen Smith (1), Jennifer Donelson, Rebecca Fox, David Booth

School of Life Sciences, University of Technology, Sydney. Sydney, NSW 2007, Australia. shannen.m.smith@student.uts.edu.au

Species that are shifting in distribution will often start out as a group of colonizers, where small group size can often mean individual fitness is dramatically reduced. In the case of tropical vagrant fish that are transported from the Great Barrier Reef to temperate Sydney annually via the East Australian Current (EAC), behavioural traits could assist in counteracting Allee effects and promote population establishment. In this study we examined whether the choice of shoal partner, conspecifics vs temperate resident species, impacts on the fitness of seasonally invading coral reef fish (Abudefduf vaigiensis) in Sydney. By following groups of A. vaigiensis from settlement, we aimed to determine whether shoaling with local fish species would be advantageous in terms of resource acquisition, growth and ultimately survival over conspecific shoaling. We found that behavioural traits varied depending on shoal type (conspecific vs heterospecific) across season (summer through to winter) as well as depending on site (‘inside’ and ‘outside’ Sydney Harbour). Some advantages were observed for tropical vagrants that shoaled with temperate residents prior to water temperature reaching critical thermal minimums for the species. Our results suggest that small behavioural differences could have large impacts on the range shift potential of tropical fish in the future.

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