Zooplankton Transport Research Articles

Recent Increase in Nybelinia surmenicola Prevalence and Intensity in Pacific Hake (Merluccius productus) Off the United States West Coast

A larval marine cestode was found in 82.0% of 834 Pacific hake (Merluccius productus) stomachs collected from 341 trawl stations along the United States west coast during the summers of 2008 and 2009. Morphology and DNA sequencing was used to identify the cestode as Nybelinia surmenicola. In an examination of 131 Pacific hake stomachs collected from the same region in 1999, N. surmenicola prevalence was 35.1%. The results from a general linear model suggested that their prevalence is influenced by year and latitude, Pacific hake size, and sex. Mean intensity of N. surmenicola in 2008-2009 was 20.22 (±1.13 SE) and was positively related to Pacific hake length and the latitude of collection. Year-1 Pacific hake (<27 cm length) had significantly lower prevalence and intensity of N. surmenicola compared to older and larger fish. Pacific hake collected south of Point Conception, California (32.5 to 35°N) had lower prevalence and intensity of N. surmenicola compared to those collected in northern latitudes (35.1 to 48.4°N). Higher N. surmenicola prevalence in Pacific hake in recent years suggests food-web fluctuations in the northern California current ecosystem caused by changes in ocean transport of zooplankton or pelagic fish distributions and warrants future monitoring as a metric for ecosystem change.

Ballast water transport of non-indigenous zooplankton to Canadian ports

AbstractDiBacco, C., Humphrey, D. B., Nasmith, L. E., and Levings, C. D. 2012. Ballast water transport of non-indigenous zooplankton to Canadian ports. – ICES Journal of Marine Science, 69: 483–491. Ballast water is one of the primary transport vectors for the transfer and introduction of non-indigenous zooplankton (NIZ). Regulations require vessels from overseas to conduct mid-ocean exchange before discharging ballast in Canadian ports. Intracoastal vessels from nearby ports may be exempt from exchange, whereas intracoastal vessels from more distant ports are required to exchange. Zooplankton in the ballast water of transoceanic exchanged (TOE), intracoastal exchanged (ICE), and intracoastal unexchanged (ICU) vessels arriving at Canada's west (WC) and east (EC) coasts were examined. NIZ density, propagule pressure, taxon richness, and community composition were compared among the three shipping classes. The WC ports received greater densities of NIZ and had greater NIZ propagule pressure than EC ports. Within WC vessels, NIZ propagule pressure and density were significantly greater in ICU vessels. TOE vessels on the EC had the greatest NIZ propagule pressure and density. ICU vessels entering Vancouver ports represented the greatest invasion risk to Canadian waters. These vessels likely mediate secondary invasions by facilitating the transport of unexchanged ballast directly from ports previously invaded, whereas short ICU voyage duration enhances organism survivorship and vessels transport NIZ over natural dispersal barriers.

The influence of hydrodynamic processes on zooplankton transport and distributions in the North Western Mediterranean: Estimates from a Lagrangian model

A Lagrangian module has been developed and coupled with the 3D circulation model Symphonie to study the influence of hydrodynamic processes on zooplankton transport and distributions in the North Western Mediterranean (NWM). Individuals are released every 3 days from March to August 2001 in two initial areas: around the DYFAMED sampling station in the central Ligurian Sea and in the Rhône river plume. Then the individuals are tracked for 40 days either as passive particles or with a simple diel vertical migration (DVM) pattern. The simulations suggest strong seasonal patterns in the distributions of the individuals released around the DYFAMED sampling station. Individuals spread all over the NWM basin after 40 days but different patterns occur depending on the season, the initial depths of release and the capacity of DVM. Offshore-shelf transport only occurs in April and May with particles ending up in the Gulf of Lions (GoL) in low concentrations. In other months, the Northern Current (NC) can be considered as a barrier for particles entering the GoL from the offshore sea. A quarter to a half of passive individuals released in the Rhône river plume remain in the GoL. The rest is transported by the NC towards the Catalan Sea. Applying a simple DVM scheme does not increase the retention of particles on the shelf.

