Two's company, three's a crowd: Food and shelter limitation outweigh the benefits of group living in a shoaling fish

Abstract

Identifying how density and number-dependent processes regulate populations is important for predicting population response to environmental change. Species that live in groups, such as shoaling fish, can experience both direct density-dependent mortality through resource limitation and inverse number-dependent mortality via increased feeding rates and predator evasion in larger groups. To investigate the role of these processes in a temperate reef fish population, we manipulated the density and group size of the shoaling species Trachinops caudimaculatus on artificial patch reefs at two locations with different predator fields in Port Phillip Bay, Australia. We compared mortality over four weeks to estimates of predator abundance and per capita availability of refuge and food to identify mechanisms for density or number dependence. Mortality was strongly directly density dependent throughout the experiment, regardless of the dominant predator group; however, the limiting resource driving this effect changed over time. In the first two weeks when densities were highest, density-dependent mortality was best explained by refuge competition and the abundance of benthic predators. During the second two weeks, food competition best explained the pattern of mortality. We detected no effect of group size at either location, even where pelagic-predator abundance was high. Overall, direct density effects were much stronger than those of group size, suggesting little survival advantage to shoaling on isolated patch reefs where resource competition is high. This study is the first to observe a temporal shift in density-dependent mechanisms in reef fish, and the first to observe food limitation on short temporal scales. Food competition may therefore be an important regulator of postsettlement reef fish cohorts after the initial intense effects of refuge limitation and predation.

Interactive effects of shelter and conspecific density shape mortality, growth, and condition in juvenile reef fish

“…Hulafish use rocky reef structure as refuge from both pelagic and benthic predators (Ford and Swearer 2013b). Competition for shelter drives density-dependent mortality on small patch reefs (Ford and Swearer 2013a). Given these observations and similar patterns observed for other species , Forrester and Steele 2004, White et al 2010, we hypothesized that shelter is also a limiting resource at the landscape scale, here defined as the scale of the regional metapopulation (i.e., Port Phillip Bay).…”

“…Based upon prior studies (Hunt et al. , Ford and Swearer , b ) we determined these to be represented by (1) the number of small holes and cracks (<2.5 cm diameter), (2) the number of large holes and cracks (2.5–15 cm diameter) and (3) a measure of reef rugosity. We quantified rugosity following Wilson et al.…”

“…Density-dependent, post-settlement mortality is common in marine reef fish populations , where very high densities often drive intense competition for shelter from predators (White et al 2010). Most of the evidence for density dependence has come from manipulative field experiments observing mortality on small spatial (within patch) and short temporal (days to weeks) scales (Hixon andCarr 1997, Ford andSwearer 2013a), with limited evidence on larger spatial (among patches) and temporal (months to years) scales (but see Steele 2004, Johnson 2006). Spatial variability in habitat quality often correlates strongly with the strength of density-dependent mortality among reef fish populations (Shima 2001, Schmitt andHolbrook 2007), and habitat quality in some cases is the strongest predictor of survival (e.g.…”

Genetic diversity affects the strength of population regulation in a marine fish

“…Although our main hypothesis concerned the effects of genetic diversity, other factors may affect the strength of population regulation. For example, for other temperate reef fishes the strength of density dependence may be affected by the abundance of predators, the structural complexity of the habitat, or a combination of both (Nitschke et al 2002, Johnson 2006, 2007, White and Caselle 2008, Ford and Swearer 2013. In this study, we used correlation analyses to test whether the strength of regulation was related to several factors including habitat complexity (measured by the densities of both understory and canopy kelp), density of an interspecific competitor (striped surfperch, Embiotoca lateralis), and density of predators (see Appendix S2 for details).…”

Juvenile convict cichlids shoaling decisions in relation to shoal size and age

“…The evolutionary benefits that drive this behavior include increased efficiency in foraging (Lachlan, Crooks, & Laland, ; Potts, Baken, Ortmann, Watts, & Wrangham, ), higher vigilance (Godin, Classon, & Abrahams, ), and general advantages related to predator avoidance (reviewed in Krause & Ruxton, ). Despite these benefits, there are also costs associated with congregating in large groups, such as greater competition for limited resources, including mates and food (Ford & Swearer, ), or an increased risk of disease (Seppälä, Karvonen, & Valtonen, ).…”