Southwestern Gulf Of Maine Research Articles

Concentration and condition of American lobster postlarvae in small‐scale convergences

AbstractInvertebrate larvae are often abundant in the surface ocean, which plays a key role in their dispersal and connectivity. Pelagic microhabitats characterized by small‐scale hydrographic variability are complex and ubiquitous in the coastal ocean, but their study is challenging, and they have been largely neglected in meroplankton ecology. Surface convergences, i.e., surface microhabitats featuring convergent horizontal currents, may aggregate the last larval stage of the American lobster and could provide shelter and food for Stage IV postlarvae and thus enhance their condition. We tested these hypotheses by conducting a series of cruises in the southwestern Gulf of Maine in summer 2021, sampling 15 paired sets of potential convergences and off‐convergence unstructured habitat. We measured postlarval abundance, surface hydrography, acoustic backscatter, and circulation. Experiments and image analysis compared condition, color, and morphology of postlarvae sampled inside and outside potential convergences. Potential convergences varied in near‐surface hydrographic patterns, with most displaying consistency among two transects and diverse patterns in salinity and temperature (e.g., across‐convergence gradients with equal or different signs). While the highest postlarval abundances were found in convergences, abundance patterns on and off convergences were not consistent, and another analysis indicated higher abundance in convergences than in a 7‐year untargeted surface ocean data set. Experiments indicated no survivorship differences among convergence and non‐convergence individuals at two temperatures, while image analyses revealed differences in color and size. Physical measurements and qualitative neuston community analyses indicated substantial heterogeneity among potential convergences. Our results reinforce that small‐scale heterogeneities are highly variable but important to the ecology of meroplankton, including the pelagic and neustonic habitats where lobster postlarvae are abundant.

Influences of Tides, Winds, and Bathymetry on Surface Transport Through a Complex Nearshore System: Measurements from GPS Drifters and an Uncrewed Aerial Vehicle

Bathymetric features such as dredged channels, constricted-inlet basins, and nearshore islands have known influences on coastal transport, which are typically studied on an individual basis. However, their interactive effects on flow when found in the same area are poorly characterized. A multi-season study was conducted to characterize surface transport in and around Wood Island Harbor, in the southwestern Gulf of Maine, where these features all exist in close proximity. Patterns of surface transport were assessed with repeated series of Uncrewed Aerial Vehicle flights and deployments of GPS drifters across tidal conditions and winds typical to the area. Transport followed a major axis through the harbor, as a result of the combined effects of the linked inlet-channel system. Despite the complex topography, the individual dynamics of transport are maintained. Tidal variations explained a majority of displacement (R2 = 0.752) along the channel axis, while across-channel wind velocity explained the majority of orthogonal displacement (R2 = 0.646). Flow in the area was friction-dominated, and both inter-island passages and the deep channel served to direct flow. The offshore transport of tidally driven flows from a constricted-inlet basin was enhanced by deep channels, with flow primarily following bathymetric contours. Our observations suggest that surrounding topography lessened the role of wave energy in governing transport. The co-occurrence of bathymetric features around a constricted-inlet basin can disrupt the typical influences of physical mechanisms on surface transport, leading to a greater importance of local conditions and the larger-scale circulation of the area.

A hierarchical modelling approach to estimating humpback whale abundance from sand lance abundance

The primary prey of humpback whales in the southwestern Gulf of Maine is sand lance. Despite this established relationship, we lack models to further understand the influence of sand lance on humpback whales or to predict humpback abundance or distribution in response to climate-related changes in sand lance abundance or distribution. We used a subset of long-term standardized survey data (2013–2019) from Stellwagen Bank National Marine Sanctuary and a Bayesian hierarchical modeling approach to explore the influence of sand lance on humpback whales at multiple spatial and temporal scales while accounting for sampling variability and propagating uncertainty. We developed zero-inflated Poisson mixed effects models for both sand lance and humpbacks, using modelled sand lance abundance as a predictor in the whale model. Results showed a statistically clear positive correlation between sand lance and humpback whales. Regional mean abundances of both species increased from north to south, though site-level variation within regions showed more variability. Results suggest annual variation in abundance of both species, with potentially different influences. We demonstrate one management application of our method by examining entanglement risk for humpback whales. Whale aggregations were more likely to occur in a high density area of fixed fishing gear that overlaps with an area of higher sand lance abundance. Our work suggests that humpback whale distribution in the larger Gulf of Maine may be impacted by climate-related fluctuations in sand lance abundance. Predicting future distributions of humpback whales is important for ecosystem-based management, including mitigation of human impacts, and our work serves as a foundation for further model development.

