Scallop Stock Research Articles

Brief results of research on scallops off the northern Kuril Islands in autumn 2022

The purpose of the work: to collect materials for assessing the state and dynamics of scallop stocks in the North Kuril zone, to study the spatial structure of settlements, to monitor the dynamics, distribution and composition of commercial catches in 2022.Materials and Methods: The work was carried out on a specialized rowing vessel “Vladimir Nikora” performed 210 dredging in the area limited by coordinates 49°20'N 49°30'N. and 154°57'E154°53'E at depths of 79-170 meters. The data collection tool is an industrial scallop dredge with a working width of 3.7 m (scallop dredge 1.04 / 3.74 m). The material type of the scallop dredge bag is kapron. The length of the dredging was 0.4-0.5 miles. The research covered the water area of 23 square meters. miles. Biological analysis was carried out for 5 thousand specimens of chlamy scallops. The biological state of hydrobionts was studied according to the methods generally accepted in fisheries research. For comparability of data on the distribution of hydrobionts, the catchability coefficient of the scallop dredge was used equal to 1. The linear dimensions of the scallops were measured with an accuracy of 1 mm.The practical significance of the research: to provide commercial and biological information materials on the total allowable catches of scallops in the North Kuril.

Recovery trajectories and management responses for three scallop stocks over ten years following an extreme marine heatwave in Western Australia

Saucer scallop, Ylistrum balloti, fisheries along central Western Australia was a key region impacted by the 2010/11 extreme marine heatwave (MHW) when mean sea surface temperature anomalies reached up to 5°C between December and March. The extreme intensity and duration of this event had adverse impacts on survival of larval, juvenile, and adult scallops in the three major stocks in Shark Bay and at the Abrolhos Islands. The severe decline, observed from low commercial landings and verified through fishery-independent surveys required strong and rapid management response, and so in consultation with industry these commercial fisheries were closed to protect the remaining population and allow stock recovery. This provided an opportunity to examine other factors, in the absence of fishing, that may impact recovery. Fishing recommenced in Shark Bay after 3.5 years with one of the two stocks recovering more slowly whilst fishing recommenced in the Abrolhos Islands after 5 years. Differences in recovery rates between regions may be attributed to differences in life-history dynamics which vary with latitude. In Shark Bay, the austral summer is pre-spawning with peak spawning in the autumn/winter and therefore higher temperatures negatively impacted the spawning stock directly. In the Abrolhos Islands however, scallops have already spawned by summer and therefore the larvae and/or the early juveniles were impacted as well as post-spawned adults. Post 2011 MHW, continued warmer ocean conditions were experienced, followed by 4 years (2016-2019) of a “marine cold spell” then “short-lived” moderate MHWs during the summers of 2019/20 to 2021/22. Each stock had, over these years, responded differently to the conditions and fishing impacts and management responses also varied. We describe the science, management and industry response to severe stock declines, recovery rates, the drivers of recruitment and recovery and effects of recent summer MHWs on the current scallop stock status. Secondly, we describe the management arrangements and harvest strategies implemented. We highlight the importance of pre-season stock monitoring that provide a basis for catch/recruitment prediction and adaptive harvest strategies to ensure timely responses to stock declines in the event of extreme events expected to become more frequent in a changing global climate.

Effect of water mass mixing on phytoplankton dynamics in the scallop culture areas off Okhotsk Sea: A microcosm experiment

