Small cryptic fishes on coral reefs: ecological effects of extreme life history traits

On Indo-Pacific coral reefs, small cryptic fishes are extremely diverse, and represent approximately 50 % of individuals in reef fish assemblages. Yet, many of these species have been little studied, particularly in the Indo-Pacific, and the influence of their often extreme life history traits on population dynamics and the broader implications for the composition and resilience of coral reef fish assemblages as a whole, remain unclear. While past research has shown that many of these small species have short and challenging generation times of 47–74 days, we are yet to determine how small cryptic fishes overcome these extreme life history characteristics to successfully persist on coral reefs. The overall goal of this thesis is, therefore, to provide a better understanding of the demographics of short-lived taxa on the Great Barrier Reef (GBR), Australia. More specifically, the present study considers the consequences of extreme life history characteristics, and especially short lifespans, with respect to their ability to recolonise after disturbance, and maintain populations throughout the year. The findings are then discussed in relation to the potential importance of these short-lived species for coral reef trophodynamics. An understanding of the responses of fish assemblages to disturbance events is central to the on-going management of coral reefs habitats. Recovery of fish communities can occur by movement of fishes or recruitment processes. However, small cryptobenthic fishes have extremely limited post-settlement mobility, with home ranges often less than 2 m² and their responses to habitat disturbance has been seldom investigated. In order to understand the patterns of recovery of short-lived taxa, and in particular, to assess their ability to recover after a disturbance, the recolonisation of populations of small cryptic fishes was examined following experimental removal. After removing resident cryptobenthic reef fish assemblages from otherwise undisturbed coral rubble areas, a rapid recovery was observed. Within 8 weeks fish assemblages were similar to their pre-removal structure in terms of fish abundance, species diversity and species richness. However, species differed in the speed and nature of their return. The return of larger species (e.g. Parapercis cylindrica) was largely mediated by recolonisation through movement of adults, while smaller, less mobile, species (e.g. Eviota spp. and Enneapterygius spp.) relied primarily on recruitment from the plankton. Although patterns of settlement and recruitment are ultimately responsible for the replenishment of local populations, this study suggests that mobility may play a strong role in restoring fish assemblages in the short-term. The majority of coral reef goby species are short-lived, with a large proportion of small taxa, such as Eviota, living less than 100 days. Despite these exceptionally short lifespans presenting the demographic challenges of very little time to mature and reproduce, these sub-annual taxa persist and are highly abundant on reefs. To understand the role and consequences of extreme life history in shaping coral reef fish populations, the structure of small reef fish populations was quantitatively documented over a 26 month study period (> 14 short-lived fish generations). Fish density varied over time, but species richness did not change significantly. These patterns were driven by the differential trends in density shown among species within the fish community. Most species with lifespans > 1 year, such as pomacentrids, exhibited a peak in recruitment during the Austral summer. In contrast, despite exceptionally short lifespans, species of Eviota showed consistent densities of both adults and newly settled recruits over the 26 months. These different temporal patterns among taxa drove changes in the overall fish community composition among seasons but there were few apparent differences in the composition of the Eviota populations over time. These results provide evidence of on-going recruitment of these small cryptic fishes, which appears to compensate for an exceptionally short life span on the reef. This study suggests that gobiid populations are able to overcome demographic limitations, and by maintaining reproduction, larval survival and recruitment throughout the year, they may avoid population bottlenecks. These findings also underline the potential trophodynamic importance of these small species; because of extremely high turn-over, and year-round recruitment, Eviota species and other short-lived fishes may be particularly valuable contributors to the flow of energy on coral reefs, underpinning the year-round trophic structure. In examining previously unstudied aspects of the demography of small cryptic fishes on the GBR, this thesis presents evidence of a rapid recruitment of small cryptic fishes to reefs after a disturbance event. Furthermore, this thesis highlights temporal stability (over 26 months) in the size structure of short-lived taxa, specifically for the gobiid genus Eviota. These findings have significant implications for the proposal that small cryptic fishes play an important, yet often forgotten, role in coral reef energetics. Sub-annual taxa provide a constant food resource and supply of reproductive energy to coral reefs trophodynamics throughout the year via predatory pathways. Although not as conspicuous as other coral reef fishes, the unique life-history of short-lived fishes and their proposed important contribution to coral reef trophodynamics suggests these taxa warrant further studies. Two important future research directions are: teasing apart the relative effects of larval supply and post-settlement mortality on abundance of recruits, and assessing the generality of these results for other short-lived taxa. Coral reef ecosystems operate over a broad range of temporal and spatial scales. The exceptional life history features and resulting population dynamics of short-lived species form an integral part of the diversity and dynamics of reef systems and thus are an important research focus.