Great South Channel Research Articles

Simultaneous in-air versus in-water measurements of humpback whale breathing sounds

Humpback whales produce an assortment of sounds ranging from moans and songs to surface-breaching, breathing, and foreflipper flaps. During a sea trial of Northeastern University’s coherent hydrophone array at the Great South Channel in US Northeastern coastal waters, the HF subaperture consisting of 32 hydrophone elements at 0.375 m spacings was deployed vertically with half of the hydrophones in air and the other half submerged underwater on September 6th, 2021. Many instances of humpback whale breathing sounds were recorded over several hours of observation. Visual sightings and video recordings were used to coregister sounds recorded on individual hydrophones of the HF subaperture. Here, we examine and compare the simultaneously recorded humpback whale breathing sound spectra, bandwidth, and duration for in-air versus underwater hydrophones. Whale distances to the vertical HF subaperture could be calculated from curved time of arrival differences due to close proximity of the array. These distances are applied to correct the received underwater sound pressure level for transmission loss and estimate underwater recorded whale breathing sound source level. A consistent broadband dip in the measured underwater sound spectra with null centered at 15 kHz is investigated by propagation modelling, considering both modal interference and attenuation from loud bubbles.

Acoustic diversity, bearing estimation, and localization of sound sources in shallow water and deep ocean off the U.S. Northeast coast using a coherent hydrophone array

An in-house developed 160-element large-aperture hydrophone array from Northeastern University was utilized for ocean monitoring in both the shallow waters of the Great South Channel (GSC) and the deep-water area off the continental slope south of Rhode Island in September 2021. Utilizing passive ocean acoustic waveguide remote sensing (POAWRS) technology, the system enabled real-time, broad-area surveillance of the oceanic environment. A novel multistage clustering was introduced for annotating the vast volume of underwater acoustic data, drawing inspiration from the human cognitive process of categorizing sounds. Initially, sounds are separated into broad groups, akin to a first human listening pass. Subsequent stages of clustering refine these categories, utilizing different sets of features to achieve a granularity of annotation that closely mirrors expert human annotation processes. It was found that the acoustic landscape of the GSC was primarily composed of diverse marine mammal sounds, such as fin whale 20 Hz pulses, humpback whale songs, minky whale buzz sequences, sperm whale and dolphin echolocation clicks, and unidentified whale calls. Conversely, the deep-water area off the continental slope featured predominantly fish sounds, chorus-like sounds, and dolphin vocalizations. Detailed analyses of the most prevalent vocalizations are presented, including their bearing-time trajectories and localizations.

Vocalisation rates of the North Atlantic right whale (Eubalaena glacialis)

Vocalisation rates were measured from North Atlantic right whales (Eubalaena glacialis) in spring 1999-2000 in the Great South Channel and off Cape Cod, USA, and in summer 1999-2000 in the Bay of Fundy, Canada. Vocalisations were classed as either ‘moans’, ‘low-frequency (LF) calls’ or ‘gunshots’. Towed hydrophone recordings (36.1 hours) were made in 21 encounters where loose aggregations of right whales were within about 1,000m. Recordings were also made using acoustic tags attached by suction cups to ten different whales (29.5 hours). Tags also recorded depth data. Moan rates (sounds per aggregation per hour) were correlated with size of whale aggregation. Individual whales produced moans at ~ 0-10 per hour (recorded from tags and the towed hydrophone). Small aggregations (2-10) gave higher moan rates (usually < ~ 60 per hr) and larger aggregations ( > 10) higher still ( ~ 70-700 per hr) (recorded from towed hydrophone). Results from the Bay of Fundy indicate high moan rates at night. Moans were usually produced in clusters. Tag data showed that moans were usually produced when whales were within about 10m of the surface. A passive acoustic system could potentially provide supplementary information on the distribution of aggregations of right whales. This could be useful for management (1) in the long term, by aiding the prediction of right whale distribution, or (2) as a real-time tool for helping to route shipping away from concentrations of right whales. The empirical evidence presented here on vocalisation rates will assist in assessing feasibility. The clustering of moans and the tendency to produce them near the surface could hamper detection and localisation efforts. Further research is underway to investigate other important practical issues such as detectability and source levels.

