Great South Channel Research Articles

Model-based estimates of right whale habitat use in the Gulf of Maine

Balancing human uses of the marine environment with the recovery of protected species requires accurate information on when and where species of interest are likely to be present. Here, we describe a system that can produce useful estimates of right whale Eubalaena glacialis presence and abundance on their feeding grounds in the Gulf of Maine. The foundation of our system is a coupled physical-biological model of the copepod Calanus finmarchicus, the preferred prey of right whales. From the modeled prey densities, we can estimate when whales will appear in the Great South Channel feeding ground. Based on our experience with the system, we consider how the relationship between right whales and copepods changes across spatial scales. The scale-dependent relationship between whales and copepods provides insight into how to improve future estimates of the distribution of right whales and other pelagic predators.

Probability and mitigation of vessel encounters with North Atlantic right whales

Successful mitigation of vessel-whale encounters requires quantitative estimates of vessel strikes, how strike rates change over time, where strikes are most likely to occur, and options for minimizing strikes. In addressing these issues, we first demonstrate a 3- to 4-fold increase in the number of reported large whale-vessel strikes worldwide from the early 1970s to the early 2000s, corresponding to a 3-fold increase in the number of vessels in the world fleet that is paralleled by an increase in vessel tonnage and speed. Second, we estimate a 50% chance of 14 or more annual ves- sel-strike reports worldwide between 1999 and 2002. For North Atlantic right whales Eubalaena glacialis, we estimate a 60% chance of observing at least 1 right whale death from vessel strike. Adjusting for undetermined causes of death and unobserved deaths, we estimate a 10-fold increase (from 1 to 10) in the expected annual number of fatal ship strikes. Third, we evaluate the eastern United States geographic distribution of right whales and vessels to calculate relative probabilities of vessel-whale encounters among 3 major right whale habitats. We determine that the Southern Calv- ing Ground poses the greatest threat of a vessel strike: 1.6- and 7-fold greater than in Cape Cod Bay and the Great South Channel, respectively. Finally, for the Great South Channel region we present a quantitatively determined vessel-traffic routing option that would achieve a 39% reduction in ves- sel-whale encounter probabilities. The methods employed in assessing encounter probabilities and vessel-routing options can be applied elsewhere to enhance the conservation of endangered and threatened species that suffer vessel-strike mortality.

Drifter observations of the Gulf of Maine Coastal Current

Two-hundred and twenty seven satellite-tracked drifters were deployed in the Gulf of Maine (GoM) from 1988 to 2007, primarily during spring and summer. The archive of tracks includes over 100,000 km logged thus far. Statistics such as transit times, mean velocities, response to wind events, and preferred pathways are compiled for various areas of the coastal GoM. We compare Lagrangian flow with Eulerian estimates from nearby moorings and evaluate drifter trajectories using Ekman theory and 3-D ocean circulation models. Results indicate that the Gulf of Maine Coastal Current is a strong and persistent feature centered on the 94±23 m isobath, but that particles: (a) deviate from the seasonal-mean core fairly regularly, and are often re-entrained; (b) follow a slower (9 cm/s), less-constrained path in the western portion off the coast of Maine relative to the eastern (16 cm/s) section; and (c) can be affected by wind events and small-scale baroclinic structures. Residence times calculated for each 1/2° grid cell throughout the GoM depict some regions (Eastern Maine and Western Nova Scotia) as being relatively steady, flow-through systems, while others (Penobscot, Great South Channel) have more variable, branching pathways. Travel times for drifters that are retained within the coastal current along the entire western side of the Gulf of Maine are typically less than two months (55 days).

