100 120 140 leO 180 200 220 

 Caropoo* Width In Miiljmitari 



Figure 3 . - -Length frequency of the carapace 

 width of horseshoe crabs captured and measured 

 in Plum Island Sound, Mass., during 1952 

 through 1954. 



The peak abundance occurred earlier in 

 the lower part of Plum Island Sound than in the 

 upper. The largest average number per collec- 

 tion in the lower part of the sound, 176.3 crabs, 

 was made in June (table 1). On the flats of the 

 upper part of the sound the largest average 

 number per collection was only 50.2 crabs and 

 this did not occur until the month of August. 



From September through December the 

 number of crabs within the sound decreased. 

 This was noticed in September on the flats and 

 later in the channels . After October only lone 

 individuals were uncovered on the flats or dredg- 

 ed up from the channels . As is indicated in 

 tables 1 and 2, this decrease first occurred in 

 the upper part of the sound. 



In general the recovery of horseshoe 

 crabs tagged in Plum Island Sound follows the 

 same pattern of seasonal abundance as was 

 described above for untagged crabs encountered 

 in the intertidal zone. The earliest recovery of 

 a tagged crab was made on March 3, 1954 on 

 Horseshoe Flat and only 8 crabs were recovered 

 during April and May (table 5) . The recovery 

 of tagged crabs increased to 27 in June and con- 

 tinued hig^ for the months of July and August. 

 Only 10 tagged crabs were recovered in the last 

 4 months of the year . 



Nearly one -third (32) of the recoveries 

 were made in areas outside Plum Island Sound 

 and all of these were found by nonalerted per- 

 sons. All but 4 of these recoveries were made 

 on beaches facing Ipswich Bay and the open ocean. 

 Cranes Beach and Plum Island Beach, which are 

 just south and north of the entrance to Plum Is- 

 land Sound, were the sites of recovery for 21 of 

 the 32 tagged crabs recovered outside Plum Is- 

 land Sound. 



Less than one -sixth (7) of the recoveries 

 was made in the upper part of the Sound, even 

 though 504 tagged crabs were released in this 

 area . The poor recovery record for this area 

 may be partly due to the lack of popular beaches 

 which result in fewer searchers on the flats, or 

 the number of crabs venturing into this area may 

 be less whether they be tagged or not. We made 

 nearly twice as many collection trips to this area 

 as to the lower part of Plum Island Sound and 

 utilized this area for other experiments on claitis, 

 but failed to recover any tagged crabs. 



Eight recoveries were listed as doubtful 

 in table 5 because the nonalerted persons who 

 sent them in did not give a specific location of 

 recovery. In some cases the date of recovery 

 was also omitted. Although the locations given 

 were not specific enough for this study, all were 

 reported to have been found within the nearby 

 Plum Island Sound area . 



Recoveries of tagged crabs during the 

 same year they were released were surprisingly 

 low; about 1 percent. During the calendar year 

 after tagging, recoveries in excess of 3 percent 

 were made from all three groups (table 6). Re- 

 coveries of tagged crabs out two and more years 

 decreased slightly, but not to the low percentage 

 for the year tagged. Natural mortality has not 

 been considered in arriving at these percentages 

 The percentage recoveries are minimum estimates 

 because the number of tags out would actually be 

 less than the number tagged. 



The recoveries of individual crabs (tabl$ 3) 

 indicate a fairly rapid oceanward migration 

 throu^out the summer. Eleven recoveries dur- 

 ing the same year they were tagged were made In 

 areas below or seaward of the place of release. 

 Two other crabs moved across the sound but in 

 an outgoing or downstream direction, while two 



12 



