The role of wind in controlling the connectivity of blue mussels (Mytilus edulis L.) populations

Estimating blue mussel (Mytilus edulis) connectivity and settlement capacity in mid-latitude fjord regions

The mussel industry faces challenges such as low and inconsistent levels of larvae settlement and poor-quality spat, leading to variable production. However, mussel farming remains a vital sustainable and environmentally responsible method for producing protein, fostering ecological responsibility in the aquaculture sector. We investigate the population connectivity and larval dispersion of blue mussels (Mytilus edulis) in Scottish waters, as a case study, using a multidisciplinary approach that combined genetic data and particle modelling. This research allows us to develop a thorough understanding of blue mussel population dynamics in mid-latitude fjord regions, to infer gene-flow patterns, and to estimate population divergence. Our findings reveal a primary south-to-north particle transport direction and the presence of five genetic clusters. We discover a significant and continuous genetic material exchange among populations within the study area, with our biophysical model's outcomes aligning with our genetic observations. Additionally, our model reveals a robust connection between the southwest coast and the rest of the west coast. This study will guide the preservation of mussel farming regions, ensuring sustainable populations that contribute to marine ecosystem health and resilience.

Long-term survey discloses a shift in the dynamics pattern of an emerging disease of cockles Cerastoderma edule, marteiliosis, and raises hypotheses to explain it

Drivers of marine disease outbreaks are poorly understood in spite of their growing impact. We present here results from a unique case study examining how cockles Cerastoderma edule have responded to the introduction of the novel protistan Marteilia cochillia, which led in 2012 to cockle fishery collapse in Galician rias. Based on intensive survey for eight years (2011-2019) of two affected shellfish beds, inner and outer in the Ría de Arousa, involving monthly evaluation of cockle health status and estimation of mortality, detailed information is provided of the declining impact of marteiliosis over a wild cockle population with evidence suggesting its increasing resistance. Disease dynamics involved an annual "breaking wave" of prevalence and subsequent cockle mass mortality, causing the near extinction of every recruited cohort. A shift in this pattern, from a severe epidemic towards an endemic profile, was observed in the inner shellfish bed since the cohort that was recruited in 2016, suggesting the hypothesis of increasing marteiliosis resistance through natural selection. Risk factors that may contribute to trigger marteiliosis outbreaks were analysed. Host age and sex did not influence susceptibility to marteiliosis. No clear relationships between environmental conditions (temperature, salinity and upwelling index) or cockle density and disease dynamics were found. Spatial differences in disease dynamics could be due to differences in the abundance of infective stages hypothetically linked to spatial differences in the population dynamics of a putative planktonic intermediate host. All these findings have potential implications for the management of diseased populations.

Identifying the source populations supplying a vital economic marine species for the New Zealand aquaculture industry

Aquaculture of New Zealand's endemic green-lipped mussel (Perna canaliculus) is an industry valued at NZ$ 336 M per annum and is ~ 80% reliant on the natural supply of wild mussel spat harvested at a single location-Te Oneroa-a-Tōhē-Ninety Mile Beach (NMB)-in northern New Zealand. Despite the economic and ecological importance of this spat supply, little is known about the population connectivity of green-lipped mussels in this region or the location of the source population(s). In this study, we used a biophysical model to simulate the two-stage dispersal process of P. canaliculus. A combination of backward and forward tracking experiments was used to identify primary settlement areas and putative source populations. The model was then used to estimate the local connectivity, revealing two geographic regions of connectivity in northern New Zealand, with limited larval exchange between them. Although secondary dispersal can double the dispersal distance, our simulations show that spat collected at NMB originate from neighbouring mussel beds, with large contributions from beds located at Ahipara (southern end of NMB). These results provide information that may be used to help monitor and protect these important source populations to ensure the ongoing success of the New Zealand mussel aquaculture industry.

Numerical study of hydrodynamic effects on Manila clam population distribution and transport in the Southwest Laizhou Bay, China

Pelagic larvae of marine organisms are abundant on many continental shelves. Due to their importance as a biological resource and contribution to marine biodiversity, the dynamics of pelagic larvae have received wide attention. Numerical models are an efficient approach to assessing pelagic larval transport driven by various ocean dynamics. However, few studies have focused on the pelagic larval distribution of Manila clam populations and related dynamic factors in the southwestern Laizhou Bay (SLB) during late spring. A hydrodynamic model including realistic meteorologic and oceanic boundary conditions was coupled with a larval transport model to describe the pelagic larval distribution and transport processes. The hydrodynamic model was validated with observational and satellite-derived data. Effects of hydrodynamics and biological behavior on pelagic dynamics were further simulated with numerical experiments. During the early planktonic period, most clam larvae in the SLB were transported eastward or northeastward. Two potential larval settlement areas (LSAs) were recognized: the east side of the dike outside Guangli port and the Lao River estuary. The dominant controlling factor was southerly wind, and larvae's response to river plume or estuarine circulation played a secondary role. Larval transport depended on bottom subtidal currents when late-stage larvae settled near the bottom layer. The hydrodynamic and larval transport model established in this study can be applied to other bays and coastal waters. Understanding the clam larval distribution and transport dynamics in the SLB can provide a reference for predicting LSAs and biological resource management in coastal aquaculture areas.

Genetic and particle modelling approaches to assessing population connectivity in a deep sea lobster

The emergence of high resolution population genetic techniques, such as genotyping-by-sequencing (GBS), in combination with recent advances in particle modelling of larval dispersal in marine organisms, can deliver powerful new insights to support fisheries conservation and management. In this study, we used this combination to investigate the population connectivity of a commercial deep sea lobster species, the New Zealand scampi, Metanephrops challengeri, which ranges across a vast area of seafloor around New Zealand. This species has limited dispersal capabilities, including larvae with weak swimming abilities and short pelagic duration, while the reptant juvenile/adult stages of the lifecycle are obligate burrow dwellers with limited home ranges. Ninety-one individuals, collected from five scampi fishery management areas around New Zealand, were genotyped using GBS. Using 983 haplotypic genomic loci, three genetically distinct groups were identified: eastern, southern and western. These groups showed significant genetic differentiation with clear source-sink dynamics. The direction of gene flow inferred from the genomic data largely reflected the hydrodynamic particle modelling of ocean current flow around New Zealand. The modelled dispersal during pelagic larval phase highlights the strong connectivity among eastern sampling locations and explains the low genetic differentiation detected among these sampled areas. Our results highlight the value of using a transdisciplinary approach in the inference of connectivity among populations for informing conservation and fishery management.