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Marine invaders, including the green crab (Carcinus maenas), cause widespread disruption in their invaded ranges. In a Marine Protected Area in Atlantic Canada, the concurrent collapse of a protected strain of Irish moss (Chondrus crispus) and establishment of green crabs prompted a removal program in 2016–2018. We evaluated the outcome of that program using surveys within and outside (control) the MPA that preceded (2015 and 2016) and ran parallel to the removal program. We used generalized linear models to assess effects on catch rates and demographics and then compared those results to the outcome of other removal programs using the framework of recruitment and growth overfishing. Green crab abundance increased between 2015 and 2017 despite removal efforts, but the increase of both males and females was suppressed compared to control sites, to 29% and 36% of expected numbers, respectively. Between 2017 and 2018, males became significantly smaller relative to control sites. Although green crab eradication is not achievable, benefits would accrue if recruitment overfishing could suppress populations to levels that could be maintained by modest harvesting. Comparing among studies showed that removal did not reduce populations or that supressed populations could rebound quickly. However, within-year population reductions and shifts to smaller sizes, which are consistent with growth overfishing, show that removal programs may achieve short-term/local benefits but seem unable to sustainably suppress populations. At larger scales, the green crab's status as one of the most resilient invaders is likely to remain. [ABSTRACT FROM AUTHOR]

A robust assessment of the American eel (Anguilla rostrata) stock, required to guide conservation efforts, is challenged by the species' vast range, high variability in demographic parameters and data inadequacies. Novel ideas and underutilised resources that may assist both analytic assessments and spatially oriented modelling include (1) species and environmental databases; (2) mining of data from scattered sources; (3) infilling of data gaps by spatial analysis; (4) age estimation from measurements of DNA methylation; evaluation of eel abundance by (5) larval, (6) glass‐bottom boat, (7) net enclosure and (8) eDNA surveys; (9) accounting for dam‐induced habitat increases in eel watercourse modelling; (10) spatially oriented modelling with and without temporal components; (11) geographically nested modelling of glass eel recruitment; (12) spawner per recruit modelling and (13) life cycle modelling to examine larval allocation effects. Eel biologists are too few to gather the required assessment data across all of the species' range. Public posting of electrofishing and eDNA metabarcoding data sets and the use of machine learning techniques to comprehensively inventory small dams will help meet some data needs. These approaches address only a small proportion of the assessment challenges that face American eels. Worldwide collaboration amongst Anguilla scientists is a key enabler of progress towards stock assessment goals. [ABSTRACT FROM AUTHOR]

Oyster recruitment to spat collectors can result in the clumping of two or more individuals, whereby they naturally cement to each other's shells. On‐bottom oyster farmers typically separate these oysters and re‐deploy them on the seafloor, potentially exposing them to predators. This raises the question of whether refraining from separating these oysters might provide an advantage against predation. We addressed this question using eastern oysters (Crassostrea virginica) and the non‐indigenous European green crab (Carcinus maenas) from part of their invaded range in Atlantic Canada. We assessed the influence of oyster arrangement (single vs. paired) and oyster size (small‐ vs. mid‐size) on mortality caused by two sizes of this invasive predator. Mortality rates were assessed with 24 h trials in which 30 oysters of a given size and arrangement were exposed to green crab predation. We found a significant influence of oyster arrangement (paired

Sustained periods of anoxia, driven by eutrophication, threaten coastal marine systems and can lead to mass mortalities of even resilient animals such as bivalves. While mortality rates under anoxia are well‐studied, the specific mechanism(s) of mortality are less clear. We used a suite of complementary techniques (LT50, histology, 16S rRNA amplicon sequencing, and valvometry) to show that the proliferation of anaerobic bacteria within eastern oysters (Crassostrea virginica) accelerates mortality rate under anoxic conditions. Manipulative laboratory experiments revealed that oyster survival under anoxic conditions was halved when bacteria were present compared to when they were excluded by the broad‐spectrum antibiotic chloramphenicol. Histological assessments supported this mechanism and showed infiltration of bacteria in oysters that were not treated with antibiotics compared to a general lack of bacteria when oysters were treated with antibiotics. 16S rRNA amplicon sequencing failed to identify any particular genera of bacteria responsible for mortality, rather a diversity of endogenous anaerobic and/or sulfate‐reducing bacteria were common among oysters. In addition, monitoring of oyster valve gaping behavior in the field revealed that oysters showed remarkable valve closure synchrony when first exposed to anoxia. However, oysters periodically opened throughout anoxia/hypoxia in both the lab and field, suggesting that the infiltration of exogenous bacteria from the environment may also influence mortality rates under natural settings. Coupled with previous studies, we posit that mass mortality events in a wide range of coastal bivalves are likely the result of co‐morbidity from asphyxiation and bacterial processes. [ABSTRACT FROM AUTHOR]