Upslope transport of near-bed zooplankton

Abstract Zooplankton residing just above the deep-sea floor is an important component of the benthic/benthopelagic food chain. Consuming planktonic particulates and organisms, holoplankton and meroplankton are prey for fish and large invertebrates. Mechanisms controlling their abundances have been explored over relatively long time scales (months to years). Here, zooplankton were sampled every 2 h for 2.2 d using a moored, automated, serial zooplankton pump. The physical regime (currents and temperature) 1–100 m above bottom was measured during an inclusive 24-d period. The study site was located on the upper continental slope (750 m) of the Mid-Atlantic Bight, between the productive shelf and more impoverished rise and abyss. The coupled biological and physical records indicated tidally driven, net upslope transport of the holoplankton. The copepod (74.5% of collections) time series showed marked periodicity with a peak frequency of ∼13 h, approximately the diurnal tide (Fourier analysis). Local maxima corresponded with minimal water temperatures. Moreover, tidal cross-slope flow was highly coherent and 90° out of phase with temperature. Thus, maximal copepod concentrations, originating in colder deeper water, would be transported up the slope by the tide. Estimated net displacement of ∼1 km/d would deliver the animals to continental-shelf depths within a couple weeks. Time series of the much less abundant larvaceans (urochordates) (15.3%) and polychaete larvae (8.9%) showed periodicities with peak frequencies of 8–9 h. Statistical significance of the periodic signals could not be determined due to low numbers. Revealing holoplankton dynamics on scales of hours, this study may contribute to understanding of, for example, copepod feeding and aggregation near the deep-sea floor.

Influence of the Columbia River plume on cross‐shelf transport of zooplankton

A high‐resolution sampling strategy along a cross‐shelf transect near the mouth of the Columbia River was used to determine the distribution, abundance, and spatial relationship between plankton and a tidally modulated river plume. During a strong ebb tide, zooplankton‐sized particles were concentrated on the seaward side of the plume front and transported 15 km across the shelf at a rate of 38 cm s−1, roughly fivefold faster than typical wind‐driven transport in the region. Physical processes associated with the developing plume vertically depressed relatively dense layers of phytoplankton and zooplankton an average of 7 m deeper into the water column beneath the plume and up to 10 km beyond the extent of the plume front. The enhanced cross‐shelf transport by the Columbia River plume is unique for this region of the Northern California Current. Its impact on the vertical and horizontal distribution of zooplankton shifts prey availability for higher trophic levels, such as larval fish, that primarily feed in the upper water column.

Mesoscale physical processes and zooplankton transport-retention in the northern Norwegian shelf region

Abstract Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100 m on the shelf in the survey region is approximately 6.4×103 tonnes wet weight day−1 northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100 m over Malangsgrunnen was approximately 0.08 day−1, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort.

Dynamic patterns of zooplankton transport and migration in Catuama Inlet (Pernambuco, Brazil), with emphasis on the decapod crustacean larvae

Dynamic patterns of zooplankton transport and migration in Catuama Inlet (Pernambuco, Brazil), with emphasis on the decapod crustacean larvae

Dynamic patterns of zooplankton transport and migration in Catuama Inlet (Pernambuco, Brazil), with emphasis on the decapod crustacean larvae

Esta investigacion tiene como objetivo cuantificar y modelar el transporte de zooplancton, con enfasis en las larvas de Crustacea Decapoda. El muestreo se realizo a tres profundidades con una bomba de plancton acoplada a una red con malla de 300 /?m. Se obtuvieron los datos de corrientes con un ADCP. Los datos mostraron la existencia de campos de corrientes y de transporte vertical y horizontal heterogeneos. Se identificaron 27 grupos de Decapoda (larvas de Sergestidae, Porcellanidae, Upogebiidae, Caridea, Brachyura). La mayoria de las especies y fases larvales mostraron patrones de migracion vertical caracteristicos, en fase con los ciclos de la marea diurnos, favoreciendo la retencion o exportacion del estuario.