Slocum Gliders Provide Accurate Near Real-Time Estimates of Baleen Whale Presence From Human-Reviewed Passive Acoustic Detection Information

Mitigating the effects of human activities on marine mammals often depends on monitoring animal occurrence over long time scales, large spatial scales, and in real time. Passive acoustics, particularly from autonomous vehicles, is a promising approach to meeting this need. We have previously developed the capability to record, detect, classify, and transmit to shore information about the tonal sounds of baleen whales in near real time from long-endurance ocean gliders. We have recently developed a protocol by which a human analyst reviews this information to determine the presence of marine mammals, and the results of this review are automatically posted to a publicly accessible website, sent directly to interested parties via email or text, and made available to stakeholders via a number of public and private digital applications. We evaluated the performance of this system during two 3.75-month Slocum glider deployments in the southwestern Gulf of Maine during the spring seasons of 2015 and 2016. Near real-time detections of humpback, fin, sei, and North Atlantic right whales were compared to detections of these species from simultaneously recorded audio. Data from another 2016 glider deployment in the same area were also used to compare results between three different analysts to determine repeatability of results both among and within analysts. False detection (occurrence) rates on daily time scales were 0% for all species. Daily missed detection rates ranged from 17 to 24%. Agreement between two trained novice analysts and an experienced analyst was greater than 95% for fin, sei, and right whales, while agreement was 83-89% for humpback whales owing to the more subjective process for detecting this species. Our results indicate that the presence of baleen whales can be accurately determined using information about tonal sounds transmitted in near real-time from Slocum gliders. The system is being used operationally to monitor baleen whales in U.S., Canadian, and Chilean waters, and has been particularly useful for monitoring the critically endangered North Atlantic right whale throughout the northwestern Atlantic Ocean.

Influence of a River Plume on the Distribution of Brachyuran Crab and Mytilid Bivalve Larvae in Saco Bay, Maine

Larvae of most coastal marine invertebrates develop through a series of pelagic stages in the inner shelf regions where they are subject to strong velocity fields, including buoyancy-driven flows from river plumes. Taxon-specific larval behavior can interact with flow fields to determine dispersal trajectories. In this study, we examined the hydrodynamic features of the Saco River plume (in the southwestern Gulf of Maine) from July to August and explored how larval behavior may alter the distribution of mytilid (i.e., Mytilus and Modiolus) bivalve larvae and three genera of brachyuran (Carcinus, Hemigrapsus, and Cancer) crab larvae in and around that plume. Hydrographic surveys (via conductivity-temperature-depth casts) and larval sampling (via plankton tows) were conducted to assess temporal and spatial variation in the horizontal and vertical distribution of larvae. The horizontal extent of the Saco River plume varied little during our study and was governed by both inertial and rotational effects. Late stage mytilid larvae were relatively homogeneously distributed in and out of the plume, while the distribution of brachyuran larvae varied among different locations, species, and larval stages. We conclude that mytilid larvae entered the plume through physical entrainment and/or upward swimming processes and could tolerate salinities associated with the plume (<25). By contrast, brachyuran larvae avoided the plume via downward swimming to avoid osmotic stress, or had perished prior to sampling.