Sea scallop mass mortality in a Marine Protected Area

A mass mortality of the sea scallop Placopecten magellanicus occurred between 2004 and 2005 in the Nantucket Lightship Closed Area (NLCA), a Marine Protected Area (MPA) in the Great South Channel, USA. On a broad scale (3951 km 2 ), scallop density (ind. m -2 ) in the MPA declined by 35%. On a fine scale (504 km 2 ), in the northeast corner of the MPA where most scallops occur, density declined by 50%. The scallops that perished were large and probably old; 80% had shell heights >130 mm (H ∞ = 155 mm). Only 6% of the decline in the northeast corner was explained by fishing mortality. The mass mortality equaled approximately 6484 metric tons of harvestable scal- lop meat (equivalent to US $100 million ex-vessel). Closing the NLCA to fishing in 1994 removed fish- ing mortality, increasing scallop survival rates. Scallops that recruited in 1994 reached 10 yr of age in 2004. Individuals of this age may suffer the effects of senescence, including parasitism by shell bor- ers and prokaryotic infection, which likely caused the mass mortality. If MPAs are used to manage fisheries, surveys with the accuracy and precision to resolve population dynamics on the spatial scale of the MPA are required or the benefits may be offset by increases in natural mortality.

Geolocation of Atlantic cod (Gadus morhua) movements in the Gulf of Maine using tidal information

AbstractInformation derived from archival tags (digital storage tags, DSTs) were used to backtrack the migration of 11 tagged Atlantic cod (Gadus morhua) during 2001 in Massachusetts Bay, the Gulf of Maine, and Georges Bank. The DST tags continuously recorded time, temperature and depth. To geolocate fish positions during its time at large, we first extracted the tidal signal from the pressure recordings on the DST tags, and then compared the resulting data to data predicted with a Massachusetts Bay tidal model that provided us with geographical coordinates at a given date and time. Using least‐squares criteria within an estimated geographical region of confidence that was constrained by biological and statistical information (e.g. swimming speed, known release and recapture location, and bottom depth) we were able to geolocate the fish. The resultant geolocated migration tracks indicate a large degree of movement of Atlantic cod in the region and an elevated importance of the Great South Channel (GSC) migration corridor between Massachusetts Bay and the western Georges Bank and Nantucket Shoals region. This observation contrasts strongly with inferences of limited movements by Atlantic cod based on conventional tag recapture methods (mean of 1200 km traveled versus 44 km traveled as measured by conventional tagging and geolocation, respectively). This study demonstrates that geolocation methodologies applied to archival tag studies hold great promise of becoming an important new tool for fisheries managers to quantify the movements of fishes. It also points out the need for greater collaboration between fisheries scientists and oceanographers, and particularly for the development of improved tidal models to cover stock regions more accurately and with higher precision.

Stock Identification of Atlantic Cod in U.S. Waters Using Microsatellite and Single Nucleotide Polymorphism DNA Analyses

AbstractManagement of Atlantic cod Gadus morhua in U.S. waters is based on a two‐stock model composed of stocks from (1) the Gulf of Maine (GOM) and (2) Georges Bank (GB) and areas south; however, evidence suggests a more fine‐scale structuring. We used microsatellite and single nucleotide polymorphism (SNP) analyses to investigate the stock structure of Atlantic cod in U.S. waters. In year 1, we analyzed microsatellite loci in larvae from GB, age‐0 juveniles from multiple locations in the GOM, and adults of unknown maturity from GB, Jeffreys Ledge, and the Great South Channel (GSC). In year 2, Atlantic cod collected from spawning aggregations in Ipswich Bay in the spring and at GB, Stellwagen Bank, Chatham, and Ipswich Bay in the winter, along with adults of unknown maturity from Long Island, New York, were surveyed at a modified battery of microsatellites and three SNPs. In year 1, we saw no significant differences in allelic frequencies between our composite sample of adult and juvenile cod from the GOM and that from GB nor between the collections from the GSC and any other site. However, a composite sample of juveniles from Massachusetts Bay was significantly different from the GB collection and juveniles from Maine. In year 2, we found highly significant differences in allelic frequencies between Atlantic cod collections from the GOM and GB. The spring collection from Ipswich Bay was highly distinct from the collection from GB and all other sites, including the winter‐spawning Ipswich Bay collection. The Long Island sample was genetically distinct from the GB and the spring collections from Ipswich Bay, but not from other collections. Our study indicates that there is genetic heterogeneity of stocks in U.S. waters, but its structure is complex and to better understand it many more spatially and temporally separated samples must be characterized.