The influence of diel vertical migration on zooplankton transport and recruitment in an upwelling region: estimates from a coupled behavioral‐physical model

AbstractDiel vertical migration (DVM) is a common zooplankton behavior in which organisms reside in surface or near‐surface waters at night and at deeper depths during the day. In many upwelling regions, DVM reduces the transport of organisms away from the region. It is unclear, however, what role DVM plays in recruitment (the arrival of larvae or juveniles to locations where they will become reproducing adults) to upwelling regions. In this study, we estimate the influence of DVM on zooplankton transport, the level of recruitment of locally produced propagules (self‐recruitment), and sources of recruits in the upwelling region near Monterey Bay, California, by simulating the trajectories of fixed‐depth and vertically migrating organisms with a drifter‐tracking algorithm driven by climatological velocity fields from a three‐dimensional hydrodynamic model. Our simulations suggest that DVM into subsurface poleward and onshore currents during the day does not fully compensate for equatorward and offshore transport in the surface Ekman layer at night and does not retain zooplankton in the Monterey Bay region. Our simulations also suggest that DVM decreases the ability of zooplankton to return to the region after being transported away and shifts source regions for recruits closer to the bay. While DVM does not appear to substantially increase the potential for self‐recruitment to the region, this study indicates that other mechanisms, such as transport during non‐upwelling periods, continuous transport below the surface, increases in mean transport depth over time, or seasonal changes in hydrography, may still enable relatively high levels of self‐recruitment to this highly advective region.

How internal waves influence the vertical distribution of zooplankton

Summary1. We present data with a high spatio‐temporal resolution from a 72‐h field survey in Bautzen Reservoir (Saxony, Germany). The aims of this survey were to observe hydrophysical processes during a period of unstable stratification in spring and investigate the effect of wind‐induced internal waves on the vertical distribution of zooplankton.2. Wind velocities up to 10 m s−1 caused a strong downwelling event of warm water at the sampling site and led to the generation of internal waves with an amplitude of 4 m.3. The zooplankton community, which was dominated by Daphnia galeata, inhabited epilimnetic waters. Downwelling enlarged the thickness of the epilimnetic layer and, hence, led to high zooplankton abundances down to relatively deep water strata indicating lateral transport of zooplankton. As a consequence, area‐specific zooplankton abundances increased considerably (max. fourfold) during downwelling.4. We conclude that classical limnological field sampling, such as for monitoring purposes, can lead to severely biased estimates of zooplankton abundance due to the interfering effects of hydrophysical processes like internal waves.5. Backscattering strengths measured by a simultaneously deployed Acoustic Doppler Current Profiler (600 kHz) were found to be correlated with estimated zooplankton abundances based on plankton samples.

Application of a lower trophic level model to a coastal sea ecosystem

Two years of environmental and plankton monitoring data observed at a station located in the coastal zone of Peter the Great Bay (Japan Sea) are used to study the seasonal development of a pelagic ecosystem. The functioning of the lower trophic levels was approximated with a semi-empirical ecosystem model based on the set of differential equations of NEMURO LTL model including nutrients, phytoplankton, and zooplankton. Advective fluxes were parameterised because of the strong cross-shelf circulation in the area driven by monsoon winds. Realistic results were obtained with variable coefficients of growth, grazing and mortality instead of constant coefficients. The coefficients’ variability takes into account seasonal fluctuations of species composition and plankton ontogenic changes. Lack of a bacterial component in the NEMURO-based model was compensated by very high production of phytoplankton. Horizontal advection was important for nutrients export from the coastal zone in summer, for herbivorous zooplankton transport into the area in spring and autumn, and controlled predatory zooplankton abundance throughout the period of investigation.

