Larval Dispersal Research Articles

When does spillover from marine protected areas indicate benefits to fish abundance and catch?

Spillover is a term commonly applied to the dispersal of fish and/or larvae from inside a closed area to areas open to fishing. The presence of spillover is often quantified by measuring gradients in attributes such as abundance or catch rates near the boundaries of closed areas or by measuring higher abundance inside closed areas compared to outside. It is commonly assumed that such gradients or ratios indicate that the closed area has benefitted the fishery and the total abundance of fish. We explore this assumption using a spatially explicit model of closed areas with different intensities of fishing and fish movement, and we find that such gradients will be expected any time there is higher abundance inside the closed area. However, such gradients do not necessarily indicate a benefit to the fishery either in terms of total catch or catch rate, and unless pre-closure fishing was intense, total abundance is not expected to rise significantly. We examine case studies that argue that spillover exists and leads to fishery benefits. We then evaluate the evidence for net benefits in these case studies and find those with evidence of net benefits all come from places where fishing pressure was intense. While most analysis come from quite small coastal closed areas, two studies of very large open-ocean closed areas are discussed, and we find that both suggest little overall impact on the tuna populations that support the main commercial fisheries affected by the closures in question.

Population genetics of Macrognathus siamensis (Synbranchiformes: Mastacembelidae): Implications for non-migratory fishery resources in the Mekong River basin

The spotted spiny eel, Macrognathus siamensis is an economically important freshwater fish in the Mekong River basin, which is undergoing dramatic biodiversity changes due to anthropogenic impacts. The species is non-migratory, with a moderate larval duration that facilitates downstream genetic connectivity. Restriction site-associated DNA sequence datasets that includes 3736 and 1244 single nucleotide polymorphisms representing putatively neutral loci, from all geographic sites and without the Tachileik site, indicate strong population structure in this sedentary fish, with a significant isolation-by-distance signature. This structure reflects isolation of tributary populations and downstream dispersal of these distinct populations to mix in the Mekong mainstem and contributes to increased genetic diversity in the lower reaches of the Mekong basin. Genetic data indicates that the dispersal of eggs and larvae downstream is not impeded by Khone Falls and that the falls prevent upstream dispersal of eggs and larvae. Despite this downstream connectivity, there is a strong population structure among above- and below-Khone Falls groups. An analysis of outlier loci putatively under selection provides evidence that the difference between connected metapopulations above and below the falls is due to the fundamental differences in environmental regimes. We hypothesize that instead of a physical barrier, the falls simply represents a demarcation between previously recognized upstream limited-floodplain environments and downstream extensive-floodplain environments. These findings emphasize the need to monitor genetic diversity of key sedentary fishery species to assess whether proposed dams along the river could affect genetic diversity downstream. This genetic diversity is particularly important for the resilience of downstream populations because of the many environmental impacts caused by dams, land use, and climate change. Early detection of reduced downstream genetic diversity could trigger remediation programs to ensure resilience and continued fisheries productivity of important non-migratory fish resources.

Implications of spawning migration patterns of the giant mud crab Scylla serrata (Forskål, 1775) on opportunities for larval dispersal

Connectivity is an essential driver for aquatic species distribution, genetic variability and stock structure. The giant mud crab (Scylla serrata) is a coastal portunid commonly associated with estuaries and mangrove systems. This species has been observed to undertake a seaward spawning migration, as the larval development is known to be more successful under the stable environmental conditions typically found in marine waters. The larvae return to the coastal areas through advection, where they are recruited and enter the estuaries after metamorphosing into the first instar. Here, we used numerical modelling to test hypotheses regarding probabilities of larval settlement of the giant mud crab and the effect of the distance offshore from which females release the eggs. Our scenarios considered the biological characteristics of larvae and oceanographic conditions for six locations for mud crabs along a complex coastline - the Queensland east coast, Australia. The models suggest that all locations tend to self-supply, and to exchange mud crab larvae with other regions, but in different magnitudes. The spawning distance offshore considerably affects larval distribution and settlement. The main drivers for larval advection in areas within the continental shelf are wind patterns and coastal currents, while offshore along the Australian continental slope, the main drivers are ocean currents. Self-recruitment is predominant, although we also observed a significant degree of connectivity between each location and the surrounding coastline. Short spawning migrations benefit self-recruitment in all scenarios, but long offshore migrations favour connectivity among different locations. This source/sink balance seems to depend on the local oceanographic features. Nevertheless, offshore spawning by the giant mud crab has the potential to provide for successful recruitment in a variety of environmental contexts. This study provides novel predictions of the probabilities of larval settlement for mud crab populations considering ocean advection that can be applied to different contexts.