The American eel (Anguilla rostrata) occupies a vast range in the West Atlantic Ocean and inflowing waters. Despite its presumed panmictic status, management of this species is geographically fragmented. There have been widespread calls for internationally coordinated efforts towards a range-wide stock assessment, but such an objective faces obstacles of a high degree of heterogeneity in major life history characteristics and the near-absence of such data in the northern, western, and southern parts of the species' range. This paper reviews novel and underutilized methods and data sources that may aid progress to an eventual range-wide assessment. Methods for obtaining information on distribution and abundance include mining of extant data and field surveys by glass bottom boat, electrofishing boat, net enclosures, environmental DNA (eDNA) and ocean larval tows. Analytic resources and tools include environmental databases, fetch as a covariate of abundance, accounting for the net effects of small ponds, GIS-oriented habitat modeling, glass eel-oriented population modeling, estimation of age structure from length structure, use of life history parameter clines to fill gaps in assessment input values, and life cycle modeling. eDNA is a cost-effective technique that has the potential to clarify American eel distribution, and possibly relative abundance, over the very large areas where data for this species are sparse. Most other techniques reviewed in this paper have substantial costs, which may constrain their use in the southern part of the American eel range where resources of aquatic science are often limited. [ABSTRACT FROM AUTHOR]

Eutrophication, the biological response to nutrient enrichment, is a well-studied process that poses a threat to coastal ecosystems around the world. Despite an understanding of the mechanisms that contribute to nutrient impacts, there is no universal method for their quantification or monitoring. Herein, a monitoring indicator is presented and evaluated for estuaries that empty into the southern Gulf of St. Lawrence. Two monitoring indicators were assessed, the first evaluating responses of the regionally dominant seagrass, eelgrass (Zostera marina), and the second examining changes in dissolved oxygen (DO). Eelgrass coverage as a percentage of available habitat was previously quantified for each estuary and related to a suite of environmental variables. The only variable for which a significant relationship was found was nitrogen loading which explained 53.0% of the variability in eelgrass coverage using a nonlinear response function. The relationship shows that -63% of available habitat occupied is the maximum for estuaries within the region. Interpolations using that relationship show eelgrass declines correspond to estuarine external nitrate-N. Evidence is also presented that DO is suited to be a primary indicator of nutrient impacts in systems with high levels of both benthic and pelagic primary production. DO loggers were deployed in the upper estuary, defined by a salinity range of 15-25 PSU and an average water depth of 1-2.5 m of 27 distinct estuaries from 2013-2020 across the southern Gulf of St. Lawrence. Time series data were parsed into biologically relevant metrics that captured symptoms of eutrophication: hypoxia, dissolved oxygen supersaturation, and coefficient of variability. These metrics were analyzed using multivariate statistics and distinct groups formed around habitat type, with algae-dominated habitat reflecting these eutrophication symptoms. The combination of hypoxia and dissolved oxygen supersaturation metrics led to a new metric termed "Eutrophic Time" (the percentage of time an estuary spends <4 mg/L and >10 mg/L dissolved oxygen) which effectively captured estuaries with a tendency towards hypoxia and those towards supersaturation. This new measure was then analyzed through multiple regression with nitrate-N loading and water residence time as predictor variables which were ultimately well correlated (R2 = 0.81). This relationship can be exploited to provide estuary-specific nitrate-N loading targets for nutrient impacted systems at management's discretion. [ABSTRACT FROM AUTHOR]