Real-time instantaneous wide-area ocean acoustic monitoring in the shallow Great South Channel and deep ocean south of Rhode Island with Northeastern University coherent hydrophone array

A large-aperture 160-element coherent hydrophone array system, developed in-house at Northeastern University, was deployed and tested at sea in shallow water Great South Channel and deep ocean south of Rhode Island during September 2021. The system implements the passive ocean acoustic waveguide remote sensing technology for real-time instantaneous wide area ocean monitoring. The array data sampled at 100 kHz per element were processed in real-time at sea to provide beamformed spectrogram imagery in multiple frequency subbands from 10 Hz to 50 kHz and spanning 360 degree horizontal azimuths about the receiver array. A wide variety of acoustic detections were made in real time, including marine mammal vocalizations and motion-related signals, fish grunts and other fish-generated signals, tonal and broadband signals from distant ships and own tow ship. Real-time detections of marine mammal vocalizations include fin whale 20 Hz pulses, humpback whale songs and social sounds, minke whale buzz sequences, sperm whale echolocation and social clicks, dolphin whistles and echolocation clicks, as well as calls from unidentified baleen and toothed whale species. A database of automatically detected signals with bearing-time information along with extracted time-frequency characteristics was generated in real-time at sea.

Larval transport pathways from three prominent sand lance habitats in the Gulf of Maine

AbstractNorthern sand lance (Ammodytes dubius) are among the most critically important forage fish throughout the Northeast US shelf. Despite their ecological importance, little is known about the larval transport of this species. Here, we use otolith microstructure analysis to estimate hatch and settlement dates of sand lance and then use these measurements to parametrize particle tracking experiments to assess the source–sink dynamics of three prominent sand lance habitats in the Gulf of Maine: Stellwagen Bank, the Great South Channel, and Georges Bank. Our results indicate the pelagic larval duration of northern sand lance lasts about 2 months (range: 50–84 days) and exhibit a broad range of hatch and settlement dates. Forward and backward particle tracking experiments show substantial interannual variability, yet suggest transport generally follows the north to south circulation in the Gulf of Maine region. We find that Stellwagen Bank is a major source of larvae for the Great South Channel, while the Great South Channel primarily serves as a sink for larvae from Stellwagen Bank and Georges Bank. Retention is likely the primary source of larvae on Georges Bank. Retention within both Georges Bank and Stellwagen Bank varies interannually in response to changes in local wind events, while the Great South Channel only exhibited notable retention in a single year. Collectively, these results provide a framework to assess population connectivity among these sand lance habitats, which informs the species' recruitment dynamics and impacts its vulnerability to exploitation.

Impact of larval behaviors on dispersal and connectivity of sea scallop larvae over the northeast U.S. shelf

Sea scallops (Placopecten magellanicus) are a highly fecund species that supports one of the most commercially valuable fisheries in the northeast U.S. continental shelf region. Scallop landings exhibit significant interannual variability, with abundances widely varied due to a combination of anthropogenic and natural factors. By coupling a pelagic-stage Individual-Based scallop population dynamics Model (hereafter referred to as Scallop-IBM) with the Northeast Coastal Ocean Forecast System (NECOFS) and considering the persistent aggregations over Georges Bank (GB)/Great South Channel (GSC) as source beds, we have examined the dispersion and settlement of scallop larvae over 1978–2016. The results demonstrated that the significant interannual variability of larval dispersal was driven by biophysical interactions associated with scallop larval swimming behaviors in their early stages. The duration, frequency, and stimulus of larval vertical migration in the ocean mixed layer (OML) affected the residence time of larvae in the water column over GB. It thus sustained the persistent aggregations of scallops in the GB/GSC and Southern New England region. In addition to larval behavior in the OML, the larval transport to the Middle Atlantic Bight (MAB) was also closely related to the intensity and duration of northeasterly wind in autumn. There was no conspicuous connectivity of scallop larvae between GB/GSC and MAB in the past 39 years except in the autumn of 2009. In 2009, the significant larval transport to the MAB was produced by unusually strong northeasterly winds. Ignoring larval behavior in the OML could overestimate the scallop population’s connectivity between GB and the MAB and thus provide an unrealistic prediction of scallop larval recruitment in the region. Both satellite-derived SST and NECOFS show that the northeast U.S. shelf experienced climate change-induced warming. The extreme warming at the shelfbreak off GB tends to intensify the cross-isobath water temperature gradient and enhance the clockwise subtidal gyre over GB. This change can increase the larval retention rate over GB/GSC, facilitating enhanced productivity on GB.

