Marine Metapopulations Research Articles

Offshore oil and gas infrastructure plays a minor role in marine metapopulation dynamics

Decommissioning consequences of offshore oil and gas infrastructure removal on marine population dynamics, including connectivity, are not well understood. We modelled the connectivity and metapopulation dynamics of three fish and two benthic invertebrate species inhabiting the natural rocky reefs and offshore oil and gas infrastructure located in the Bass Strait, south-east Australia. Using a network approach, we found that platforms are not major sources, destinations, or stepping-stones for most species, yet act as modest sources for connectivity of Corynactis australis (jewel anemone). In contrast, sections of subsea pipelines appear to act as stepping-stones, source and destination habitats of varying strengths for all study species, except for Centrostephanus rodgersii (long-spined sea urchin). Natural reefs were the main stepping-stones, local source, and destination habitats for all study species. These reefs were largely responsible for the overall metapopulation growth of all study species (average of 96 % contribution across all species), with infrastructure acting as a minor contributor (<2 % average contribution). Full or partial decommissioning of platforms should have a very low or negligible impact on the overall metapopulation dynamics of the species explored, except C. australis, while full removal of pipelines could have a low impact on the metapopulation dynamics of benthic invertebrate species and a moderate impact on fish species (up to 34.1 % reduction in the metapopulation growth). We recommend that the decision to remove offshore infrastructure, either in full or in-part, be made on a platform-by-platform basis and consider contributions of pipelines to connectivity and metapopulation dynamics.

Quantifying larval dispersal portfolio in seabass nurseries using otolith chemical signatures

The temporal asynchronies in larvae production from different spawning areas are fundamental components for ensuring stability and resilience of marine metapopulations. Such a concept, named portfolio effect, supposes that diversifying larval dispersal histories should minimize the risk of recruitment failure by increasing the probability that at least some larvae successfully settle in nursery. Here, we used a reconstructive approach based on otolith chemistry to quantify the larval dispersal portfolio of the European seabass, Dicentrarchus labrax, across six estuarine nursery areas of the northeast Atlantic Ocean. The analysis of natal and trajectory signatures indicated that larvae hatch in distinct environments and then dispersed in water masses featured by contrasting chemical signatures. While some trace elements appeared affected by temporal changes (Mn and Sr), others varied spatially during the larval stage but remained poorly affected by temporal fluctuation and fish physiology (Ba, Cu, Rb and Zn). We then proposed two diversity metrics based on richness and variations of chemical signatures among populations to reflect spatio-temporal diversity in natal origins and larval trajectories (i.e., estimates of dispersal portfolio). Along the French coast, the diversity estimates were maximum in nurseries located at proximity of offshore spawning sites and featured by complex offshore hydrodynamic contexts, such as the Mont St-Michel bay. Finally, our findings indicate that the dispersal portfolio was positively related with the local abundance of seabass juveniles, supporting the assumption that heterogeneity in dispersal history contributes to promote recruitment success in nurseries.

Assessing regional connectivity patterns of bivalvia in fragmented archipelagos: Insights from biophysical modeling in French Polynesia

Larval dispersal and connectivity are key processes that drive marine metapopulation dynamics, and therefore should be well characterized when designing effective management strategies. While temperature and food availability can structure marine species connectivity patterns, their contribution has not been thoroughly investigated in highly fragmented archipelagos. We used biophysical modeling of larval dispersal to explore the connectivity patterns of species with complex life-cycles across French Polynesia (FP), a territory formed by more than a hundred small, geographically isolated islands covering an area as large as Europe. We first simulated ten years of larval dispersal to investigate the spatial and temporal (seasonal and interannual) variability in larval dispersal pathways for different hypothetical species exhibiting a range of Larval Precompetency Period (LPP) values. Then, using the black-lip pearl oyster (Pinctada margaritifera) as a model species, we accounted for variability in the LPP induced by temperature and food availability, as derived from a Dynamic Energy Budget (DEB) model. The model showed that food availability and mesoscale turbulence (eddies) in the Marquesas jointly constrained larval dispersal, reducing its potential connectivity with other archipelagos in FP. The DEB simulations also revealed seasonal and interannual variability in connectivity driven by environmental conditions. However, accounting for food and temperature effects on larval development, barely changed the connectivity pattern at regional scale due to the remoteness of this archipelago. Our study thus provides appropriate management units definition at regional scale for the species across FP.

