Abstract
Climate change is resulting in rapid poleward shifts in the geographical distribution of tropical and subtropical fish species. We can expect that such range shifts are likely to be limited by species-specific resource requirements, with temperate rocky reefs potentially lacking a range of settlement substrates or specific dietary components important in structuring the settlement and success of tropical and subtropical fish species. We examined the importance of resource use in structuring the distribution patterns of range shifting tropical and subtropical fishes, comparing this with resident temperate fish species within western Japan (Tosa Bay); the abundance, diversity, size class, functional structure and latitudinal range of reef fishes utilizing both coral reef and adjacent rocky reef habitat were quantified over a 2 year period (2008–2010). This region has undergone rapid poleward expansion of reef-building corals in response to increasing coastal water temperatures, and forms one of the global hotspots for rapid coastal changes. Despite the temperate latitude surveyed (33°N, 133°E) the fish assemblage was both numerically, and in terms of richness, dominated by tropical fishes. Such tropical faunal dominance was apparent within both coral, and rocky reef habitats. The size structure of the assemblage suggested that a relatively large number of tropical species are overwintering within both coral and rocky habitats, with a subset of these species being potentially reproductively active. The relatively high abundance and richness of tropical species with obligate associations with live coral resources (i.e., obligate corallivores) shows that this region holds the most well developed temperate-located tropical fish fauna globally. We argue that future tropicalisation of the fish fauna in western Japan, associated with increasing coral habitat development and reported increasing shifts in coastal water temperatures, may have considerable positive economic impacts to the local tourism industry and bring qualitative changes to both local and regional fisheries resources.
Highlights
The world’s oceans have substantially warmed since 1955 [1], and there is increasing evidence that the geographic range of marine organisms has shifted in accordance with this warming [2,3,4]
Within each location 2 separate study sites were designated according to the dominant benthic community composition: coral habitats were designated as areas where live coral cover exceeded .60%, whereas rocky reef sites were composed of 100% rock habitat (Fig. 3)
Salinity was similar at each location and season. Despite both surveyed locations being temperate in climate (33uN, 133uE) the fish fauna was both numerically, and in terms of richness, dominated by fishes with a tropical origin. This is the first study worldwide to show that a temperate reef habitat is numerically dominated by a tropical fauna [4]
Summary
The world’s oceans have substantially warmed since 1955 [1], and there is increasing evidence that the geographic range of marine organisms has shifted in accordance with this warming [2,3,4]. There is increasing evidence that tropical fish populations may be expanding into temperate regions, but there is still little information on the factors which may facilitate or constrain such expansion [4,7,8]. As approximately 10% of coral reef fishes are classified as coral dependent throughout some part of their life stage [12], for these species a reliance on live coral resources is likely to constrain shifts into temperate reef habitats [13,14,15]. Within regions devoid of high live coral cover, fish communities are expected to comprise species with little reliance on coral habitats and the resources they provide [4], independent of increased coastal temperatures [9]
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