Larval Transport Pathways for Lutjanus peru and Lutjanus argentiventris in the Northwestern Mexico and Tropical Eastern Pacific

Understanding how ocean currents influence larval dispersal and measuring its magnitude is critical for conservation and sustainable exploitation, especially in the Tropical Eastern Pacific (TEP), where the larval transport of rocky reef fish remains untested. For this reason, a lagrangian simulation model was implemented to estimate larval transport pathways in Northwestern Mexico and TEP. Particle trajectories were simulated with data from the Hybrid Ocean Coordinate Model, focusing on three simulation scenarios: (1) using the occurrence records of Lutjanus peru and L. argentiventris as release sites; (2) considering a continuous distribution along the study area, and (3) taking the reproduction seasonality into account in both species. It was found that the continuous distribution scenario largely explained the genetic structure previously found in both species (genetic brakes between central and southern Mexico and Central America), confirming that the ocean currents play a significant role as predictors of genetic differentiation and gene flow in Northwestern Mexico and the TEP. Due to the oceanography of the area, the southern localities supply larvae from the northern localities; therefore, disturbances in any southern localities could affect the surrounding areas and have impacts that spread beyond their political boundaries.

Migratory life cycle of Anguilla anguilla: a mirror symmetry with A. japonica.

The European and Japanese eel populations have declined significantly in recent decades. To effectively manage and conserve them, gaining a better understanding of their migratory life cycles is important. Previous research on the spawning ecology and larval dispersal of European and Japanese eels has led to many significant discoveries and advancements for their migratory life cycles. However, different scholars hold varying views on their migratory life cycles, especially concerning the European eel, therefore this article aims to provide a comprehensive review of research from multiple disciplines concerning the spawning ecology and larval dispersal of European and Japanese eels and to propose migratory life cycles of these two species. The migratory life cycle of the European eel is as follows: European silver eels undertake a year-long spawning migration from September to January to reach the Sargasso Sea for spawning before the next spawning season, typically between December and May. After hatching, European eel leptocephali are transported by the Gulf Stream, Frontal Countercurrents, North Atlantic Current, and Azores Current and return to Europe and North Africa for growth. Recruitment of European glass eels mainly occurs between October and June of the following year, and the recruiting season is more concentrated in countries closer to the spawning area and more dispersed in countries farther away. The consistent recruitment pattern and the growth rate of leptocephali suggest a larval transport period, also called larval duration, of around 1 year. Understanding the migratory life cycle of European eels can facilitate the evaluation or development of their conservation measures.

Larval dispersal and physical connectivity of Pheronema carpenteri populations in the Azores

The study of larval dispersal and connectivity between deep-sea populations is essential for the effective conservation and management of deep-sea environments and the design and implementation of Marine Protected Areas. Dense sponge aggregations, known as “sponge grounds”, are a key component of marine benthic ecosystems, by increasing the structural complexity of the sea floor and providing structure and habitat for many other species. These aggregations are characteristic of the Azores deep-sea environment. These sessile organisms rely primarily on larval dispersal for their reproduction. Connectivity between specific Pheronema carpenteri sponge aggregations in the Azores was studied using a 3-D biophysical dispersal model. Different biological trait scenarios were analyzed, considering spawning seasonality and pelagic larval duration. Model results indicate that regional circulation patterns drive larval dispersion, shaping population connectivity of P. carpenteri sponge aggregations in the Azores, particularly among aggregations in the Central Group of Azorean islands. Some areas present high retention rates, receiving larvae from several sponge aggregations while also being important larval source aggregations. In contrast, aggregations from the Eastern Group may be isolated from the others. Larval dispersal and connectivity patterns were analyzed concerning the current configuration of Marine Protected Areas (MPAs) in the Azores. The results underscored the importance of maintaining protection efforts in existing MPAs and identified stepping-stone locations and specific sites where additional measures could enhance species connectivity in the Azores.