Seasonal ambient noise levels and impacts on communication in the North Atlantic right whale

The North Atlantic right whale is a highly endangered species of baleen whale. Acoustic communication plays an important role in the social behavior of these whales. Right whales are found in coastal waters along the east coast of the United States, an area characterized by high levels of human activity. Most of these activities generate noise that is propagated into the coastal marine environment. The goals of this project are to characterize the noise, both natural and anthropogenic, in right whale habitat areas to determine what levels of noise the whales are regularly exposed to, and whether the acoustic behavior of right whales changes in response to increased noise. Continuous recordings were made from autonomous bottom-mounted recorders in three major habitat areas in 2004 and 2005; Cape Cod Bay (December–May), Great South Channel (May), and the Bay of Fundy, Canada (August) to passively detect right whales by recording their vocalizations. Here, we describe the ambient noise levels in these recordings to describe the daily acoustic environment of right whales, how noise varied over diel, weekly, and seasonal time scales, and whether noise levels correlated with any observed changes in acoustic behavior of the whales.

Linking sound production and dive behavior in feeding humpback whales

Acoustic studies of baleen whales are becoming increasingly common. However, a minority of studies combines acoustic data with technologies that allow sound production to be placed in a detailed behavioral context. Noninvasive digital acoustic recording tags (DTAGS) were attached to humpback whales (Megaptera novaeangliae) on the western North Atlantic’s Great South Channel feeding grounds to study foraging and acoustic behavior. Acoustic records totaling 48.4 data hours from four attachments were aurally and automatically analyzed, and occurrences of several sound types were correlated with body orientation and dive behavior of the tagged subject. Whales produced a wide variety of sound types, which differed in depth and context of production. Long, tonal groans and moans (3-s average duration, 500-Hz peak frequency) occurred more frequently in upper portions of the water column, in contrast to broadband paired bursts (50-ms average duration, 300-Hz peak frequency), which appeared associated with bottom feeding. Virtual behavioral study using GEOZUI4D is used to visualize how sounds may be associated with foraging behavior. Overall results indicate sophisticated, situation-dependent use of sound. Behavioral implications of masking effects by surrounding vessel traffic noise are discussed.

Shell growth of sea scallops (Placopecten magellanicus) in the southern and northern Great South Channel, USA

Abstract Shell growth of sea scallops in two commercially productive regions of the Great South Channel (GSC) (41°4′N 69°16′W) was studied using tag–recapture experiments. Commercial fishers captured and returned 9.7% of the 11 704 sea scallops tagged in the southern GSC study area, and 7.9% of the 18 274 sea scallops tagged in the northern GSC study area. Scallop density and shell height distribution were sampled with underwater video in the two study areas. In the southern GSC tagged scallops grew faster, reached larger asymptotic size, and had higher growth performance (Φ′) than in the northern GSC study area. Mean sea scallop density in the southern GSC was 0.117 scallops m−2 (s.e. = 0.01), and 2.601 scallops m−2 (s.e. = 0.28) in the northern GSC. Environmental factors, fishing pressure, and sea scallop density all influence shell growth on a fine geographic scale (1–100 km2) and should be considered in area-specific management strategies, such as that currently used in the USA sea scallop fishery.

Sound production patterns from humpback whales in a high latitude foraging area

Numerous studies have been conducted on humpback whale song, but substantially fewer have focused on the acoustic properties of non‐song sound production (i.e., feeding and social sounds). Non‐invasive digital acoustic recording tags (DTAGS) were attached to humpback whales on the western North Atlantics Great South Channel feeding grounds during July 2004. Acoustic records totaling 48.4 data hours from four of these attachments were aurally analyzed for temporal trends in whale signal production. A custom automatic detection function was also used to identify occurrences of specific signals and evaluate their temporal consistency. Patterns in sound usage varied by stage of foraging dive and by time of day. Amount of time with signals present was greater at the bottom of dives than during surface periods, indicating that sounds are probably related to foraging at depth. For the two tags that recorded at night, signals were present during a greater proportion of daylight hours than night hours. These results will be compared with previously published trends describing diel patterns in male humpback whale song chorusing on the breeding grounds. Data from the continuation of this research during the summer of 2005 will also be included.