Next-generation matrices for marine metapopulations: The case of sea lice on salmon farms.

Classifying habitat patches as sources or sinks and determining metapopulation persistence requires coupling connectivity between habitat patches with local demographic rates. While methods to calculate sources, sinks, and metapopulation persistence exist for discrete-time models, there is no method that is consistent across modeling frameworks. In this paper, we show how next-generation matrices, originally popularized in epidemiology to calculate new infections after one generation, can be used in an ecological context to calculate sources and sinks as well as metapopulation persistence in marine metapopulations. To demonstrate the utility of the method, we construct a next-generation matrix for a network of sea lice populations on salmon farms in the Broughton Archipelago, BC, an intensive salmon farming region on the west coast of Canada where certain salmon farms are currently being removed under an agreement between local First Nations and the provincial government. The column sums of the next-generation matrix can determine if a habitat patch is a source or a sink and the spectral radius of the next-generation matrix can determine the persistence of the metapopulation. With respect to salmon farms in the Broughton Archipelago, we identify the salmon farms which are acting as the largest sources of sea lice and show that in this region the most productive sea lice populations are also the most connected. The farms which are the largest sources of sea lice have not yet been removed from the Broughton Archipelago, and warming temperatures could lead to increased sea louse growth. Calculating sources, sinks, and persistence in marine metapopulations using the next-generation matrix is biologically intuitive, mathematically equivalent to previous methods, and consistent across different modeling frameworks.

Coastal connectivity of an abundant inshore fish species: model-data comparison along the southern coast of South Africa

Biophysical models are often used to estimate larval dispersal patterns for the assessment of marine metapopulation spatial structure. However, comparisons of these models with field observations are relatively rare, and the extent to which models reproduce true marine connectivity patterns is unclear. We developed a biophysical model for larvae of the blacktail seabream Diplodus capensis (Sparidae), an abundant recreational and subsistence fishery species along the south-east coast of South Africa, and compared outputs from various configurations of that model to results from a field study conducted in a large regional embayment (Algoa Bay). Seasonal patterns of dispersal and recruitment produced by the model agreed best with field observations when thermal constraints on spawners and larvae were included. Spatial gradients in settling larvae also matched well, with the model capturing observed high settler densities within the lee of a major headland. Nevertheless, stronger spatial gradients were observed in larval densities from the field study when compared with model results, which may be explained by behavioural post-settlement processes. Model-based dispersal patterns revealed up to 5 subpopulations along the southern coast, with barriers to connectivity between subpopulations generally linked to hydrographic features. Overall, our results suggest that thermally mediated spawning behaviour, physical transport and post-settlement processes all play important roles in determining marine connectivity for the blacktail seabream. Refining physiological larval constraints may be an important component that needs to be considered going forward.

EST-Microsatellite Types and Structural Scenarios in European Hake Fisheries.