The nearshore (Soya Warm Water (SWW)) and offshore (Intermediate Cold Water (ICW) and Okhostk Sea Low Salinity Water (OSLSW)) waters in the Okhotsk Sea differ substantially in their physico-chemical characteristics. Thus, the extent to which water mass mixing impacts phytoplankton dynamics in the Japanese scallop Patinopecten yessoensis culture areas needs to be assessed to properly manage scallop stocks. This study examined the phytoplankton dynamics in nearshore (SWW) and offshore (ICW and OSLSW) waters and phytoplankton response to water mass mixing (SWW + ICW, OSLSW + ICW) for 5 days in spring (2007) and summer (2007–2009) microcosm experiments. Increased chlorophyll a concentration in ICW was observed due to higher nutrients available for phytoplankton uptake than in OSLSW and SWW at the start of each microcosm experiment. In comparison to spring microcosm, mixing of SWW + ICW promoted higher chlorophyll a concentration and faster Si(OH)4 utilization than in OSLSW + ICW in summer. In addition, the size structure of chlorophyll a differed from micro-size (>10μm) in SWW + ICW to nano-size (<10μm) chlorophyll a in OSLSW + ICW. Suspended particulate organic matter (SPOM) δ13C ratios were positively correlated with chlorophyll a in both mixed microcosms whereas SPOM δ15N isotopic ratios were inversely related to nitrate concentration in SWW + ICW, except in spring. This suggests that stable isotopes can be used in predicting phytoplankton biomass and nutrient utilization. In comparison to microcosm experiments, nearshore water column structure suggests vertical water mixing in summer to autumn when phytoplankton biomass and micro-size fraction were at maximum. Taken together, the advance of nutrient-rich ICW to nutrient-poor nearshore areas in the Okhotsk Sea resulted in increased chlorophyll a biomass and dominance of micro-size phytoplankton in summer, potentially enhancing nutrient and food supply to bottom-cultured scallops as suggested by both microcosm experiments and field observations.

Biological Aspects, Fisheries, and Aquaculture of Yesso Scallops in Russian Waters of the Sea of Japan

Scallops are bivalve filter-feeding mollusks that can attain a large size and have great importance in terms of their ecological roles and commercial value. Overfishing has led to a decrease in scallop stocks worldwide, leading to intense development of the aquaculture industry. The latter is well-established in Asian countries in the East Sea and Sea of Japan. In this paper, we summarized information regarding the biology, stock dynamics, and fishery of yesso scallops inhabiting Russian waters of the Sea of Japan. This species has relatively rapid growth rates and reaches a marketable size of 100 mm in shell height at age 3–5 years. In Russian waters, the total commercial stock of yesso scallops in 2021 was estimated at 2784 t. Commercial fisheries have been banned since 2020 due to the depletion of natural stocks as a result of illegal fishing and overexploitation. The total allowable catch for yesso scallops has been set at 3 t for monitoring and recreational purposes. The cultivation of the scallops includes spat collection, spat intermediate culture, and final grow-out on the seafloor or in suspended cages. In the past decade, this industry has demonstrated rapid growth and currently accounts for &gt;16,000 t per year. Environmental fluctuations and epizootics seem to be the most important challenges for the scallop aquaculture sector in the Sea of Japan.

Beach strandings of Tehuelche scallop, Aequipecten tehuelchus, in Patagonia: Multi-scale processes and management implications

The magnitude and causal mechanisms of a massive beach stranding of Tehuelche scallops that occurred in November 2017 in San José Gulf, Argentina, were investigated with the long-term goal of improving the assessment and management of the scallop fishery. The biomass of scallops washed ashore and deposited over a 10-km stretch of coast was estimated by quadrat sampling and compared with the results of a scallop stock assessment survey conducted three months prior to the stranding event. The resulting estimate of total biomass loss was in the order of 200 t, representing 10% of the estimated total scallop biomass in the San José gulf. The stranding coincided with persistent strong southerly winds (13 m/s) blowing for 24 h in San José Gulf, and large-scale windstorms that affected the southern tip of South America. Surface waves predicted under such windstorm conditions could generate strong bottom orbital velocities at shallow waters (<10 m depth), sufficient to drag and transport ashore scallops by Stokes drift (600–2000 m in 24 h). Analysis of local wind data recorded over a 6.8-year period indicated that such windstorm conditions occurred with an average frequency of 7.7 times per year, implying that beach strandings could have a significant impact on the scallop resource and its fishery. The actual impact of windstorms would depend on the location, depth and size composition of scallop beds, shallow beds (<10 m depth) being more susceptible to stranding risks. The use of spatial harvest control rules, instead of the global total allowable catch used at present, could reduce the risks of yield loss by directing the harvest to the more vulnerable scallop beds.