Animal life in the shallow subseafloor crust at deep-sea hydrothermal vents

It was once believed that only microbes and viruses inhabited the subseafloor crust beneath hydrothermal vents. Yet, on the seafloor, animals like the giant tubeworm Riftia pachyptila thrive. Their larvae are thought to disperse in the water column, despite never being observed there. We hypothesized that these larvae travel through the subseafloor via vent fluids. In our exploration, lifting lobate lava shelves revealed adult tubeworms and other vent animals in subseafloor cavities. The discovery of vent endemic animals below the visible seafloor shows that the seafloor and subseafloor faunal communities are connected. The presence of adult tubeworms suggests larval dispersal through the recharge zone of the hydrothermal circulation system. Given that many of these animals are host to dense bacterial communities that oxidize reduced chemicals and fix carbon, the extension of animal habitats into the subseafloor has implications for local and regional geochemical flux measurements. These findings underscore the need for protecting vents, as the extent of these habitats has yet to be fully ascertained.

Advances in understanding physical and biological controls on eggs and larval distribution in Pacific fisheries: A review

The early stages of fish, comprising eggs and larvae, are exceptionally fragile and sensitive to environmental dynamics and climate change. Pacific Ocean (PO) currents play an important role in shaping the distribution of marine organisms, influencing global climate patterns, heat distribution, coastal temperatures, and nutrient redistribution. These currents reveal significant changes within the climate system. Their variability across different timescales can be attributed to the complex interplay of physical forces. These currents are subjected to diverse anthropogenic factors, exerting detrimental effects on the dispersal of fish larvae. Furthermore, climate change variables, including alterations in tropical PO temperature associated with the ENSO cycle, Atlantic Nino modes influencing equatorial Atlantic temperature, changes in ocean salinity, polar ice cap melting, increasing greenhouse gases, marine heatwaves, and fluctuations in subsurface flows, directly impact the distribution, abundance, and species composition of early life stages. Major Pacific fisheries, such as those targeting Pacific sardines, saury, and anchovies, undergo population booms and declines due to significant alterations in the current dynamics of currents and fronts within the PO. The anticipated intensification of the ENSO cycle, characterized by more frequent and severe El Niño (warm) and La Niña (cold) events as a result of climate change, is expected to significantly impact the early developmental stages of important commercial fish stocks regularly. This review synthesizes the current understanding of the physical and biological parameters driving changes in ocean currents and their implications for early fish dispersion, emphasizing the critical need for research in this area to inform global conservation efforts, fisheries management, and food security.

Mesoscale ocean eddies determine dispersal and connectivity of corals at the RMS Titanic wreck site

The sinking of the RMS Titanic on 15 April 1912 remains one of most iconic maritime disasters in history. Today, the wreck site lies in waters 3800 m deep approximately 690 km south southeast of Newfoundland, Atlantic Canada. The wreck and debris field have been colonized by many marine organisms including the octocoral Chrysogorgia agassizii. Because of the rapid deterioration of the Titanic and the vulnerability of natural deep-sea coral populations to environmental changes, it is vital to understand the role the Titanic as well as other such structures could play in connecting ecosystems along the North American slope. Based on Lagrangian experiments with more than one million virtual particles and different scenarios for larval behavior, given the uncertainties around the biology of chrysogorgiids, the dispersal of larvae spawned at the Titanic wreck is studied in a high-resolution numerical ocean model. While the large-scale bathymetry shields the Titanic from a strong mean flow, mesoscale ocean eddies can considerably affect the deep circulation and cause a significant speed up, or also a reversal, of the circulation. As a consequence, the position of upper and mid-ocean eddies in the model largely controls the direction and distance of larval dispersal, with the impact of eddies outweighing the importance of active larval swimming in our experiments. Although dependent on larval buoyancy and longevity, we find that the Titanic could be reached by larvae spawned on the upper slope east of the Grand Banks. Therefore, the Titanic could act as a stepping stone connecting the upper to the deep continental slope off Newfoundland. From the Titanic, larvae then spread into deep Canadian waters and areas beyond national jurisdiction.