Simple SummaryThe conservation of the maximum genetic background from all genomic regions matters for sustainability in the changing global scenario. This study focused the importance of integrating such genetic variation in order to infer meaningful units for management and sustainability. We show that knowledge that can be obtained from different marker types should be goal-oriented, i.e., remnant patterns of selection pressures and extreme drift episodes (directional markers, i.e., the evolutionary genetic background), current evolutionary novelty and adaptive potential for fisheries’ sustainability (balanced markers, i.e., the potential genetic drive of the species), and demographic dynamics of genetic relevance for fisheries’ management (neutral markers, i.e., the present-day fishery structure and connectivity). In particular, special attention should be paid to the variation in supergenes or balancing markers, which are a rich source of evolutionary novelty and can be crucial for species welfare and survival.A fishery’s structure and connectivity are priors to its effective management. A successful description of such processes depends on both the sampling design and the choice of adequate genetic markers. EST markers are perfusing the studies of marine metapopulations and are believed to provide access to functional polymorphisms. However, the assumed adaptive role of outlier EST loci might not be generalizable. EST-microsatellites represent the upper polymorphic boundary in these regions because of their high mutation rate. We have subclassified the polymorphisms of EST-microsatellites to assess their structural contribution in the European hake, a paradigmatic and highly mobile marine species (HMMS). Because of the counterbalanced forces between directional markers (15%) and balanced markers (23%), the whole marker set offers the same structural situation as the one observed with neutral markers (62%), i.e., k = 2 gene pools. In contrast to outlier EST- microsatellites, neutral EST subsets allow one to measure crucial population phenomena for fisheries’ management. The high inter-population divergence of outlier EST-microsatellites is compatible with drifted post-selection genomic regions rather than with ongoing local selective pressures. The structural scenario in hake is explainable by a limited gene flow across the Almería-Oran Front (AOF) and by the within-basin IBD pattern of connectivity plus drift-related demographic events. This study highlights how polymorphic properties of EST-microsatellite types can be useful to address mutually excluding research tasks in fisheries, i.e., to address its evolutionary history (directional markers or FAPS: Fossil Adaptive Polymorphic Systems); to delineate management units (neutral markers or NAPS: Non Adaptive Polymorphic Systems); or to ensure sustainability (balanced markers or APS: Adaptive Polymorphic Systems).

Reactivity, Attenuation, and Transients in Metapopulations

Transient dynamics often differ drastically from the asymptotic dynamics of systems. In this paper we analyze transient dynamics in birth-jump metapopulations where dispersal occurs immediately after birth (e.g., via larval dispersal). We address the choice of appropriate norms as well as the effect of stage structure on transient dynamics. We advocate the use of the $\ell_1$ norm, because of its biological interpretation, and extend the transient metrics of reactivity and attenuation to birth-jump metapopulations in this norm. By way of examples we compare this norm to the more commonly used $\ell_2$ norm. Our focus is the case where transient dynamics are very different than asymptotic dynamics. We provide simple examples of metapopulations where this is the case and also show how increasing the number of habitat patches can increase this difference. We then connect the reactivity and attenuation of metapopulations to the source-sink classification of habitat patches and demonstrate how to meaningfully measure reactivity when metapopulations are stage-structured, with a focus on marine metapopulations. Our paper makes three primary contributions. First, it provides guidance to readers as to the appropriate norm and scalings for studying transients in birth-jump metapopulations. Second, it provides three examples of transient behavior in metapopulations involving slow-fast systems, crawl-bys, and high dimensionality. Third, it connects the concepts of reactivity and attenuation to the source-sink classification of habitat patches more commonly found in marine metapopulations.

Time or Space? Relative Importance of Geographic Distribution and Interannual Variation in Three Lineages of the Ascidian Pyura chilensis in the Southeast Pacific Coast

Spatial and temporal variation of environmental parameters can affect dispersal, recruitment and population persistence of marine benthic species. Studies including inter-annual comparisons of genetic structure often indicate high/moderate temporal heterogeneity in marine invertebrates, which may be a prevailing pattern. This suggests that temporal studies are necessary to understand the dynamics of marine metapopulations. In this study, we analyzed the spatio-temporal genetic structure of the ascidianPyura chilensis, a low dispersal sessile marine species endemic from the Southeast Pacific coast and highly demanded for human consumption. We sequenced a fragment of the mitochondrial gene Cytochrome Oxidase I (COI) from 1,005 individuals of six locations (30–40 individuals per site and year) spanning a wide latitudinal range (24°–42°S) and sampled over 5 years (2012, 2014, 2015, 2016, and 2017). The genetic structure of COI indicates the presence of three monophyletic lineages (haplogroups 1–3) previously described for the species, being one of them highly divergent and geographically restricted (~39°S, Los Molinos). Considering the whole dataset, a picture of strong spatial differentiation but temporal stability emerged inPyura chilensis.However, detailed studies of the two main lineages revealed important differences in the extent of spatio-temporal variation. Analyses using haplotype frequencies sorted by site and year showed that, for haplogroup 1, genetic variation was explained mainly by differences between sites, while for haplogroup 2 differences between years were prevailing. Haplogroup 3 was restricted to the most southern sites, and also showed inter-annual variability in its frequency. These results point to disparate patterns of genetic differentiation, which may reflect different adaptive scope or variation in reproductive and dispersal features and could be a response to extreme events such as El Niño (2015–2016). This work calls for caution when obtaining general trends in species clearly differentiated in lineages, and prompts instead for separate analyses of sub-specific genetic lineages whenever possible.

