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LOBSTER DISTRIBUTION MAPS

Finally, we come to the goal of this exercise, which is to understand the spatial and temporal (place and time) distribution of lobsters in relation to the available substrates and possible Essential Lobster Habitats. To accomplish this goal several different approaches were attempted, but all were eventually shown to have severe limitations. Obviously, the best source of information on where lobsters are present during their various life stages is the lobstermen themselves. Of course there will always be a slight difference of opinion as to who actually qualifies to represent the legitimate lobstermen.

Knowing the reluctance which lobstermen understandably show about revealing the positions of their favorite lobster grounds, we adopted a different strategy. First, we tried to determine the least likely places where lobsters would be found. This effort failed due to the limited time constraints on the project.

Next, we attempted to determine where the lobstermen thought that some of the historic breeding grounds were located and where some of the rare habitats such as clay pipe used to be found. We got the same negative result.

We attempted to determine trap density along the coast based on aerial photos. Unfortunately, we soon realized that some lobstermen leave their traps for long periods in areas they do not fish. They also sometimes leave traps on lobster grounds in order to preserve their territory and prevent others from fishing there.

Lastly, we tried to determine which areas used to be considered highly prized lobster grounds, but now are no longer as attractive. Although we tried several methods from questionaires, to oral histories to posting maps, none were successful in the time allotted.

Another objective for this report was to involve several academic institutions and scientific laboratories in the data collection. This was to ensure that the latest findings were incorporated into the study. This objective was only partially met by focusing on currently published articles and relying heavily on personal contacts and the Internet. Also, several Lobster Summit proceedings were reviewed to fully understand the thrust of current (as yet unpublished) research projects. Later, personal contacts with other researchers active in the field proved invaluable in the final draft of this report.

Additionally, it would have been desirable to work with agencies which are responsible for management of the lobster fishery such as ASMFC, NMFS and NEFMC. Undoubtedly, their resources and input could have provided useful insights into these problems. However, direct contact was discouraged until the project was initially completed.

In spite of these handicaps, a picture of adult lobster distribution seems to be emerging from the data at hand. Put simply, lobsters appear to concentrate at the edges of habitats and near the substrate contacts. The evidence comes from the published NMFS trawl survey data. Of course, it is well known that the trawl surveys do not fully sample all habitats. They are limited by shallow depths, do not sample coastlines, and tend to avoid so-called "hangs" and "hard ground." Furthermore, trawling is impossible in areas where there are too many traps. Nevertheless, when all of these factors are taken into consideration, an adult lobster distribution is revealed that appears surprisingly predictable based on the substrate maps.

One of the earlier published maps of lobster distribution (Fig. 49) was compiled by NMFS as part of a much larger study of Fish Distribution in the NY Bight Region (Grosslein and Azarovitz 1982). Since 1967, continental shelf waters more than 27 m (about 89 ft) from Nova Scotia to Cape Hatteras have been surveyed at least twice each year in spring and autumn. To provide a more generalized picture of density distribution for autumn and spring, the 1973 and 1974 offshore data were combined in a single plot. The survey area was subdivided into 76 different regions or "strata" based on geographic and hydrographic factors. Trawl stations were randomly selected in each stratum, with approximately one station for every 1,000 sq. km (300 sq. nmi). Nets used were modified commercial otter trawls designed for fishing on the bottom. There is no indication that the particular habits of lobsters were in any way considered in this sampling program.

These published maps reveal only part of the story since they show only where lobsters were retrieved rather than where they were absent. Still, when these maps are overlain to the revised substrate map, there is a general correspondence between the relative lobster distribution in spring and the coarser sediment contacts. This pattern is repeated in autumn when lobsters move across the Georges Bank into relatively warm shoal waters. While it is dangerous to generalize from such a large-scale biased survey with fewer than 150 points of control, these results are intriguing and warrant further investigation.

The other published NMFS trawl survey map (Fig. 50 A and B) was included in the Atlas of the Georges Bank (Backus 1987). Again the trawls were random with no attention given to American lobster sites. Here we have a compilation of several years of surveys conducted between 1968 and 1981. We also have more than 200 points for two seasons where lobster were retrieved (ranging from 0.1-10 kg or 0.22-22 lbs in landings). Perhaps of greater importance are the nearly 300 control points for each season where no lobsters or at least less than 0.1 kg (0.22 lbs.) were found. This allows us to overlay the lobster distribution maps to depth and substrate maps and begin to investigate areas where lobsters are actually scarce.

From the overlays we see that no significant numbers of lobsters were taken during the fall in the vast central area of the Gulf of Maine. In this region covering an area of more than 40000 sq. km (nearly a million acres) over 50 trawl attempts were made without success. The situation is similar in the spring with only two trawls recovering lobsters in the region. When we examine the area carefully we find that depths generally range from 150-200 meters (roughly 500-650 ft). The substrate map shows that the bottom consists mostly of fine-grained sediments including sand silt and clay with an abundance of sandy silt and clayey silt. Although a few shallow ledges characterized as gravely-sand appear to have been sampled, no lobsters were recovered in the trawls.

Looking more closely at the Georges Bank area, we find the expected concentration in both seasons along the shelf edge, but find that very few lobsters are encountered on the central bank area in spring. In the fall, there are several more successful trawls, but these appear to be localized around a few gravely sand areas shown on the south side of the bank.

Returning to the 1973 maps, the same gap in the central Gulf of Maine is present, with no lobsters obtained in fall and only two successful trawls in spring. On Georges Bank we find the same relationship to the gravely areas.

The agreement is uncanny between two sets of data obtained nearly a decade apart. From this trawl data, it appears that lobsters are more frequently found on the boundaries between two different substrates. It has previously been suggested that local population density may correspond with the availability of shelters (Bologna et al. 1993). Additional evidence indicates that boulder shelters are also a necessary ingredient (Hudon and Lamarche 1989). These findings suggest that the American lobster may be a key indicator species, which is much more laterally restricted than some of the literature would indicate. This could either be due to the lobsters' preference for maximum information and environmental choices (as we saw with kelp) or it could be due to sample bias.

Unfortunately, no further conclusions can be drawn from such small maps, but it is imperative that this overlay procedure be duplicated with full-scale maps and a more complete and current database. There are anecdotal reports that lobsters in recent years have become more concentrated in soft bottom areas, but maps to support this argument have not yet been published. If possible, these types of maps and input from lobstermen should be integrated into a GIS (Geographical Information System) and then statistics could be applied to determine significant correlations. Only then will we begin to see to what extent lobsters have become dependent on a few areas of prime habitat in the offshore region for their future survival.