others moved upstream from the point of re- 

 lease. One crab was recovered at the place of 

 release . 



Some of the tagged crabs moved sea- 

 ward fairly rapidly. One particular individual, 

 tagged on Lufkins Flat August 11, 1952, was 

 recovered at Cranes Beach, 2 to 3 miles away 

 on August 14, 1952 . Thus the sharp increase 

 in recovery of crabs during the next calendar 

 year after tagging and the recovery of crabs 

 from areas in an oceanward direction from the 

 point of release seems to indicate that the in- 

 dividual crabs stay in Plum Island Sound for 

 only a short time each summer. Therefore 

 they might be considered as transient visitors 

 to the seashore which would explain the low 

 rate of recovery during the year a group was 

 tagged. 



The apparent migration to and from the 

 Plum Island Sound area indicates that the horse- 

 shoe crabs under observation are a local 

 population. The recovery of more crabs during 

 the calendar year after tagging, and recoveries 

 one, two and three years after tagging within 

 an area having a radius less than 13 miles from 

 the point of tagging supports this view . 



The crabs tagged in Plum Island Sound do 

 not necessarily return to this area the following 

 year . Ten crabs tagged in Plum Island Sound 

 were recovered in or very near the estuaries 

 south of this area. Exploration of these estu- 

 aries showed that horseshoe crcibs were 

 abundant and characteristic depressions on the 

 flats were seen and "cast" or molted shells 

 were collected. The recovery of four crabs 

 tagged from these estuaries, although from a 

 very small group of tagged and released crabs, 

 would indicate that these areas are utilized by 

 some horseshoe crabs. No figure has been in- 

 cluded for these crabs as so few were released 

 and recovered. Although no collections were 

 made on the southern shores of Cape Ann, one 

 recovery from Gloucester Harbor and another 

 in the Annisquam River indicates that Plum Is- 

 land Sound crabs may reach this shore by way 

 of the Annisquam River canal. 



Exploration of an area north of the sound 

 --Hampton Harbor and Black Water Creek, N.H. 

 --uncovered very few horseshoe crabs, and 



their characteristic depressions in the flat v/ete 

 rare . In addition, no recoveries of tagged 

 crabs have been made within Hampton Harbor, 

 and only one crab tagged in Plum Island Sound 

 was recovered on beaches outside Hampton 

 Harbor even though the entire shoreline from 

 Cape Ann to Rye Harbor, N.H., is well populated 

 by summer people who might find tagged crabs . 



Measurements of adult male and female 

 horseshoe crabs show that the mean carapace 

 width remained nearly constant during the threfe 

 years, and that there is a definite difference be- 

 tween the sexes, the females averaging 47 mm., 

 wider than the males (table 7 and fig. 3). The 

 observed difference in size between the sexes 

 did not seem to appreciably affect the recovery 

 of tagged crabs . The mean carapace width and 

 standard deviation for recovered tagged males 

 was 120.7 mm. and 9.04, and for females 156,0 

 mm . and 9 . 45 . 



The techniques employed to recover 

 tagged crabs did not permit an accurate estimate 

 of the population . The tagging and recoveries 

 of crabs were extended over such a long period 

 that natural mortality and recruitment mi^t 

 introduce large sources of error. There was 

 also no segment of the tagging program that 

 could be treated separately because of the long 

 time between tagging and recoveries . Even so, 

 roug^ estimates of the population might be madfe . 

 On July 20, 1953, the staff recovered 6 tagged 

 crabs on Point Peter while capturing a total of 

 1,018 untagged crabs. An estimate derived from 

 these figures would yield a figure of 164,000 

 crabs. On June 4, 1954, an alerted searcher re- 

 covered 4 tagged crabs and 495 untagged crabs . 

 An estimate based on these figures would be 

 151,000 crabs. During the summer of 1955, an- 

 other alerted searcher recovered 15 tags after 

 capturing and killing 9,541 crabs. An estimate 

 would yield about 1,000,000 crabs. Even with 

 the inaccuracies mentioned, the figures indicatt; 

 that the population is of considerable magnitude. 



CONCLUSIONS 



1 . An onshore migration of Plum Islaid 

 Sound horseshore crabs begins early in March 

 and continues to a peak in June. 



2 . The largest number of horseishoe 



13 



