"A comprehensive knowledge of the geological environment is required to answer questions regarding the health of a fishery, the effects of ocean dumping and pollutant dispersal, and the impact of energy exploration activities."
During the past 15 years, many Georges Bank fishery populations including cod, haddock, herring, and sea scallops have declined while populations of non-commercial species have expanded rapidly.
Northeastern fishery landings are valued at approximately $800 million dockside, of which a large proportion is produced on Georges Bank. Recently, scientists of the U.S. Geological Survey (USGS) and the National Marine Fisheries Service (NMFS) have undertaken an interdisciplinary effort to document direct interactions between physical environmental factors and the abundance and distribution of fishery species.
Geological processes have resulted in the evolution of habitats favorable to commercial fishery species.
The varied nature of sedimentary environments on Georges Bank is a key element in the development of the biological community. Seafloor sediment originally was transported to the bank by glaciers. During and after glacial retreat, the rise of sea level and the action of tidal and storm currents marked the start of an erosional episode on the bank that continues today. The winnowing and redistribution of glacial sediment has formed modern habitats, which are altered as geological processes continue to operate. For example, a large area of seabed surfaced by gravel pavement is expanding westward as sand is transported off the bank.
USGS studies and maps provide knowledge of the sedimentary environment that is fundamental to understanding the habitat and life cycle of important commercial fishery species.
It is well known that sediment type is one of several important factors governing the abundance and distribution of fishery species; however, the nature and geographic distribution of sediments on Georges Bank are poorly known. These first-of-a-kind maps of the seafloor environment are a basis for formulating new approaches to studying the Bank's living resources and environmental processes.
USGS recently discovered and mapped a gravel pavement that covers an area of more than 3000 square kilometers extending along the northern part of the bank.
The pavement forms as a residual deposit where strong tidal and storm currents winnow sand from coarse glacial sediment. The gravel is an important habitat for the spawning and survival of several fishery species. For instance, distribution patterns of juvenile cod indicate that the gravel habitat is where they are best able to avoid predators and to find food sources. Therefore, the gravel may be essential for their survival and recruitment to the fishery. Also, sea scallops are abundant on the gravel or gravelly sand habitats and apparently are unable to colonize large areas of the bank where shifting sand is a major environmental factor.
New USGS maps of the seafloor environment show that the historic herring spawning grounds of eastern Georges Bank are located on gravel pavement where the strongest tidal currents occur between sand ridges.
This region has an unique environment where herring eggs can attach to a firm substrate and hatch in clean, oxygenated water. The herring population was abundant in the 1960's but collapsed in the early 1970's. During a joint USGS/NMFS survey of the spawning grounds in 1991, no herring eggs or larvae were found, indicating that the Georges Bank herring population has not yet recovered.
Physical disturbance of the seafloor alters habitats, and is caused by natural processes (tides, storms) and by commercial fishing activities.
USGS/NMFS sidescan sonar surveys of fishing grounds show extensive scarring of the bottom by groundfish trawls and scallop dredges. It is not known whether disturbance of the seabed by fishing has a positive or a negative effect on the fishery itself. However, we know that fishing activity causes mixing of the upper part of the seabed. It impedes colonization of the gravel bottom by attached organisms that flourish in untrawled areas, and thus reduces local biological roughness and overall species diversity and abundance.
Future USGS studies in collaboration with NMFS will determine how sedimentary environments are utilized by fishery species and their predators, prey, and associated fauna.
Detailed studies will focus on physical factors such as sediment type, topography, current strength, water chemistry, habitat disturbance, and on the abundance, distribution, and life histories of species.