The contribution of nearshore oceanography to temporal variation in larval dispersal.

Patterns of population connectivity shape ecological and evolutionary phenomena from population persistence to local adaptation and can inform conservation strategy. Connectivity patterns emerge from the interaction of individual behavior with a complex and heterogeneous environment. Despite ample observation that dispersal patterns vary through time, the extent to which variation in the physical environment can explain emergent connectivity variation is not clear. Empirical studies of its contribution promise to illuminate a potential source of variability that shapes the dynamics of natural populations. We leveraged simultaneous direct dispersal observations and oceanographic transport simulations of the clownfish Amphiprion clarkii in the Camotes Sea, Philippines, to assess the contribution of oceanographic variability to emergent variation in connectivity. We found that time-varying oceanographic simulations on both annual and monsoonal timescales partly explained the observed dispersal patterns, suggesting that temporal variation in oceanographic transport shapes connectivity variation on these timescales. However, interannual variation in observed mean dispersal distance was nearly 10 times the expected variation from biophysical simulations, revealing that additional biotic and abiotic factors contribute to interannual connectivity variation. Simulated dispersal kernels also predicted a smaller scale of dispersal than the observations, supporting the hypothesis that undocumented abiotic factors and behaviors such as swimming and navigation enhance the probability of successful dispersal away from, as opposed to retention near, natal sites. Our findings highlight the potential for coincident observations and biophysical simulations to test dispersal hypotheses and the influence of temporal variability on metapopulation persistence, local adaptation, and other population processes.

Fitness consequences of marine larval dispersal: the role of neighborhood density, arrangement, and genetic relatedness on survival, growth, reproduction, and paternity in a sessile invertebrate.

Dispersal can evolve as an adaptation to escape competition with conspecifics or kin. Locations with a low density of conspecifics, however, may also lead to reduced opportunities for mating, especially in sessile marine invertebrates with proximity-dependent mating success. Since there are few experimental investigations, we performed a series of field experiments using an experimentally tractable species (the bryozoan Bugula neritina) to test the hypothesis that the density, spatial arrangement, and genetic relatedness of neighbors differentially affects survival, growth, reproduction, paternity, and sperm dispersal. We manipulated the density and relatedness of neighbors and found that increased density reduced survival but not growth rate, and that there was no effect of relatedness on survival, growth, or fecundity, in contrast to previous studies. We also manipulated the distances to the nearest neighbor and used genetic markers to assign paternity within known mother-offspring groups to estimate how proximity affects mating success. Distance to the nearest neighbor did not affect the number of settlers produced, the paternity share, or the degree of multiple paternity. Overall, larger than expected sperm dispersal led to high multiple paternity, regardless of the distance to the nearest neighbor. Our results have important implications for understanding selection on dispersal distance: in this system there are few disadvantages to the limited larval dispersal that does occur, and limited advantages for larvae to disperse further than a few 10s of meters.

Unexpected mismatches in population structure among marine mussel life‐history stages reveal the true scales of planktonic larval dispersal

AbstractStudies investigating gene flow in sessile or sedentary marine species typically draw conclusions about larval dispersal by investigating genetic structure of adults. Here, we generated microsatellite data from adults, recruits, settlers and planktonic larvae of the brown mussel, Perna perna, from the southeast coast of South Africa, and identified a consistent mismatch in genetic structure between the adults and all earlier life stages. While adults could be assigned to two major geographical groups (western and eastern), most of the early‐stage mussels were strongly affiliated with the eastern group. This suggests that few of the early‐stage individuals present in the western portion of the sampling range will eventually establish themselves in the adult population, highlighting the importance of post‐recruitment processes as drivers of population structure. Our findings caution against the exclusive use of genetic data generated from adults to assess population connectivity facilitated by the dispersal of planktonic propagules.