Diel Reproductive Periodicity of Haddock in the Southwestern Gulf of Maine

AbstractWe studied a population of Haddock Melanogrammus aeglefinus in the Gulf of Maine to determine whether it exhibits diel spawning periodicity. Commercial fishing vessels were chartered for 25 dedicated long‐lining trips to collect sexually mature Haddock in the Stellwagen Bank National Marine Sanctuary at locations identified by commercial fishers as having spawning aggregations. In order to examine diel effects on Haddock reproduction, the changes in CPUE and percentages of male and female Haddock of all reproductive maturity stages, as well as gonadosomatic index (GSI), were observed across a 24‐h diel cycle. The comparison of diel patterns among different maturation stages was used to differentiate diel periodicity due to spawning from feeding behavior. Diel periodicity was observed for the proportion of males and females in the catch, the proportion and CPUE of male and female maturation stages most indicative of imminent spawning, and for female GSI. Although spawning occurred throughout the diel cycle, peak spawning of females occurred during the evening hours (1700–2100 hours) and gradually declined to low values during the day at 0900 hours. In an apparent contradiction male spawning stages were uniformly high except for lows during the night between 2100 and 0100 hours. We hypothesize that this pattern is consistent with lekking behavior where strong competition among spawning‐ready males during courtship would reduce their feeding activity during the peak spawning hours. Strong annual differences in diel periodicity may have resulted from different temporal patterns in the spawning season or density dependence, or both.

A generalized baleen whale call detection and classification system

Passive acoustic monitoring allows the assessment of marine mammal occurrence and distribution at greater temporal and spatial scales than is now possible with traditional visual surveys. However, the large volume of acoustic data and the lengthy and laborious task of manually analyzing these data have hindered broad application of this technique. To overcome these limitations, a generalized automated detection and classification system (DCS) was developed to efficiently and accurately identify low-frequency baleen whale calls. The DCS (1) accounts for persistent narrowband and transient broadband noise, (2) characterizes temporal variation of dominant call frequencies via pitch-tracking, and (3) classifies calls based on attributes of the resulting pitch tracks using quadratic discriminant function analysis (QDFA). Automated detections of sei whale (Balaenoptera borealis) downsweep calls and North Atlantic right whale (Eubalaena glacialis) upcalls were evaluated using recordings collected in the southwestern Gulf of Maine during the spring seasons of 2006 and 2007. The accuracy of the DCS was similar to that of a human analyst: variability in differences between the DCS and an analyst was similar to that between independent analysts, and temporal variability in call rates was similar among the DCS and several analysts.

Diel vertical migration behavior of Calanus finmarchicus and its influence on right and sei whale occurrence

Diel vertical migration (DVM) by herbivorous copepods likely has a profound effect on the behavior and ecology of copepod predators. We characterized the DVM behavior of late-stage Calanus finmarchicus in the southwestern Gulf of Maine during the spring seasons of 2005 to 2007, and investigated the influence of this behavior on the occurrence of zooplanktivorous baleen whales. On 5 occasions, we occupied an oceanographic station for 1 to 2 d and conducted (1) a half-hourly census of whales and (2) a half-hourly cast with an instrument package measuring temperature, salinity, chlorophyll fluorescence, and copepod abundance. We observed significant variability in DVM behavior both within and among years that was unrelated to stratification or chlorophyll concentration. Instead, DVM appeared to be influenced by the vertical distribution of phytoplankton, the presence of visual predators (sand lance Ammodytes spp.), copepod developmental stage, and the feeding history of individual copepods. Migrating copepods had oil sacs that were 44% larger than non-migrating copepods at the surface after accounting for developmental stage, which suggests that well-fed copepods are more likely to vertically migrate. While the occurrence of North Atlantic right whales Eubalaena glacialis was unrelated to variability in the migration behavior of C. finmarchicus, sei whales Balaenoptera borealis were significantly less abundant during times of strong DVM behavior. We speculate that right whales do not compete directly with sand lance and herring for C. finmarchicus, but by inducing DVM behavior, these fish are likely influencing the distribution and abundance of sei whales in the southwestern Gulf of Maine.