Background: Late in the 1990s, the non-indigenous European green crab (Carcinus maenas) colonized the shorelines of eastern Prince Edward Island, in Atlantic Canada. Due to concerns of further spread into productive shellfish habitats, an annual survey was conducted between 2000 and 2013 to detect a potential range expansion of this species. We compiled and analyzed that data and document green crab expansion using records of annual occurrence and relative density. Methods: Surveys were conducted during the fall season of each year by deploying baited traps at 29 sites along the island's two main shorelines (north and south shores). These sites were selected based on areas deemed more likely to be invaded by the green crab. Raw data per site and date was transformed to catch per unit effort (CPUE) to estimate relative abundances. Results: Populations of this species showed an uneven westward expansion along the north and south shores. Expansion rates changed among years but, overall, crab abundance was higher and changes in abundance were faster along the south shore than the north shore of the island. The westward expansion continues until this day. Conclusions: Based on the information compiled we hypothesize that the dissimilarity in range expansion rate was related to the availability of suitable habitat to sustain large green crab populations along the south shore. We also discuss implications of this expansion for commercial shellfish and native coastal communities. [ABSTRACT FROM AUTHOR]

The growth and spread of non-indigenous green crabs (Carcinus maenas) in Atlantic Canada is of concern to the sustainability of shellfish resources, particularly in areas recently invaded. Commercial green crab fishing has been initiated on Prince Edward Island to help control this species and provide a new resource for inshore fishermen. Developing a soft-shell crab product modelled after the Venetian ‘Moleche’ would provide an economic incentive beyond the existing hard-shell crab bait market. However, answers to questions such as the timing and characteristics of green crab moulting are required. A pilot study conducted in 2014–2015 collected seven groups of crabs and held them in individual compartments for 2–4 weeks to record moulting rates and physical characteristics. We found that a synchronized ‘moulting window’ occurs during July for male crabs. Field experiments in 2015 had an average moulting rate of 34%, with group-specific rates as high as 60%. The same cohort of crabs held in the laboratory had an average moulting rate of 48%, with group-specific rates as high as 75%. We observed a gradual increase in moulting rates from early to mid-July, after which all crabs caught had recently moulted, with evidence of new carapaces on all crabs. In 2015, the moulting window followed a 5°C increase in water temperature. Regarding morphology, the presence of a ‘halo’ on the episternites of the carapace was an indicator that a crab would soon moult. These promising results represent the first step in assessing the feasibility of a soft-shell, green crab industry. [ABSTRACT FROM PUBLISHER]

The Department of Fisheries and Oceans in Canada is experimenting with a commercial fishery on the European green crab (Carcinus maenas), an invasive species in North America, to help reduce the negative impact this animal has on ecosystems and native shellfish populations. We determined the break-even price that fishers would require for green crabs under different fishing scenarios (i.e. different gear and catch per trap per day). We also determined, for a 21 day season, the minimum catch per trap per day for fishermen to break even at market prices of $0.50/lb,$1/lb, and $3.50/lb. Several scenarios were profitable, but our results suggest the price of crab (dockside) would have to be sufficiently high to motivate fishers to continue the fishing pressure needed to reduce populations of this invasive species. The most economically profitable scenario was a fyke net by-catch fishery, similar to what currently exists on Prince Edward Island during the eel fishing season. [ABSTRACT FROM AUTHOR]

Atlantic Canada’s American oyster (Crassostrea virginica) beds, while economically and ecologically important, have been in decline over the past few decades. Predation by crabs, in particular by the European green crab (Carcinus maenas), has been proposed as one of the potential causes of such decline. Hence, this study examined oyster mortality levels in multiple beds across Prince Edward Island (PEI) and then experimentally assessed the contribution of green crab predation to oyster mortality. Results from surveys conducted in 10 estuaries across PEI in 2014 indicate that the probability of mortality for small oysters was significantly higher when green crabs were present then in areas without green crabs. This probability of mortality was significantly less when there was the presence of alternative prey like natural mussel beds (Mytilus edulis). The odds of oyster mortality were also higher when beds had rock crabs (Cancer irroratus) compared to beds with no rock crabs. Given the potential importance of green crab predation, its influence was assessed in 2015 using two field experiments with tethered oysters. Our results indicate that odds of small oyster mortality occurring were much higher in green crab inclusion cages than in the open environment and the exclusion cages. These results reaffirm that oysters up to ~40 mm SL are vulnerable to predation, and at least some of the mortality affecting these oysters can be causally attributed to green crab predation. Green crab predation rates upon small oysters are relevant given the economic benefits and ecosystem services provided by these bivalves. They highlight the need for the industry to consider mitigation measures and potentially adapt their oyster growing strategies. [ABSTRACT FROM AUTHOR]