Oceanographical-driven dispersal and environmental variation explain genetic structure in an upwelling coastal ecosystem

The seascape comprises multiple environmental variables that interact with species biology to determine patterns of spatial genetic variation. The environment imposes spatially variable selective forces together with homogenizing and diverging drivers that facilitate or restrict dispersal, which is a complex, time-dependent process. Understanding how the seascape influences spatial patterns of genetic variation remains elusive, particularly in coastal upwelling systems. Here, we combine genome-wide SNP data, Lagrangian larval dispersal simulated over a hydrodynamic model, and ocean environmental information to quantify the relative contribution of ocean circulation and environmental heterogeneity as drivers of the spatial genetic structure of two congeneric intertidal limpets, Scurria scurra and S. araucana, along the central coast of Chile. We find that a genetic break observed in both limpet species coincides with a break in connectivity shown by the Lagrangian dispersal, suggesting that mean ocean circulation is an important seascape feature, in particular for S. scurra. For S. araucana, environmental variation appears as a better predictor of genetic structure than ocean circulation. Overall, our study shows broad patterns of seascape forcing on genetic diversity and contributes to our understanding of the complex ecological and evolutionary interactions along coastal upwelling systems.

Does rafting promote contemporary gene flow? Global and regional patterns of population genetic diversity and structure on the false limpet Siphonaria lateralis in the Southern Ocean

Rafting has been proposed as an effective mechanism for species without free-living pelagic larvae to achieve long-distance dispersal, theoretically preventing population differentiation over wide distributional ranges. Moreover, rafting has been advocated as a main dispersal mechanism for marine invertebrates with sub-Antarctic distributions, because of abundant buoyant kelps, driven by the Antarctic Circumpolar Current. Nonetheless, little attention has been given to the role of rafting to establish regular gene flow across the sub-Antarctic, and the geographic and temporal scales at which it occurs. Aiming to unravel these major questions about the extent of genetic connectivity across the Southern Ocean (SO), we studied the pulmonate limpet Siphonaria lateralis, a benthic species with encapsulated larvae, found on the rocky intertidal of sub-Antarctic islands and southern South America. Since S. lateralis is closely associated with D. antarctica, dispersal by rafting is plausible, as revealed by the absence of phylogeographic structure across the sub-Antarctic. We sampled 116 individuals from eight localities across the SO, and used 5,515 SNPs obtained through Genotyping-by-Sequencing, to determine contemporary genetic diversity, structure, and gene flow at two spatial scales; global, across the SO, and regional, within Kerguelen. Results identified substantial genetic structure, differentiating Patagonia, Falklands/Malvinas Islands, South Georgia and the Kerguelen archipelago, and low levels of contemporary gene flow. The most notable genetic differentiation was found between Patagonia/Falklands and South Georgia/Kerguelen. Structure was also significant between Patagonia and the Falkland/Malvinas Islands. Conversely, South Georgia and Kerguelen exhibited closer genetic affinity, and indications of recent but limited gene flow. Moreover, historical gene flow estimates between the four populations were low. At regional scale, noteworthy genetic structure persisted, and gene flow was insufficient to prevent genetic differentiation within Kerguelen. Consequently, rafting’s potential may be overestimated as a contemporary mechanism promoting gene flow across the SO, as these events may be sporadic, irregular, and unpredictable for marine invertebrates lacking a larval dispersal stage, since contemporary dispersal events don’t seem to facilitate high gene flow at both scales. Accordingly, other oceanographic factors or processes may hinder the establishment of species associated with macroalgae, and as consequence, contemporary genetic connectivity in the sub-Antarctic.

Integrating Habitat Suitability and Larval Drift Modeling for Spawning‐To‐Nursery Functional Habitat Connectivity Analysis in Rivers