The conundrum of biont-free substrates on a high-energy continental shelf: Burial and scour on Nantucket Shoals, Great South Channel

A survey of the adjacent waters east of Nantucket, Massachusetts provided an opportunity to evaluate the epibiota attached to cobbles, rocks, boulders, and Atlantic surfclam shell in a region of high tidal current velocity and sand scour where burial, exhumation, and scour may limit epibiont coverage on exposed, and thus otherwise highly preferred for attachment, substrates. Such conditions may confute the expectation that substrate complexity always adds significantly to ecosystem value by expanding the range of habitat options and consequently increasing species richness and trophic linkages. Sedimentary particles potentially providing good attachment substrate for erect sessile epibiota included surfclam shells, abundant at many locations, cobbles, nearly ubiquitous, rocks, routinely encountered, and occasional boulders. The attached epibiota fell into three categories based on their biases for particle types. Some preferred the largest particles or evidence of their occupation was best preserved on these particles: these included sponges, mussels, and barnacles and their scars. Some preferred intermediate and smaller terrigenous particles; these included tunicates and encrusting bryozoans. Some preferred surfclam shells, namely the slipper shells and erect hydroids. Slow-growing attached epibionts were exceedingly rare and all soft-bodied attached epibionts were rare. Only barnacles and their taphonomic scars and hydroids were common. The frequency of barnacle scars relative to intact barnacles suggests sediment scour under a high-flow regime. Mussels were rarely attached to larger sedimentary particles such as cobbles and rocks, though commonly occurring locally as mussel beds on sand and pebble, further supporting the ephemerality of exposed unscoured attachment sites. The absence of attached epibionts demonstrates that edaphic processes minimize the importance of cobbles, rocks, boulders, and shells in community structure in some subtidal high-energy regimes, defying expectations from their contribution to substrate complexity. Their apparent contribution to habitat complexity belies their resultant much more minor role in determining community composition, thereby limiting their ecosystem value.

Anomalous 1992 spring and summer right whale (Eubalaena glacialis) distributions in the Gulf of Maine

No right whales (Eubalaena glacialis) were sighted during aerial surveys in May-July 1992 in the Great South Channel region of thesouthwestern Gulf of Maine. This was the first year that spring surveys failed to detect right whales in this region. During the latespring/early summer season when right whales would normally be expected in the Great South Channel, a few were sighted in the centralGulf of Maine, none were found in their usual late summer/early autumn feeding areas near Nova Scotia and a few were seen inMassachusetts Bay. The absence of right whales in the Great South Channel in 1992 can be attributed to a shift in the regional zooplanktoncommunity. The usual spring zooplankton of the region is strongly dominated by the calanoid copepod Calanus finmarchicus, verticallyand horizontally aggregated into dense patches which are the preferred foraging areas of right whales. The 1992 zooplankton was dominatedby pteropods, distributed evenly throughout the water column. It is possible, although unlikely, that pteropods are unacceptable prey forright whales. A more likely explanation is that their local densities within small-scale patches were below the energetic threshold requiredfor successful right whale feeding. The shift in zooplankton dominance in 1992 is likely related to significantly reduced water temperaturesand a delay in the development of the usual hydrographic structure of the region. The 1992 temperature and hydrographic anomalies, inturn, can be attributed principally to an unusually large influx of colder and fresher Scotian Shelf Water, and may have been enhanced bywidespread cooling of the Northern Hemisphere caused by sulphuric acid haze in the atmosphere from the June 1991 eruption of Pinatubovolcano in the Philippines.

Predicting bycatch hotspots based on suitable habitat derived from fishery-independent data

Bycatch remains a global problem in managing sustainable fisheries. A critical aspect of management is understanding the timing and spatial extent of bycatch. Fisheries management often relies on observed bycatch data, which are not always available due to a lack of reporting or observer coverage. Alternatively, analyzing the overlap in suitable habitat for the target and non-target species can provide a spatial management tool to understand where bycatch interactions are likely to occur. Potential bycatch hotspots based on suitable habitat were predicted for cuskBrosme brosmeincidentally caught in the Gulf of Maine American lobsterHomarus americanusfishery. Data from multiple fisheries-independent surveys were combined in a delta-generalized linear mixed model to generate spatially explicit density estimates for use in an independent habitat suitability index. The habitat suitability indices for American lobster and cusk were then compared to predict potential bycatch hotspot locations. Suitable habitat for American lobster has increased between 1980 and 2013 while suitable habitat for cusk decreased throughout most of the Gulf of Maine, except for Georges Basin and the Great South Channel. The proportion of overlap in suitable habitat varied interannually but decreased slightly in the spring and remained relatively stable in the fall over the time series. As Gulf of Maine temperatures continue to increase, the interactions between American lobster and cusk are predicted to decline as cusk habitat continues to constrict. This framework can contribute to fisheries managers’ understanding of changes in habitat overlap as climate conditions continue to change and alter where bycatch interactions could occur.