Structure and timing of recirculation around Georges Bank: an observational and modeling study at the Great South Channel: Part I––ensemble smoother

An ensemble smoother is used to assimilate moored temperature, salinity, and velocity data into a local area primitive equation model. The overall goal of the analysis is to estimate variability of Georges Bank recirculation, i.e., northward flow through the Great South channel in support of the US Global Ocean Ecosystem Dynamics (GLOBEC) Georges Bank experiment. Here, identical twin experiments are carried out to test the ensemble smoother with a finite-element circulation model of the Great South Channel, based on a previous formulation designated QUODDY. The ensemble smoother utilizes a finite number of Monte Carlo model simulations to estimate model error covariance. The prior distribution from which the ensemble members are simulated is implicitly defined by the forward model by adding spatially correlated Gaussian random variables to the initial conditions, and time-dependent boundary elevations. Atmospheric forcing (wind stress) is derived from buoy measurements and is assumed to be known with certainty. The accuracy of the estimator depends on the state space variable being estimated and proximity to the data. In these twin experiments the domain-wide mean error variance of temperature, salinity, and velocity were reduced 96%, 93%, and 89%, respectively. The prediction statistics for the estimate are accurate throughout the domain. Non-linearity of the forward model and subsequent skewness of the posterior probability density function (pdf) are investigated. It is found that the posterior distribution is sufficiently Gaussian to use Gaussian confidence intervals. These results give confidence for using the numerical formulation and ensemble smoother to examine variability in circulation at Great South Channel with available data.

Variation in acoustic activity of North Atlantic right whales in three critical habitat areas in 2004

The North Atlantic right whale is a critically endangered species. Five major habitat areas have been defined based on the presence of groups of right whales during different seasons of the year. Autonomous seafloor acoustic recorders, ‘‘pop-ups,’’ have been used over the past five years to identify the presence/absence of right whales in Cape Cod Bay, Massachusetts by detection of right whale contact calls. These recordings have shown the effectiveness of using acoustics to monitor right whale behavior. In 2004, multi-unit arrays of pop-ups were deployed in three critical habitat areas; Cape Cod Bay (January–April), Great South Channel (May), and the Bay of Fundy (August). The presence/absence of right whale calls within range of the array, estimates of the minimum number of individuals calling, the types of calls recorded, and the diurnal patterns of sound production were measured to compare the acoustic repertoire use of right whales in these three habitats. These data were compared to visual survey data in the Bay of Fundy. These results can be used to compare diurnal and nocturnal behavioral patterns of the whales and provide information on behavioral differences between these three critical habitat areas.

Characterization of the mean and seasonal flow regime on Georges Bank from shipboard acoustic Doppler current profiler data

From 1995 through 1999 the Northwest Atlantic GLOBEC program sponsored over 100 oceanographic cruises to Georges Bank and the surrounding waters. About 12,000 hours of shipboard acoustic Doppler current profiler (ADCP) data were collected during the program on more than 80 of these cruises, forming one of the most extensive three‐dimensional current measurement data sets ever collected. These data have been used here to examine the seasonal variability of the bank’s barotropic flow field. The variability has been described using the 5 years of ADCP observations to derive both small area statistics and seasonal least squares fits, and a more flexible description based upon interpolating functions describing the spatial and temporal dependence of the bank‐wide stream functions. Both methods show the seasonal acceleration of the bank gyre in the summer with larger velocities along the north flank and smaller seasonal variations over the southern flank. The stream functions show the seasonal change from a winter condition with on‐bank flow from the northwest through to the acceleration and ecologically critical closure of the anticyclonic Georges Bank gyre during the summer. On the basis of annual and semiannual decompositions of the seasonal progression, the closure of the gyre in the area of the Great South Channel appears to occur by June, first in the shallow waters and then progressively southward, but it does not extend as far as the middle of the southern flank. A series of monthly Lagrangian drift predictions based upon the stream function analysis, suggest that the observed seasonal movement of the larval fish from their spawning grounds over the Northeast Peak in March, through to their late summer concentration over the central bank, can generally be ascribed to the seasonal barotropic flow over the inner portion of the bank. The analysis also showed that those larvae found over the outer part of the bank are, almost certainly, doomed to be swept out of the banks’ ecosystem, to either the west or south.