AbstractHabitat suitability modeling is a commonly used methodology to plan and assess in‐stream habitat enhancement in rivers, such as for the key fish life stages spawning and juvenile development. However, their use only allows modeling the spatial distribution of habitats, but not their connectivity. By integrating micro‐scale habitat modeling and a larval drift model, we assess the functional connectivity between spawning and larval nursery habitats for four rheophilic and litophilic fish species in a channelized and hydropower impacted reach of the lower Inn River (Bavaria, Germany), in which two restoration measures, a bypass channel and an island side‐channel system, have been constructed to improve longitudinal connectivity and habitat conditions. The study aims to (a) map spawning and larval nursery habitats, (b) quantify their connectivity, and (c) optimize functional habitat connectivity through an alternative bypass channel location. Results show that the channel's morphological complexity influences quantity and quality of available spawning and nursery habitats and their connectivity. Despite the presence of nursery habitats across the analyzed channel, the slow lateral larval dispersion during drift limits their accessibility to only the left river bank, making only 33% of available habitats useable. The degree of functional connectivity and hence the percentage of useable habitats can however be increased up to 95.3 % when considering different spatial configurations of habitats, which were explored in two alternative restoration scenarios. The results demonstrate the importance of considering functional habitat connectivity in habitat assessments and restoration planning.

Coupling of potential habitat models with particle tracking experiments to examine larval fish dispersal and connectivity in deep water regions.

Computing Lagrangian trajectories with ocean circulation models is a powerful way to infer larval dispersal pathways and connectivity. Defining release areas and timing of particles to represent larval habitat realistically is critical to obtaining representative dispersal pathways. However, it is challenging due to spatial and temporal variability in larval density. Forward-tracking particle experiments were conducted to study larval connectivity of four species (neritic or mesopelagic) in the Gulf of Mexico's (GoM) deep-water region. A seasonal climatology coupled with predicted potential larval habitat models based on generalized additive models was used to delimit the particle dispersal origin. Two contrasting mesoscale circulation patterns were examined: (1) high Loop Current (LC) intrusion, absence of recently detached LC anticyclonic eddies (LC-ACE), and no interaction between LC-ACEs and the semi-permanent cyclonic eddy (CE) in the Bay of Campeche (BoC), and (2) limited LC intrusion, a recently detached LC-ACE, and interaction between LC-ACEs and the BoC's CE. To simulate larval transport, virtual larvae were randomly released in the potential habitats and advected for 30 days with the velocity fields of the HYbrid Coordinate Ocean Model with hourly-resolution assimilation. Potential habitat location and size played a major role in dispersal and connectivity. A greater percentage of particles were retained in potential habitats restricted to the southern BoC, suggesting lower connectivity with other GoM regions than those encompassing most of the BoC or the central Gulf. Mesoscale feature interactions in the western GoM and BoC led to greater dispersion along the western basin. By contrast, the absence of ACE-CE interaction in the BoC led to greater retention and less connectivity between the southern and northern GoM. Under high LC intrusion, particles seeded north of the Yucatan Shelf were advected through the Florida Straits and dispersed within the GoM. Coupling potential habitat models with particle experiments can help characterize the dispersal and connectivity of fish larvae in oceanic systems.

Genetic diversity of the Ruditapes decussatus and evidence of its hybridization with the alien R. philippinarum in the Western Mediterranean Sea

The introduction of alien species in marine ecosystems is often driven by the increasing demand of fishery resources. This is the case of the Manila clam (R. philippinarum), imported in Europe from Japan since the 1970s, to meet the growing demand for clams that the native species, the grooved carpet shell clam (Ruditapes decussatus), could not satisfy. Alien species introduction could threaten the genetic diversity and integrity of the native clam, also causing hybridization (i.e., gene flow from one species into the gene pool of another). Since R. philippinarum recently spread in a few important Mediterranean coastal areas, a combined approach based on morphological characteristics, length differences of two nuclear species-specific markers (ITS2, 5SrDNA) and the sequence of the mitochondrial gene cytochrome c oxidase subunit I (COI), was used to investigate the presence of hybrids in six Mediterranean wetlands (Sardinia, Italy). Eight individuals morphologically identified as R. decussatus were hybrids, having sequences specific to both R. decussatus and R. philippinarum in their nuclear DNA (ITS2 and 5SrDNA). Most of these individuals were found to be post-first generation (F1) hybrids indicating that F1-hybrids may be fertile. Secondly, to study the genetic diversity of R. decussatus in the Sardinian wetlands as well as in its whole distribution area, >380 new COI sequences from the eastern Atlantic Ocean and Mediterranean Sea were analysed along with those available from public databases. Mitochondrial COI data revealed variable haplotype and nucleotide diversities in different areas, which were not dependent on sample sizes. The aquaculture breeding activities and clam transplantation between different countries, along with the long pelagic larval dispersal and the commercial import of other bivalve species might have promoted gene exchange between different sites and thus higher diversity levels in a few wild populations. Our research, evaluating the genetic makeup of wild and hatchery stocks and clarifying the degree of hybridization, can contribute to develop further recommendations for conserving the genetic integrity of R. decussatus.

