Eastern Scotian Shelf Research Articles

Modelling the bottom-up effects of climate change on primary production in the Gulf of St. Lawrence and eastern Scotian Shelf

Modelling the bottom-up effects of climate change on primary production in the Gulf of St. Lawrence and eastern Scotian Shelf

Marine Spatial Planning in Canadian Arctic Shipping Governance: Exploring Its Application in the Northern Low-impact Shipping Corridors Initiative

Abstract This article examines different integrated management frameworks and area-based measures in Canada’s integrated ocean management regime, including their definitions, features and practices. It explicitly analyzes ma-rine spatial planning (MSP) and its practice in Canada’s Atlantic, Pacific and Arctic coasts. Case studies, including the Eastern Scotian Shelf Integrated Management (ESSIM) initiative, Pacific North Coast Integrated Manage-ment Area (PNCIMA) initiative, and Marine Plan Partnership (MaPP), show that MSP has brought additional value to comprehensive ocean planning and facilitated integrated shipping governance. Arctic marine traffic gov-ernance requires an integrated and holistic framework to address the inti-mate relationship and complex interaction between humans and the envi-ronment. The potential application of MSP in developing and governing the Northern Low Impact Shipping Corridors initiative is examined. MSP is con-sidered as a framework or an approach to inform better decision-making for the Corridors initiative from five perspectives: 1) improving interdepart-mental and cross-jurisdictional collaboration; 2) enhancing Inuit engage-ment; 3) encouraging knowledge co-production; 4) supporting data collec-tion and spatial analysis; and 5) facilitating implementation and adaptation of the Corridor initiative.

All roads lead to Rome: inter-origin variation in metabolomics reprogramming of the northern shrimp exposed to global changes leads to a comparable physiological status

Impacts of global ocean changes on species have historically been investigated at the whole-organism level. However, acquiring an in-depth understanding of the organisms’ cellular metabolic responses is paramount to better define their sensitivity to environmental challenges. This is particularly relevant for species that experience highly different environmental conditions across their distribution range as local acclimatization or adaptation can influence their responses to rapid global ocean changes. We aimed at shedding light on the cellular mechanisms underpinning the sensitivity to combined ocean warming (OW) and acidification (OA) in the northern shrimp Pandalus borealis, from four different geographic origins defined by distinctive environmental regimes in the northwest Atlantic: i.e. St. Lawrence Estuary (SLE), Eastern Scotian Shelf (ESS), Esquiman Channel (EC) and Northeast Newfoundland Coast (NNC). We characterized targeted metabolomics profiles of the muscle of shrimp exposed to three temperatures (2, 6 or 10°C) and two pH levels (7.75 or 7.40). Overall, shrimp metabolomics profiles were modulated by a significant interaction between temperature, pH and origin. Temperature drove most of the metabolomics reprogramming, confirming that P. borealis is more sensitive to OW than OA. Inter-origin differences in metabolomics profiles were also observed, with temperature*pH interactions impacting only shrimp from SLE and ESS, pH affecting only shrimp from SLE and temperature impacting shrimp from all origins. Temperature influenced metabolomics pathways related to the tricarboxylic acid cycle (TCA) and amino acid metabolism, resulting mainly in an accumulation of TCA intermediates and tyrosine. Temperature*pH and pH in isolation only affected amino acid metabolism, leading to amino acids accumulation under low pH. However, the ratio of ATP : ADP remained constant across conditions in shrimp from all origins suggesting that their energetic status is not affected by OW and OA. Still, the accumulation of TCA intermediates and tyrosine suggests the possible enhancement of immune responses under future OW and OA conditions. Our findings suggest that shrimp from SLE are more sensitive at the molecular level, compared to others, to future complex environmental conditions. This underlines the importance of investigating intraspecific variation in mechanisms of responses to combined drivers when trying to define species’ sensitivity to global ocean changes.

