John R. Ford

Identifying the key biophysical drivers, connectivity outcomes, and metapopulation consequences of larval dispersal in the sea

BackgroundPopulation connectivity, which is essential for the persistence of benthic marine metapopulations, depends on how life history traits and the environment interact to influence larval production, dispersal and survival. Although we have made significant advances in our understanding of the spatial and temporal dynamics of these individual processes, developing an approach that integrates the entire population connectivity process from reproduction, through dispersal, and to the recruitment of individuals has been difficult.We present a population connectivity modelling framework and diagnostic approach for quantifying the impact of i) life histories, ii) demographics, iii) larval dispersal, and iv) the physical seascape, on the structure of connectivity and metapopulation dynamics. We illustrate this approach using the subtidal rocky reef ecosystem of Port Phillip Bay, were we provide a broadly-applicable framework of population connectivity and quantitative methodology for evaluating the relative importance of individual factors in determining local and system outcomes.ResultsThe spatial characteristics of marine population connectivity are primarily influenced by larval mortality, the duration of the pelagic larval stage, and the settlement competency characteristics, with significant variability imposed by the geographic setting and the timing of larval release. The relative influence and the direction and strength of the main effects were strongly consistent among 10 connectivity-based metrics.ConclusionsThese important intrinsic factors (mortality, length of the pelagic larval stage, and the extent of the precompetency window) and the spatial and temporal variability represent key research priorities for advancing our understanding of the connectivity process and metapopulation outcomes.

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

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.

Ecological determinants of recruitment to populations of a temperate reef fish, Trachinops caudimaculatus (Plesiopidae)

Although recruitment is often influenced by microhabitat characteristics that affect larval settlement and post-settlement growth and survival, the influence of some habitat features, such as the presence of conspecifics and the accessibility of food, are poorly understood, particularly on temperate reefs. We investigated the ecological determinants of recruitment in the southern hulafish (Trachinops caudimaculatus; McCoy, 1890), a small zooplanktivorous reef fish in Port Phillip Bay, Australia. We hypothesised that T. caudimaculatus would show positive relationships with microhabitat characteristics that provide greater access to food and shelter for newly settled recruits. To test this hypothesis, we surveyed T. caudimaculatus populations and associated microhabitat characteristics on shallow reefs. Overall, habitat characteristics explained 65% of the variation in recruitment, with recruitment greatest to reefs with abundant (1) adults, suggesting positive settlement cues and benefits to survival through shoaling, (2) accessible food (numerous prey), suggesting enhanced survival because of faster growth, and (3) shelter, suggesting enhanced survival through greater availability of refuges from predation. As T. caudimaculatus is an important prey species and sensitive to changes in pelagic productivity, mobile predators and water quality, we suggest it may be a suitable bioindicator of changes to temperate reef ecosystems.

The forgotten shellfish reefs of coastal Victoria: documenting the loss of a marine ecosystem over 200 years since European settlement

Victoria has lost vast areas (>95%) of native flat oyster (Ostrea angasi, Sowerby 1871) and blue mussel (Mytilus edulis galloprovinicialis, Lamarck 1819) reefs from estuarine and coastal waters since European settlement. We document the decline of these reefs by examining indigenous use of shellfish, the decimation of oyster reefs by dredge fishing in early colonial days (1840s–1860s) and later removal of mussel reefs by the mussel and scallop dredging industry (1960s‒1990s). Review of current scientific information reveals no notable areas of continuous oyster reef in Victoria and we consider this habitat to be functionally extinct. While the large-scale removal and destructive fishing practices that drove the rapid declines have not occurred since the mid-1990s, a natural recovery has not occurred. Recovery has likely been hampered historically by a host of factors, including water quality and sedimentation, lack of shell substrate for settlement, chemical pollution impacts, disease of native flat oysters (Bonamia), and more recently introduced species that compete with or prey on shellfish. However, research in the United States has demonstrated that, by strategic selection of appropriate sites and provision of suitable settlement substrates, outplanting of aquaculture-reared oysters and mussels can re-establish shellfish reefs. While a long-term sustained and structured approach is required, there is potential to re-establish shellfish reefs as a functioning ecological community in Victoria’s coastal environment.