The seasonal distribution and abundance of harbor seals occurring south of Maine were documented by counting the number of seals at traditional haulout locations. The average number of seals counted during each survey in Massachusetts and New Hampshire was 3,560 ± 255 (95% CI), 1983–1987. The maximum number of seals counted on any individual survey was 4,736 individuals. Fifty percent of all the surveys since January 1985 have resulted in counts greater than 4,000 seals reflecting a 27% increase in the abundance of seals in our study area since that date. Seventy-five percent of the seals in southern New England are located at haulout sites on Cape Cod and Nantucket Island. The largest aggregation of seals in the eastern United States occurs mid-winter at Monomoy Island and adjacent shoals. A single high count of 1,672 seals occurred at this site during the study period. An additional 271–374 seals were also counted in Rhode Island, Connecticut and eastern Long Island Sound during surveys conducted in March 1986 and 1987.
The American sandlance Ammodytes americanus was the single dominant prey item of harbor seals in waters adjacent to Cape Cod based on the modified frequency of occurrence of each prey species in scat samples collected from three haulout sites on Cape Cod between 1984–1987. During January and February sandlance was the near exclusive prey item at Monomoy (99%, n= 80). During March and April, the frequency of Atlantic herring Clupea harengus increased in the scat samples at this site. Regional differences in the diet of seals reflect distinct prey communities throughout the study area. Since 1986, the percent occurrence and importance of sandlance in the diet of seals has decreased, reflecting an overall decrease in abundance of this prey species in waters adjacent to Cape Cod. In spite of fluctuations in abundance, and regional differences in the diet of seals throughout the study area, sandlance still comprised a minimum 55% of the total prey species of harbor seals throughout the study area.