Diel periodicity in both sei whale vocalization rates and the vertical migration of their copepod prey observed from ocean gliders

The daily activity cycles of marine predators may be dictated in large part by the timing of prey availability. For example, recent studies have observed diel periodicity in baleen whale vocalization rates that are thought to be governed by the diel vertical migration of their zooplanktonic prey. We addressed this hypothesis by studying associations between sei whale (Balaenoptera borealis) vocalization rates, oceanographic conditions, and the vertical distribution of the whales' prey, the calanoid copepod Calanus finmarchicus, during May 2005 in the southwestern Gulf of Maine using an array of autonomous ocean gliders. Each of the four gliders was equipped with sensors to measure temperature, salinity, and chlorophyll fluorescence. Three of the four gliders carried a digital acoustic recorder and the fourth carried a 1‐MHz acoustic Doppler current profiler. We observed strong diel periodicity in the acoustic backscatter measured by the current profiler that we attribute (based on a corroborating shipboard study) to the diel vertical migration of C. finmarchicus. Sei whale vocalization rates also exhibited diel periodicity, with more calls detected during the daytime when C. finmarchicus was observed at depth. We found no evidence to suggest that the observed patterns in sei whale calling rates were attributable to diel periodicity in background noise or acoustic propagation conditions. Sei whales are adept at foraging on nearsurface aggregations of C. finmarchicus; therefore we expect that the whales were feeding at night. We hypothesize that calling rates are reduced at night while the whales are feeding, but increase with social activity during the day when copepods are either more difficult or less efficient to capture at depth. The gliders' persistence during adverse weather conditions experienced during the study allowed continuous collocated observations of whale vocalization behavior and oceanographic conditions that have not been previously possible with traditional shipboard techniques.

Zooplankton feeding behavior and particle selection in natural plankton assemblages containing toxic Alexandrium spp.

Laboratory experiments suggest that toxic Alexandrium spp. cells are unpalatable to zooplankton grazers, and that toxic cells should be selectively avoided by zooplankton when feeding in mixtures of different prey species. Such avoidance, if practised in the wild, might contribute to harmful bloom formation by reducing losses of Alexandrium spp. due to grazing. In the spring of 1998 and 1999, during 'red tide' outbreaks in the southwestern Gulf of Maine, weekly experiments were performed using field collected natural water samples with ambient phytoplankton and dominant mesozooplankton grazers. The feeding response of Acartia hudsonica, Semibalanus balanoides nauplii, and Calanus finmarchicus was tested during various weeks in natural water samples with low concentrations of Alexandrium spp. (~1000 cells l -1 , typical natural concentrations for this region). Semibalanus sp. nauplii consistently avoided toxic Alexandrium spp. and other dinoflagel- lates. C. finmarchicus selectively fed on diatoms when they were abundant, and fed non-selectively on all dinoflagellates (except Ceratium spp.) when the spring bloom declined and dinoflagellates dominated. A. hudsonica non-selectively cleared Alexandrium spp. throughout the study periods. During spring Alexandrium spp. bloom formation, if non-selective grazers such as A. hudsonica dom- inate the zooplankton, Alexandrium spp. losses from grazing depend on grazer abundance (biomass); if selective feeders such as S. balanoides nauplii dominate, then Alexandrium spp. benefits from reduced grazing losses relative to alternative prey.

Upper ocean carbon export, horizontal transport, and vertical eddy diffusivity in the southwestern Gulf of Maine

The naturally occurring radionuclides 234 Th and 7 Be were used to investigate the magnitude of upper ocean particulate organic carbon export and the rate of vertical eddy diffusion in the southwestern Gulf of Maine. Sampling occurred during the spring (March, April) and summer (July, August) of 1997. Both non-steady-state and horizontal transport models were assessed and found to be very important for the accurate determination of 234 Th export. Upper ocean particulate organic carbon export was estimated using modeled 234 Th export and the ratio of particulate organic carbon to 234 Th on GF/F filters. Our measurements demonstrate that the southwestern Gulf of Maine is typical of many coastal regimes, having an organic carbon export ratio (particulate export/primary production) which ranges from 9 to 49% depending on the depth of integration and primary productivity estimate. 7 Be derived estimates of vertical eddy diffusivity into summer surface waters ranged from 0.5 to 1.5 cm 2 sec −1, and show that this mechanism is sufficient to support the amount of `new’ nitrogen required for the measured particulate carbon export.