Feature-oriented regional modeling and simulations in the Gulf of Maine and Georges Bank

The multiscale synoptic circulation system in the Gulf of Maine and Georges Bank (GOMGB) region is presented using a feature-oriented approach. Prevalent synoptic circulation structures, or ‘features’, are identified from previous observational studies. These features include the buoyancy-driven Maine Coastal Current, the Georges Bank anticyclonic frontal circulation system, the basin-scale cyclonic gyres (Jordan, Georges and Wilkinson), the deep inflow through the Northeast Channel (NEC), the shallow outflow via the Great South Channel (GSC), and the shelf–slope front (SSF). Their synoptic water–mass ( T– S) structures are characterized and parameterized in a generalized formulation to develop temperature–salinity feature models. A synoptic initialization scheme for feature-oriented regional modeling and simulation (FORMS) of the circulation in the coastal-to-deep region of the GOMGB system is then developed. First, the temperature and salinity feature-model profiles are placed on a regional circulation template and then objectively analyzed with appropriate background climatology in the coastal region. Furthermore, these fields are melded with adjacent deep-ocean regional circulation (Gulf Stream Meander and Ring region) along and across the SSF. These initialization fields are then used for dynamical simulations via the primitive equation model. Simulation results are analyzed to calibrate the multiparameter feature-oriented modeling system. Experimental short-term synoptic simulations are presented for multiple resolutions in different regions with and without atmospheric forcing. The presented ‘generic and portable’ methodology demonstrates the potential of applying similar FORMS in many other regions of the Global Coastal Ocean.

On the geographic and seasonal patterns of the near-surface circulation on Georges Bank — from real and simulated drifters

We examine the near-surface circulation on Georges Bank by comparing data collected from satellite-tracked drifters in the real ocean and data computed for simulated drifters in a virtual ocean. The observed trajectory data set was obtained from drifters drogued at 10 m during the 1995–1997 US GLOBEC Northwest Atlantic/Georges Bank field program. The simulated drifter trajectories were computed based upon the seasonal circulation as predicted by the Dartmouth Circulation Model, using bimonthly climatological forcing. The observed and simulated drifter patterns indicate well-organized anticyclonic around-bank flow on the northern flank, Northeast Peak, and southern flank throughout the year. The key to recirculation around the Bank is the seasonality of the northward flow in the Great South Channel. Winter months are characterized by little northward flow, while there is significant northward flow in the Great South Channel in summer. In late summer, both observed and numerical drifters indicate a minimum recirculation time on Georges Bank of roughly 40 days. The simulated drifter trajectories generally predict a seasonal climate consistent with the observed drifters, though the effects of weather events on the observed drifters are not captured by the numerical simulations.

Flow-induced aggregation of plankton at a front: a 2-D Eulerian model study

We use a simple 2-D Eulerian numerical model to illustrate a mechanism whereby depth-keeping swimming behavior on the part of an organism, combined with a convergent flow field at a surface front, can create dense populations. In this model, a steady flow field and vertical diffusivity are prescribed, along with an initial plankton distribution. The plankton swim vertically with speeds that depend only on depth, where the form of that dependence may reflect such factors as the vertical variation in light level or the concentration of some prey organism. The Eulerian nature of the model enables diffusion to be treated explicitly. The numerical model describes the evolution of an initial plankton patch in time. A parameter study illustrates the roles played by diffusion, water velocity and details of the animals’ swimming behavior on patch evolution. Output from the model is compared with data taken at a dense plankton patch observed near a small-scale front in the northern Great South Channel in early June 1989.

Stock Identification of HaddockMelanogrammus aeglefinuson Georges Bank Based on Otolith Shape Analysis

Abstract Otolith shape analysis was examined to determine its utility for stock identification of haddock Melanogrammus aeglefinus on Georges Bank. Otolith samples were collected from the Northeast Peak (eastern Georges Bank) and the Great South Channel (western Georges Bank) spawning components to examine stock continuity across Georges Bank. Otolith shape was described using 20 Fourier harmonics, four morphometric characteristics (area, length, width, and perimeter), and two shape indices (circularity and rectangularity). Potential confounding sources of variation (fish length, otolith position, age-group, and year-class) were examined and accounted for in the analyses before interpretation of spatial/stock differences. Significant age-specific differences in otolith shape between eastern and western Georges Bank haddock indicated stock separation across the Bank. Classification success for each spawning component ranged from 63 to 80% across age-groups and appeared to depend on growth rate differences....

