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HABITAT CHARACTERISTICS
A complete inventory of all available habitats is beyond the scope of this paper. Only those environments which have been identified or suggested as potential lobster habitat are evaluated below:
The estuaries of the Gulf of Maine are fed by a vast watershed (Fig. 22) covering 178,000 sq. km (69,000 sq. miles) which empties 950 billion liters (251 billion gallons) of freshwater into near shore environments each year. Within the US, the 17 major estuary systems comprise some of the most productive ecosystems on the planet (Platt 1998). Lobsters reportedly utilize the following habitats in these areas: (see Cooper and Uzmann 1980)
Mud base with burrows - These occur primarily in harbors and quiet estuaries where currents are usually not a major factor. Lobster shelters are formed from excavations in soft substrate. Cooper and Uzmann (1980) reported lobster burrows at depths of 30-60 m (about 100-200 ft) in the Sheepscot Estuary. Lobsters excavate burrows 60-80 cm (about 24-32 in) below the sediment water interface at a relatively steep angle of 40-90 degrees. During the winter lobsters may take shelter in a mud burrow and close off the entrance with a partition of sediment and debris; they can remain enclosed in this shelter for weeks or months at a time. To the north, off Prince Edward Island, burrows reportedly had a diameter about the size of the lobster and were spaced about 2 m apart. (Densities up to 20 lobsters per sq. m or about 17 per sq. yd for small juveniles. Adults probably less than 0.01 lobsters per sq. m or 0.008 lobster per sq. yd) (Cooper and Uzmann 1980).
Rock, cobble and gravel - Investigators at the University of New Hampshire (Brown et al. unpub.) have modeled lobster habitats in the Great Bay Estuary (Fig. 23A and 23B). They have found that in Great Bay Estuary, juvenile lobsters apparently occupy a relatively small stretch about 6.5 km (4 mi) long near the mouth of the Piscataqua river system. Furthermore, it is evident that these juvenile lobsters prefer shallow bottoms with gravel and gravely sand substrates (Fig. 24) where salinity remains high and temperature is relatively moderate throughout the year. All of these conditions occur in a contiguous area covering less than 1000 acres. Another tiny area in Great Bay Estuary may provide limited shelter for juveniles among eelgrass.
In Maine, researchers at the Darling Marine Center (Steneck and Wilson 1998) have found that lobster population densities appear greater west of Penobscot Bay than they are to the east (Fig. 25). This could be due to currents, distribution of gravel/cobble substrates or a combination of these and other factors.
Near the mouth of Penobscot Bay, they have also shown a high concentration of lobsters. Settlement densities (Fig. 26) totaled more than 1 lobster per sq. meter or roughly 0.8 per sq. yd. Young lobsters (Fig. 27) were also observed at high densities (over 0.5 juvenile lobsters per sq. m or 0.4 per sq. yd and more than 0.75 adolescent lobsters per sq. m or about 0.63 per sq. yd).
In Rhode Island, one of the primary estuarine surveys was done by Wahle (1993). His study showed that "Based on direct benthic censuses along a 22 km (about 14 miles) length of Narragansett Bay Estuary, new benthic recruits were absent from featureless sedimentary habitats that form the majority of the bottom in this shallow bay (generally less than 10 m or about 30 ft deep). Rocky habitat and cobble /boulder habitat supported both new recruits and older lobsters; mechanisms for an apparent restriction of recruitment at upper-bay sites were suggested to be reduced larval supply and physiological stress" (Lawton and Lavalli 1995).
Rock/shell
The adult lobsters at Great Bay Estuary also utilize the sand and gravel environment in the channels, but appear to prefer a rock/shell habitat more characteristic of the high temperature, low salinity regimes of the central bay. While this habitat in Great Bay Estuary is more extensive than that occupied by juveniles, it still involves only about 5000 acres, which is about 20% of the estuary (see Fig. 23 A, B and 24).
According to NMFS' Office of Habitat Conservation (personal communication), the relationships between wetlands and fish production are an essential and important part of the ongoing debate on wetland regulation and policy. Unfortunately, these relationships are complicated and often unappreciated. For a few fisheries, such as American lobster, the importance of wetlands has been discovered only recently, and the primary influences on productivity are still being investigated. Because of the complexity of aquatic systems, it is difficult to quantify the exact effect of the loss or degradation of a particular acre of wetland on the fishery as a whole.