Abundance and demographic parameters of humpback whales from the Gulf of Maine, and stock definition relative to the Scotian Shelf

The Gulf of Maine is one of the principal summer feeding grounds for humpback whales (Megaptera novaeangliae) in the North Atlantic, and was one focus of effort in an ocean-basin-wide study known as the Years of the North Atlantic Humpback (YoNAH) project. Data from that project and from subsequent surveys were used to assess stock boundaries, abundance and demographic parameters for Gulf of Maine humpbacks. Surveys on the Scotian Shelf in the summers of 1998 and 1999 produced the first substantial dataset of identified individual humpbacks observed in this region, which lies between the well-studied areas of the Gulf of Maine and Newfoundland. The results gave a match rate of approximately 27% (14 of 52 individuals) between the Scotian Shelf and the Gulf of Maine, with evidence that many of the matched whales were transient in the Gulf of Maine; there were no matches to any other location in the North Atlantic. These data suggest that the range of most whales from the Gulf of Maine usually does not extend as far east as the Scotian Shelf or Newfoundland. Only one whale was observed on the Scotian Shelf in both the 1998 and 1999 surveys, and another seen in 1998 had also been sighted there in 1994. This low inter-annual match rate suggests that the abundance of humpback whales on the Scotian Shelf is larger than previously recognised. Three different but overlapping estimates of abundance for the Gulf of Maine population were calculated. Mark-recapture data from 1992/93 gave an estimate of abundance of 652 (CV = 0.29); however, this estimate is likely biased because of heterogeneity in sampling and in animal distribution. Photo-id data also provided a minimum population estimate of 497 humpbacks known to be alive in 1997; this estimate is also likely to be negatively biased because of heterogeneity. Finally, line-transect surveys conducted in 1999 yielded estimates of 816 (CV = 0.45) or 902 humpback whales (CV = 0.41, including a portion of the eastern Scotian Shelf stratum); these transect-based estimates are more consistent with the number of humpbacks (1,273, including dead animals) in the current photo-id catalogue for the Gulf of Maine. Overall, the size of the Gulf of Maine population is likely to be in the high hundreds, but no more precise estimate can be calculated at this time. The growth rate for the Gulf of Maine population was estimated using an interbirth interval method using data from 1992-2000. The estimate was either 1.00 (for a calf survival rate of 0.51) or 1.04 (for a calf survival rate of 0.875). Although confidence limits are not available (because maturation parameters could not be estimated), both estimates of population growth rate are outside the 95% confidence intervals of the previous estimate of 1.065 for the period 1979-1991 (Barlow and Clapham, 1997). It is unclear whether this apparent decline is an artefact resulting from a shift in distribution or is a real phenomenon; if the latter, it may be related to known high mortality among young-of-the-year whales in the waters of the US mid-Atlantic states. However, calf survival appears to have increased since 1996, presumably accompanied by an increase in population growth.

Modeling demersal fish and benthic invertebrate assemblages in support of marine conservation planning.

Marine classification schemes based on abiotic surrogates often inform regional marine conservation planning in lieu of detailed biological data. However, these schemes may poorly represent ecologically relevant biological patterns required for effective design and management strategies. We used a community‐level modeling approach to characterize and delineate representative mesoscale (tens to thousands of kilometers) assemblages of demersal fish and benthic invertebrates in the Northwest Atlantic. Hierarchical clustering of species occurrence data from four regional annual multispecies trawl surveys revealed three to six groupings (predominant assemblage types) in each survey region, broadly associated with geomorphic and oceanographic features. Indicator analyses identified 3–34 emblematic taxa of each assemblage type. Random forest classifications accurately predicted assemblage distributions from environmental covariates (AUC > 0.95) and identified thermal limits (annual minimum and maximum bottom temperatures) as important predictors of distribution in each region. Using forecasted oceanographic conditions for the year 2075 and a regional classification model, we projected assemblage distributions in the southernmost bioregion (Scotian Shelf‐Bay of Fundy) under a high emissions climate scenario (RCP 8.5). Range expansions to the northeast are projected for assemblages associated with warmer and shallower waters of the Western Scotian Shelf over the 21st century as thermal habitat on the relatively cooler Eastern Scotian Shelf becomes more favorable. Community‐level modeling provides a biotic‐informed approach for identifying broadscale ecological structure required for the design and management of ecologically coherent, representative, well‐connected networks of Marine Protected Areas. When combined with oceanographic forecasts, this modeling approach provides a spatial tool for assessing sensitivity and resilience to climate change, which can improve conservation planning, monitoring, and adaptive management.