RIGHT WHALES (EUBALAENA GLACIALIS) ON JEFFREYS LEDGE: A HABITAT OF UNRECOGNIZED IMPORTANCE?

AbstractNorth Atlantic right whales (Eubalaena glacialis) are known to spend the majority of the year between the Great South Channel southeast of Cape Cod, and the Nova Scotian shelf. We examined sightings of right whales on and around Jeffreys Ledge, a 54‐km‐long glacial deposit off the coast of northern Massachusetts, New Hampshire, and Maine. Sightings on Jeffreys Ledge were extracted from three data sets: (1) a systematic survey of the entire northeastern continental shelf between 1979 and 1982, (2) whale‐watch and research‐cruise sighting data from 1984 to 1997, and (3) a collaborative database of sightings collected by organizations conducting right whale research and all other available sources. Each database supported two seasonal sighting peaks. During summer (especially July and August) sightings were primarily of mother‐calf pairs. Several cow‐calf pairs were seen over several days to weeks. Several females were resighted in more than one year, but only when calves were present. During October, November, and December, sightings included all age classes, surface‐feeding behavior was frequently observed, and some animals were resighted over several weeks. Given the relatively reduced sighting effort during fall, this number of sightings is surprising. During the 20 yr of observations, 52 of 374 photo‐identified North Atlantic right whales (13.9%) were seen at least once on Jeffreys Ledge. We suggest that Jeffreys Ledge may be a more important right whale habitat than previously believed, and that it may play an important role in annual movements and distribution of this population.

NOTEPROROCENTRUM LIMA (DINOPHYCEAE) IN WATERS OF THE GREAT SOUTH CHANNEL NEAR GEORGES BANK

A 0–10 m plankton tow collected at one station in the Great South Channel near Georges Bank in June 1994 was rich in dinoflagellates, with the usually epibenthic Prorocentrum lima (Ehrenberg) Dodge present as the dominant species. Temperature and salinity profiles suggest that the phytoplankton population at this station may have been advected from waters on the southern flank of Georges Bank. This is the first unequivocal report of P. lima in the plankton community of the Great South Channel and its vicinity.

The Maine coastal current: spring climatological circulation

Computational results are presented for the climatological circulation in the Gulf of Maine, with special emphasis on its coastal current in the periods March-April and May-June. The simulations use a 3-D prognostic Finite Element model. The computational domain includes the entire Gulf plus adjacent waters, with a higher resolution coastal domain nested within. A series of six bimonthly Gulf-wide simulations establishes the spring conditions within the annual climatological cycle. The distinctive Gulf of Maine cyclonic circulation is persistent, with significant bimonthly modulation of key Maine coastal current features. In all cases there is a significant barotropic pressure variation of order 5 cm along the margin of the coastal current, accompanying detailed, variable interactions between it and the interior Gulf. In the spring periods of emphasis, a well-developed coastal current is present. Numerical drifters generally confirm the existence of branch points south of Penobscot Bay, east of Cape Ann, and at Great South Channel. Important differences between the March-April and May-June circulation are apparent. In particular, both the offshore transport at the Penobscot branch point and the along-shore transport at Cape Ann are strong, dominant features in March-April; both are weak in May-June. The more highly resolved, nested calculations replicate the general structure of the Gulf-wide circulation provided the correct boundary conditions are applied, confirming the pervasive dynamical influence of the Gulf-scale circulation on the coastal current. The nested calculations also expose finer structure due to river sources, heat flux, and topographic refinement. The separate influences of local (tidal rectification, local wind, local baroclinicity, river discharges) and remote (Gulf-scale) forcing are examined. Tidal rectification in the Eastern Gulf and wind-driven return flow near-shore are both important contributors to the details of the coastal current. Local baroclinicity is critical and must be in balance with the offshore pressure conditions; otherwise unrealistic exchanges with the Gulf occur. An approximate division of local and remote dynamical regimes lies near the 100-m isobath. Implications are drawn for future modeling studies.