MIGRATIONS OF THE HORSESHOE CRAB UMULUS POLYPHEMUS IN PLUM ISLAND SOUND, MASS. Marine Biological Laboratory DECl J 1957 WOODS HOLE, MASS. ^ SPECIAL SCIENTIFIC REPORT-FISHERIES Na 220 UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE EXPLANATORY NOTE The series embodies results of investigations, usually of restricted scope, Intended to aid or direct management or utilization practices and as guides for administrative or legislative action . It is issued in limited quantities for Official use of Federal, State or cooperating agencies and in processed form for economy and to avoid delay in publication . United States Department of the Interior, Fred A. Seaton, Secretary U.S. Fish and Wildlife Service MIGRATIONS OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS, IN PLUM ISLAND SOUND, MASSACHUSETTS By John p. Baptist, Fishery Research Biologist, U.S. F.W.S., Beaufort, N.C., Osgood R . Smith, Fishery Research Biologist, U.S.F.W., Washington, D.C., and John W. Ropes, Fishery Research Biologist, Clam Investigations, U.S.F.S.W., Kingston, R. I. Special Scientific Report- -Fisheries No. 22 0. Washington, D. C. September 1957 ABSTRACT During the years 1952, 1953, and 1954 a total of 1,780 horseshoe crabs (Limulus polyphemus) were tagged and released in or near Plum Island Sound, Mass . Recovery of tagged crabs indicated an onshore migration that began early in March and reached a peak in June. The largest numbers of crabs were present in the Sound during June, July, and August, after which an offshore migration resulted in a reduction of their numbers on the flats. Low recoveries of crabs during the year they were tagged indicated that in- dividual crabs may spend a fairly short time in the Sound each summer . A population of horseshoe crabs that is probably local for Plum Island Sound and nearby estuaries was indicated by the tag recoveries . Five tagged crabs were recovered almost 4 years after tagging. CONTENTS Page Introduction 1 Methods 1 Results 2 Table 1 . --Summary of intertidal collections of horseshoe crabs in upper and lower parts of Plum Island Sound, Mass ., 1951 through 1954 2 Table 2. --Summary of subtidal collections of horseshoe crabs in upper and lower parts of Plum Island Sound, Mass., 1951 through 1955 2 Table 3. --The release of tagged horseshoe crabs from areas within Plum Island Sound,, Mass ., and their recovery during 1951 through 1955 3 Table 4 . - -The release of tagged horseshoe crabs from areas outside Plum Island Sound, Mass., and their recovery 1952 through 1955 7 Table 5. --Number of horseshoe crabs recovered by three groups of searchers 1952 through 1955, listed by areas and months g Table 6. --Number and percentage of tagged horseshoe crabs recovered 1952 through 1955 9 Table 7 .--Mean carapace width of adult male and female horseshoe crabs collected in Plum Island Sound, Mass., 1952 through 1954 9 Discussions 13 Conclusions 13 References I4 Appendix A 15 MIGRATIONS OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS, IN PLUM ISLAND SOUND, MASSACHUSETTS INTRODUCTION The predation on soft -clam (Mya aren- aria) by the horseshoe crab (Limulus polypher- mus) is of major significance in the soft -clam producing areas of Massachusetts (Turner, Ayers, and Wheeler, 1948; Shuster, 1950; Smith and Chin, 1951). The migratory habits of the horseshoe crab in Plum Island Sound, Mass., were investigated as a part of studies to develop methods for control of this predator. Observations on the distribution, abun- dance, and migration of Limulus were made in the sound from 1949 to 1955. Preliminary ob- servations indicated a seasonal pattern of abun- dance within the sound during