Novel signals of adaptive genetic variation in northwestern Atlantic cod revealed by whole-genome sequencing.

Selection can create complex patterns of adaptive differentiation among populations in the wild that may be relevant to management. Atlantic cod in the Northwest Atlantic are at a fraction of their historical abundance and a lack of recovery within the Gulf of Maine has created concern regarding the misalignment of fisheries management structures with biological population structure. To address this and investigate genome‐wide patterns of variation, we used low‐coverage sequencing to perform a region‐wide, whole‐genome analysis of fine‐scale population structure. We sequenced 306 individuals from 20 sampling locations in U.S. and Canadian waters, including the major spawning aggregations in the Gulf of Maine in addition to spawning aggregations from Georges Bank, southern New England, the eastern Scotian Shelf, and St. Pierre Bank. With genotype likelihoods estimated at almost 11 million loci, we found large differences in haplotype frequencies of previously described chromosomal inversions between Canadian and U.S. sampling locations and also among U.S. sampling locations. Our whole‐genome resolution also revealed novel outlier peaks, some of which showed significant genetic differentiation among sampling locations. Comparisons between allochronic winter‐ and spring‐spawning populations revealed highly elevated relative (FST) and absolute (dxy) genetic differentiation near genes involved in reproduction, particularly genes associated with the brain‐pituitary‐gonadal axis, which likely control timing of spawning, contributing to prezygotic isolation. We also found genetic differentiation associated with heat shock proteins and other genes of functional relevance, with complex patterns that may point to multifaceted selection pressures and local adaptation among spawning populations. We provide a high‐resolution picture of U.S. Atlantic cod population structure, revealing greater complexity than is currently recognized in management. Our genome‐scan approach likely underestimates the full suite of adaptive differentiation among sampling locations. Nevertheless, it should inform the revision of stock boundaries to preserve adaptive genetic diversity and evolutionary potential of cod populations.

Inter-annual variability of shelf water volume on the northeastern Canadian and United States continental shelf, 1973–2013

Understanding the variability of waters confined to the northeastern United States and Canadian continental shelf is essential in determining factors that influence hydrographic patterns and structure, changes in fisheries stocks and planktonic assemblages, and the impact of Arctic warming. Shelf waters (SHW) located between Newfoundland and Cape Hatteras are characterized by lower salinity (<34 PSU) and lower temperatures than the generally warmer and more saline slope waters located farther offshore. Significant inter-annual variability of SHW volume has been noted from previous studies within the more limited Mid Atlantic Bight sub-region. Multiple factors may impact the volume of SHW, including local and remote river runoff and precipitation, release and equator-ward transport of fresh water from the Arctic and sub-polar gyre, ice melt from Greenland, cross-shelf removal, and both vertical and horizontal mixing processes. Here, we estimate inter-annual variability of SHW volume over the large shelf domain located between Newfoundland and Cape Hatteras from 1973 through 2013. Objectively-analyzed (OA) annual mean SHW thickness and error (as a percent of data variance) is computed for each of eleven sub-regions spanning our study domain for the 41-year period. Results show significant decreasing trends in SHW volume within 5 of the 11 sub-regions (The Grand Banks through the Eastern Scotian Shelf sub-regions) in agreement with previous work which has shown a decreasing trend of northern Labrador Current transport. OA annual mean SHW volume anomalies were cross-correlated with annual mean shelf slope front (SSF) position anomalies for corresponding sub-regions, displaying a highly significant relationship for the Georges Bank, the Mid Atlantic Bight, and the DelMarVa Hatteras shelf, with higher volumes corresponding to more offshore SSF positions. Annual mean near-surface salinity anomalies, when cross-correlated with OA annual mean SHW volume anomalies, displayed significant positive correlations from the Gulf of St. Lawrence northeast to the Newfoundland shelf and displayed primarily negative correlations from the Gulf of Maine to the DelMarVa Hatteras sub-region. The North Atlantic Oscillation annual indices displayed significant cross-correlations with annual mean OA SHW volume anomalies only for the Tail of the Grand Banks and Georges Bank., There were no significant cross correlations computed between the OA annual mean SHW volume anomalies and the NAO winter index for corresponding years.

