Convictfish on the move: variation in growth and trophic niche space along a latitudinal gradient

Abstract

The range expansion of tropical fish into temperate waters is increasing markedly in response to climate change. Range-expanding fish encounter novel diets and environments, but we know little about how these conditions facilitate or hinder distribution shifts. Here, we quantified relative growth rate, morphometric condition and trophic niche of juvenile Acanthurus triostegus, a dominant range-expanding tropical surgeonfish, at four locations across 10° of latitude off eastern Australia. We related these metrics to differences in temperature and nutritional quality of dominant seaweeds and the epilithic algal matrix. Temperate food sources were richer in nitrogen than tropical diets. Stable isotope analysis (δ13 carbon and δ15 nitrogen) of fish muscle revealed a large trophic niche breadth at the highest latitude indicating a generalist foraging strategy, and more nitrogen-enriched isotopic signatures compared to tropical regions. Fish length was strongly correlated to δ13C in all regions, suggesting an ontogenetic shift in diet independent of latitude. Despite temperature differences of 4°C, fish growth and body condition were similar across tropical and temperate regions. These results suggest that more nutritious temperate diets may compensate for the effects of cooler water temperatures. Neither summer water temperatures nor dietary factors appear to limit the success of juvenile tropical vagrants as they continue to expand their range along eastern Australia.

Behavioural generalism could facilitate coexistence of tropical and temperate fishes under climate change

“…Optimal foraging theory predicts that individuals modify their behaviour to maximise their physiological integrity (fitness) as prey availability changes (Sih & Christensen, 2001; Stephens & Krebs, 1986). A study using stable isotopes performed at the same sites and on the same species showed that prey types consumed by tropical and temperate species differed regionally (from warmer to cooler latitudes; Kingsbury et al., 2019; Miranda et al, 2019), which can explain the changes in their foraging behaviour as observed in the present study.…”

Multiple spawning events promote increased larval dispersal of a predatory fish in a western boundary current

“…Similar implications exist for broadcast spawners in other boundary current systems which are also changing (Saba et al, 2016; Sakamoto et al, 2005). Increased poleward dispersal of many species is already being observed in many boundary current systems including coastal south eastern Australia, through increased poleward transport of tropical larvae and subsequent “tropicalisation” of temperate areas (Miranda et al, 2019; Vergés et al, 2014).…”

Tropicalization unlocks novel trophic pathways and enhances secondary productivity in temperate reefs

“…Some of these nuances therefore may not have been captured by our functional classification approach. For example, range‐expanding herbivores may exploit novel resources in temperate reefs (Miranda et al., 2019), and examining the exact pathways exploited by each fish in a novel environment will require a more thorough analysis of their diet. Further research is also needed to understand the effect of other fine (e.g.…”