The intermingling of benthic macroinvertebrate communities during a period of shifting range: The “East of Nantucket” Atlantic Surfclam Survey and the existence of transient multiple stable states

AbstractA survey of the region eastward of Nantucket provided an opportunity to examine the cold temperate–boreal boundary along the high‐energy Great South Channel. Here described are the benthic macroinvertebrate community types encountered, with a focus on the influence of climate change on the range boundaries of the benthic biomass dominants and the potential existence of transient multiple stable states. The survey identified three primary community types. The shallowest sites were occupied by a surfclam‐dominated community, comprising an abundance of large (≥150 mm) surfclams, and a few common attached epibiota primarily attached to exposed surfclam shell. Two communities exist at intermediate depths, one dominated by submarket and small market‐size surfclams (<150 mm) and the other, created by mussel mats and their attendant epibiota, crabs, sea urchins, and other mobile epifauna. Mussels are a foundational species, establishing a hard‐bottom terrain conducive to these other denizens in soft‐bottom habitat. Cobbles were nearly ubiquitous, rocks were routinely recovered, and boulders were encountered occasionally. Slow growing attached epibionts were exceedingly rare and mobile epifauna were not obviously associated with these large sedimentary particles; nor were the surfclam or mussel communities. The frequency of barnacle scars suggests sediment scour under the high‐flow regime characteristic of the surveyed region, which voids the habitat potential of these sedimentary particles. The abundance of surfclam shell indicates that surfclams have inhabited the shoaler depths for an extended time; limited shell at deeper sites supports the inference from the absence of large animals that these sites are relatively newly colonized and represent further evidence of an offshore shift in range brought on by increasing bottom water temperatures. The dichotomous nature of the two primary community types at mid‐depths suggests that these two communities represent multiple stable states brought on by the interaction of an invading cold temperate species with the receding boreal fauna resulting in a transient intermingling of species, which, however, structure the habitat into exclusionary stable states rather than overlapping in a co‐occurrence ecotone.

Sea scallops exhibit strong local spatiotemporal structure associated with seabed stability and high flows

AbstractSea scallops (Placopecten magellanicus) live along the Northwest Atlantic continental shelf and are aggregated over a broad range of spatial scales (m2–105 km2). However, little is known about the spatial distribution of local scallop neighborhoods, the scale at which spawning, intra‐specific competition, and predator–prey interactions occur. We surveyed 30,995 km2 of Georges Bank and the Great South Channel annually from 2003 to 2010 counting the number of scallops in 54,016 locally replicated 3.24‐m2 quadrats with underwater video. There were about 4 billion scallops occupying 11,200 km2 of the study area in concentrations from 1 to 41 scallops per scallop. Scallop spatial distribution switched from dispersed to aggregated at a concentration of 3–4 scallops in every year. High concentrations (>3) occurred in 11% but were only persistent in 4% (449 km2) of scallop habitat. There were 13 persistent high‐concentration aggregations (7–217 km2), and all but two occurred on gravel‐dominated sediments left by prehistoric glacial retreat. Model‐derived benthic boundary shear stresses in the persistent aggregations averaged two times higher than in scallop habitat, but the seabed was about two times more stable because it had significantly less sand and more granule–pebble and cobble sediment. The area occupied by scallops each year varied little (9%) despite a 49% increase in total scallop abundance between 2005 and 2007, suggesting Georges Bank scallops have a proportional density population structure. Most scallops occurred alone or at low concentrations (≤2) where fertilization success is probably poor. The persistent high‐concentration aggregations we identified may be critical for successful reproduction and sustainable harvest. Observing organisms at scales corresponding to their individual interactions reveals important processes shaping their landscape‐scale spatiotemporal distributions. These processes are obscured by the methods typically employed to estimate abundances of commercial fishery species. The spatiotemporal structure of sea scallop distribution has important implications for the design of abundance surveys and the assumptions underlying stock assessment methods. These should be investigated. Finally, this work suggests that natural disturbance (sediment stability) may play a substantial role in shaping spatiotemporal distributions and dynamics of benthic marine populations.