Size at sexual maturity of waved whelk (Buccinum undatum) on the Eastern Scotian Shelf

The waved whelk (Buccinum undatum) is commercially fished in the North Atlantic Ocean; the fishery is growing in Atlantic Canada. The species’ oviparous reproduction limits dispersal, posing a challenge to fishery management at the meta-population level. The Minimum Landing Size (MLS) for B. undatum in Nova Scotia is set at 65 mm with intent to maintain an adequate spawning stock. The implicit assumption that the MLS approximates a size at which half the population has reached reproductive maturity (LM50) is unjustified. We measured the LM50 of the whelks sampled from commercial fishing catches during 2016 and 2017 at five sites on the Eastern Scotian Shelf located roughly 11–210 km apart. There were significant differences among all measured parameters including total shell length, LM50, sex ratio and parasitic prevalence. The latter was measured to investigate its effects on calculations of LM50 and fishery regulations. The LM50 was smaller than the MLS at all sites. The LM50 for males ranged 45–64 mm, and 53–65 mm for females. The age at LM50 was found to be consistent with other studies: 5–6 years for both sexes. The size-frequency distributions in most samples were approximately normally distributed. The sex ratio was within the range found in other studies but varied among sampling sites. The prevalence of castrating trematodes was found to be 2–26% in males and 5–25% in females. We suggest that the area-based, effort management model for the fishery should adopt locale-specific MLS values based on the LM50 determined for sub-populations being exploited within specific areas of fishing access.

Exploitation drives an ontogenetic-like deepening in marine fish

Virtually all studies reporting deepening with increasing size or age by fishes involve commercially harvested species. Studies of North Sea plaice in the early 1900s first documented this phenomenon (named Heincke's law); it occurred at a time of intensive harvesting and rapid technological changes in fishing methods. The possibility that this deepening might be the result of harvesting has never been evaluated. Instead, age- or size-related deepening have been credited to interactions between density-dependent food resources and density-independent environmental factors. Recently, time-dependent depth variations have been ascribed to ocean warming. We use a model, initialized from observations of Atlantic cod (Gadus morhua) on the eastern Scotian Shelf, where an age-dependent deepening of ∼60 m was observed, to assess the effect of size- and depth-selective exploitation on fish distribution. Exploitation restricted to the upper 80 m can account for ∼72% of the observed deepening; by extending exploitation to 120 m, all of the deepening can be accounted for. In the absence of fishing, the model indicated no age-related deepening. Observations of depth distributions of older cod during a moratorium on fishing supported this prediction; however, younger cod exhibited low-amplitude deepening (10-15 m) suggestive of an ontogenetic response. The implications of these findings are manifold, particularly as they relate to hypotheses advanced to explain the ecological and evolutionary basis for ontogenetic deepening and to recent calls for the adoption of evidence of species deepening as a biotic indicator or "footprint" of warming seas.

Fine-scale temperature-associated genetic structure between inshore and offshore populations of sea scallop (Placopecten magellanicus).