Estuaries depend on their wetlands to maintain water quality and provide the basis for food chains that culminate in human consumption of seafood. These wetlands are the source of runoff and nutrients for many of the estuarine and coastal habitats. They influence the temperature and salinity regimes for these areas and indirectly influence the habitat selections of the American lobster.
In the late 1970's and early 1980's, this country was losing wetlands at an estimated rate of 300,000 acres per year. The Clean Water Act and state wetland protection programs have helped to decrease wetland losses to an estimated 70,000 to 90,000 acres per year.
New England is continuing to lose wetlands at an alarming rate. In Maine, the extensive coastal rivers, bays and estuaries support both recreational and commercial fisheries for finfish and shellfish. Key commercial species, such as the American lobster, depend on Maine's extensive estuarine and marine wetlands for food and protection as juveniles and adults. Maine, while not subject to the intense development pressure of its neighbors to the south, nevertheless had lost approximately 20% of its estimated original wetlands base by the mid 1980's. This compares with 9% for New Hampshire, 28% in Massachusetts, 38% in Rhode Island and 74% in Connecticut.
Lobster shelters are formed from excavations that cut deep into peat; they are often obscured by dense algal growth. The reef forms from blocks of salt marsh peat that break and fall into adjacent marsh creeks and channels (Fig. 28). Additional research is needed to relate the satellite imagery directly to known lobster areas. However, we do know that peat reefs are often associated with marsh grass (Spartina) and appear to provide moderate protection for lobsters against fishes and crabs. This is presumably because lobsters blend in with root structures (Barshaw and Lavalli 1988). (Densities up to 5.7 individuals per square m or about 4.8 ind. per sq. yd.)
The coast of the Gulf of Maine is over 7000 miles long and is littered with islands. It has long been recognized that the character of the coastline and associated islands is strongly influenced by the underlying geology (Fig. 29). The state of Maine alone has over 4500 separate islands which are supported mainly by granites. As the satellite photo of the central Maine coastline shows (Fig. 30), conditions exist for enhanced productivity along the island margins. Each island influences currents which help to mix, oxygenate, and enrich the water. The islands also cause local up welling of deeper, colder, nutrient rich waters. It has long been recognized that island margins can provide ideal conditions for lobsters. "It is no accident that Maine's largest lobster harvests are found along the section of coast with the largest number of islands" (Conkling 1995).
The relationship between lobsters and kelp habitats provides important clues about how American lobsters adapt to altered habitats and their ultimate carrying capacities. In order to fully understand these associations, we must first investigate the kelp forests themselves.
Kelp beds in New England consist primarily of seaweed species of Laminaria longicruris and L. saccharina (locally referred to as kelp.). These kelps inhabit sub tidal areas from Canada to Long Island Sound (Egan and Yarish 1990). Kelps possess a fairly low maximum temperature tolerance and certain forms begin dying off at temperatures above 160-180C (610-640F). Its optimum temperature range is even more restrictive and at its southern limit, the most rapid growth period occurs during spring when water temperatures are 100-120C (500-540F) (Egan and Yarish 1990).
Kelp distribution is known for only a few isolated areas in Maine. In Penobscot Bay, kelp beds develop on some of the nutrient enriched rocky bottom habitats which are prime lobster grounds (Platt 1998). In addition, kelp forests have been mentioned as "one of the preferred habitats where lobsters hide during the period they are shedding their old shells" (Conkling 1995). Unfortunately, we did not have access to an accurate map showing the distribution of kelp beds along the entire New England coast. Therefore, it was not possible to map this habitat in relation to lobster abundance in general. Nevertheless, lobster behavior in kelp beds could tell us much about how lobsters adapt to habitat changes.
In recent years, field studies have been conducted to determine how lobsters use kelp beds as habitat (Bologna and Steneck 1993). The site chosen was the so-called "Thread of Life" located between Rutherford Island and Crow Island off the Central Maine coast. Although natural kelp beds were found throughout this region growing on scattered hard substrate in the featureless sediment, extensive kelp beds do not naturally occur here (Bologna and Steneck 1993).
Experimental kelp beds were transplanted and control plots were established on relatively featureless silty-sand substrate in sub tidal regions 10-15 m (33-49 ft) deep. Water temperatures at this location ranged from 20-30C (360-370F) in winter to 150-160C (590-610F) during the summer. They used only kelp greater than 50 cm (about 20 in) total length because "only large kelp were commonly observed sheltering lobsters Ö Beginning in late spring and lasting through early fall the region is commercially fished for lobsters" (Bologna and Steneck 1993).
