1. Changes in Closure Frequency Per Year
Shellfish Closure Trends Near Mt. Desert Island, Maine
By Marianne Ferguson ‘14
Abstract
Shellfish are a popular commercial fishery harvest in Maine. However, they are susceptible to biotoxins transmitted by toxic phytoplankton, which can lead to paralytic or amnesic shellfish poisoning of humans that consume them. The Maine Department of Marine Resources has a phytoplankton monitoring program in place to detect such biotoxins and to close areas of shellfish harvesting need be. Since 2001, they have archived key data from every closure. One of Maine’s shellfish species, the Blue Mussel, is sensitive to environmental changes and is known as a bio-monitor of pollution. Therefore, a method will be developed to analyze the changes in shellfish closures between Stockton Springs and Machiasport (closure area 64-A) including infected Blue Mussels over the time spanning the years 2001 to 2013.
Results
Figure 1 shows a distribution of changes in closure frequencies from year to year. The pattern seems to suggest that the frequency of closures increased in the southwestern region of closure area 64-A, especially off the coast of Mount Desert Island and between Swan’s Island and Deer Isle. There were no negative slopes in the frequency of closures, suggesting that over the course of time from 2001 to 2013, every location in the study area experienced an increase in closures. However, to ensure that these trends were statistically significant the p-value raster was used to mask out non-significant slopes. Figures 2 and 3 show changes in frequency trend values for locations having a p-value less than .05 and 0.10 respectively. Although not many closed regions had statistically significant increases, as can be seen by the three closed regions shown in Map 2, it is still interesting to note that there were no decreases in closure frequency.
Changes in Closure Frequency Per Year
Figure 1. Changes in closure frequencies in closure area 64-A from 2001 to 2013.
Methods
Biotoxin related shellfish fishery closure GIS data for the years 2001 through 2013 were generated by the Maine Department of Marine Resources and downloaded from the Maine Office of GIS. All data are projected using NAD 1983 UTM Zone 19N. Closures associated with Blue Mussels for area 64-A were extracted from the data for each individual year. Total number of closures within each year were tallied and mapped using ArcGIS’ union and dissolve tools. These closure summaries (stored as polygons) were then converted to rasters for each year for subsequent analysis. Using the R programming environment, the change in the frequency of closures from year to year was analyzed. For each pixel location, a regression analysis was performed where changes in closure frequency was regressed against year. This generated two raster outputs: a regression slope and a p-value for each pixel location. The p-value raster was used to isolate all slope pixels having p-values less than or equal to 0.05 and 0.10.
Conclusion
In the future, this method can be used to visualize and analyze all shellfish closures off the coast of Maine to determine if there are any important spatial and temporal trends in areas of high closure frequencies that stand out. One important implication for closure trends is the placement of shellfish aquaculture sites, which need high water quality and healthy conditions in order to ensure safe human consumption. If possible, it would be interesting to compare closure trends to changes in land use or terrestrial water quality over time. This information can be used to improve the management of Maine’s shellfish industry and could potentially lead to a reduction in closures.
Changes in Closure
Frequency Per Year
Figure 2. Changes in closure frequencies in closure area 64-A from 2001 to 2013 for which p-value≤ 0.05.
Acknowledgments
The data for this study was compiled by the Maine Department of Marine Resources and accessed through the Maine Office of GIS. I would like to thank Manny Gimond for his support throughout this project.
Changes in Closure
Frequency Per Year
Figure 3. Changes in closure frequencies in closure area 64-A from 2001 to 2013 for which p-value≤0.10.