We examine the hypothesis that reef fish larvae have some direct influence on their own dispersal and ability to recruit to their natal reef by tracking cohorts of bicolor damselfish (Stegastes partitus) from hatching to settlement onto the reef, about 30 d later. We conducted high-resolution sampling during two consecutive years in a small area (15 km 3 20 km) off the west coast of Barbados, extending from depths of 0 to 100 m. Observations of discrete stage-specific larval patches of mean size of 29.4 and 13.2 km 2 for preflexion (1‐5-d old) and flexion/postflexion (.5-d old) stages extending ca. 30 m in the vertical indicated that larvae initially dispersing as patches tend to stay in coherent patches throughout their pelagic duration. Highest concentrations of preflexion larvae within a patch were in the upper 20 m, while those of older larvae were always deeper. Downward migration of about 60 m throughout ontogeny within stratified currents represented a retention mechanism for locally spawned larvae. Most of the variability in estimated retention rates between daily cohorts occurred during the earliest stages as a result of the dynamic nature of surface currents experienced by larvae prior to the onset of vertical migration. Differences in residence time between experiments were consistent with observed intermonthly variability in recruitment strength, implying that pelagic processes can explain recruitment rates. These results provide empirical evidence for larval retention of coral reef fishes and stress the role of active behavior in larval transport.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2004.49.6.1964

Direct evidence of a biophysical retention mechanism for coral reef fish larvae

http://www.churaumi.net/onihitode/Brodie%20et%20al%20COTS%20in%20press.pdf

Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence

Population outbreaks of the coral-eating crown-of-thorns seastar, Acanthaster planci, continue to kill more coral on Indo-Pacific coral reefs than other disturbances, but the causes of these outbreaks have not been resolved. In this study, we combine (1) results from laboratory experiments where larvae were reared on natural phytoplankton, (2) large-scale and long-term field data of river floods, chlorophyll concentrations and A. planci outbreaks on the Great Barrier Reef (GBR), and (3) results from A. planci—coral population model simulations that investigated the relationship between the frequency of outbreaks and larval food availability. The experiments show that the odds of A. planci larvae completing development increases ~8-fold with every doubling of chlorophyll concentrations up to 3 μg l−1. Field data and the population model show that river floods and regional differences in phytoplankton availability are strongly related to spatial and temporal patterns in A. planci outbreaks on the GBR. The model also shows that, given plausible historic increases in river nutrient loads over the last 200 years, the frequency of A. planci outbreaks on the GBR has likely increased from one in 50–80 years to one every 15 years, and that current coral cover of reefs in the central GBR may be 30–40% of its potential value. This study adds new and strong empirical support to the hypothesis that primary A. planci outbreaks are predominantly controlled by phytoplankton availability.

Three lines of evidence to link outbreaks of the crown-of-thorns seastar Acanthaster planci to the release of larval food limitation

Benthic marine organisms are characterized by a bipartite life history in which populations of sedentary adults are connected by oceanic transport of planktonic propagules. In contrast with the terrestrial case, where ‘long distance dispersal’ (LDD) has traditionally been viewed as a process involving rare events, this creates the possibility for large numbers of offspring to travel far relative to the spatial scale of adult populations. As a result, the concept of LDD must be examined carefully when applied in a marine context. Any measure of LDD requires reference to an explicit ‘local’ scale, often defined in terms of adult population demography, habitat patchiness, or the average dispersal distance. Terms such as ‘open’ and ‘closed’ are relative, and should be used with caution, especially when compared across different taxa and systems. We use recently synthesized data on marine propagule dispersal potential and the spread of marine invasive species to draw inferences about average and maximum effective dispersal distances for marine taxa. Foremost, our results indicate that dispersal occurs at a wide range of scales in marine communities. The nonrandom distribution of these scales among community members has implications for marine community dynamics, and for the implementation of marine conservation efforts. Second, in agreement with theoretical results, our data illustrate that average and extreme dispersal scales do not necessarily covary. This further confounds simple classifications of ‘short’ and ‘long’ dispersers, because different ecological processes (e.g. range expansion vs. population replenishment) depend on different aspects of the dispersal pattern (e.g. extremes vs. average). Our findings argue for a more rigorous quantitative view of scale in the study of marine dispersal processes, where relative terms such as ‘short’ and ‘long’, ‘open’ and ‘closed’, ‘retained’ and ‘exported’ are defined only in conjunction with explicit definitions of the scale and process of interest. This shift in perspective represents an important step towards unifying theoretical and empirical studies of dispersal processes in marine and terrestrial systems.

