Data and Results
Initial Results Prior to the Closure to Bottom Fishing in December 1994 (Collie et al., 1997; Collie et al., 2000): During the year before a large section of Georges Bank was closed to mobile fishing gear, two research cruises were conducted. Data collected during these expeditions revealed that both bottom fishing disturbance and depth exert a large influence on the area's benthic megafauna. The numerical abundance of megafauna, biomass, species richness, and species diversity were all statistically greater at sites that were classified as undisturbed by bottom fishing. In contrast, evenness was higher at disturbed sites, indicating that these sites were less likely to be dominated by a few very abundant species. Compared to shallow sites (40-50 m depth), deeper sites (80-90 m depth) on Georges Bank had a greater number of benthic organisms and increased biomass, species richness, and species diversity.
Since the percent cover of sessile epifauna could not be quantitatively analyzed using the Naturalist dredge samples, bottom photographs and videotapes of the sea floor were examined. In the year 1994, the types of sessile epifauna that covered the largest area of the sea floor were the tube-forming polychaete worm Filograna implexa, hydroids, bushy bryozoans, and encrusting bryozoans. Compared to other sites, deep, undisturbed sites typically had a higher percent cover of Filograna implexa, bushy bryozoans, and hydroids. In contrast, encrusting bryozoans tended to be more abundant at disturbed sites. One potential explanation for this finding is that, due to their encrusting morphology, these bryozoans cannot easily be detached from the substrate by trawling or dredging. Since bottom fishing has reduced the abundance of more fragile organisms at the disturbed sites, encrusting bryozoans may be freed from inter-specific competition for space and can, thus, become more abundant.
TWINSPAN: A statistical technique known as TWINSPAN (Two-Way Indicator Species Analysis) was used to classify the most commonly observed species into 7 groups based upon their abundance at sites with varying depths and disturbance levels. The results of this analysis are presented below.
Astarte montagui Astarte subaequilatera Crossaster papposus Hyas coarctatus Natica clausa Pagurus pubescens Phyllodoce groenlandica
Aspidophoroides monopterygius Calliostoma occidentale Cerastoderma pinnulatum Chlamys islandicus Chone infundibuliformis Eunice norvegica Lebbeus groenlandicus Musculus discors Ophiopholis aculeata Protula tubularia Sinum perspectivum Spirontocaris lilljeborgii Thelepus cincinnatus
Eualus pusiolus Henricia sanguinolenta Modiolus modiolus Nemertinea unident. Pandalus motagui Potamilla reniformis
Astarte elliptica Astarte undata Cyclocardia borealis Pagurus arcuatus Pholis gunnellus Urticina felina
Buccinum undatum Placopecten magellanicus Strongylocentrotus droebachiensis
Crangon septemspinosa Dichelopandalus leptocerus Flabellina sp. Harmothoe imbricata Liparis atlanticus Myoxocephalus aeneus Nereis zonata
Astarte borealis Asterias vulgaris Cancer irroratus Colus spp. Crepidula plana Neptunea decemcostata Pagurus acadianus Long-term Trends Following the Bottom Fishing Closure (Collie et al., 2004; Hermsen et al., 2003)
Deep Sites: Between the years 1994-2000, two sites located at deep depths (80-90 m) were surveyed on five occasions. One of these sites (Site 20) had previously been classified as undisturbed by bottom fishing, whereas the other, deep site (Site 13) had been heavily fished with otter trawls and scallop dredges. Compared to Site 13, the benthic megafauna at Site 20 were significantly more abundant and had a greater biomass. These differences between Sites 13 and 20 increased throughout the duration of this study. Similarly, the rate of megafaunal production at Site 20 (174-256 kcal m-2 yr-1) was significantly higher than the production rate at Site 13 (30-52 kcal m-2 yr-1). Nevertheless, at these two sites, there was no significant difference in terms of the production:biomass ratio, which is a measure of the rate of biological turnover.