Bloom dynamics of toxic Alexandrium species in the northeastern U.S

Coastal waters of the northeastern U.S. are subject to recurrent outbreaks of paralytic shellfish poisoning (PSP) caused by toxic dinoflagellates in the genus Alexandrium. PSP is not uniform across the large region, but instead reflects Alexandrium growth and toxin accumulation in five separate habitats or zones defined by circulation patterns and the discontinuous distribution of the dinoflagellates. Each of these habitats has a unique set of environmental and oceanographic forcings that determine the timing and extent of bloom development and transport and that regulate the extent of genetic exchange with adjacent populations. Several habitats (e.g. the southwestern Gulf of Maine, Massachusetts Bay, and Georges Bank) are linked hydrographically and may share the same Alexandrium population via large‐scale transport in a. coastal current, whereas the other two habitats (eastern Maine and southern salt ponds‐embayments) seem to be isolated and have little or no hydrographic or genetic linkage to adjacent regions during bloom seasons. My paper provides an overview of the regional ecology and oceanography of Alexandrium through a focus on these five subpopulations. Issues that relate to PSP and Alexandrium dynamics throughout the world are highlighted, including species dispersal, the role of cysts and “initiation zones” in bloom development, and the influence of large‐ and small‐scale hydrography on population development and transport. The ability of Alexandrium to colonize multiple habitats and to persist over a large region is emphasized in recognition of the adaptability and resilience of this important organism.

The Formation of High Nutrient–Low Salinity Water in the Gulf of Maine: a Nutrient Trap?

Waters with low salinity and high nitrate were identified in the south-western Gulf of Maine by Townsend and Christensen (1992). Their extreme characteristics were attributed to local remineralization processes, and the operation of a nutrient trap. This paper uses a newly compiled data set, and an algorithm relating nitrate to hydrographic properties to examine the occurrence and characteristics of this type of water. Observations of low salinity–high nitrate water are not uncommon, and represent approximately 20% of the database. Based on the occurrence of the most extreme waters of this type identified by the algorithm, it is concluded that two related mechanisms cause its formation. The mixing of low salinity Maine Surface Water with remnant Maine Intermediate Water in the fall starts this process. The interaction of newly formed Maine Intermediate Water with the spring freshet of Maine rivers completes it. These mechanisms can explain the formation of low salinity–high nitrate water, including ‘ nutrient trap ’ water, in general.

Springtime nutrient and chlorophyll a concentrations in the southwestern Gulf of Maine

We report nutrient and phytoplankton pigment concentrations during three hydrographic surveys in the southwestern Gulf of Maine (GOM) during the South Channel Ocean Productivity Experiment (SCOPEX). During the first survey in mid-March 1988, winter conditions prevailed in the deeper central waters of the survey area, with a deep mixed layer, high surface nutrient concentrations (NO 3 > 8 μM), and low Chl a concentrations (<0.5 μg l −1). In shallower areas along the western flank of the southwestern GOM and over Georges Bank, the spring bloom had begun with Chl a concentrations >2 μg 1 −1. During the second survey in late April 1988, the spring bloom was declining in the southwestern GOM, and considerable spatial heterogeneity in the stage of the bloom and the levels of nutrients occurred. Near the sill of the Great South Channel, and east of Cape Cod where a low salinity surface plume had stabilized the water column, nutrient and phytoplankton concentrations were low (NO 3 < 0.5 μM, Chl a 1–2 μg 1 −1. In the deeper offshore waters in some areas, the bloom still continued (NO 3 1–2 μM, Chl a > 4 μg 1 −1), while in the northeastern part of the study area it had just begun (NO 3 > 5 μM, Chl a 1–2 μg l −1 The spring bloom had terminated throughout the southwestern GOM during the third survey in early June 1989. A low salinity surface plume that entered the study area along the western flank extended east from Cape Cod to Georges Bank, resulting in a strong pycnocline at 15–20 m. Nutrient and phytoplankton concentrations in this surface plume were low in the deeper central region of the survey area (NO 3 <0.25 μM, Chl a < 1 μg 1 −1 Beneath this surface plume, subsurface maxima of ammonia and Chl a occurred (NH 3 > 2 μM, Chl a > 1 μg 1 −1). Over Nantucket Shoals and Georges Bank, the water column was well mixed due to enhanced tidal dissipation, resulting in higher nutrient and phytoplankton concentrations. The nutrient and phytoplankton data collected in these hydrographic surveys suggest that phytoplankton abundance during spring is controlled by seasonal hydrographic changes. These observations do not support the SCOPEX hypothesis that dense surface aggregations of the copepod Calanus finmarchicus in this region in late spring are due to unusually high phytoplankton productivity.