the summer months. During 1951 a program of marking the crabs by cutting off half of the tail and cer- tain abdominal spines was started. This system gave some returns, but many were not definite enough to be useful because it soon became evi- dent that some individuals had lost a tail or spines before we marked them . A program of tagging with Petersen - disk tags was initiated in 1951 and continued through 1954. A total of 1,639 horseshoe crabs was tagged and released from flats within Plum Island Sound during these years. An additional 141 horseshoe crabs were released from areas outside Plum Island Sound making a total of nearly 1,800 Limulus tagged in or near Plum Is- land Sound. The authors were assisted in the recov- ery of tagged crabs and the collection of untagged crabs by Lionel Sheppard, Clam Commissioner, Charles Bayley, Deputy Clam Commissioner, of Ipswich, Mass., and by Daniel Pierce, Conser- vation Officer, Massachusetts Department of Marine Fisheries. METHODS Most of the horseshoe crajjs tagged were captured and released in the intertidal zone of Plum Island Sound. A few, encountered while searching for previously tagged individuals, were tagged in the Annisquam River near Glou- cester, Mass., and in Hampton Harbor, N.H. Subtidal collections were made with a 30-inch-wide scallop dredge or beam trawl hauled by a 16 -foot outboard motorboat. The warp was made fast in the boat, but by holding on to it one could feel whether the dredge was scraping the bottom . Although the dredge did not catch large numbers of crabs, it did provide some observations in the subtidal zone. Intertidal collections were made while crabs were moving to and from the flats during a flooding tide or sometimes an ebbing tide. We observed that fairly large numbers of horseshoe crabs began their movements onto the flats al- most as soon as did the tide and would stay until just before the flats became exposed at low tide. Crabs that had buried themselves in the flat dur- ing some previous tide, rose out of the soil and moved about as the tide flooded the flat . Several hundred of these crabs could be harvested by three men during one tide. Best results were obtained at the narrow entrance to a channel . By wading in water 2 to 3 feet deep the three of us picked the crabs off the bottom and placed them in the boat which was towed behind or an- chored nearby. Of the techniques used, this one yielded the largest number of crabs . During low tide, the presence of Limulus was indicated by depression that liberally pock- marked the flats . These depressions, which were circular, 6 to 8 inches wide and 2 to 3 in- ches deep, were made by crabs in search of food or in preparation for laying eggs . By digging with a clam hoe in or at the edge of fresh pits we often uncovered a crab or a mating pair, but this method was not nearly as productive as col- lections made during a flooding tide . Petersen tags were attached to the craba through a hole pierced by an awl at the right rear point of the prosoma or "head" . The numbered white disk was placed on a nickel pin and diis pushed through the hole so that this disk was under the prosoma. The red disk with a notice of reward and address for mailing was placed on the upper side. The width of the crab was measured at its widest point and the sex noted. This information was recorded with the serial number of the tag. Usually a day's catch could be tagged, measured and released the same day it was caught, but some large collections were held overnight in the boat or laboratory. This ap- parently did the crabs no harm as they were lively when released and some of the highest returns