Quantitative Analysis of the Fishery-Independent Queensland Saucer Scallop (Ylistrum balloti) Trawl Survey

The Queensland saucer scallop Ylistrum balloti fishery is a significant component of the Queensland East Coast Otter Trawl Fishery. In 1996, the saucer scallop stock collapsed, and by 2006, had partially recovered. In 2016, the stock again collapsed, when it was declared, and remains, overfished. In response to the collapses, fishery managers implemented an annual fishery-independent trawl survey of scallop abundance, although funding for the survey has been inconsistent. This study examined 8 y of discontinuous fishery-independent scallop trawl survey data (1997 to 2000, 2017 to 2020). A two-stage approach was introduced to firstly calibrate catch rates for differences between survey vessels for each year, and then derive adjusted mean densities (number ha–1) for the 0+ and 1+ scallop age classes and total scallops, for each year and strata. Quasi-Poisson generalized linear models were used for both stages. Results showed that adjusted mean total scallop densities declined by approximately half from the early survey years (1997 to 2000) to the later survey years (2017 to 2020). Adjusted mean total scallop density and adjusted mean densities of the 0+ and 1+ age classes were lowest in 2020. The 1+ age class numerically dominated survey catch rates.

Using satellite remote sensing to improve the prediction of scallop condition in their natural environment: Case study for Georges Bank, Canada

The continuous, synoptical and high spatio-temporal resolution of thermal and visible satellite observations constitute an asset when characterizing and monitoring biogeochemical cycles in the oceans. In particular, they provide a unique insight into the hydrodynamics of the surface ocean and phytoplankton phenology. This information can be combined with in-situ observations of higher trophic levels to understand the functioning of the marine ecosystem, with potential use for fisheries and aquaculture. In the Gulf of Maine, off the eastern seaboard of North America, Georges Bank is one of the world's most productive natural sea scallop (Placopecten magellanicus) fisheries, which sustains economically valuable fisheries in Canada and the US. In Canada, the relationship between the scallop meat weight and its shell height (known as scallop condition) is used to obtain biomass estimates for stock assessment and thereby influences the fishing quotas. On Georges Bank, for a 100 mm shell height scallop, scallop condition has varied significantly interannually between 10.2 g and 19.5 g. Establishing a relationship between environmental factors and scallop condition would improve the forecasting of scallop biomass and reduce the uncertainty when determining the total allowable catch for the subsequent fishing season. In this study, satellite remote-sensed sea-surface temperature (SST) and chlorophyll-a concentration (Chla) coupled with in-situ measurements of scallop condition (SC) on Georges Bank were used to assess the relationship between SC and satellite-derived environmental conditions between 1985 and 2019 for SST and between 1998 and 2019 for Chla. The inter-annual variation of SC in May was found to be positively correlated (p-value <0.001) with the variation in SST in winter-spring months of the current and previous year of the SC survey. An empirical linear model for the estimation of SC was developed and validated using two independent datasets of satellite SST (r2 = 0.64, MAPD = 7%). Another model using cumulative SST in the previous year only was also provided (r2 = 0.58, MAPD = 7.7%), which could serve as an alternative choice when the SST products in the current year are not yet available for use. There was a weak relationship observed between SC and Chla; this is likely due to the influence of water-column vertical mixing on transport of phytoplankton to the benthic zone where the sea scallop resides. Our study indicates that a cost-effective satellite-derived SST product can be used to improve predictions of SC and provide useful information and decision support tools for managing scallop stocks.