The average size of the 1423 individual lobsters which were attracted to the kelp beds during this two year study ranged from 51-61 mm CL (approx. 2-2.5 ft CL) indicating that most of the lobster were adolescents (Bologna and Steneck 1993). While physiologically mature, these individuals were probably not functionally mature.
The results showed that "population density and biomass inside transplanted kelp beds (1.20 to 1.68 ind. per sq. m or 1.0 to 1.4 ind. per sq. yd) were significantly higher than in control regions (0.14 ind. per sq. m or about 0.12 ind. per sq. yd). Lobsters did not burrow into the sediment, but sought shelter beneath the kelp. No difference in lobster density was observed between live and artificial [plastic] kelp treatments, but both treatments maintained lobster densities and biomasses that were an order of magnitude greater than adjacent control regions" (Bologna and Steneck 1993). Perhaps of more importance is the fact that lobster density was significantly greater in the smallest patches. "Moreover, lobsters typically occupied the edges of kelp beds and their abundance within kelp patches increased" as the exposed perimeters increased (Bologna and Steneck 1993). It was theorized that "lobsters use edges of kelp beds to maximize their sensory input while still allowing them to remain under cover." This suggests that "edge effects influence the local carrying capacity for lobsters by influencing the lobster's choice of kelp beds as habitat" (Bologna and Steneck 1993). Furthermore, these edge effects appear to be limiting lobster abundance along the bed perimeter.
Rockweed, (Ascophyllum) is a species of macroalgae common to Gulf of Maine marine and estuarine intertidal areas. Distribution is influenced by tidal elevation, salinity, wave energy, exposure to ice, and substrate (Fig. 31). While it typically adheres to rocky substrate, a form of rockweed (Ascophyllum nodosum ecad scorpioides) grows in salt marshes, over organic and sandy soils. The various ecological functions of the extensive rockweed beds are just beginning to be understood, and its importance to lobsters has not yet been assessed (Conkling 1995).
Eelgrass (Zostera) is a submergent, vascular plant typically growing in sub tidal inshore waters along the middle and northern Atlantic seaboard. It requires a muddy to sandy sediment, which is usually associated with moderate water currents and limited wave action (Fig. 32). Eelgrass beds serve as structure and cover for marine and estuarine vertebrates and invertebrates, and as a primary producer of organic matter (Conkling 1995). Lobster shelters are sometimes formed from excavations into rhizomes of eelgrass meadows like those found off Nantucket (Fig. 33). Curiously, eels are often seen occupying these lobster shelters and they might be partly responsible for decreases in lobster densities over a seasonal period.
In Great Bay Estuary, some lobsters have been associated with eelgrass beds (Brown et al. unpub.). However, it does not appear that eelgrass provides any more protection from predators than does mud substrate (Barshaw and Lavalli 1988). (Densities fewer than 0.04 ind. per sq. m or about 0.03 per sq. yd; mostly juveniles and adolescents.)
Recently volunteers at the Lobster Conservancy (Diane Cowan, personal comm.) have conducted a census of lobster nursery grounds along the coastline of Harpswell, Maine. This survey called the Intertidal Lobster Monitoring Program (ILMP) was conducted under a grant from the Davis Conservation Foundation. The ILMP confirmed the presence of early settlement, postlarval and juvenile lobsters in the lower intertidal zone in that area. Similar sampling efforts in New Hampshire, Massachusetts, Rhode Island, and Connecticut show that young-of-the-year and juvenile lobsters do use this habitat and that it may be more important than previously expected. This intertidal sampling effort underscores the importance of mapping the lobster nursery grounds in detail before they are altered or destroyed by human activities. (Densities up to 4.33 individuals per sq. m or about 3.25 per sq. yd; mostly juveniles and adolescents.)
Sand base with rock - This is the most common inshore rock type where the depth is generally greater than 40 m (approx. 130 ft). It consists of sandy substrate overlain by flattened rocks, cobbles and boulders. Lobsters are associated with abundant sponges, Jonah and rock crabs. Shelters are formed by excavating sand under a rock to form U-shaped, shallow tunnels. (Density of 3.2 lobsters/sq. m or about 2.7 per sq. yd with avg. CL = 40 mm or 1.57 in) (Cooper and Uzmann 1980).
Boulders overlying sand - This rock type is best described off McNutt Island Nova Scotia. It is relatively rare for inshore New England (Densities range from 0.13 to 0.09 ind. /sq. m or about 0.11 to 0.08 ind. per sq. yd.)