came from crabs so held. A case in point is a collection of 38 made on June 26, 1952, which was brought to the laboratory, kept out of water overnight, tagged and released the next day. Seven have been recovered; 4 in 1953, 1 in 1954, and 2 in 1955. Also, 92 collected on August 14, 1952 and released the next day, have yielded 14 recoveries; 4 in 1952, 2 in 1953, 5 in 1954, and 3 in 1955 . The recovery of tags may be roughly divided into three groups, according to who found them: (1) recoveries made throughout the year by our staff while collecting crabs for' tagging and other data; (2) recoveries by "alerted" persons who knew of our tagging program and were deliberately searching for tagged crabs within Plum Island Sound, either for the reward or to help us; and (3) purely accidental recover- ies by visitors to the seashore. This last group includes many people from inland towns and the general surprise and curiosity is quite evident from the letters. These may be considered chance or random recoveries, which might be expected from any popular beach or shoreside area. The first two- groups, those found by the staff and "alerted" persons, are not random in geographical distribution because most of the searching was done within Plum Island Sound. RESULTS Results are tabulated in tables 1 to 7 and shown in figures 1 to 3 . Table 1. --Summary of intertidal collections of horseshoe crabs in upper and lower parts of Plum Island Sound, Mass., 1951 through 1954 Average Number of Total number number of c: rabs collections of crabi s per coUectic )n Month Uppe r Lower Upper Lower Upper Lower April 1 0 20 20.0 May 10 2 282 25 28.2 12.5 June 22 4 537 705 24.4 176.3 July 12 7 271 1,171 22.6 167.3 August 5 9 251 798 50.2 88.7 September 1 5 2 100 2.0 20.0 October 1 1 1 17 1.0 17.0 Total 52 28 1,364 2,816 26.2 100.6 Table 2. --Summary of subtidal collections of horseshoe crabs in upper and lower parts of Plum Island Sound, Mass., 1951 througji 1955 Average number Number of Total nunr iber of crabs collections of crabs per collection Month Upper Lower Upper Lower Upper Lower March 2 3 5 2 2.5 0.7 April 4 4 15 39 3.8 9.8 May 4 5 46 74 11.5 14.8 June 2 2 7 2 3.5 1.0 July 1 1 4 10 4.0 10.0 August 1 2 14 15 14.0 7.5 September 3 6 16 48 5.3 8.0 October 6 5 8 19 1.3 3.8 November 3 4 3 1 1.0 0.3 December 4 1 0 1 0.0 1.0 Total 30 33 118 211 3.9 6.4 Table 3. --Release of tagged horseshoe crabs from areas within Plum Island Sound, Mass., and their recovery during 1951 through 1955. Released Date Place Mature ImmatT M F ure Total Recovered Date Place Sear - Sex and Size cher(4) mm . 8/14/51 Doles Island 2 5/29/52 Roger Island R. 14 6 9 29 6/16/52 Hales Cove 13 7 20 7/17/52 6/20/54 Cranes B. Sargents B., Gloucester N N F M 171 ? 6/27/52 Nelsons Island 25 9 4 38 5/10/53 Beach, mo. Essex R. N F 160 5/16/53 Wingaersheek B. N F 163 8/ 8/53 1/2 mile mo. Essex R. N M 117 12/ -/53 Ipswich(l and 2) N M 120 8/11/54 Cranes B. N F 157 8/12/54 Cranes B. (No letter) N M 111 4/ 8/55 Cranes B. N M 121 9/ 5/55 Eagle Hill Jl. A F 147 7/1/52 Hales Cove 1 1 7/14/52 Hales Cove 6 2 8 8/ 3/52 Ipswich Bay(l) N M 112 8/ 1/52 Hales Cove 1 1 8/11/52 Nighways Creek 4 10 14 7/20/53 Point Peter(3) S F 155 8/11/52 Lufkins Flat 22 11 3 36 8/13/52 Lufkins Flat(3) s M 107 8/ 14/52 Cranes B, N M 117 Table 3 . - -Continued Released Recovered Mature Immat- Total Sear- Sex and Size Date Place M F ure Date Place 1 c;her(4) mn^. 8/11/52 Lufkins Flat (cont'd.) 8/ 5/53 Third Cr. A F 146 8/ 6/53 Middleground A M 110 9/26/54 Cranes B . N M 127 8/12/52 Ipswich Yacht Club 28 3 31 9/ 3/55 Eagle Hill R. A M 108 8/13/52 Lufkins Flat 11 8 2 21 8/14/52 Lufkins Flat 126 48 4 178 8/ -/52 Cranes B. N M 136 9/ 2/52 Lufkins Flat(l) N M 120 11/ 1/52 Roger Island R. A F 150 12/29/52 Plum Island B.