Automatic counting methods in aquaculture: A review

AbstractObject counting in aquaculture is an important task, and has been widely applied in fish population estimation, estimation of lobster abundance and scallop stocks, and so forth. However, underwater object counting is challenging for biologists and marine scientists because of the diversity of backgrounds of the lake or ocean, the uncertainty of the object motion, and the occlusion between objects. With the rapid development of sensor, computer vision, and acoustic technologies, advanced and efficient counting methods are available in aquaculture. We reviewed underwater object counting methods in aquaculture, provided a survey including more than 50 articles in the recent 10 years, and analyzed the pros and cons of the counting methods and the applicable scenarios of those methods. Finally, the major challenges and future trends of underwater object counting in aquaculture are discussed.

Estimation of the capture efficiency and abundance of Atlantic sea scallops (Placopecten magellanicus) from paired photographic–dredge tows using hierarchical models

The efficiency of survey gear is an important measure that can be used to estimate the absolute scale of populations in assessment models. We develop a general hierarchical model for estimating the efficiency of a New Bedford-style sea scallop (Placopecten magellanicus) survey dredge from paired dredge and Habcam camera system tows. Habcam data for each tow consist of multiple images that give information on within-tow variability of scallop density. The model accounts for these multiple observations as well as the possibility of differences between the true densities in dredge and Habcam pairs due to the pairs not covering exactly the same ground. We fit several models with alternative assumptions to observations on Atlantic sea scallops and compare the relative performance using Akaike’s information criterion. The best performing model estimated higher dredge efficiency on fine substrates than on coarse ones (approximately 0.40 and 0.27, respectively). Our results inform the scale of annual abundance estimates from dredge surveys and reduce uncertainty in the sea scallop stock assessments.

Pigment profiling of ‘QN Orange'scallop and ‘Bohai Red’ scallop adductor muscles by liquid chromatography coupled with triple quadrupole mass spectrometry

Two new scallop strains, the ‘QN Orange’ scallop and the ‘Bohai Red’ scallop, were selected from the hybrid cohort between the bay scallop (Argopecten irradians irradians) and the Peruvian scallop (Argopecten purpuratus) to improve the bay scallop stock, which has suffered from significant inbreeding depression characterized by poor growth and high mortality. Although both strains were originated from the same hybrid cohort, the ‘QN Orange’ scallops possess orange adductor muscles while the ‘Bohai Red’ scallops have white adductor muscles. To explore the pigment profiles of adductor muscle coloration in these two scallop strains, an optimized method based on liquid chromatography coupled with triple quadrupole mass spectrometry (HPLC-QqQ-MS) with atmospheric pressure chemical ionization (APCI) was established to determine chlorophyll (chlorophyll a and chlorophyll b) and carotenoids (including α-carotene, β-carotene, ε-carotene, γ-carotene, ζ-carotene, lutein, fucoxanthin, zeaxanthin, canthaxanthin, pectenoxanthin, and pectenolone). The results showed that the total contents of carotenoids in the adductor muscle, gill, and mantle of ‘QN Orange’ scallops were 82.68, 105.49, and 100.49 μg g−1, which were 49.88, 3.26, and 4.18 times higher than those in the ‘Bohai Red’ scallops, respectively. In particular, the contents of the pigments pectenolone, pectenoxanthin, fucoxanthin, zeaxanthin, and lutein in the ‘QN Orange’ scallops were 171.58, 61.67, 3.87, 22.16 and 23.27 μg g−1, which were 6.92, 3.38, 27.64, 15.75, and 13.00 times higher than those in the ‘Bohai Red’ scallops, respectively. These results suggest that the orange coloration in the ‘QN Orange’ scallops might be caused by the accumulation of carotenoids.