In the northwest Atlantic Ocean, sea scallop (Placopecten magellanicus) has been characterized by a latitudinal genetic cline with a breakpoint between northern and southern genetic clusters occurring at ~45°N along eastern Nova Scotia, Canada. Using 96 diagnostic single-nucleotide polymorphisms (SNPs) capable of discriminating between northern and southern clusters, we examined fine-scale genetic structure of scallops among 27 sample locations, spanning the largest geographic range evaluated in this species to date (~37-51°N). Here, we confirmed previous observations of northern and southern groups, but we show that the boundary between northern and southern clusters is not a discrete latitudinal break. Instead, at latitudes near the previously described boundary, we found unexpected patterns of fine-scale genetic structure occurring between inshore and offshore sites. Scallops from offshore sites, including St. Pierre Bank and the eastern Scotian Shelf, clustered with southern stocks, whereas inshore sites at similar latitudes clustered with northern stocks. Our analyses revealed significant genetic divergence across small spatial scales (i.e., 129-221 km distances), and that spatial structure over large and fine scales was strongly associated with temperature during seasonal periods of thermal minima. Clear temperature differences between inshore and offshore locations may explain the fine-scale structuring observed, such as why southern lineages of scallop occur at higher latitudes in deeper, warmer offshore waters. Our study supports growing evidence that fine-scale population structure in marine species is common, often environmentally associated, and that consideration of environmental and genomic data can significantly enhance the identification of marine diversity and management units.

Singing through winter nights: Seasonal and diel occurrence of humpback whale (Megaptera novaeangliae) calls in and around the Gully MPA, offshore eastern Canada

AbstractHumpback whale use of areas off eastern Canada is poorly understood, a knowledge gap that could impact future conservation efforts. We describe the acoustic occurrence of humpback whales in and around the Gully Marine Protected Area (MPA), an eastern Scotian Shelf submarine canyon. Near‐continuous acoustic recordings sampling at 16 kHz were collected from the MPA and nearby slope areas from October 2012 to September 2014 using near‐bottom recorders. In an offshore region where humpbacks were thought to be rare, we observed calls from October to June with a peak in song and nonsong calls in December and January. This suggests that some individuals occur in Canadian waters in winter and the Gully region may be a North Atlantic humpback whale migratory corridor. Calls were predominantly songs indicating potential mating activities. Song and nonsong calls occurred more at sunset and during hours of darkness than during daylight. This study improves our understanding of the seasonal occurrence of humpback whales on the Scotian Slope and, more specifically, their use of an offshore protected area.

Spatio-temporal distribution of beaked whales on Canada’s East Coast

Beaked whales represent some of the least understood marine mammals worldwide with the movements and distribution of many species largely unknown. Around eastern Canada current knowledge is limited to the eastern Scotian shelf and northern bottlenose whales. The acoustic signals of the beaked whale species are recognizable and sufficiently unique to be candidates for passive acoustic monitoring. Thirteen deep-water recorders located along the shelf break off Eastern Canada from Bonnécamps Canyon (42.5 N) to Southern Labrador (55.3 N) collected acoustic data near-continuously from Aug 2015 to July 2016. A minimum of one of every 20 minutes were recorded at 250,000 samples per second to monitor for the presence echolocations clicks of odontocetes, including beaked whales. An automated detector, validated by manual analysts, identified the presence of endangered Northern Bottlenose whales (Hyperoodon ampullatus), Cuvier’s (Ziphius cavirostris), and special concern Sowerby’s beaked whales (Mesoplodon bidens). The presence data were analyzed to determine the occurrence and residency durations of beaked whales throughout the geographic range studied. We then studied the influence of currents, sea ice, surface temperatures, chlorophyll, distance to the 1000 m isobath, background noise and anthropogenic noise on the whales’ acoustic occurrence. Acoustic studies such as this allow us to gain further insights on the occurrence of these notoriously difficult to study species.