(l) N F 154 6/23/53 Richards Ground A F 143 7/20/53 Point Peter (3) S M 115 5/ 1/54 Plum Island Shore N M 119 11 5/54 Richards Ground A M 123 7/24/54 Gloucester Harbo r N M 102 8/ 5/54 Richards Ground A M 123 8/ 8/54 The Nobbs N F 141 6/ 1/55 1 mile S. Cranes B. N F 159 6/15/55 Cranes B. N M 112 7/ 4/55 Ipswich River mo i. N F 152 8/15/52 Hales Cove 43 43 6 92 9/ 2/52 Sandy Point (1) N M 115 7/20/53 Point Peter (3) S M 124 8/ 6/53 Middleground A F 159 9/ 2/53 S. end Plum Is.(l) N F 160 9/ 7/55 Point Peter A F 140 8/21/52 Hales Cove 35 51 4 90 6/11/53 Hampton Beach, N.H. N F 164 6/18/53 The Nobbs N F 168 6/21/53 The Nobbs N M 131 8/ 6/53 Middleground A F 153 6/ 4/54 Hales Cove (3) A F 160 7/ 4/55 Grape Island N F 161 8/22/52 North of Rowley MiddlegTound31 50 3 84 3/29/53 Horseshoe Flat N F 149 6/19/53 Mo. Ipswich R. N M 130 7/ 5/53 The Nobbs N M 119 7/20/53 Point Peter (3) S F 154 9/ 4/54 Ipswich (1 and 2) N M 114 6/11/55 Parker River N F 145 7/ 7/55 Lufkins Flat A F 152 7/ 7/55 Davis Pt., Gloucester N M 119 7/17/55 Third Creek A F 146 8/ 2/55 Nighways Creek A F 163 8/25/52 Point Peter 81 104 7 192 6/23/53 Richards Ground K F 149 Table 3 . - -Continued Released Date Place Mature Immat- M F ure Total Recovered Date Sear- Sex and Size Place cher(4) mm. 8/25/52 Point Peter (cont'd.) 8/27/52 Nelsons Island 34 28 66 8/28/52 Horseshoe Flat 43 33 79 6/24/53 Richards Ground 7/22/53 Third Creek (See 2nd, (3) 8/26/53 Plum Island Sound(2) 9/ 1/53 Lanesville, Gloucester 9/21/53 Ipswich B. (2) 6/10/54 Eagle Hill R. Cove 6/10/54 Cranes B. 6/ -/54 Richards Ground 6/ -/54 Eagle Hill R. 7/ 3/54 Richards Ground 7/ 6/54 Cranes B. 7/ 8/54 Plum Island B. 8/21/54 Eagle Hill R. 8/23/54 Plum Island B. 5/21/55 Plum Island (2nd. recovery) 8/ 1/55 Lufkins Flat 8/ 4/55 Highways Creek 8/31/52 Rowley Shore of Plum Island R. 5/30/53 Between Great and Little Neck 6/25/53 Annisquam R. 7/20/53 Point Peter (3) 6/ 4/54 Hales Cove (3) 8/28/54 Ipswich Bay (2) 6/17/55 Cranes B. 7/20/53 Point Peter (3) 7/ 3/54 Sandy Point 8/23/54 Great Neck A F Recovery) S F N F M M F F M F F M F F F F M F N M F F F M F M M M F 159 159 142 126 137 166 165 125 142 148 123 155 183 156 159 133 153 111 153 152 150 122 149 116 106 129 172 Total released in 1952 517 414 49 980 Total recovered of this group 36 43 2 81 7/20/53 Point Peter 21 20 7/20/53 Parker River 9 7/21/53 Nelsons Island 22 17 1 7/31/53 Point Peter 32 44 42 9 39 76 7/5/54 Richards Ground A M 120 8/2/54 S.endPlum Island(l) N F 149 5/29/55 Plum Island B. N F 155 8/20/53 Cranes B. N F 169 8/30/53 Lufkins Flat AM 114 5/31/54 Eagle Hill R. N F ? 6/ 4/54 Hales Cove (3) AM 124 6/ 4/54 Hales Gove A F 159 7/ 5/53 WiirgaersheefcB. N F 13^ Table 3 . - -Continued Released Recovered Mature Immat- Sear- Sex and Size Date Place M F ure Total Date Place cher(4) mm. 9/1/53 Point Peter 28 41 69 6/29/54 Point Peter(3) S F 156 6/29/54 Point Peter(3) s F 156 7/21/55 Middleground A F 151 8/15/55 Eagle Hill R. A M 115 9/10/53 Broad Sound 5 1 2 8 9/16/53 Point Peter 4 1 5 6/10/54 Cranes B. N M 124 9/17/53 Buoy 4 to Eagle Hill R. 2 2 4 9/17/53 Third Creek 4 9 1 14 11 4/54 Richards Ground A F 152 9/29/53 Point Peter 1 1 10/2/53 Bluff Creek 7 5 3 15 Total released in 1953 134 139 9 282 Total recovered of this group 5 10 0 15 6/17/54 Doles Island 11 9 20 7/11/54 Mo. Ipswich R. N M 120 7/ 8/55 Lufkins Flat A F 156 8/17/55 Point Peter A M 113 6/29/54 Point Peter 99 240 6 345 7/ 5/54 Sandy Point N F 160 8/ 3/54 Eagle Hill R. A F 151 8/ 7/54 Little Neck A F 153 6/ 7/55 Cranes B. A F 162 6/16/55 Eagle Hill R. N