In addition, differences in the benthic community structure at Sites 13 and 20 were noted. Despite the fact that species richness was greater at Site 20, Site 13 was reported to have a higher species diversity and evenness. In the years 1997-1999, the low evenness and diversity at Site 20 were largely due to the fact that a single species of polychaete worm, Thelepus cincinnatus, accounted for up to 50% of the individuals present. The differences in community structure at the two deep sites were principally related to the increased abundance at Site 20 of two polychaete species (T. cincinnatus and Potamilla neglecta), one brittle star (Ophiopholis aculeata), one crab (Hyas coarctatus), and six shrimp species (Dichelopandalus leptocerus, Lebbeus groenlandicus, Pandalus montagui, Eualus pusiolus, Crangon septemspinosa, and Spirontocaris spinus). In contrast, the benthic community at Site 13 was dominated by the sea star Asterias vulgaris, the sea urchin Strongylocentrotus droebachiensis, and two types bivalve mollusks (Astarte spp. and Cyclocardia borealis). Since sampling occurred during different months of the year, inter-annual changes in community composition at the deep sites may be related to seasonal patterns of species abundance.
Shallow Sites: Following the two initial surveys conducted in 1994, Site 17, which had originally been classified as a shallow, disturbed site, became incorporated into one of the closed areas on Georges Bank. Shortly after this occurrence, bottom fishing disturbance markedly increased at Site 18, which had been previously classified as a shallow, undisturbed site. Beginning in 1997, samples were also collected at Site 17W, which is a heavily disturbed site located adjacent to the closed area.
Prior to the bottom fishing closure at Site 17, this area's biomass was significantly lower than Site 18. However, this situation was quickly reversed as the megafaunal biomass at Site 17 increased from 51 g wet weight m-2 in 1994 to 2,474 g wet weight m-2 in 2000. In contrast, there was comparatively little variation in biomass at Site 18 during this time period. Other changes occurring at Site 17 include a 4-fold increase in the numerical abundance of benthic organisms and a 12-fold increase in megafaunal production. The main factor contributing to the increased production at Site 17 was the growth of the sea scallop Placopecten magellanicus and the sea urchin Strongylocentrotus droebachiensis. As sea scallops at this site matured, their biomass increased, but their production slowed down, leading to a decline in the production:biomass ratio. At disturbed Site 18, production varied between 32-57 kcal m-2 yr-1, but did not show any definite temporal trends.
During the time period of this study, species diversity increased at both Sites 17 and 18. However, this increase occurred at a much faster rate at Site 17. Evenness did not different significantly between these two sites. Compared to the shallow, disturbed sites where community composition was fairly stable, Site 17 was characterized by increases in the abundance of species of crabs (Cancer irroratus, Pagurus acadianus, and Hyas coarctatus), shrimp (Dichelopandalus leptocerus), mollusks (Crenella glandula, Astarte borealis, Placopecten magellanicus, and Buccinum undatum), polychaetes (Nereis zonata), and echinoderms (Asterias vulgaris, Strongylocentrotus droebachiensis, and Ophiopholis aculeata). Site 17W was distinguished from all other shallow sites by its high abundance of sand shrimp (Crangon septemspinosa), while Site 18 was characterized by an elevated abundance of rock crabs (Cancer irroratus).
Graphs. Click on the thumbnails below to see a larger version of these graphs. Please note that these graphs differ slightly from those included in Collie et al. (2004), because taxa too small to be retained consistently by a 5-mm sieve were not filtered out of the dataset.
- Megafaunal abundance at shallow sites
- Biomass at shallow sites
- Species diversity at shallow sites
- Evenness at shallow sites
- Megafaunal abundance at deep sites
- Biomass at deep sites
- Species diversity at deep sites
- Evenness at deep sites
Data Downloads. Three data tables are available for download in a tab-delimited text format. A brief explanation of the type of information included in each of these tables is also provided below. These data tables contain information on Naturalist dredge samples collected between the years 1994-2000. Recent dredge data collected in the years 2002 and 2003 and all data describing the abundance of colonial epifauna in bottom photographs are not available at this time.
Important: When including any information derived from these data tables in a publication or presentation, proper citation of the data source is mandatory.