Visual predation during springtime foraging of the North Atlantic right whale (Eubalaena glacialis)

To assess the role that vision plays in the ability of the North Atlantic right whale (Eubalaena glacialis) to detect its primary prey species, the calanoid copepod Calanus finmarchicus, we have compared the absorbance spectrum of the E. glacialis rod visual pigment, the transmittance spectra of C. finmarchicus carotenoid pigments, as well as the downwelling irradiance and horizontal radiance spectra collected during springtime at three locations in the western Gulf of Maine. The E. glacialis rod visual pigment absorbs light maximally at 493 nm, while microspectrophotometric measurements of the C. finmarchicus carotenoid pigments reveal transmission spectra with minima matching very well with the E. glacialis rod visual pigment absorbance spectra maximum. Springtime spectral downwelling irradiance and horizontal radiance values from the surface waters of Cape Cod Bay and at all depths in Great South Channel overlap the E. glacialis rod absorbance spectrum, allowing C. finmarchicus to appear as a high-contrast dark silhouette against a bright background spacelight, thus facilitating visually guided contrast foraging. In contrast, spectral downwelling irradiance and horizontal radiance at depth in Cape Cod Bay, and all depths in Wilkinson Basin, do not overlap the E. glacialis rod absorbance spectrum, providing little if any useful light for contrast vision.

Diel and Spatial Dependence of Humpback Song and Non-Song Vocalizations in Fish Spawning Ground

The vocalization behavior of humpback whales was monitored over vast areas of the Gulf of Maine using the passive ocean acoustic waveguide remote sensing technique (POAWRS) over multiple diel cycles in Fall 2006. The humpback vocalizations comprised of both song and non-song are analyzed. The song vocalizations, composed of highly structured and repeatable set of phrases, are characterized by inter-pulse intervals of 3.5 ± 1.8 s. Songs were detected throughout the diel cycle, occuring roughly 40% during the day and 60% during the night. The humpback non-song vocalizations, dominated by shorter duration (≤3 s) downsweep and bow-shaped moans, as well as a small fraction of longer duration (∼5 s) cries, have significantly larger mean and more variable inter-pulse intervals of 14.2 ± 11 s. The non-song vocalizations were detected at night with negligible detections during the day, implying they probably function as nighttime communication signals. The humpback song and non-song vocalizations are separately localized using the moving array triangulation and array invariant techniques. The humpback song and non-song moan calls are both consistently localized to a dense area on northeastern Georges Bank and a less dense region extended from Franklin Basin to the Great South Channel. Humpback cries occur exclusively on northeastern Georges Bank and during nights with coincident dense Atlantic herring shoaling populations, implying the cries are feeding-related.

Stock identification of Atlantic cod (Gadus morhua) in US waters: an interdisciplinary approach

Abstract Mismatches between biological population structure and management unit boundaries often violate the unit-stock assumption, which can reduce the accuracy and relevance of stock assessment results and lead to ineffective fishery management. Since 1972, Atlantic cod (Gadus morhua) have been managed in US waters as two units: the Gulf of Maine and the Georges Bank stocks, both of which have experienced recent difficulties in rebuilding. An interdisciplinary review of available biological information was conducted to investigate cod population structure in US waters and to evaluate the biological appropriateness of the current two-stock model. Our review demonstrates that spawning components in the Great South Channel, Nantucket Shoals, southern New England, and Middle Atlantic are more connected with spawning components in the Gulf of Maine than on eastern Georges Bank, with which they are currently managed. Therefore, a modification of current stock boundaries is recommended to provide a more accurate representation of biological population structure. Proposed alternatives divide inshore and offshore spawning components into separate management units, thereby separating the current Georges Bank stock longitudinally. Continued research, including stock composition analysis, is required to evaluate uncertainties, delineate biological stocks, and develop sustainable management practices that account for intrastock diversity (e.g. winter and spring-spawning components that overlap spatially).