/pdf/propagule-dispersal-and-the-scales-of-marine-community-1ukn2296go.pdf

Propagule dispersal and the scales of marine community process

An improved understanding of the dispersal patterns of marine organisms is a prerequisite for successful marine resource management. For species with dispersing larvae, regional-scale hydrodynamic models provide a means of obtaining results over relevant spatial and temporal scales. In an effort to better understand the role of the physical environment in dispersal, we simulated the transport of reef fish larvae among 321 reefs in and around the Cairns Section of the Great Barrier Reef Marine Park over a period of 20 years. Based on regional-scale hydrodynamics, our models predict the spatial and temporal frequency of significant self-recruitment of the larvae of certain species. Furthermore, the results suggest the importance of a select few local populations in ensuring the persistence of reef fish metapopulations over regional scales.

/pdf/the-structure-of-reef-fish-metapopulations-modelling-larval-1c9gjovg0k.pdf

The structure of reef fish metapopulations: modelling larval dispersal and retention patterns.

Numerical models capable of simulating the hydrodynamics associated with large assemblages of reefs are applied to the larval dispersal of Acanthaster planci. Larval distributions which result from passive transportation have been simulated and the resulting patterns of dispersal are compared with the distribution of adults and the observed pattern of Acanthaster population spread. Three forms of asymmetry in reef connectivity are identified within the southern portion of the Cairns Section of the Great Barrier Reef Marine Park. They confirm that larval dispersal within the region cannot be considered as an istropic random process and the recruitment of reefs is not from a homogeneous larval pool. The three forms of asymmetry in the transport of larvae, together with the the longshore currents within the main lagoon, provide a mechanism which results in the passive concentration of larvae adjacent to reefs that border the main lagoon. The three forms of asymmetry are able to account for: (1) the apparent southward spread of Acanthaster populations; (2) the high incidence of “outbreaks” on mid-shelf reefs; and (3) the susceptibility of particular reefs to repeated recruitment. The high incidence of outbreaks on the inner matrix reefs bordering the main lagoon can be explained as a purely physical phenomenon.

Modelling the larval dispersal of Acanthaster planci

Models capable of simulating large-scale hydrodynamics and Acanthaster larval dispersal are presented for the Cairns Section of the Great Barrier Reef Marine Park. The models identify asymmetries in the patterns of larval dispersal which can explain many observed features of the recent episodes of Acanthaster activity. The northern part of the Cairns Section, between the ribbon reefs and the mainland, is seen to be hydrodynamically distinctive, with the potential to be self-seeding, and therefore the source of Acanthaster activity. Implications of the emerging pattern of stochastic connectivity among reefs are explored using a simple population model. This model is based on a hypothetical assemblage of reefs, with patterns of connectivity derived from the results of the larval dispersal simulation. Experiments with the model demonstrate the potential for the long-term maintenance of populations in the northern region. By simulating the propagation of population outbreaks through the system, it is shown how such a model could be used to test certain hypotheses concerning the cause of outbreaks and to identify the most likely primary outbreak areas.

M. K. James

Papers

Modelling the larval dispersal of Acanthaster planci

Great Barrier Reef Hydrodynamics, Reef Connectivity and Acanthaster Population Dynamics