Cobbles - shelters formed in the interstitial spaces between rocks, pebbles and boulders making up the bed. (Densities up to 16 ind. per sq. m or about 13 ind. per sq. yd.)
Bedrock base with rock and boulder overlay - This rock type is relatively common inshore from low tide to 15-45 m (or about 50-150 ft). Burrowing is generally not possible on this substrate. Shelters are formed by rock overhangs or crevices. Encrusting coraline algae and attached organisms such as anemones, sponges and mollusks cover exposed surfaces. Green sea urchins and starfish are common. Cunner, tautog, sculpin, sea raven and redfish are the most abundant fish occupying the bedrock-rock habitat. (Density of 0.1 lobsters per sq. m or about 0.08 per sq. yd up to 0.3 lobsters per sq. m or 2.5 per sq. yd in summer closures).
Mud-shell/rock substrate - Best described off Rhode Island. (Density of 0.15 lobsters per sq. m or about 0.13 per sq. yd). Usually found where sediment discharge is low and shells make up the majority of the bottom.
OFFSHORE LOBSTER HABITATS (Fig. 35 A and B )
Sand base with rock - consists of sandy substrate overlain by flattened rocks, cobbles and boulders. Although common inshore, this habitat is rather restricted in the offshore region except along the north flank of Georges Bank.
Clay base with burrows and depressions - common to the outer shelf and upper slope. Burrows excavated by lobsters up to 1.5 m (nearly 5 ft) and frequently having an object in the center such as a boulder, rock or cast-off debris. Large bowl-like depressions range in size from 1.0 to 5.0 m (roughly 3-15 ft) in diameter and may shelter several lobsters at a time offshore. (Minimum density of 0.001 lobsters per sq. m in summer). (Cooper and Uzmann 1980)
Mud-clay base with anemones - common habitat for lobsters on the outer shelf or upper slope. Anemones are 20-35 cm (about 8-14 in) tall with a diameter of 5-10 cm (about 2-4 in) at their base. Forests of mud anemones may reach densities of 3 or 4 per square meter (roughly 2.5-3.3 per sq. yd). Depressions serve as shelter for relatively small lobsters. Generally within the 50-80 mm (roughly 2-3 in) CL size range. (Minimum density of 0.001 lobsters per sq. m or 0.0008 per sq. yd. in summer.) (see Cooper and Uzmann 1980).
Mud base with burrows - occurs offshore mainly in the deep basins (up to 250 m or about 820 ft). This environment is extremely common offshore. There are, as yet, no estimates available of lobster density in this setting.
Clay Pipes - characterized by hollowed out pieces of hardened sediment of irregular shapes and sizes that lie littered on the seabed in selected areas. The origin of clay pipe is uncertain. However, one possible explanation is that burrows made by worms, crabs, lobsters and other organisms have become hardened by iron rich waters percolating through the burrow walls (Valentine personal comm.). Another theory is that clay pipes are the remains of tree roots. Whatever their origin, these habitats appear to be shrinking. Anecdotal evidence suggests that clay pipe areas have been disturbed by fishing gear.
In Massachusetts, clay pipes were once a prominent feature of the sea floor on the northern edge of Stellwagen Bank. This is consistent with the large percentage of clay bottom in this region. For reasons of their own, lobstermen have been reluctant to reveal the location of historic or current clay pipe areas. As a result, it is not known whether they are found in other predominantly clay substrates.
The following summary is based heavily on the unsurpassed descriptions of the actual scientists who have participated in more than 200 dives in submersibles in the canyons off Georges Bank (Cooper et al. 1987). We have frequently quoted directly from them in order to capture the flavor of their first-hand sightings and the depth of their experience.
There are more than 15 submarine canyons that cut into the shelf edge on the south side of Georges Bank (Fig. 36). These are V-shaped, sinuous valleys which resemble canyons of fluvial (river) origin. These canyons were first surveyed in the 1930's, but they were not fully explored until manned submersibles were used extensively in the early 1980's. Since that time, one of the largest, Oceanographer Canyon, has been studied most thoroughly (Fig. 37). Fortunately, the results are generally applicable to the other major canyons.
These canyons present a diverse group of habitat types made possible by the rapid changes in substrate and the abundance of nutrients distributed by the strong currents. The canyons present habitats which are recognized as important nursery grounds for a number of bottom animals including lobster, crabs, tilefish and hakes. Juveniles of these species have been observed in naturally occurring and excavated shelters in the bottom, in both the semi-consolidated sandy silts and in the boulder fields. "Concentrations of lobsters (adolescents and adults), for example are substantially greater in submarine canyons than in areas nearby; lobsters seen inside the canyons are usually adolescents (<80 mm or less than about 3.15 in CL) while those nearby but outside the canyons are usually adults" (Cooper et al. 1987).