Eyes in the sky: linking satellite oceanography and biotelemetry to explore habitat selection by basking sharks

Satellite-based oceanographic data products are a valuable source of information on potential resource availability for marine species. Satellite oceanography data may be particularly useful in biotelemetry studies on marine species that feed at low trophic levels, such as zooplanktivorous whales, sharks, and rays. The basking shark, Cetorhinus maximus, is a well-documented zooplanktivore in the western North Atlantic, yet little is known of its movements and spatial ecology in this region. A combination of satellite tag technologies were used to describe basking shark movements with respect to concurrent satellite-observed oceanographic conditions in order to test for selection of these environmental variables. Satellite-linked ‘smart’ position only transmitting tags (SPOTs, N = 10) were used to assess horizontal movements, activity space, and habitat selection, while pop-up satellite archival tags (PSATs, N = 7) were used to describe depth preferences of basking sharks during summer and fall. The duration of SPOT tracks ranged from 5 to 45 days. Basking sharks used relatively small activity spaces in three focal areas off Massachusetts: Vineyard Sound, the Great South Channel, and Cape Cod Bay. These sharks appeared to select areas with shallow bottom depths, high primary production and chlorophyll concentrations, and steep surface gradients, but significant selection for these variables was only detected between mid-August and mid-October when the sharks were primarily located in Cape Cod Bay. Basking sharks in the southern Gulf of Maine during summer and fall focus their activities in discrete areas likely to support high primary and secondary productivity. Habitat selection may also be influenced by mating and social activity at times, but further research is needed to differentiate these behaviors from foraging activity. Satellite-based biotelemetry and oceanography are powerful tools that together can provide valuable new insights into habitat selection patterns of highly mobile marine species.

Spatial and temporal distribution of Atlantic mackerel (Scomber scombrus) along the northeast coast of the United States, 1985–1999

Abstract Radlinski, M. K., Sundermeyer, M. A., Bisagni, J. J., and Cadrin, S. X. 2013. Spatial and temporal distribution of Atlantic mackerel (Scomber scombrus) along the northeast coast of the United States, 1985–1999. – ICES Journal of Marine Science, 70: 1151–1161. The distribution of Atlantic mackerel (Scomber scombrus) during their spring migration along the Mid-Atlantic Bight and into the Gulf of Maine has historically been associated with spring warming along the continental shelf. Variations in mackerel distributions based on National Marine Fisheries Service spring surveys were compared with variations in sea surface temperature (SST) from satellite remote sensing for the eastern US continental shelf for the period 1985–1999. The mackerel stock was first analysed as a unit, then separated into three size classes to assess differences in distribution among years and individuals of various lengths. Results showed an across-shelf correlation between catch and March SST in the Mid-Atlantic Bight for both the entire population and each size class. Along-shelf catch variations were correlated with SST for large mackerel, but not total stock or smaller size classes. Finally, the distribution of mackerel length in the Gulf of Maine was negatively correlated with March SST in the Great South Channel. Results suggest surface temperature along the northeast continental shelf may be used to predict certain, but not all, aspects of annual migration along the shelf, and that factors in addition to temperature are also important in controlling distributions of Atlantic mackerel.

Surficial sediment stability on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals

Surficial sediment stability was estimated on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals (36,699km2) by determining where benthic shear stresses derived from an ocean model matched or exceeded the critical shear stress of the observed surficial sediments. The shear stress resulting from M2 and S2 semi-diurnal tides was estimated with the Finite-Volume Community Ocean Model. Mixed-sediment critical shear stress levels were calculated for sediment compositions ranging from sand to boulder-dominated using 67,400 underwater video quadrats sampled from 1999 to 2010. Stresses matched or exceeded the sediment critical levels in 16,926km2 (46%) of the study area, and were inversely related to water depth (r2=69.1%). In depths >50m (10,953km2) all sediments were stable due to weak flow (≤0.4Nm−2). In the shallower higher flow areas (>0.4Nm−2, 25,716km2) only sediments containing gravel remained stable. The largest stresses occurred on Nantucket Shoals and central and northeastern Georges Bank (≥2Nm−2); in these areas only sand with cobbles or sediments dominated by gravel remained stable. Outcrops of these stable sediments were surrounded by highly unstable areas with stresses 2 to 9 times higher than the sediment critical levels. This analysis identifies the locations which likely remain stable even under the high shear stresses typical of Georges Bank, the Great South Channel and eastern Nantucket Shoals. Further, we provide the map products needed to begin investigating the influences of natural sediment disturbance on the spatial and temporal patterns of the benthos including the resilience of stable versus unstable areas to anthropogenic disturbances.