M 114 6/25/55 Cranes B. N F 159 7/26/55 Ipswich Bay(2) N F 159 7/30/55 Middleground A M 140 7/31/55 Lufkins Flat A F 149 8/ 2/55 Lufkins Flat A M 142 8/ 4/55 Cranes B. N F 154 8/ 9/55 Staceys Creek A F 168 8/11/55 Ipswich Bay (1 and 2) N F 160 8/14/55 Sandy Point N F 165 9/ 3/55 Eagle Hill R. A F 152 7/27/54 Plum Island Bridge 1 9 10 Total release in 1954 HI 258 6 375 Total recovered in this group 5 13 0 18 Grand total released 762 811 64 1639 Grand total recovered 46 66 2 114 Table 3. --Continued (1) Indicates letter or postmark date was used; actual date of recovery not supplied by searcher. (2) Indicates poor location of recovery was supplied by searcher. (3) Indicates these crabs were re-released by the staff. (4) Symbols used in table: Searcher - N = non-alerted, A = alerted, S = staff. (5) Includes 2 released in 1951 for which sex was not recorded. Table 4. --Release of tagged horseshoe crabs from areas outside Plum Island Sound, Mass., and their recovery 1952 through 1955. Released Date Recovered Place Mature Immat- Total Date M F ure Place Sear- Sex & Size cher(l) mm. 8/12/52 Haskells Landing, Essex, Mass. 25 14 6/10/54 Black Water Cr . , N.H. 2 3 6/25/54 ConomoPt., Essex, Mass. 47 27 7/15/54 Hampton River, N.H. 1 8/ 2/54 Black Water Cr., N.H. 2 1 8/17/54 Annisquam R., Mass. 10 6 39 77 I 3 16 6/14/53 Wingaersheek B. N F 180 7/20/53 Ferris Landing, Conomo Pt., Essex, Mass . N F 150 4/10/55 Wingaersheek B. N M 114 8/ -/55 Essex River N M 101 Total released in these areas 86 52 3 141 Total recovered of this group 2 2 4 (1) Searcher - N = nonalerted 3 O m o- ^^ u (U X u u Hi CO 09 Q. 3 O feS, 9) X) m ID > o o V l-l JJ « l-l o o _ tn >« l-l A i -a ■^ (U VH *-> O to u — • ■a S a 1 ^_^ ^ M g in •o in lU ■ IH u •* > o in o 0) l-l l-l (4 V >< ■ 1 IH M < IH rt C 4) o CO Z -^ . 0) O _3 l-l in o o 0) l-l CO in u nj OJ >- in •s c o s fo ■* in f». rx i? PC t^ es M CO -H m o \o t^ cs -J tN in -^ -^ -H CO ■* M -^ ts ts ts -H HO —I «N -H es (S t^ -^ vO O f^ ^ -H «o CN ■* f^ vO CN CS ■* ■* ■* — < -H CN CN -H 00 00 ■* -H in 00 vo -H vo 00 r^ CO 00 CO 2 05 CO Afc 2 ■< 2 2.a.-< cS^ 0 z a Tf 0) *-* o IH cs ^ *« CN II CN < •^ ■«*' § to B l-l 4-1 II lU J3 •§: = : CO a o o o IH T3 Z to C - - •f-* - : t^ D. ■a to : 3 «-> 1 IH cd rea : ^H 0) u ich a l-l >, X! - - " ■* 1) Xt ^ to ■a iH 0) O : : ; l-l IH t*H ^H & > o ^ in -H t— ) to to o ^ ^H •9 u _3 : : = CO < 5 ^^ 3 ^ •— 1 c? CO ^ in 2 Table 6. --Number and percentage of tagged horseshoe crabs recovered 1952 through 1955 Number tags out 5/ Number tags recovered Percentage recovery tagged in •52 '53 '54 tagged in tagged in Recovered •52 •53 ^54 '52 '53 '54 1952 980 loL/ 1.02 1953 971 282 302/ 1^/ 4 3.09 0.71 1954 948 280 375 241/ 2.53 3.57 1.07 1955 926 273 371 16 4 14 1.73 1.47 3.77 1956 910 269 357 1/ 1 of these re- released 2/ 7 " " »• 3/ 2 " ■• " 4/ 3 •' " It 5/ Neglecting natural mortality . Table 7. --Mean carapace width of adult male and female horseshoe crabs collected in Plum Island Sound, Mass., 1952 through 1954 Males Females Year Number Mean width Standard deviation Number Mean width Standard deviation 1952 1953 1954 608 655 204 118.1 118.6 116.7 9.08 8.30 9.82 491 601 295 156.1 155.1 155.5 10.82 11.27 10.75 Total 1,467 118.1 8.95 1,387 155.5 11.04 DISCUSSION Horseshoe crabs were encountered in Plum Island Sound every month of the year ex- cept January and February (table 2) . As no dredg- ing was done during these months, it is possible that some may have been present in the subtidal zone . Dredgings made during the month of March indicated that some may be active at this time. Although crabs were present in channels of the sound all or nearly all year, they