Zooplankton distributions and transport across the northeastern tidal front of Georges Bank

Tidal fronts are important sites of contact and mixing between zooplankton populations on and off Georges Bank in the northwest Atlantic. This study, a component of the US GLOBEC program, examined crossfrontal distributions of zooplankton, especially the copepod Calanus finmarchicus, across the northeastern tidal front of Georges Bank relative to the tidal cycle, frontal structure, flow field, and day/night cycle. A unique feature was a precisely executed sampling strategy designed to isolate spatial, tidal, and day/night effects. Vertically stratified zooplankton sampling was done along tidally coordinated crossfrontal transects and during time series studies at specific locations. Zooplankton taxa differed in their spatial and vertical distributions relative to the front and the hydrographic structure of the water column. There were significant associations between zooplankton assemblages and spatial location, depth, and water mass. Zooplankton distributions generally tracked water masses but usually showed crossbank abundance gradients. Consistent differences between day and night distributions were not apparent for most taxa. Meroplankton (larvae of benthic animals) were derived primarily from onbank populations. The zooplankton community was advected with the tidal flow. Crossfrontal distributions characteristic of onbank and offbank flow regimes clearly differentiated tidal phases and interacted with the flow field and topography. Salinity (a proxy for water mass) and depth interacted in controlling vertical distributions. As tidal forcing sloshed water masses up and down the bank slope, some taxa followed isohalines through depth changes while other taxa maintained themselves within a relatively limited depth or salinity regime. Surface convergence and downwelling, tidal upwelling of bottom water, and mid-depth tidal pumping influenced crossfrontal exchange.

Krill aggregation in the St. Lawrence system, and supply of krill to the whale feeding grounds in the estuary from the gulf

Persistent high-density krill aggregations make the St. Lawrence Estuary and the Gulf of St. Lawrence important feeding-grounds for large marine mammals. To estimate the effects of the circulation on the seasonal krill distribution, a krill biomass-concentration equation with active verti- cal migration was coupled to a 3D regional sea ice-ocean circulation model. The results show recur- rent spatial patterns of aggregation and advection controlled by the circulation and a high sensitivity to the parameters of the biological model. The time spent in the surface layer is crucial for the reten- tion of organisms in the estuary. The simulated krill aggregation areas are associated with 3 pro- cesses (tidal interactions with bathymetry, wind-driven upwelling and mean circulation). Zooplank- ton generally aggregate near the edges of the Laurentian Channel and other secondary channels, at locations that are consistent with the sparse synoptic information on the distributions of large marine mammals in the gulf. The simulations also indicate that changes in the seasonal circulation signifi- cantly affect the krill distribution within the gulf through gyre intensities, the seasonal thermocline and the strength of the estuarine circulation. Finally, the variability of zooplankton transport to the estuary from the gulf appears to be controlled by processes acting on the circulation mode at the mouth of the estuary and estuarine pumping of the krill layer towards the head of the Laurentian Channel. The simulated krill biomass imported into the estuary changed by a factor of 2 over the 3 simulated years.

Shelf–offshore exchange processes, and their effects on mesozooplankton biomass and community composition patterns in the northeast Pacific

Abstract The local zooplankton communities that inhabit a several-hundred kilometer-wide band along the eastern margin of the North Pacific typically include mixtures of species endemic to both nearshore and open-ocean environments. Many have behavioral and life-history adaptations that enhance their probability of survival and retention in either nearshore or offshore regions. Transects across the continental margin typically show a gradient in community composition from primarily nearshore to primarily oceanic, and this gradient is usually steepest near the continental shelf break. However, the transition is often neither monotonic nor complete. Barriers to exchange of water and organisms across this transition are leaky and variable in their location and effectiveness. In this paper, we review mechanisms and consequences of shelf–offshore exchange of zooplankton. Many of these processes vary in sign and intensity with latitude and also in time (wind events, seasonal cycle, and interannual). In general, surface-layer transport of zooplankton is more likely to be seaward in summer and in lower-latitude “upwelling” regions, more likely to be shoreward in winter and in higher-latitude “downwelling” regions. Because zooplankton life cycles and depth distributions are also strongly seasonal, there are important interactions that could potentially drive large match–mismatch variations in distribution and survivorship of both zooplankton and their predators.