SPRAT: A spatially-explicit marine ecosystem model based on population balance equations

To successfully manage marine fisheries using an ecosystem-based approach, long-term predictions of fish stock development considering changing environmental conditions are necessary. Such predictions can be provided by end-to-end ecosystem models, which couple existing physical and biogeochemical ocean models with newly developed spatially-explicit fish stock models. Typically, individual-based models (IBMs) and models based on advection-diffusion-reaction (ADR) equations are employed for the fish stock models. In this paper, we present a novel fish stock model called SPRAT for end-to-end ecosystem modeling based on population balance equations (PBEs) that combines the advantages of IBMs and ADR models while avoiding their main drawbacks. SPRAT accomplishes this by describing the modeled ecosystem processes from the perspective of individuals while still being based on partial differential equations.We apply the SPRAT model to explore a well-documented regime shift observed on the eastern Scotian Shelf in the 1990s from a cod-dominated to a herring-dominated ecosystem. Model simulations are able to reconcile the observed multitrophic dynamics with documented changes in both fishing pressure and water temperature, followed by a predator–prey reversal that may have impeded recovery of depleted cod stocks.We conclude that our model can be used to generate new hypotheses and test ideas about spatially interacting fish populations, and their joint responses to both environmental and fisheries forcing.

Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a “pelagic outburst” of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a “suprabenthic habitat occupation” thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.

Influence of Benthic Macrofauna as a Spatial Structuring Agent for Juvenile Haddock (Melanogrammus aeglefinus) on the Eastern Scotian Shelf, Atlantic Canada.

We examined the habitat of juvenile haddock on the eastern Scotian Shelf (off Nova Scotia, Canada) in relation to grab-sampled benthic macrofaunal invertebrate species assemblages in order to determine whether there were significant differences in benthic macrofauna between areas of historically persistent high and low juvenile haddock abundance. Our analyses were conducted over two spatial scales in each of two years: among banks (Emerald, Western and Sable Island), approximately 60 km distant from each other, and between areas of high and low juvenile haddock abundance at distances of 10 to 30 km–all in an area that had not experienced groundfishing in the decade prior to sampling. We also examined fine-scale (10s of metres) within-site variability in the macrofauna and used surficial sediment characteristics, along with hydrographic variables, to identify environmental correlates. PERMANOVA identified statistically significant differences in biomass, density and composition of the benthos associated with juvenile haddock abundance; however it was difficult to determine whether the results had biological relevance. Post hoc tests showed that these differences occurred only on Sable Island Bank where both fish and benthos may have been independently responding to sediment type which was most different there (100% sand in the area of low haddock abundance vs. 22% gravel in the area of high haddock abundance). In total, 383 benthic taxa representing 13 phyla were identified. Annelida was the most specious phylum (36.29% of taxa, representing 33 families), followed by Arthropoda (with Crustaceans, mostly Amphipoda, accounting for 25.07% of the total number of taxa). The strongest pattern in the macrofauna was expressed at the largest scale, between banks, accounting for approximately 25% of the variation in the data. Emerald Bank, deeper, warmer and saltier than the Western and Sable Island Banks, had a distinctive fauna.

Salinity variability along the eastern continental shelf of Canada and the United States, 1973–2013

