GIS-based Analysis of Harp and Hooded Seal Sighting Locations On Shore In The Southern Gulf of Maine
by
David E. Harris
Lewiston-Auburn College, University of Southern Maine
Coauthors: Matthew Bampton, Rosemary Mosher, Sat Gupta, Cathleen McAnneny

Harp seals (Phoca groenlandica) and hooded seals (Cystophora cristata) pup and breed on the ice in the Gulf of St. Lawrence and at the “Front” (off Newfoundland) but otherwise have somewhat different natural histories. Published reports (Stevick & Fernald 1998; McAlpine et al. 1999; Harris et al. 2001; Harris et al. 2003) show increasing numbers of these ice seals being sighted on the Maine coast since 1990, with more sightings reported in the southern as compared to the northern Maine coast. In an effort to understand the behaviors of these species, we used vector GIS to analyze 421 harp and 163 hooded seal sightings on the southern Gulf of Maine shore between Kittery (43°05’ N, 70°44’W) and Rockland, Maine (44°06’N, 69°06’W) from 1/1/97 to 12/31/02. Records of these sightings were retrieved from the database of the Marine Animal Lifeline, Westbrook, ME and confirmed by photo and/or direct observation. For each sighting we determined the date, species, health status, location (in UTMs), nearest subtidal and intertidal shoreline type, distance to 30 m and 60 m ocean depth, distance to the closest public land, human population within 30 km (with census block data), and distance to the following Gulf features: Cashes Ledge, Jeffreys Ledge, Wilkinson Basin, Georges Basin, and Jordan Basin. Using a Pearson Chi-Square test for categorical variables and either a Mann Whitney test or a Kruskal Wallis test for numerical variables, we performed group-to-group comparisons by species and health status. We then constructed a step-wise linear regression model using the number of seal sightings within a 1 km radius of each sighting location as the dependent variable and the independent variables listed above. Our group-to-group results show that, compared to harp seals, hooded seals were sighted closer to 30 m ocean depth (p=0.002), trended toward being sighted in proximity to low rather than high-energy subtidal habitats (p=0.051), and were less healthy when sighted (p=0.016). The reasons for these differences are not known, but may reflect differences in foraging or other behaviors by species. We also found that sicker animals (both species combined) were more likely than healthier animals to be sighted in proximity to high-energy intertidal habitats (p=0.005). This may indicate that healthier animals sought out easier landfalls, while sicker animals came ashore without exercising this option. Our linear regression analysis showed that closer proximity to Wilkinson Basin was the strongest predictor of high seal sighting density. Closer proximity to public land, higher human population within 30 km, being further south, and being further from Jeffreys and Cashes Ledges were also significant predictors of high seal sighting density. For the completed model, rsq = 0.40. These findings suggest that ice seals in the Gulf of Maine may be seeking or avoiding certain geographical features in a search for foraging habitat. They also suggests that human variables influencing the likelihood that a seal is reported could be impacting seal sighting densities and raises the possibility that there are many more ice seals on the beach in the Gulf of Maine than are reported to stranding organizations.

Date received: August 25, 2003