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HUMAN IMPACTS ON HABITAT
Any discussion of human impacts on marine habitats is inherently controversial. The only thing most people can agree on is that different habitats are impacted differently. In an effort to quantify the impact of fishing activities on habitats, Peter Auster has prepared a conceptual model (Auster and Langdon 1998). The graph (Fig. 51) shows the relative level of complexity of various substrates and how that changes with increasing fishing effort. They showed that piled boulders are most complex and are the most sensitive to fishing effort. Next comes dispersed boulders and cobbles followed by pebble-cobbles with epifauna. Bedforms such as sand and silt are not surprisingly shown as the least complex and the least sensitive to alteration. Significantly, they have not included any mention of bedrock or rocky ledges and shorelines. Intuitively, these also would be very insensitive to fishing effort regardless of their complexity.
Auster and Langdon (1998) provide a map of nearly 15,000 trawl and scallop dredge tows (Fig. 52) conducted between 1989 and 1994. When this map is overlain to substrate, it is clear that the gravel bottoms are being heavily trawled. If this map is representative of commercial tows in general, then we should find evidence of reduced complexity in the most heavily towed gravel and cobble areas.
This is precisely what Valentine and Lough (1991) have documented in photos of eastern Georges Bank (Fig. 53). Untrawled areas with gravel pavement show abundant worm tubes and other attached organisms. In trawled areas the rocks are bare and both complexity and productivity is decreased.
Cooper and others (1987) have shown similar effects in the submarine canyons where blocks of the thin gravel pavement have been knocked off the walls and fallen to the canyon floor
Habitats on Stellwagen Bank are impacted to a lesser extent due to differences in substrate. The sidescan sonar images (Fig. 54A and 54B) reveal numerous otter trawl and dredge marks on the sandy bottom, but these activities apparently have little impact on the habitat complexity.
From these examples, we can cautiously conclude that bottom trawling has a significant impact on lobster habitats in coarse-grained sediments like boulders, cobble and gravel, but little effect on finer grained sediments. However, we must not ignore the potential for damage to extremely fragile habitats like clay pipe and anemone burrows in the mud/clay setting. This still leaves a few habitats where bottom trawling probably does little damage.
Adult lobsters are also subject to man's influence. Lobsters have historically been known to live for more than 30 years. Even under contemporary patterns of commercial exploitation, lobsters up to 217 mm (about 8.5 in) CL and weighing about 9 kilos (nearly 20 lbs.) have been encountered in shallow water during the summer months.
Lobsters are sensitive to chemicals and have been known to vacate areas that have become polluted. However, not all of these impacts are negative. Some of the largest lobsters have been known to occupy man-made shelters such as discarded cans and barrels.
Other important human activities which may lead to pollution and lobster habitat destruction include landfills, dredging, dumping, industrial wastes, spills and sewage outfalls. Point sources of pollution come from industrial plants, such as pulp and paper mills, fish processing plants, textile mills, metal fabrication and finishing plants, municipal sewage treatment plants, and chemical and electronic factories.
Non-point sources are not as easily located. Rainwater runoff often contains pesticides from agricultural and forested areas along with hydrocarbons, heavy metals and organics from urban areas. It is not unusual for older cities to combine their storm drainage system with the sewer system which results in raw sewage discharges during times of overflow. All of these pollution sources can have a tremendous impact on water quality, habitat preservation and ultimately human health. These problems can be multiplied when the contaminants get into the sediments and then are disturbed by dredging. When the contaminants are suspended in the water column they become available for uptake by many species (including lobsters) and can accumulate throughout the food chain.
Ocean dumping has been identified as another major problem for lobster especially when it results in burying gravel beds. "Ocean dumping of silt-clay over gravel may increase spatial competition among juvenile lobsters for shelter in remaining gravel habitat" (Potter and Elner 1982).
Considerable research has already been done on the effects of hydrocarbons and drilling fluids on lobsters (Atema et al. 1982). These studies show that "both the chemical toxicity in the water column and the physical effect of covering the substrate with drilling mud interfere with normal lobster behavior." Some of these tests resulted in the death of adult lobsters within hours. For postlarval lobsters sublethal effects included feeding and molting delays, severe delays in shelter construction, increased walking and swimming difficulties, and lethargy. Atema and others (1982) concluded "perhaps as little as 1 mm (about 0.04 in) covering of drilling mud may cause increased exposure to predators and currents, resulting in the substrate becoming unsuitable for lobster settling and survival."
Recently, concern has been raised about the effects of sewage outfall pipes on lobsters. As we have seen, environmental conditions which are not toxic to lobsters can still cause them to shift or vacate an area. These include changes in temperature, salinity, currents, substrate, pH, dissolved oxygen, nutrients, pesticides and other contaminants. Much work is still needed to understand exactly why lobsters have apparently abandoned certain areas impacted by man.
There have been few documented examples of lobster habitat enhancements in the Gulf of Maine, but there may yet be significant potential. We have already mentioned the favorable results obtained by planting artificial kelp beds. Similar results were obtained when artificial shelters made of PVC pipe or concrete blocks were positioned on the sea floor (Ojeda and Dearborn 1991). So far, the evidence seems to indicate that these methods merely serve as gathering points for lobsters in the surrounding area. Many believe that overall lobster density is not increased, but this may be an oversimplification.
Anecdotal evidence of lobsters being attracted to discarded rubble may point the way forward. This enhanced image on the flank of Stellwagen Bank (Fig. 55) purportedly captures a boulder pile that was dumped some time ago. Lobstermen now report good catches in this vicinity. If lobsters do concentrate on the edges of habitat boundaries, it is likely that here they would be afforded additional protection from predators. It is therefore reasonable to assume that their overall survival rate would increase. Furthermore it is possible that some artificial lobster shelters might be adapted to increase reproductive success as well. In addition, we have just begun to explore the possibility of artificial clay pipe habitats. Clearly more work is needed in this area before any firm conclusions can be drawn.