Dynamic larval dispersal can mediate the response of marine metapopulations to multiple climate change impacts

Climate change is having multiple impacts on marine species characterized by sedentary adult and pelagic larval phases, from increasing adult mortality to changes in larval duration and ocean currents. Recent studies have shown impacts of climate change on species persistence through direct effects on individual survival and development, but few have considered the indirect effects mediated by ocean currents and species traits such as pelagic larval duration. We used a density‐dependent and stochastic metapopulation model to predict how changes in adult mortality and dynamic connectivity can affect marine metapopulation stability. We analyzed our model with connectivity data simulated from a biophysical ocean model of the northeast Pacific coast forced under current (1998–2007) and future (2068–2077) climate scenarios in combination with scenarios of increasing adult mortality and decreasing larval duration. Our results predict that changes of ocean currents and larval duration mediated by climate change interact in complex and opposing directions to shape local mortality and metapopulation connectivity with synergistic effects on regional metapopulation stability: while species with short larval duration are most sensitive to temperature‐driven reduction in larval duration, the response of species with longer larval duration are mostly mediated by changes in both the mean and variance of larval connectivity driven by ocean currents. Our results emphasize the importance of considering the spatiotemporal structure of connectivity in order to predict how the multiple effects of climate change will impact marine populations.

Local connections and the larval competency strongly influence marine metapopulation persistence.

The relationship between metapopulation stability and connectivity has long been investigated in ecology, however, most of these studies are focused on theoretical species and habitat networks, having limited ability to capture the complexity of real‐world metapopulations. Network analysis became more important in modeling connectivity, but it is still uncertain which network metrics are reliable predictors of persistence. Here we quantify the impact of connectivity and larval life history on marine metapopulation persistence across the complex seascape of southeast Australia. Our work coupled network‐based approaches and eigenanalysis to efficiently estimate metapopulation‐wide persistence and the subpopulation contributions. Larval dispersal models were used to quantify species‐specific metapopulation connectivity for five important fisheries species, each summarized as a migration matrix. Eigenanalysis helped to reveal metapopulation persistence and determine the importance of node‐level network properties. Across metapopulations, the number of local outgoing connections was found to have the largest impact on metapopulation persistence, implying these hub subpopulations may be the most influential in real‐world metapopulations. Results also suggest the length of the pre‐competency period may be the most influential parameter on metapopulation persistence. Finally, we identified two major hot spots of local connectivity in southeast Australia, each contributing strongly to multispecies persistence. Managers and ecologists would benefit by employing similar approaches in making more efficient and more ecologically informed decisions and focusing more on local connectivity patterns and larval competency characteristics to better understand and protect real‐world metapopulation persistence. Practically this could mean developing more marine protected areas at shorter distances and supporting collaborative research into the early life histories of the species of interest.

Yellow snapper (Lutjanus argentiventris) connectivity in the Southern Gulf of California

We analysed the genetic connectivity and larval transport routes of Lutjanus argentiventris to test if eddies could transport coastal-demersal fish larvae between the peninsular and mainland coasts of the Southern Gulf of California. Larval transport was estimated using the ROMS oceanographic model during the main spawning period (July–August). We used 12 microsatellite loci to assess genetic diversity, population structure and gene flow estimates in 233 L. argentiventris samples from nine sites. The oceanographic model suggested the existence of a stream flow and eddies that maintain connectivity in the Southern Gulf of California. The global AMOVA and paired FST showed no significant genetic differentiation among the sites, and the estimations of the number of migrants indicated moderate to high gene flow among locations. However, after testing five demographic scenarios of connectivity with a coalescent sampler, our results supported the presence of a metapopulation structure with source-sink dynamics. We discuss the challenges to reconcile our results considering the assumptions of the different analyses and the characteristics of marine metapopulations. Connectivity of L. argentiventris could be representative of other costal-demersal species with a similar life history and spawning season.

