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The Maine Horseshoe Crab Surveys were begun in 2001 to establish quantitative baseline data and determine whether horseshoe crab (Limulus polyphemus) populations are stable or declining. Spawning counts were conducted at sites from Casco Bay (southern Maine) to Frenchman’s Bay (downeast), to establish an index of relative abundance. Few (relatively) abundant populations remain in Maine at Middle Bay (Harpswell/Brunswick), Thomas Point (Brunswick), and in the Damariscotta River. Healthy but less abundant populations persist in the Bagaduce River (Brooksville/ Sedgwick) and in Taunton Bay (Hancock/Franklin/Sullivan).

An intensive tagging study has been conducted on Taunton Bay, which offers a natural population with no known history of harvest, and which is effectively closed to immigration and emigration by the physical characteristics of the Bay. During four field seasons, 6176 observations have been logged on 3883 individuals (2595 males, 1288 females; 66.8%, 33.2%). Returns by the 2001 tag year class have varied between 8.7 and 7.5 % in subsequent years. Females comprised 28% (of 116) of the returning 2001 tagged animals in 2002, and 33% (of 113) in 2003, but declined to only 13% (of 100) returning in 2004—just 1% of the original 2001 tag year class of 1333 (individuals). A cause has not yet been attributed, but adult molting is suspected. However, unless adult molting is confirmed, mortality must be assumed instead.

Return rates of individuals tagged in 2001 were analyzed to evaluate spawning site fidelity. While return rates from year to year did not exceed 8.7% of the original tag year class of 1333 individuals, 22% of the individuals were observed again in the years from 2002 to 2004.

Similarly, observations for 2004 were analyzed to determine the ratio of new individuals to those observed that had been tagged in the three prior years. In 2004, there were 1384 observations of 915 individuals (592 males, 323 females) at the tagging site, of which 25% had been tagged in prior field seasons. This suggests that a significant number of the adult horseshoe crabs in the vicinity of Shipyard Point may now be tagged, and each additional season of data will increase the value of the existing data for understanding the population dynamics of horseshoe crabs in Taunton Bay, and other sites in Maine.


Funded by the Maine Department of Marine Resources, Maine Coastal Program, Casco Bay Estuary Project and Bar Mills Ecological.


Invertebrates; Casco Bay


Casco Bay watershed, Taunton Bay



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