Sediments in Oceanographer Canyon and most of the other canyons are influenced by the strong clockwise currents on Georges Bank which flow westward across the shelf and by tidal currents which flow up and down the axes. As a result, the canyon sediments are strongly asymmetrical (uneven) with gravel pavements and boulders along the eastern canyon rim and with mostly sand along the western wall. On the east wall, the thin gravel pavement predominates from about 150 to 300 meters (about 500-1000 ft) and the underlying silts or muds are frequently exposed in scattered burrows. Cobbles and boulders up to 1 m. (3.3 ft) in length occur in the gravel pavement. Currents as strong as 1-2 knots distribute sand and sandy-silt along the western canyon wall and the canyon floor is covered with ripples and dunes of slightly gravely sand. These sand dunes which are 1-3 m (about 3-10 ft) in height, were only observed in Oceanographer, Hydrographer and Gilbert canyons; the other canyons south of Georges Bank had only rippled sand. From about 300 to 500 m (about 1000-1500 ft) a clay-like sandy silt or mud is riddled with burrows of lobsters, crabs, fish or anemones. These burrows are often found on steep cliff faces and occasionally these structures collapse, dropping angular slabs onto the canyon floor and leaving vertical walls 10-15 m high (about 30-50 ft).
"Faunal diversity and, to some extent, abundance in the Georges Bank canyon heads appear to be closely tied to the presence of cobbles and boulders on the ocean floor and to exposures of the consolidated sandy silt into which various animals tunnel and burrow. Commercial concentrations of lobsters occur in depths of 100-300 m (about 300-1000 ft) in and near the canyons and over the slopes between them" (Cooper et al. 1987).
The following classification of submarine canyon habitat types is modified from Cooper and Uzmann 1980 and Cooper et al. 1987. The faunal characterization is based on the shallow 150-229 m (492-751 ft) depth zone only.
Type I - Canyon rim and walls consists of sand or semi-consolidated silt substrate (clay-like consistency) with less than 5% overlay of gravel. Characterized by a burrowing mud anemone. Relatively featureless except for conical sediment mounds along canyon axis. (Densities of 0-2 lobsters per 10,000 sq. m or about 2 1/2 acres; mostly adolescents and adults.)
Type II - Canyon walls consist of gravely sand, sand or semi-consolidated silt substrate (clay-like consistency) with more than 5% overlay of gravel. Relatively featureless. Burrowing mud anemones live in tubes that project 10 cm (about 4 in) or more above the bottom and the animal itself may extend another 10-15 cm (about 4-6 in) above that. Also associated with Jonah crabs, ocean pout, starfish, rosefish, squirrel hake. (Densities up to 10 lobsters per 10,000 sq. m or about 2 1/2 acres; mostly adolescents and adults.)
Type III - Rim and Head of Canyons and at base of walls. Sand or semi-consolidated silt (clay-like consistency) overlain by siltstone outcrops and talus up to boulder size. Featured, very rough bottom with erosion by animals and scouring. Lobsters associated with, rock anemones, Jonah crabs, ocean pout, tilefish, starfish, conger eels and white hake. (Densities of 5-1260 lobsters per 10,000 sq. m or about 2 1/2 acres; mostly adolescents and adults.)
Type IV - Pueblo villages- Submarine canyon clay wall with burrows. Heads of Canyons to middle canyon walls consolidated clay substrate, heavily burrowed and excavated. Slope 5 degrees to 70 degrees but generally more than 20 degrees and less than 50 degrees. Juvenile and adult lobsters and associated fauna create borings up to 1.5 m (nearly 5 ft) in width, 1 m (about 3 ft) in height and 2 m (about 6 ft) or more in depth. Between 5 to 10% of borings have multiple openings. Lobsters associated with Jonah crabs, tilefish, shell-less hermit crab, ocean pout, starfish, and conger eels (Fig. 38). (Densities of 5-1260 lobsters per 10,000 sq. m or about 2 1/2 acres; mostly adolescents and adults.) This habitat may well contain the highest densities of lobsters found offshore.
Type V - Sand dune substrate located along the axis. It is associated with white hake, Jonah crab and goosefish. Few, if any, resident lobsters are found in this environment.