A Next-Generation Approach to Calculate Source-Sink Dynamics in Marine Metapopulations.

In marine systems, adult populations confined to isolated habitat patches can be connected by larval dispersal. Source-sink theory provides effective tools to quantify the effect of specific habitat patches on the dynamics of connected populations. In this paper, we construct the next-generation matrix for a marine metapopulation and demonstrate how it can be used to calculate the source-sink dynamics of habitat patches. We investigate the effect of environmental variables on the source-sink dynamics and demonstrate how the next-generation matrix can provide useful biological insight into transient as well as asymptotic dynamics of the model.

Hidden similarities in the dynamics of a weakly synchronous marine metapopulation

Populations of many marine species are only weakly synchronous, despite coupling through larval dispersal and exposure to synchronous environmental drivers. Although this is often attributed to observation noise, factors including local environmental differences, spatially variable dynamics, and chaos might also reduce or eliminate metapopulation synchrony. To differentiate spatially variable dynamics from similar dynamics driven by spatially variable environments, we applied hierarchical delay embedding. A unique output of this approach, the "dynamic correlation," quantifies similarity in intrinsic dynamics of populations, independently of whether their abundance is correlated through time. We applied these methods to 17 populations of blue crab (Callinectes sapidus) along the US Atlantic coast and found that their intrinsic dynamics were broadly similar despite largely independent fluctuations in abundance. The weight of evidence suggests that the latitudinal gradient in temperature, filtered through a unimodal response curve, is sufficient to decouple crab populations. As unimodal thermal performance is ubiquitous in ectotherms, we suggest that this may be a general explanation for the weak synchrony observed at large distances in many marine species, although additional studies are needed to test this hypothesis.

Dispersal and population connectivity are phenotype dependent in a marine metapopulation.

Larval dispersal is a key process determining population connectivity, metapopulation dynamics, and community structure in benthic marine ecosystems, yet the biophysical complexity of dispersal is not well understood. In this study, we investigate the interaction between disperser phenotype and hydrodynamics on larval dispersal pathways, using a temperate reef fish species, Trachinops caudimaculatus. We assessed the influence of larval traits on depth distribution and dispersal outcomes by: (i) using 24-h depth-stratified ichthyoplankton sampling, (ii) quantifying individual phenotypes using larval growth histories extracted from the sagittal otoliths of individual larvae, and (iii) simulating potential dispersal outcomes based on the empirical distribution of larval phenotypes and an advanced biological-physical ocean model. We found T. caudimaculatus larvae were vertically stratified with respect to phenotype, with high-quality phenotypes found in the bottom two depth strata, and poor-quality phenotypes found primarily at the surface. Our model showed high- and average-quality larvae experienced significantly higher local retention (more than double) and self-recruitment, and travelled shorter distances relative to poor-quality larvae. As populations are only connected when dispersers survive long enough to reproduce, determining how larval phenotype influences dispersal outcomes will be important for improving our understanding of marine population connectivity and persistence.

Accounting for ocean connectivity and hydroclimate in fish recruitment fluctuations within transboundary metapopulations.

