Biogeography and ecology of baldchin groper Choerodon rubescens in a changing climate

Climate change is causing shifts in the distribution of marine species. Thus adaptive management of fisheries resources and conservation of marine biodiversity needs to understand how population demographics respond to climate change across a species’ range. The waters off Western Australia (WA) have been identified as a hotspot for warming oceans due to climate change, and recently experienced rapid warming during the 2011 marine heat wave (2011-Mhw). Average temperatures have remained higher than long-term average since then, and are predicted to result in poleward distributional shifts of most fish species in the region. However, empirical data for assessing these predictions and the underlying demographic changes are lacking.

WA is also recognised as a global hotspot for endemic marine species and therefore provides an ideal system to study climate-induced changes in population demographics of endemic species. Endemic marine species represent the majority of WA’s fisheries value and changes in the distribution and abundance of endemics will have important consequences to this industry. Furthermore, because of the consistent temperature and habitat gradient along the WA coastline, driven by a lack of upwelling events and a strong poleward boundary current, this region is ideal for studying climatic impacts on species distributions.

The baldchin groper Choerodon rubescens is an endemic wrasse in WA that is highly targeted by recreational and commercial fisheries. It inhabits shallow waters (1–40 m) and is distributed across a latitudinal (21-34 °S) gradient in habitat (coral to rocky-kelp reefs) and temperature (18-24 °C). It is especially at risk of being negatively impacted by rapidly changing environmental conditions because of the combination of its narrow range (1,400 km), vulnerable life history characteristics (long lifespan, protogynous hermaphrodite) and fishing effects. Because C. rubescens shares a similar distribution with many fish species in WA and occurs across a tropical to temperate gradient, this work may serve as a useful global model.

This thesis assesses the demographic responses of C. rubescens across its range to rapid warming associated with the 2011-Mhw. I present data on distribution, abundance, recruitment, size structure, ecology and genetic connectivity across the geographic range of C. rubescens to evaluate population level responses during elevated water temperatures following the 2011-Mhw.

Subsequent to the 2011-Mhw, recruitment at the cooler (southern) end of C. rubescens distribution was unusually high in shallow water lagoons at locations where the species was previously absent or in low abundance (Chapter 2). Warmer temperatures likely made conditions favourable for recruits to survive in greater numbers at these locations. There were differences in the abundance of adults and juveniles across the range of C. rubescens, with adult abundance peaking at the range centre and juvenile abundance peaking at the cooler end (Chapters 3 and 4). At the warmer (northern) end, there was no evidence of recruitment during periods of elevated water temperature (Chapters 3 and 4). Range-wide surveys in shallow-water recruitment habitats (lagoons < 3 m in depth) also showed that higher juvenile abundance at the cooler end of the range continued for three years, indicating that environmental conditions were suitable for ongoing recruitment and survival in this area (Chapter 4). These collective differences in abundance, size composition and recruitment provide the first empirical evidence of distributional change in response to warming oceans in C. rubescens.

As sea temperatures continue to warm, the survival of C. rubescens recruits at and beyond the cooler range edge will be influenced by availability of suitable resources. Abundance surveys across a range of habitats (reef, reef margin, and macrophyte beds), behavioural data and dietary analyses identified that juveniles rely on habitat margins between reef and sand, where soft sediments provide sand-associated invertebrate prey and adjacent reef provides protection from predators. Thus, as oceans warm, the number of recruits arriving and surviving beyond the cooler range edge will be determined by larval delivery mechanisms together with availability of reef margin habitat and associated prey.

Larval connectivity along C. rubescens distribution was partially identified by using genetic population structure in outlier loci from single-nucleotide polymorphisms (SNPs) to trace the origin of recruits at the cooler end of the species’ range to populations in warmer waters 400 km to the north (Chapter 5). The presence of poleward larval connectivity indicates that C. rubescens is likely to colonize novel habitats further south. However, the mismatch in local adaptations of recruits and their settling environment suggests they are under selective pressure, and likely to have low survival probabilities.

In conclusion, by combining abundance, ecological and genetic data I found supporting lines of evidence for a poleward distributional shift in response to warming oceans in C. rubescens along the WA coast. Under a warming ocean scenario, C. rubescens will likely continue to shift its range southwards if resources are available and settling juveniles can adapt to local environmental conditions. However, this study highlights the important uncertainties and complexities surrounding future predictions of distributional change, reinforcing the need for empirical assessments across the geographic ranges of species. This study provides a set of useful guidelines for designing research strategies that better evaluate predicted range shifts, which will help inform the management and conservation of marine species in a changing climate.