Acoustically-inferred zooplankton distribution in relation to hydrography west of the Antarctic Peninsula

Abstract The relationship between the distribution of zooplankton, especially euphausiids (Euphausia and Thysanoessa spp.), and hydrographic regimes of the Western Antarctic Peninsula continental shelf in and around Marguerite Bay was studied as part of the Southern Ocean GLOBEC program. Surveys were conducted from the RVIB N.B. Palmer in austral fall (April–June) and winter (July–August) of 2001. Acoustic, video, and environmental data were collected along 13 transect lines running across the shelf and perpendicular to the Western Antarctic Peninsula coastline, between 65°S and 70°S. Depth-stratified net tows conducted at selected locations provided ground-truthing for acoustic observations. In fall, acoustic volume backscattering strength at 120 kHz was greatest in the southern reaches of the survey area and inside Marguerite Bay, suggestive of high zooplankton and micronekton biomass in these regions. Vertically, highest backscattering was in the 150–450 m depth range, associated with modified Circumpolar Deep Water (CDW). The two deep troughs that intersect the shelf break were characterized by reduced backscattering, similar to levels observed off-shelf and indicative of lower zooplankton biomass in recent intrusions of CDW onto the continental shelf. Estimates of dynamic height suggested that geostrophic circulation likely caused both along- and across-shelf transport of zooplankton. By winter, scattering had decreased by an order of magnitude (10 dB) in the upper 300 m of the water column in most areas, and high backscattering levels were found primarily in a deep (>300 m) scattering layer present close to the bottom. The seasonal decrease is potentially explained by advection of zooplankton, vertical and horizontal movements, and mortality. Predictions of expected backscattering levels based on net samples suggested that large euphausiids were the dominant source of backscattering only at very particular locations and depths, and that copepods, siphonophores, and pteropods were more important in many locations.

Internal tidal bores and bottom nepheloid layers

During a littoral optical environment project field study in shallow water off Oceanside, California, large spikes in optical attenuation (dominated by suspended particles) were observed in a near-bottom mooring. Using moored current meters together with aircraft remote imaging overflights and profiling anchor stations, we established that the spikes were associated with the trailing edges of internal solitary waves of depression of tidal frequency. Each trailing edge had a bore-like face with strong convergent bottom currents followed by a rank-ordered packet of solitons. The near bottom optical attenuation spikes were correlated with these bottom convergent currents at the trailing edge and were not correlated with high bottom stress. Higher optical attenuation was also found near surface at the leading edge of the internal solitary wave of depression. We suggest that in a similar manner to the accumulation and shoreward transport of particles in a surface convergence area (e.g., slick lines), the bottom convergence area at the trailing edge also accumulates and transports particles shoreward. Bottom convergence must be associated with vertical motions which can help in lifting the nepheloid layer off the bottom. This scenario has implications for near bottom optical searches, for transport of non-buoyant zooplankton and for sediment redistribution.

Habitat-specific distribution of Pacific ocean perch ( Sebastes alutus) in Pribilof Canyon, Bering Sea

Shelf edge canyons are well-known sites of enhanced biomass due to on-shore transport and concentration of zooplankton along their axes, both of which contribute to the high densities of nekton frequently found in these canyons. Using a combination of acoustics, trawling, and in situ observations with a remotely operated vehicle (ROV), the distribution of pelagic and demersal biota within Pribilof Canyon in the Bering Sea was examined in September of 1995 and 1997. Near-bottom acoustic scattering patterns in the 38 kHz data showed high concentrations of biomass beginning around the 180 m bottom depth contour and continuing to about 220 m, which were presumed to be adult fish based on their target strength distributions. The 120 kHz data also showed very strong scattering in the water column between 150 and 175 m, which was absent from the 38 kHz data, and therefore attributed mainly to zooplankton. The dominant taxa collected in bottom trawls and mid-water plankton tows were adult rockfishes (Pacific ocean perch, Sebastes alutus) and euphausiids ( Thysanoessa spp.), respectively. In situ videos revealed dense aggregations of these rockfishes inhabiting a “forest” of attached sea whips, Halipteris willemoesi, during night deployments of the ROV, while areas with damaged sea whips had far fewer rockfish, and areas without this biotic habitat structure had no rockfish. During the day, the rockfishes were seen above the “forest”, where they were apparently feeding on dense swarms of euphausiids. It appears that these rockfish utilize this predictable and abundant food resource in the canyon during the day and are associated with the sea whip habitat at night during periods of inactivity. More research is needed on these slow-growing biotic habitats and how fishing activities in the Bering Sea and elsewhere may impact these habitats.