Marine resources stewardships are progressively becoming more receptive to an effective incorporation of both ecosystem and environmental complexities into the analytical frameworks of fisheries assessment. Understanding and predicting marine fish production for spatially and demographically complex populations in changing environmental conditions is however still a difficult task. Indeed, fisheries assessment is mostly based on deterministic models that lack realistic parameterizations of the intricate biological and physical processes shaping recruitment, a cornerstone in population dynamics. We use here a large metapopulation of a harvested fish, the European hake (Merluccius merluccius), managed across transnational boundaries in the northwestern Mediterranean, to model fish recruitment dynamics in terms of physics-dependent drivers related to dispersal and survival. The connectivity among nearby subpopulations is evaluated by simulating multi-annual Lagrangian indices of larval retention, imports, and self-recruitment. Along with a proxy of the regional hydroclimate influencing early life stages survival, we then statistically determine the relative contribution of dispersal and hydroclimate for recruitment across contiguous management units. We show that inter-annual variability of recruitment is well reproduced by hydroclimatic influences and synthetic connectivity estimates. Self-recruitment (i.e., the ratio of retained locally produced larvae to the total number of incoming larvae) is the most powerful metric as it integrates the roles of retained local recruits and immigrants from surrounding subpopulations and is able to capture circulation patterns affecting recruitment at the scale of management units. We also reveal that the climatic impact on recruitment is spatially structured at regional scale due to contrasting biophysical processes not related to dispersal. Self-recruitment calculated for each management unit explains between 19% and 32.9% of the variance of recruitment variability, that is much larger than the one explained by spawning stock biomass alone, supporting an increase of consideration of connectivity processes into stocks assessment. By acknowledging the structural and ecological complexity of marine populations, this study provides the scientific basis to link spatial management and temporal assessment within large marine metapopulations. Our results suggest that fisheries management could be improved by combining information of physical oceanography (from observing systems and operational models), opening new opportunities such as the development of short-term projections and dynamic spatial management.

Insights from genetic and demographic connectivity for the management of rays and skates

Studying demographic and genetic connectivity can help assess marine metapopulation structure. Rays and skates have no larval phase; hence, population connectivity can only result from active movement of individuals. Using thornback ray (Raja clavata) in European waters as a case study, demographic and genetic connectivity were studied for 11 putative populations with unequal population abundances and two hypotheses of dispersal rates. Genetic simulation results highlighted three large metapopulations: in the Mediterranean, around the Azores, and on the Northeast Atlantic shelf. Demographic results highlighted a finer population structure indicating that several pairs of putative populations might be demographically linked. Results were highly sensitive to dispersal assumptions and relative population abundances, which provided insights into the potential magnitude of genetic and demographic connectivity differences. Accounting for demographic connectivity appears to be crucial for managing and conserving rays and skates, while genetic connectivity provides a longer-term perspective and less subtle spatial structures. Moreover, accounting for heterogeneity in population abundances is a key factor for determining or interpreting metapopulation connectivity.

Integrating larval connectivity with local demography reveals regional dynamics of a marine metapopulation.

Many ocean species exist within what are called marine metapopulations: networks of otherwise isolated local populations connected by the exchange of larval offspring. In order to manage these species as effectively as possible (e.g., by designing and implementing effective networks of marine protected areas), we must know how many offspring are produced within each local population (i.e., local demography), and where those offspring disperse (i.e., larval connectivity). Although there is much interest in estimating connectivity in the relatively simple sense of identifying the locations of spawning parents and their settling offspring, true measures of demographic connectivity that account for among-site variation in offspring production have been lacking. We combined detailed studies of local reproductive output and larval dispersal of a coral reef fish to quantify demographic connectivity within a regional metapopulation that included four widely spaced islands in the Bahamas. We present a new method for estimating demographic connectivity when the levels of dispersal among populations are inferred by the collection of genetically "tagged" offspring. We estimated that 13.3% of recruits returned to natal islands, on average (95% CI = 1.1-50.3%), that local retention was high on one of the islands (41%, 95% CI = 6.0-97.0%), and that larval connectivity was appreciable, even between islands 129km apart (mean = 1.6%, 95% CI = 0.20-8.8%). Our results emphasize the importance of properly integrating measurements of production with measurements of connectivity. Had we not accounted for among-site variation in offspring production, our estimates of connectivity would have been inaccurate by a factor as much as 6.5. At a generational timescale, lifetime offspring production varied substantially (a fivefold difference among islands) and the importance of each island to long-term metapopulation growth was dictated by both larval production and connectivity. At the scale of our study (local populations inhabiting 5-ha reefs), the regional metapopulation could not grow without external input. However, an exploratory analysis simulating a network of four marine protected areas suggested that reserves of >65ha each would ensure persistence of this network. Thus, integrating studies of larval connectivity and local demography hold promise for both managing and conserving marine metapopulations effectively.