The role of intraspecific trait variation in driving post-metamorphic survival: Implications for recruitment in open populations.

Most ecological studies attempting to understand causes of population dynamics and community structure disregard intraspecific trait variation. We quantified the importance of natural intra-cohort variation in body size and density of juveniles for recruitment of a sessile marine organism, the barnacle Semibalanus balanoides. Barnacles are representative of species organised in metapopulations, that is, as open local populations connected by larval dispersal. We tracked the individual growth and survival of a cohort of juvenile barnacles from two shores of North Wales. Barnacles settled as larvae in spring of 2002 on previously cleared rock. The density of these new recruits was experimentally manipulated in June and randomly selected individuals were monitored from June to October to evaluate the role of barnacle size and density in predicting survival. In doing so we characterised density at three spatial scales (quadrat: 25 cm2, cells within quadrats: 25 mm2 and neighbourhood: number of neighbours in physical contact with the target barnacle). At all scales, variations in juvenile body size exacerbated the effect of density-dependent mortality on population size. While density-dependent mortality was very intense in the small-sized individuals, large-sized individuals experienced very weak density-dependent mortality and showed high survival rates. Using the concept of 'Jensen inequality', we show that important biases in estimations of survival, based on population size only, occur at high barnacle densities, where survival is low. Our study highlights the role of body size variation in understanding dynamics of open populations.

Unidirectional dispersal of blow fly larvae following decomposition fluids from a pig carcass

The decomposition of a body, and the associated gaseous and liquid discharges emanating from it, attract gravid female blow flies which lay their eggs in or on the body. After the eggs have hatched, the emerging larvae start feeding on the body. As decomposition progresses, the blow fly larvae often migrate away, typically in a random manner in search of favourable conditions for pupation. In this paper, we report on a rarely described phenomenon of unidirectional mass migration of blow fly larvae and postulate on the factors which may drive this process. A decomposition trial utilising a 60-kg pig carcass, deployed in the summer months in Table Mountain National Park, Cape Town, South Africa, was conducted in 2022. On the fifth day of the trial, simultaneous unidirectional mass dispersal of blow fly larvae was observed. The larvae moved downhill in a southeasterly direction, following the flow of decomposition fluids oozing out from the pig carcass. The ‘larval migration stream’ had a length of approximately 1.5 m with a width of 40 cm, tapering to 17 cm at the terminal point. The larval migration stream consisted of the larvae of Chrysomya albiceps and Chrysomya chloropyga. This study demonstrates the importance of understanding the timing and pattern of dispersal of post-feeding blow fly larvae in each geographical region. This is crucial as the minimum post-mortem interval can be miscalculated if older immature insects dispersing from the corpse are not considered and collected during crime scene investigations.

Comparison of in situ black-lipped oyster spat collection and larval dispersal modelling results in semi-closed pearl-farming lagoons of the Tuamotu Archipelago

Spat collection of the pearl oyster Pinctada margaritifera in atoll lagoons of French Polynesia is the fundamental sustain of black pearl farming. Spat collection has always yielded variable results in space and time, but obvious signs of steady decreases, even collapses, have emerged in several lagoons. Spat collection materializes the ecological connectivity pathways between wild spawning populations and the location of artificial larval settlement substrates. To assess if oyster larval dispersal modelling could capture such pathways, we compared four six-week long spat collector deployment periods with dispersal simulations in two different lagoons. Spat collectors displayed wide spatial and temporal variations. Numerical modelling and field experiments were generally not in agreement. Although both methods have limitations, they can still approximate each other. But the accuracy of model simulations cannot be ascertained with spat collection data only. Using a SWOT (Strength-Weakness-Opportunities-Threats) analysis, we emphasize the complementarity of both approaches for management decisions.