The South Channel Ocean Productivity EXperiment

The South Channel Ocean Productivity EXperiment (SCOPEX) was a multidisciplinary study of a whale-zooplankton predator-prey system in the southwestern Gulf of Maine, focusing on the oceanographic factors responsible for the development of dense patches of the copepod Calanus finmarchicus, which comprise the major prey resource for right whales ( Eubalaena glacialis). Three non-mutually exclusive hypotheses underlay the study: patch development is due to (1) extremely high in situ primary and secondary productivity; (2) large numbers of Calanus advected into the region and concentrated by hydrographic processes; and/or (3) a behavioral tendency of the copepods themselves to aggregate. The results confirmed the cooccurrence of right whales with high density Calanus patches, and also demonstrated that right whales fed on patches with higher proportions of larger lifestages. The physical oceanographic studies supported the advection hypothesis, possibly augmented by a tendency of Calanus to aggregate, but there was little evidence to support the productivity hypothesis.

Discarding of Commercial Groundfish Species in the Gulf of Maine Shrimp Fishery

Abstract The objective of the research was to quantify discards of American plaice Hippoglossoides platessoides, witch flounder Glyptocephalus cynoglossus, yellowtail flounder Pleuronectes ferrugineus, winter flounder Pleuronectes americanus, goosefish Lophius americanus, silver hake Merluccius bilinearis, and Atlantic cod Gadus morhua in the Gulf of Maine shrimp fishery. Data were obtained from 50 tows made by commercial trawlers fishing in the southwestern Gulf of Maine, Mean discard percentages (100 × fish discarded/fish caught) per tow, on a weight basis, were 93% for silver hake, 81% for American plaice, 76% for witch flounder, 56% for Atlantic cod, 41 % for goosefish, 20% for yellowtail flounder, and 11% for winter flounder. Mean discard percentages per tow on a numerical basis were slightly higher. Neither catch condition nor tow duration affected discard percentage, but discard percentage decreased with increasing depth for American plaice.

Toxic phytoplankton blooms in the southwestern Gulf of Maine: testing hypotheses of physical control using historical data

Blooms of the toxic dinoflagellate Alexandrium tamarense Lebour have been nearly annual features along the coasts of southern Maine, New Hampshire and Massachusetts, USA, since 1972. In 1990 two hypotheses which have been used to explain the initiation of these blooms in the southwestern portion of the Gulf of Maine were tested using historical records of shellfish toxicity, wind, and river flow. The first hypothesis states that the blooms were initiated or advected to shore by wind-driven coastal upwelling. The second states that established blooms were advected from north to south alongshore in a coastally trapped buoyant plume of water. Of the eleven years examined (1979 to 1989), we found seven cases inconsistent with the wind-driven upwelling hypothesis, and only one case (1985) which contradicts the plume-advection hypothesis. 1985 was an unusual year in many respects, and we suggest that some other mechanism was responsible for the toxic outbreaks. In addition, the wind-driven upwelling hypothesis could not explain the observed north-to-south temporal progression of toxicity each year. The plume-advection hypothesis was found to best explain the datails of the timing and spread of shellfish toxicity in Gulf of Maine waters to the south of Penobscot Bay, Maine. These include the variable north-to-south progression with time, the presence of a toxin-free zone south of Cape Ann, Massachusetts, the sporadic nature of toxic outbreaks south of Massachusetts Bay, and the apparently rare occurrence of high toxicity levels well offshore on Nantucket Shoals and Georges Bank.