Resource Abundance, Fisheries Management, and Fishing Ports: The U.S. Atlantic Sea Scallop Fishery

The Atlantic Sea Scallop fishery has grown tremendously over the past twenty years. The location and magnitude of harvestable biomass fluctuates dramatically due to both natural variation and the explicitly spatial management system designed to allow small individuals to grow larger and more valuable. These fluctuations in natural advantages can have profound effects on fishing ports. We use methods from economic growth literature to show that ports with lower initial scallop landings have grown the fastest. Furthermore, good access to biomass influences long-run changes in landings, although this effect exhibits considerable variability across ports. We also find evidence of returns to scope, suggesting that ports with other fishing activities could be well positioned to attract new scalloping activity when stock conditions are favorable. Further investigation of the largest ports using time-series methods also shows a high degree of variability; there are long-run relationships between scallop fishing and harvestable scallop stock in some ports, short-run relationships in some ports, and no relationship between the two in others. We interpret this as evidence that heterogeneity in the natural productivity of the ocean combined with explicitly spatial fisheries management has induced a spatial component to the port-level response to changes in biomass availability.

Structure and long-term dynamics of zoobenthos communities in the areas of scallop Chlamys islandica beds at Kola Peninsula

Current state of zoobenthos in the areas of scallop Chlamys islandica beds in the southern Barents Sea and eastern White Sea (Voronka) is described. The scallop stock is ex­ploited there with the landing by specialized vessels, without separation of catch, so bycatch of other species is considerable. As the result, significant changes occurred in structure of benthic communities during the times of scallop fishery. To assess the fishery effect, species composition of benthos, its dominant species and trophic structure is considered separately for 4 regions with different degree of exploitation. Dynamics of zoobenthos abundance is traced over the last 25 years. Changes in trophic structure of the benthic community at Cape Svyatoy Nos (southern Barents Sea) are analyzed in details. Although up to ¼ of its total biomass is presented by carnivorous species and the biomass of C. islandica has decreased, the portion of sestonophages has increased in this community. Recently a recovery of zoob­enthos is observed in its southern part, supposedly because of the animals redistribution from adjacent areas unaffected by fishery. On the contrary, extremely low biomass of zoobenthos, in particular scallop, is observed in the northern part of the community. Biomass of benthos in the Voronka area is high, but it is formed mainly by mussels. There is concluded that more than a decade is required for full recovery of the scallop stock and accompanying zoobenthos communities to the native state.

Modeling larval dispersal and connectivity for Atlantic sea scallop (Placopecten magellanicus) in the Middle Atlantic Bight

Larval Atlantic sea scallops (Placopecten magellanicus) simulations in the Middle Atlantic Bight (MAB) from 2006 to 2012 were performed to investigate annual and inter-annual dispersal and connectivity patterns among stock regions. These simulations used a circulation model based on the Regional Ocean Modeling System (ROMS) and an individual-based larval model (IBM) that included larval behavior. The circulation model used realistic dynamical forcing (e.g., winds, tides, and open ocean boundary conditions), thermo-dynamical fluxes (e.g., solar radiations, sensible and latent heating), and hydrological forcing; the larval IBM included vertical swimming and sinking behaviour, temperature-dependent growth, and settlement. Simulated larvae that reach settlement size and suitable habitat in 45 days are considered ‘successful’, and two regions are considered ‘connected’ by larval dispersal when larvae successfully disperse from one region to the other. In general, simulated larval dispersal patterns varied seasonally (28% higher in September and October compared to May and June), among years (2007 through 2009 had 5% lower larval success during August and September compared to other years), and spatially, with larvae released from the northern regions like Long Island acting as a substantive larval source with 14% greater dispersal success and 15% greater connectivity with other regions than those released elsewhere. Over the seven years simulated, the MAB scallop stocks showed high rates of connectivity to regions to the south and more limited and variable connectivity to regions to the north. In species like sea scallops with limited adult mobility, larval dispersal supplies recruits, enables range expansion, and connects populations. Thus, appreciation of dispersal patterns are essential for fishery management of this economically valuable stock.