Continental shelf waters located off the east coast of Canada and the United States are part of a long shelf current system that is partly comprised of colder, less-saline waters originating from high latitudes, including waters from the North Atlantic sub-polar gyre, along with ice-melt and freshwater input from local rivers. A 41-year analysis (1973–2013) of near-surface salinity (NSS) using three hydrographic datasets (Bedford Institute of Oceanography “Climate”, NOAA/ESDIM, and Canadian Marine Environmental Data Service (MEDS)) allowed an examination of NSS variability within 11 continental shelf sub-regions, extending from the southern Newfoundland Shelf of eastern Canada to the DelMarVa/Hatteras Shelf of the United States. Although the periods of record containing sufficient data vary between sub-regions, regional mean NSS values are lowest within the Gulf of St. Lawrence and highest on the DelMarVa/Hatteras shelf, with largest annual variability within the Gulf of St. Lawrence. After removal of outliers, long-term linear trends computed from annual mean NSS were detected along the Newfoundland Shelf (+0.011y−1), Western Scotian Shelf (−0.007y−1), Gulf of Maine (−0.014y−1), Georges Bank (−0.011y−1), and DelMarVa/Hatteras Shelf (+0.024y−1). A long-term quadratic fit to annual mean NSS from the Eastern Scotian Shelf displays a salinity increase through 1992 of +0.026y−1, decreasing thereafter until 2013 by −0.028y−1. A quadratic fit for the Western Grand Banks displays a NSS increase through 2007 of +0.022y−1, decreasing thereafter through 2013 by −0.006 y−1. Annual mean NSS from the Eastern Grand Banks, Tail of the Grand Banks, Gulf of St. Lawrence, and Middle Atlantic Bight display no long-term trends. Inter-annual variability (IAV) of NSS residuals shows similar small mean squared error (mse) of 0.02–0.04 for the four northern-most sub-regions (Newfoundland Shelf, Eastern, Tail and Western Grand Banks) and are correlated at 0-year lag. IAV of NSS residuals (mse) are larger for the Gulf of St. Lawrence (~0.19), Eastern and Western Scotian Shelf (~0.09–0.06), Gulf of Maine and Georges Bank (~0.08–0.06), Middle Atlantic Bight (~0.19), and maximal for the DelMarVa/Hatteras Shelf (~0.36), and are also correlated at 0-year lag, but are uncorrelated with the four northern-most sub-regions. Consideration of a simple “flux variation” model that includes along-shelf, altimeter-derived velocity anomalies measured upstream on the Western Scotian Shelf and the positive along-shelf mean salinity gradient between the Eastern Scotian Shelf and the DelMarVa/Hatteras Shelf, may explain the synchronous nature of NSS residuals for the southern-most 6 sub-regions. Furthermore, the flux variation model results in calculated NSS residuals that are within a factor of two of observed NSS residuals for the southern-most DelMarVa/Hatteras Shelf. Co-varying broad-scale coastal sea level and shelf break front position anomalies also support the flux variation model, as do CMV Oleander temperature anomalies across a limited Middle Atlantic Bight shelf region. Overall, the relationships between along-shelf observations of NSS and other shelf parameters support an existing wind-driven dynamical shelf model. Specifically, a flux variation model is able to describe IAV of NSS along a section of the Canadian and U.S shelf for periods greater than one year. In the future, this model may be able to provide useful indices of regime change as noted within the Northeast Shelf Large Marine Ecosystem by other workers.

Evaluating trophic cascades as drivers of regime shifts in different ocean ecosystems

In ecosystems that are strongly structured by predation, reducing top predator abundance can alter several lower trophic levels—a process known as a trophic cascade. A persistent trophic cascade also fits the definition of a regime shift. Such ‘trophic cascade regime shifts' have been reported in a few pelagic marine systems—notably the Black Sea, Baltic Sea and eastern Scotian Shelf—raising the question of how common this phenomenon is in the marine environment. We provide a general methodology for distinguishing top-down and bottom-up effects and apply this methodology to time series from these three ecosystems. We found evidence for top-down forcing in the Black Sea due primarily to gelatinous zooplankton. Changes in the Baltic Sea are primarily bottom-up, strongly structured by salinity, but top-down forcing related to changes in cod abundance also shapes the ecosystem. Changes in the eastern Scotian Shelf that were originally attributed to declines in groundfish are better explained by changes in stratification. Our review suggests that trophic cascade regime shifts are rare in open ocean ecosystems and that their likelihood increases as the residence time of water in the system increases. Our work challenges the assumption that negative correlation between consecutive trophic levels implies top-down forcing.