Alongshore transport of a toxic phytoplankton bloom in a buoyancy current: Alexandrium tamarense in the Gulf of Maine

We examined the mechanisms controlling blooms of the toxic dinoflagellate Alexandrium tamarense Lebour and the concomintant patterns of shellfish toxicity in the southwestern Gulf of Maine, USA. During a series of cruises from 1987 to 1989, hydrographic parameters were measured to elucidate the physical factors affecting the distribution and abundance of dinoflagellates along this coast. In 1988 and 1989 when toxicity was detected in the southern part of this region, A. tamarense cells were apparently transported into the area between Portsmouth, New Hampshire, and Cape Ann, Massachusetts, in a coastally trapped buoyant plume. This plume appears to have been formed by the outflow from the Androscoggin and Kennebec Rivers. Flow rates of these rivers, hydrographic sections, and satellite images led us to conclude that the plume persisted for about a month, and extended alongshore for several hundred kilometers. The distribution of cells followed the position of the plume as it was influenced by wind and topography. When winds were downwelling-favorable (to the southwest), cells were moved alongshore to the south, and were held to the coast; when winds were upwelling-favorable (to the northeast),the plume sometimes separated from the coast, advecting the cells offshore. In 1987 when no plume was present, A. tamarense cells were scarce, and no toxicity was recorded at the southern stations. The alongshore advection of toxic cells within a coastally trapped buoyant plume can explain the details of the temporal and spatial patterns of shellfish toxicity along the coast. We hypothesize that (1) the source of the A. tamarense populations is in the north, possibly associated with the Androscoggin and Kennebec estuaries, that (2) toxicity patterns follow a predictable relationship with river flow volume and timing of flow peaks and that (3) wind stresses directly influence the distribution of low salinity water and the dinoflagellate cells. Local, in situ growth of dinoflagellates can be an important factor initiating toxic dinoflagellate blooms. However, these data demonstrate the significant role of alongshore transport of established populations of A. tamarense in controlling the location and timing of paralytic shellfish poisoning (PSP) outbreaks in May and June along the southwestern coast of the Gulf of Maine.

Factors Influencing the Size of American Lobster (Homarus americanus) Stocks Along the Atlantic Coast of Nova Scotia, Gulf of St. Lawrence, and Gulf of Maine: A New Synthesis

The general aspects of fluctuating American lobster (Homarus americanus) stocks are considered in an attempt to identify common principles controlling lobster populations in the Gulf of Maine, Gulf of St. Lawrence, and the Atlantic coast of Nova Scotia. Inshore fishing mortality is known to be precariously high in both Canadian and American waters yet only in the central Northumberland Strait region and along the Atlantic coast of Nova Scotia are stocks experiencing steep declines. Existing regulations allow fishing of lobsters well below the size at which maturity can be expressed. In western and central Northumberland Strait female lobsters are not protected by the 'berried' law because of the timing of the fishing season. Climatic change, expressed as sea-surface temperature, is closely associated with the success of lobster recruitment in the Gulf of Maine, but nowhere else. Huntsman's hypothesis that warm surface water of sufficient duration is essential for the successful completion of the larval stages of the lobster is found to have wide application. Warmer waters are also found to have a preponderance of plankton in the size categories required for feeding by the first two larval stages. Both the southern Gulf of St. Lawrence and the southwestern Gulf of Maine, including Georges Bank, are believed to be prolific nursery grounds for the early larval lobsters because warm water with plenty of food stimulates rapid development and settling. We calculated that larvae released on Georges Bank supply recruits to southern Nova Scotia, Bay of Fundy, and Maine, whereas some larvae from the southern Gulf of St. Lawrence are thought to settle as far as Cape Breton Co., N.S., and before the closure of the Strait of Canso, Guysborough Co., N.S. The past importance of lobster recruitment to the Atlantic coast of Nova Scotia through the Strait of Canso is estimated to have been as high as 60% of the average landings in Chedabucto Bay during the 11 best years of this fishery. The failure of the Atlantic coast lobster fishery off Nova Scotia is believed to have been initiated by a larval recruitment failure caused by the synchronous closure of the Strait of Canso and the start of a general climatic cooling. Possibly a critical temperature was reached along the Atlantic coast of Nova Scotia in the 1950s below which larval survival is much reduced by the shortened season. Reduced lobster abundance along the Atlantic coast of Nova Scotia may have contributed to the imbalance in the benthic ecosystem in which an uncontrolled urchin population consumed the kelp beds and until recently greatly reduced the primary production input to the system.Key words: American lobster, Homarus americanus; stocks, recruitment, ecology, larval transport