A spatiotemporal Evaluation of Atlantic Sea Scallop Placopecten magellanicus Habitat in the Gulf of Maine Using a Bioclimate Envelope Model

AbstractA bioclimate envelope model was developed to evaluate the impacts of climate variability on spatiotemporal availability of suitable habitat for the Atlantic sea scallop Placopecten magellanicus in the Gulf of Maine (GOM). Bioclimate envelopes were established through habitat suitability indices (HSIs) based on bottom temperature, bottom salinity, current velocity, depth, and bottom composition. The relationship between Atlantic sea scallop abundance and each environmental variable was quantified using suitability indices, which were generated based on standardized scallop abundance sampled over 10 years of dredge survey data. Boosted regression tree models were used to determine the relative importance of each environmental variable to scallop abundance, thereby establishing a weighting scheme within the HSI. A regional circulation model was coupled with the weighted HSI to hindcast spatiotemporal dynamics of suitable habitat for Atlantic sea scallop in coastal and offshore waters of the GOM from 1978 to 2013. Higher habitat suitability was found along inshore areas compared with offshore areas. Model predictions indicated an increasing trend in habitat suitability in inshore waters since 1978 and decreasing habitat suitability in offshore waters. This research provides a novel modeling framework with which to enhance research and management of commercially valuable Atlantic sea scallop stocks over broad spatiotemporal scales in the climatically altered GOM.

Evidence of clandestine harvest and failure of conservation policies for Argopecten purpuratus in the Rinconada Marine Reserve (Chile)

Abstract The study of Argopecten purpuratus reproduction, post‐larval settlement, stock size, and population size structure and shell growth was undertaken in the 2000s in the Rinconada marine reserve (Chile) to evaluate the effectiveness of scallop recruitment and self‐sustainability. The results highlight strong seasonal and inter‐annual variations of environmental conditions and scallop gonadosomatic index, spat collection, benthic distribution, total abundance and population size structure. The Von Bertalanffy growth parameters were L∞ = 120.12 mm and K = 0.9681; commercial size would be reached in about 17.2 months in the bay. Substrate availability, meteorological conditions, hydrodynamics and illegal harvesting explain spatial and temporal variations in scallop distribution and abundance. Recruitment strength depends on one main cohort from year to year. While recruitment made up 81% and 94% of total abundances in May 2002 and May 2003, respectively, there were no overall density‐dependent relationships between stock size and recruitment. Spawning asynchrony in the bay supports the hypothesis that multiple gamete releases form part of a reproductive strategy in response to environmental variability. The massive disappearance of large‐sized scallops during the study periods was direct evidence of overfishing owing to clandestine harvesting within the marine reserve. The failure of current legislation, strategies and policies for scallop conservation requires new regulations to restore scallop stock size, maintain its reproductive performance and limit illegal harvesting in the Rinconada. A scenario allowing harvesting regulated by stock‐dependent fishing quotas would more efficiently ensure stock recovery and self‐sustainability. The modalities of this new policy are discussed.

Is an Apicomplexan Parasite Responsible for the Collapse of the Iceland Scallop (Chlamys islandica) Stock?

