Fish Communities

From NOAA’s NMFS Ecology of the Northeast Continental Shelf Toward an Ecosystem Approach to Fisheries Management, courtesy of Michael Fogarty.

The relative abundance of different groups of fishes on the Northeast Continental Shelf has shown dramatic fluctuations over the last forty years
The relative abundance of different groups of fishes on the Northeast Continental Shelf has shown dramatic fluctuations over the last forty years

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Both fish harvesters and researchers recognize that there are a number of areas where fish species consistently occur together. We find that there are recognizable fish communities found in the Gulf of Maine, Scotian Shelf, Georges Bank, the Northern Mid-Atlantic Bight, Southern Mid-Atlantic Bight, on the edge of the continental shelf, and in the transition zone between the Gulf of Maine and Georges Bank. Finer subdivisions can be identified within each region, but the broad-scale patterns provide important insights into fish community structure.

Analysis of NEFSC bottom trawl survey data indicates that groupings of fish occur in well-defined regions.
Analysis of NEFSC bottom trawl survey data indicates that groupings of fish occur in well-defined regions. The major areas identified all consistently show distinct assemblages of fish*

Biodiversity

We can also see that there are geographical regions with higher numbers of species (or species richness). For example, an examination of research vessel trawl catches shows that certain areas support more fish species than others, and that these sites tend to be related to topographic features such as sharp depth gradients. If we measure the average number of species caught in trawl surveys over time, we see a generally stable overall pattern for the shelf as a whole. However, when we examine subareas, some differences emerge. For example, on Georges Bank, we see an increase in the number of species caught over the last decade – in part reflecting an increase in more southern species found on the bank.

Trends in Species Groups

Dramatic changes in the relative abundance of different species groups have been observed over time. During the early 1960s, the abundance of northeastern groundfish species began a period of sharp decline as a result of overexploitation. Under a number of new management actions starting in 1994, some stocks have started to improve. These actions included the establishment of large-scale closed areas, restrictions on the days-at-sea allowed for each vessel, and gear regulations such as increased mesh-size. Small pelagic fishes, notably herring and mackerel, also declined in the region under intensive exploitation by the distant water fleets in the 1960s. These species have since undergone a tremendous increase in abundance. During the period of decline for groundfish, a large-scale increase in abundance of certain elasmobranchs (dogfish and some skates) was observed.

Why Does it Matter? – Fish Scales

School of fish

When we look at things from an ecosystem perspective, different types of scales are important relative to a view that focuses on one species at a time. For example, the spatial scales of relevance may become the areas where identifiable groups (or assemblages) of species occur rather than the distribution patterns of an individual species. Or areas that have particularly high numbers of species may be of special interest. At another level, we tend to see that the overall abundance of whole groups of fish species tends to be much more stable than any one of the species making up the group. Some aspects of ecosystem-based fishery management will tend to focus on these different levels of organization and different scales.

The average number of species caught in NEFSC bottom trawl stations averaged over all seasons
The average number of species caught in NEFSC bottom trawl stations averaged over all seasons indicates some distinctive patterns indicating higher species richness along depth gradients and other features

Although the exact mechanisms underlying this increase have not been determined, one suggestion is that overall declines in abundance in certain parts of the ecosystem have resulted in the release of food and/or space that the dogfish and skates could then use. These small elasmobranchs began to decline starting in the mid- to late 1980s as fishing pressure on these species increased. Because elasmobranchs produce many fewer young than most other fish, and they tend to become mature at older ages, they are vulnerable to overexploitation. These changes in the composition of the fish communities and the relative importance of different species groups raise questions about whether the changes are reversible if we reduce overall fishing pressure or whether more direct manipulations might be needed. In some cases, the shifting patterns of fishing pressure on different groups over time may have strongly influenced patterns of decline and recovery.

*Map by Chad Keith NMFS, NEFSC