did not appear on the clam flats until April . Even at this time their mumbers were very low and pits made by the crabs were not in evidence . In May there was an obvious and abrupt increase, both in the numbers of crabs that could be picked up, and in the numbers of pits on the flats . The numbers of crabs present during the following months increased even more abruptly, and reached a peak during July and August. The total numbers collected per month (1,442 and 1,049) and the average number per collection (75.9 and 74.9) were highest for these 2 months. Figure 1. --Release of 1,135 tagged horseshoe crabs from the lower part of Plum Island Sound, Mass., during 1952, 1953, and 1954, and their recovery. 10 (T) Hompton Atlantic Ocean ■c^<^. :# Figures indicate Number of Recoveries Lj = the summer tagged 0=' year after tagging <^=2yeQrs after tagging i'3 years offer togging Esse? Sargents BeactiQ} 70l4 7 76T45' 70143' <> - »»m Figure 2. --Release of 504 tagged horseshoe crabs from the upper part of Plum Island Sound, Mass., during 1952, 1953, and 1954 and their recovery. 11 0 100 120 140 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 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 crabs are present in Plum lalmd Sound cftiring the months of June, July, and August. 3 . Individual crabs probably spend a fairly short time in the sound each year . 4. An offshore migration is most evident during September . 5 . Tag recoveries indicate that there is a fairly discrete population of horseshoe crabs in Ipswich Bay. 6. The techniques used to recover tagged crabs were not adequate for an accurate estimate of the population size. REIFEIEENCES Shuster, C. N., Jr. 1950. III. Observations on the natural history of the American horseshoe crab, Limulus polyphemus . Third report of investigations of methods of improving the shellfish resources of Massachusetts . Mass. Dept. Consv., Div. Mar. Fish., pp. 18-23. (Also Woods Hole Oceanographic Institute Contr. No. 564.) Smith, O. R., andE. Chin 1951. The effects of predation on soft clams, Mya arenaria . Conv. Add. Nat. Shell. Assoc. 1951, pp. 37-44. Turner, H. J., J. C. Ayers, andC. L. Wheeler 1948. App. II. The horseshoe crab and boring snail as factors limiting the abundance of the soft -shell clam. Report on investigations of the propagation of the soft -shell clam, Mya arenaria . Mass. Dept. Consv., CXv. Mar. Fish., pp. 43-45. (Also Woods Hole Oceanographic Institute Contr . No. 162.) 14 APPENDIX A The following data were obtained too late for inclusion in the body of the paper but do not contradict the results discussed. It is perhaps worthwhile to note that horseshoe crabs tagged in 1952 were still available for recovery in 1956. As the majority of the crabs tagged were mature, this would indicate infrequent, if any, molting of mature horseshoe crabs. Tag Recoveries During 1956 Date and location crabs were Released Date and location crabs were Recovered 8/22/52 - North of Rowley, Middleground 8/12/52 - Haskell's Landing - Essex 8/12/52 - Ipswich Yacht Club 8/25/52 - Point Peter 8/-/55 - Cranes B. Ni/- ^y - 140i/mm. (Sent in Sept. 1956) 6/16/56 - Wingaersheek Beach, Ip. N - F - 160 mm. 7/15/56 - Plum Island N - M - 111 mm. 6/16/56 - Eagle Hill R. A - M - 131 mm. 8/28/52 - Horseshoe Flat 7/13/56 - Wingaersheek Beach N - F - 160 mm. 7/21/53 - Point Peter 8/23/56 - Sargents Beach, Gloucester N - F - 154 mm. 9/ 1/53 - Point Peter 6/ 7/56 - Plum Island B. N - M - 125 mm. U Symbols for search groups: N = Nonalerted; A = alerted 2/ Symbols for sex of crabs: F = female; M = male 3/ Size in mm . 15 INT.-DUP SEC. WASH.. DC j "BL WHOI Library - Spna 5 WHSE 01154 V X