Due to the total and unexpected collapse of the Iceland scallop, Chlamys islandica, stocks around Iceland during the 2000s, a commercial fishing ban has been imposed on this valuable resource since 2003. Following the initial identification of an apicomplexan parasite in the scallops, a long-term surveillance program was established to evaluate the effect of the parasite on the population. The infections were highly prevalent in all shell sizes throughout the study. However, the parasite only impacts mature scallops where they cause severe macroscopic changes, characterized by an extensively diminished and abnormally coloured adductor muscle. A highly significant relationship was observed between infection intensity and gonad and adductor muscle indices. The first four years of the study, were characterized by high infection intensity and very poor condition of the adductor muscle and gonads, whilst during subsequent years, infections gradually decreased and the condition of the scallops improved. Histopathological changes were restricted to the presence of apicomplexan zoites which were widely distributed, causing varying degrees of pathology in all organs. In heavy infections, muscular and connective tissues were totally necrotized, destroying significant parts of numerous organs, especially the adductor muscle, digestive gland and gonads. The progression of the disease was in good synchrony with the mortality rates and the subsequent decline observed in the scallop stock and recruitment indices. Our findings strongly suggest that the apicomplexan parasite played a major role in the collapse of the Iceland scallop stock in Breidafjordur. In addition to causing mortality, the infections significantly impact gonad development which contributes further to the collapse of the stock in the form of lower larval recruitment. Furthermore, compelling evidence exists that this apicomplexan pathogen is causing serious disease outbreaks in other scallop populations. Similar abnormal adductor muscles and the parasite itself have been identified or observed in association with other mass mortality events in several different scallop species and commercial stocks in the northern hemisphere.

Calico Scallop Argopecten gibbus Abundance on the Cape Canaveral Bed and on Florida's Gulf of Mexico Shelf

AbstractAnnual landings of the calico scallop Argopecten gibbus fishery in the southeastern United States increased from less than 3,000 kg in 1959 to 19.5 million kg of adductor muscle meat in 1984. The fishery began to collapse in early 1986 in Florida and fell below 1 million kg/year in the mid‐1990s. From 2002 through 2011, the only reported landings were 550 kg in 2009, but annual landings of less than 200,000 kg beginning in 2012 indicate the resource rebounded enough in that year to create renewed interest in the fishery. When the fishery was developing, the National Marine Fisheries Service (NMFS) sponsored 59 cruises (1956 through the mid‐1970s) to map the distribution of the stock and found the calico scallop stocks on Florida's east coast principally occurred around the 40‐m depth contour near the shelf break. By the early 1980s, surveys were no longer routinely conducted. Two surveys in the mid‐1990s were dedicated to investigating the collapse of the stock. The present study consisted of eight cruises from 2004 to 2006: four each to the beds off Cape Canaveral on the eastern coast and the poorly documented beds off southwestern Florida. Cruises studying baitfish and cruises in the Gulf of Mexico by the Southeast Area Monitoring and Assessment Program (SEAMAP) yielded data that provided additional calico scallop abundance estimates. Abundant, albeit small, scallops were found on both coasts during three of the four cruises to each coast though none were detected in fall 2004 on either coast. Our results concur with those historic NMFS surveys conducted prior to the period of heaviest exploitation of the resource (1974–1999); the center of distribution for calico scallops on the Cape Canaveral beds still occurs near the 40‐m depth contour. The essential habitat for calico scallops, shell base, which was suspected to have declined during the most active fishery periods, was found to have persisted consistently at that depth. There were few scallops with a shell height greater than 40 mm, the minimal size considered acceptable for commercial exploitation.Received July 9, 2015; accepted September 15, 2015

Distribution of the non-indigenous colonial ascidian Didemnum vexillum (Kott, 2002) in the Bay of Fundy and on offshore banks, eastern Canada

The invasive colonial ascidian Didemnum vexillum (Kott, 2002) was initially discovered in the fall of 2013 near Parrsboro, Nova Scotia. A rapid response survey was conducted in April 2014 to confirm the presence of the species and to determine its distribution near the original detection site. Subsequent surveys between May and August 2014 examined the dredge contents collected during sea scallop stock assessment surveys on German Bank, northern Browns Bank, eastern Georges Bank and in the Bay of Fundy region. The presence of D. vexillum was confirmed at 22 of 30 stations sampled in four areas of the Minas Basin and Minas Channel region in the northern Bay of Fundy during the rapid response survey. The scallop surveys confirmed the presence of D. vexillum at 9 of 829 stations sampled in the Bay of Fundy region, including 7 in the Minas Basin, 1 off Digby Gut and another off the coast of Yarmouth. Due to the presence of a native species, D. albidum, in the region, a PCR assay was developed to distinguish D. vexillum from all other species in the region. Once the PCR assay was validated this assay was used to confirm all positive identifications in this study. Colonies overgrew rocks, bivalve shells and seaweeds or were retrieved as large dislodged fragments. They were in an overwintering state in April, but healthy, and observed to grow into dense mats in summer. In other regions of the world, D. vexillum has been reported to foul shellfish and aquaculture gear, smother benthic organisms such as the sea scallop (Placopecten magellanicus), and overgrow substrates, suggesting this new colonial invasive ascidian poses a potential threat to Eastern Canada aquaculture and commercial benthic fisheries.

