| July 2003
Has the Mt. Hope Bay Fish Assemblage Changed More Than Narragansett Bay Assemblages? |
Introduction
Figure 1: The
greater Narragansett Bay region including Mt. Hope Bay and the Sakonnet River,
with identification of areas used for further analysis
- Previously, research has focused on comparing winter flounder abundance in the Massachusetts portion of Mt. Hope Bay to abundances for the entire Narragansett Bay and coastal Rhode Island.
- This ignores strong spatial patterns in winter flounder catches due to location effects. Estuarine ecologists have long known that fish abundance is strongly affected by location within the estuary.
- To partially account for these biases, we conducted an analysis of winter flounder and fish species assemblages based on Rhode Island Department of Environmental Management (RIDEM) Seasonal Trawl Survey data. We divided this data into 9 area sectors within the greater Narragansett Bay.
- The RIDEM Seasonal Trawl Survey data base was chosen for this analysis because it is the only data set that has good spatial coverage of the entire greater Narragansett Bay Estuary and covers a 23 year period, including years before and after the collapse of winter flounder stocks.
Winter Flounder Abundance
Figure 2: Annual mean winter flounder catch, segregated into the 9 areas shown in Figure 1, plotted against time. The line referred to as 0 (dark blue) is the mean of all data pooled together and represents the trend for the greater Narragansett Bay region.
Figure 3: Same as Figure 2, but data has been aggregated into 5 year periods, and some areas have been dropped for clarity. As above, the line referred to as 0 (dark blue) represents the trend for the greater Narragansett Bay region.
- Figures 2 and 3 indicate that the abundance of winter flounder has decreased in all areas of the greater Narragansett Bay region over the last fifteen to twenty years.
- The decline in catches within Mt. Hope Bay appears to be similar to declines in most other areas within the greater Narragansett Bay region.
Other Fish
Hogchoker
Figure 4: Same as Figure 3, but for hogchoker.
Windowpane
Hogchoker
Figure 4: Same as Figure 3, but for hogchoker.
Windowpane
Figure 5: Same as Figure 3, but for Windowpane.
Scup
Figure 5: Same
as Figure 3, but for Scup.
- Figures
4 through 6 show plots of mean catch (within the entire greater
Narragansett Bay region)
for three other species. Two of these species (hogchoker and windowpane)
show declines during the last fifteen years, similar to winter flounder.
The mean catch for scup shows an increase over the same period.
- A total of 29 species
were analyzed across the entire region, with 10 showing a decline, 14 showing
an increase, and 5 exhibiting no identifiable trend over the last 20 years.
- Data analysis is ongoing for individual subareas, including Mt. Hope Bay.
Assemblages
Figure 7: Canonical
plots for fish assemblages in Mt. Hope Bay and Narragansett Bay showing
changes in assemblage over the last two decades.
- The plots in Figure 7 show similar trends in both Mt. Hope Bay and the entire greater Narragansett Bay region.
- A shift from benthic, or bottom dwelling, fishes (e.g. winter flounder) towards pelagic, or water column fishes (e.g., herring) has occured in both Mt. Hope Bay and greater Narragansett Bay over the last 20 years.
- In all cases, trends observed in Mt. Hope Bay appear to be similar to the other areas.
Conclusion: It's
a Narragansett Bay-Wide Pattern
- Temporal changes in winter flounder and other fish abundances in Mt. Hope Bay appear to mirror something happening throughout the greater Narragansett Bay system.
- Future research will examine potential causes of the observed fish assemblage shift in detail.
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