Transient tidal eddy motion in the western Gulf of Maine, part 1: Primary structure

High frequency radar-derived surface current maps of the Great South Channel (GSC) in the western Gulf of Maine in 2005 revealed clockwise (CW) and anticlockwise (ACW) eddy motion associated with the strong regional tidal currents. To better elucidate the kinematics and dynamics of these transient tidal eddy motions, an observational and modeling study was conducted during the weakly stratified conditions of winter 2008–2009. Our moored bottom pressure and ADCP current measurements in 13m depth were augmented by historical current measurements in about 30m in documenting the dominance of highly polarized M2 semidiurnal currents in our nearshore study region. The high-resolution finite element coastal ocean model (QUODDY) – forced by the five principal tidal constituents – produced maps depicting the formation and evolution of the CW and ACW eddy motions that regularly follow maximum ebb and flood flows, respectively. Observation versus model current comparison required that the model bottom current drag coefficient be set to at an unusually high Cd=0.01 – suggesting the importance of form drag in the study region. The observations and model results were consistent in diagnosing CW or ACW eddy motions that (a) form nearshore in the coastal boundary layer (CBL) for about 3h after the respective tidal current maxima and then (b) translate southeastward across the GSC along curved 50m isobath at speeds of about 25m/s. Observation-based and model-based momentum budget estimates were consistent in showing a first order forced semidiurnal standing tidal wave dynamics (like the adjacent Gulf of Maine) which was modulated by adverse pressure gradient/bottom stress forcing to generate the eddy motions. Observation-based estimates of terms in the transport vorticity budget showed that in the shallower Inner Zone subregion (average depth=23m) that the diffusion of nearshore vorticity was dominant in feeding the growth of eddy motion vorticity; while in the somewhat deeper Outer Zone subregion (33m) bottom current lateral shear and water column stretching/squashing was significant in modulating the eddy motion. We conclude that the transient eddy motions in the GSC region are phase eddies that accompany the change of tide across the GSC and are (1) generated by bottom stress gradients in the shallower nearshore – an issue which needs to be better understood for improved future forecasting.

Detection and Recognition of North Atlantic Right Whale Contact Calls in the Presence of Ambient Noise

In this paper, the problem of detecting and recognizing North Atlantic right whale (NARW), <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Eubalaena</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">glacialis</i> , contact calls in the presence of ambient noise is considered. A proposed solution is based on a multistage, hypothesis-testing technique that involves the generalized likelihood ratio test (GLRT) detector, spectrogram testing, and feature vector testing algorithms. The main contributions of this paper are the inclusion of noise kernels for signals likely to produce false alarms and a second stage classification algorithm which extracts parameters from candidate contact calls and constructs a scaled squared error statistic for parameters which lie outside the range of expected calls. Closed-form representations of the algorithms are derived and realizable detection schemes are developed. Test results show that the proposed technique is able to detect approximately 80% of the contact calls detected by the human operator with about 26 false alarms per 24 h of observation. Testing data set included 44 227 right whale contact calls detected by eight human operators who performed visual and aural inspection of the data spectrogram. Data were collected in different periods from March 2001 to February 2007, in Cape Cod Bay, Great South Channel, and in the coastal waters of Georgia.

Regional-scale mean copepod concentration indicates relative abundance of North Atlantic right whales

Management plans to reduce human-caused deaths of North Atlantic right whales Eubalaena glacialis depend, in part, on knowing when and where right whales are likely to be found. Local environmental conditions that influence movements of feeding right whales, such as ultradense copepod patches, are unpredictable and ephemeral. We examined the utility of using the regional-scale mean copepod concentration as an indicator of the abundance of right whales in 2 critical habitats off the northeastern coast of the United States: Cape Cod Bay and Great South Channel. Right whales are usually found in Cape Cod Bay during the late winter and early spring, and in the Great South Channel during the late spring and early summer. We found a significant positive relationship between mean concentration of the copepod Calanus finmarchicus in the western Gulf of Maine and the frequency of right whale sightings in the Great South Channel. In Cape Cod Bay we found a significant positive relationship between the mean concentration of other copepods (largely Pseudocalanus spp. and Centropages spp.) and the frequency of right whale sightings. This information could be used to further our understanding of the environmental factors that drive seasonal movement and aggregation of right whales in the Gulf of Maine, and it offers a tool to resource managers and modelers who seek to predict the movements of right whales based upon the concentration of copepods.