Yield trait improvement of bay scallops following complete diallel crosses between different scallop stocks

This study describes the complete diallel hybridization between newly introduced bay scallop stock (W) from Canada and local commercial stock (D) grown under laboratory conditions, in China. Larval survival and growth during all life stages (larvae, spat, and adult) were compared among hybrid (DW, WD) and purebred (DD, WW) populations. Significant heterosis was detected for survival during the larval stage (> 1% of the mid-parent values). The mean heterosis (H (m)) varied in growth throughout the life span. More than 50% of the H (m) values were positive and negative in the DW and WD groups, respectively. The influence of maternal effects and mating types (intrapopulation vs. interpopulation crosses) on growth for all life stages was not consistent. Larval survival did not differ significantly (P > 0.05) with maternal effect or mating type. In the harvest stage, shell length (SL), shell height (SH), shell width (SW), and total weight (TW) were larger in the hybrid compared with the inbred groups. Positive H (m) values were observed in SL (1.5%), SW (5.8%), and TW (12.3%), and were more significant in the DW groups (6.1%, 4.5%, 6.8%, and 27.2%). These results suggest that hybridization between two geographic populations is a good tool for improving bay scallop growth. However, unstable heterosis between the two populations requires further study.

Modelling larval dispersal of the king scallop (Pecten maximus) in the English Channel: examples from the bay of Saint-Brieuc and the bay of Seine

The king scallop (Pecten maximus) is one of the most important benthic species of the English Channel as it constitutes the first fishery in terms of landings in this area. To support strategies of spatial fishery management, we develop a high-resolution biophysical model to study scallop dispersal in two bays along the French coasts of the English Channel (i.e. the bay of Saint-Brieuc and the bay of Seine) and to quantify the relative roles of local hydrodynamic processes, temperature-dependent planktonic larval duration (PLD) and active swimming behaviour (SB). The two bays are chosen for three reasons: (1) the distribution of the scallop stocks in these areas is well known from annual scallop stock surveys, (2) these two bays harbour important fisheries and (3) scallops in these two areas present some differences in terms of reproductive cycle and spawning duration. The English Channel currents and temperature are simulated for 10 years (2000–2010) with the MARS-3D code and then used by the Lagrangian module of MARS-3D to model the transport. Results were analysed in terms of larval distribution at settlement and connectivity rates. While larval transport in the two bays depended both on the tidal residual circulation and the wind-induced currents, the relative role of these two hydrodynamic processes varied among bays. In the bay of Saint-Brieuc, the main patterns of larval dispersal were due to tides, the wind being only a source of variability in the extent of larval patch and the local retention rate. Conversely, in the bay of Seine, wind-induced currents altered both the direction and the extent of larval transport. The main effect of a variable PLD in relation to the thermal history of each larva was to reduce the spread of dispersal and consequently increase the local retention by about 10 % on average. Although swimming behaviour could influence larval dispersal during the first days of the PLD when larvae are mainly located in surface waters, it has a minor role on larval distribution at settlement and retention rates. The analysis of the connectivity between subpopulations within each bay allows identifying the main sources of larvae which depend on both the characteristics of local hydrodynamics and the spatial heterogeneity in the reproductive outputs.