Zooplankton vertical migration and the active transport of dissolved organic and inorganic carbon in the Sargasso Sea

The least known component of the “biological pump” is the active transport of carbon and nutrients by diel vertical migration of zooplankton. We measured CO 2 respiration and dissolved organic carbon (DOC) excretion by individual species of common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station. The inclusion of DOC excretion in this study builds on published research on active transport by respiration of inorganic carbon and allows a direct assessment of the role of zooplankton in the production of dissolved organic matter used in midwater microbial processes. On average, excretion of DOC makes up 24% (range=5–42%) of the total C metabolized (excreted+respired) and could represent a significant augmentation to the vertical flux that has already been documented for respiratory CO 2 flux by migrant zooplankton. Migratory fluxes were compared to other transport processes at BATS. Estimates of combined active transport of CO 2 and DOC by migrators at BATS averaged 7.8% and reached 38.6% of mean sinking POC flux at 150 m, and reached 71.4% of mean sinking POC flux at 300 m. DOC export by migrator excretion averaged 1.9% and reached 13.3% of annual DOC export by physical mixing at this site. During most of the year when deep mixing does not occur, diel migration by zooplankton could provide a supply of DOC to the deeper layers that is available for use by the microbial community. A carbon budget comparing migrant zooplankton transport to the balance of fluxes in the 300–600 m depth strata at BATS shows on average that the total migrant flux supplies 37% of the organic carbon remineralized in this layer, and that migrant DOC flux is more than 3 times the DOC flux gradient by diapycnal mixing. New estimates of active transport of both organic and inorganic carbon by migrants may help resolve observed imbalances in the C budget at BATS, but the magnitude is highly dependent on the biomass of the migrating community.

Horizontal zooplankton dispersal by diel vertical migration in S 2 tidal currents on the northwest European continental shelf

A mechanism is discussed wherey diel (24 h period) vertical migration of marine organisms interacts with semi-diurnal tidal currents to induce net horizontal transport. This happens because diel vertical migration, which is locked to the day-night cycle, is synchronized with the S 2 (principal solar semi-diurnal) tidal constituent which has a period of 12 h and is also sun-synchronized. Diel vertical migration has not previously been considered to be implicated in the generation of net zooplankton transport in tidal currents. The key parameter which characterizes migration-induced transport is S 2 current phase. The phase modulates the magnitude and strictly controls the direction of migration-induced transport. S 2 current phase maps determined from a hydrodynamic tidal model of the northwest European continental shelf show regions of expected convergence, divergence and retention of diel-migrating marine organisms.

Zooplankton distributions at the Georges Bank frontal system: retention or dispersion?

The relation of zooplankton distributions to the physical environment across the frontal system on the northern flank of Georges Bank is described for early summer. The goal of this study was to determine if the frontal system is transparent or a barrier to the transport of zooplankton on and off the Bank. Physical characteristics of the frontal system included a tidal front on the Bank, and processes associated with the Bank edge such as a thermohaline front and a strong residual current jet. Persistent patterns in the cross-bank distributions of zooplankton were related to the locations of these physical features such that separate zooplankton communities occurred on and off the Bank. Zooplankton on the Bank were most abundant in the mixed area and often maximal in the vicinity of the tidal front. The separation into Georges Bank and Gulf of Maine communities was related to the edge of the Bank, although some zooplankton from the latter community did occur on the Bank as far as the tidal front. A particle advection model demonstrates that the strength of the current jet at the Bank edge and slope is sufficient to act as a barrier to cross-bank exchange. However, in the lower layer at the Bank edge the along-bank current is greatly reduced, resulting in increased cross-bank transport for plankton which spend time in this deep layer. This contributes to local retention of plankton already on the Bank and provides a mechanism for transport onto the Bank of plankton in deeper waters at the Bank edge. It may represent an important mechanism for supplying zooplankton prey to the important fish habitats of northeastern Georges Bank.