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

Comparative analyses of the dynamics of exploited marine ecosystems have led to differing hypotheses regarding the primary causes of observed regime shifts, while many ecosystems have apparently not undergone regime shifts. These varied responses may be partly explained by the decade-old recognition that within-system spatial heterogeneity in key climate and anthropogenic drivers may be important, as recent theoretical examinations have concluded that spatial heterogeneity in environmental characteristics may diminish the tendency for regime shifts. Here, we synthesize recent, empirical within-system spatio-temporal analyses of some temperate and subarctic large marine ecosystems in which regime shifts have (and have not) occurred. Examples from the Baltic Sea, Black Sea, Bengula Current, North Sea, Barents Sea and Eastern Scotian Shelf reveal the largely neglected importance of considering spatial variability in key biotic and abiotic influences and species movements in the context of evaluating and predicting regime shifts. We highlight both the importance of understanding the scale-dependent spatial dynamics of climate influences and key predator–prey interactions to unravel the dynamics of regime shifts, and the utility of spatial downscaling of proposed mechanisms (as evident in the North Sea and Barents Sea) as a means of evaluating hypotheses originally derived from among-system comparisons.

Formation and distribution of sea ice in the Gulf of St. Lawrence: A process‐oriented study using a coupled ocean‐ice model

AbstractA coupled ocean‐ice model for the eastern Canadian shelf is used to examine main physical processes affecting sea ice conditions in the Gulf of St. Lawrence (GSL) and adjacent waters. The coupled model is based on NEMO and uses OPA9 as the ocean circulation component and LIM2 as the ice model. The coupled model is forced by atmospheric reanalysis fields produced by Large and Yeager (2004). The model results are used to examine the roles of thermodynamics and dynamics on sea ice distributions and patterns of ice production and melting, and the influence of ice capping on the circulation in the study region. Analysis of model results indicates that local production of sea ice is important in shallower areas over the northern and western GSL. Equatorward advection of sea ice from the St. Lawrence Estuary is affected significantly by the Gaspé Current. An index is used to quantify the relative importance of thermodynamic and dynamics of sea ice in the GSL. It is found that both thermodynamics and dynamics are important over most of the GSL, except for waters around Anticosti Island, in the southeastern Gulf, and over the eastern Scotian Shelf, where dynamics (or sea ice movements) are the most important mechanism for the presence of sea ice. The study also demonstrates that ice capping significantly reduces the strength of the winter circulation in the GSL.

Transmitting species‐interaction data from animal‐borne transceivers through Service Argos using Bluetooth communication

Summary Interactions between upper trophic‐level predators and their prey remain poorly understood due to their inaccessibility during foraging at sea. This uncertainty has fuelled debate on the impact of predation by species such as the grey seal (Halichoerus grypus) on fish stocks. The Vemco Mobile Transceiver (VMT) has provided us with new knowledge on interactions between pinnipeds and fish species. However, the necessity to recover the VMT for data retrieval has limited deployments to locations where confidence in instrument recovery is high, and has thus restricted both species and geographical sampling. To overcome these limitations, a Bluetooth link was integrated into the VMT and GPS satellite‐linked transmitter. The two‐unit design allows data collected by the VMT to be transmitted via Bluetooth to the satellite transmitter, which relays the interaction data to the ARGOS satellite system for retrieval. To evaluate in‐situ performance, units were deployed on two adult female grey seals on Sable Island, NS in October 2012 and recovered during the subsequent breeding season. Data archived by the VMT were compared with data uploaded via ARGOS. The deployment periods were 76–84 days. The total number of valid detections archived was 179. All detections archived by the first unit (n = 66) were transmitted via ARGOS, while all but two of the 113 archived detections from the second unit were transmitted. Detections recovered from both units were from other VMT‐tagged grey seals (n = 173) and moored V13 transmitters on Middle Bank, Eastern Scotian Shelf (n = 6). These preliminary results are proof‐of‐concept that integrated Bluetooth VMTs can be used on a broader variety of marine predators to collect data on species interactions in otherwise inaccessible environments and without the need to recover instruments.