Genetic and oceanographic tools reveal high population connectivity and diversity in the endangered pen shell Pinna nobilis

For marine meta-populations with source-sink dynamics knowledge about genetic connectivity is important to conserve biodiversity and design marine protected areas (MPAs). We evaluate connectivity of a Mediterranean sessile species, Pinna nobilis. To address a large geographical scale, partial sequences of cytochrome oxidase I (COI, 590 bp) were used to evaluate phylogeographical patterns in the Western Mediterranean, and in the whole basin using overlapping sequences from the literature (243 bp). Additionally, we combined (1) larval trajectories based on oceanographic currents and early life-history traits and (2) 10 highly polymorphic microsatellite loci collected in the Western Mediterranean. COI results provided evidence for high diversity and low inter-population differentiation. Microsatellite genotypes showed increasing genetic differentiation with oceanographic transport time (isolation by oceanographic distance (IBD) set by marine currents). Genetic differentiation was detected between Banyuls and Murcia and between Murcia and Mallorca. However, no genetic break was detected between the Balearic populations and the mainland. Migration rates together with numerical Lagrangian simulations showed that (i) the Ebro Delta is a larval source for the Balearic populations (ii) Alicante is a sink population, accumulating allelic diversity from nearby populations. The inferred connectivity can be applied in the development of MPA networks in the Western Mediterranean.

Step-wise drops in modularity and the fragmentation of exploited marine metapopulations

Many nearshore species are distributed in habitat patches connected only through larval dispersal. Genetic research has shown some spatial structure of such metapopulations and modeling studies have shed light onto possible patterns of connectivity and barriers. However, little is known about human impact on their spatial structure and patterns of connectivity. We examine the effects of fishing on the spatial and temporal dynamics of metapopulations of sedentary marine species (red sea urchin and red abalone) interconnected by larval dispersal. We constructed a metapopulation model to simulate abalone and sea urchin metapopulations experiencing increasing levels of fishing mortality. We performed the modularity analysis on the yearly larval connectivity matrices produced by these simulations, and analyzed the changes of modularity and the formation of modules over time as indicators of spatial structure. The analysis revealed a strong modular spatial structure for abalone and a weak spatial signature for sea urchin. In abalone, under exploitation, modularity takes step-wise drops on the path to extinction, and modules breakdown into smaller fragments followed by module and later metapopulation collapse. In contrast, sea urchin showed high modularity variation, indicating high- and low-mixing years, but an abrupt collapse of the metapopulation under strong exploitation. The results identify a disruption in larval connectivity and a pattern of collapse in highly modular nearshore metapopulations. These results highlight the ability of modularity to detect spatial structure in marine metapopulations, which varies among species, and to show early changes in the spatial structure of exploited metapopulations.

Larval fish dispersal in a coral-reef seascape.

Larval dispersal is a critical yet enigmatic process in the persistence and productivity of marine metapopulations. Empirical data on larval dispersal remain scarce, hindering the use of spatial management tools in efforts to sustain ocean biodiversity and fisheries. Here we document dispersal among subpopulations of clownfish (Amphiprion percula) and butterflyfish (Chaetodon vagabundus) from eight sites across a large seascape (10,000 km2) in Papua New Guinea across 2 years. Dispersal of clownfish was consistent between years, with mean observed dispersal distances of 15 km and 10 km in 2009 and 2011, respectively. A Laplacian statistical distribution (the dispersal kernel) predicted a mean dispersal distance of 13-19 km, with 90% of settlement occurring within 31-43 km. Mean dispersal distances were considerably greater (43-64 km) for butterflyfish, with kernels declining only gradually from spawning locations. We demonstrate that dispersal can be measured on spatial scales sufficient to inform the design of and test the performance of marine reserve networks.