BRUCE B. GOLLEHt
SEP 2 8 !S8i
Sf *TES O* h
NOAA Technical Report NMFS SSRF-747
Movement, Growth, and
Mortality of American
Lobsters, Homarus americanus,
Tagged Along the
Coast of Maine
Jay S. Krouse
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Marine Fisheries Service
NOAA TECHNICAL REPORTS
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NOAA Technical Report NMFS SSRF-747
a fe. Movement, Growth, and
Mortality of American
Lobsters, Homarus americanus,
Tagged Along the
Coast of Maine
Jay S. Krouse
U.S. DEPARTMENT OF COMMERCE
Malcolm Baldrige, Secretary
National Oceanic and Atmospheric Administration
National Marine Fisheries Service
The National Marine Fisheries Service (NMFS) does not approve, rec-
ommend or endorse any proprietary product or proprietary material
mentioned in this publication. No reference shall be made to NMFS, or
to this publication furnished by NMFS, in any advertising or sales pro-
motion which would indicate or imply that NMFS approves, recommends
or endorses any proprietary product or proprietary material mentioned
herein, or which has as its purpose an intent to cause directly or indirectly
the advertised product to be used or purchased because of this NMFS
Tagging areas 1
Results and discussion 3
Literature cited 12
1 . Maine coast showing the three tagging areas and recovery points o f American lobsters 2
2. Length-frequencies of American lobsters tagged and released at Kennebunkport, Boothbay Harbor, and Jones-
port, Maine 3
3. Size distributions of tagged American lobsters still at large after September 1977 at each tagging area 4
4. Premolt-postmolt carapace length relations of recaptured tagged American lobsters that molted at each release site 5
5. Kennebunkport, Maine, region showing dispersal of recaptured tagged American lobsters, May 1975-September
6. Boothbay Harbor, Maine, region showing dispersal of recaptured tagged American lobsters, May 1975-September
7. Jonesport, Maine, region showing dispersal of recaptured tagged American lobsters, May 1 975-September 1 977 . . 8
8. Average distances traveled by tagged American lobsters during weekly time intervals prior to recapture 9
9. Distances moved by recaptured American lobsters of various sizes tagged and released at Kennebunkport,
Boothbay Harbor, and Jonesport, Maine 10
10. Recaptures of tagged American lobsters as related to time at large at each tagging area 10
1 . Monthly tag recoveries of American lobsters by release area off Maine, 1 975-77 3
2. Comparison of the proportions of American lobsters recaptured to those tagged by two biologists at each release
area, 1975-77 4
3. Mean sizes of tagged American lobsters recaptured along with those lobsters not recaptured, 1 975-77 4
4. Comparison of the sex ratios of tagged American lobsters released with those recaptured at each release area,
5. Average distances moved by recaptured American lobsters at each tagging area 9
6. Summary of the distances traveled by recaptured tagged American lobsters at each tagging area, 1975-77 10
7. Annual instantaneous rates of apparent total and fishing mortality on American lobsters estimated from returns
grouped by different time intervals 11
8. Estimated percentage of tag loss after various time intervals for American lobsters released at Kennebunkport,
Boothbay Harbor, and Jonesport, Maine 11
Movement, Growth, and Mortality of American Lobsters, Homarus
americanus, Tagged Along the Coast of Maine 1
JAY S. KROUSE 2
During the spring of 1975, 2,882 American lobsters, Homarus americanus, were tagged at three locations off
Maine. Four months after release 65% of the lobsters had been returned and by the completion of the study in
September 1977, 2,188 (75.9%) lobsters had been recaptured. Most returns (88%) occurred within a 5 n.mi. (9.3
km) radius of the release site and only about 1% of the recaptured lobsters had moved more than 10 n.mi. (18.5
km). Movement and catchability did not vary significantly by sex nor size. The majority of lobsters traveled
shoreward or along the coast on a west to southwesterly course with minimal easterly movement. All long
distance migrants O20 n.mi. or 37.0 km) followed a south to southwesterly course. Extremely high annual
instantaneous fishing mortality rates (4.0-7.3) estimated for each release area confirm the overexploitation of the
Maine inshore lobster fishery.
During the past decade concern for the future well-being of
the Maine American lobster fishery has intensified as levels of
fishing effort have increased and catches have generally
declined. In response to this interest in Maine's most valuable
commercial fishery, the Lobster Research Project of the Maine
Department of Marine Resources (DMR), initiated in 1966
extensive studies of various facets of the fishery (Thomas 1973;
Krouse and Thomas 1975; Krouse 1978) and biology of the
lobster (Krouse 1973). Even though information from these
studies has provided some basis for scientific management of the
lobster fishery, additional research is required in many areas.
One important area with a paucity of information is that of
lobster movement along the Maine coast. To date there have
been three tagging studies with Maine lobsters. Harriman 3 and
Cooper (1970) tagged lobsters at Monhegan Island [about 10
n.mi. (nautical miles), 18.5 km offshore] and determined that
those lobsters were nonmigratory since most recaptures were
recovered within a 2 n.mi. (3.7 km) radius of the island. In con-
trast, Dow (1974) reported that 5 of 162 lobsters (23 returns in
all) tagged by commercial fishermen off the Maine coast
traveled 75-138 n.mi. (138.9-255.6 km) toward Cape Cod. Four
of these migrant lobsters were larger than the Maine maximum
legal size of 127 mm CL (carapace length) when tagged,
indicating a positive relationship between a lobster's size and
In view of the limited size and scope of these lobster tagging
studies conducted previously in Maine waters, we decided to
undertake a coastwise tagging project. Objectives of this present
study were to provide new information on growth, mortality,
and movement or migration patterns of legal-sized lobsters
(81-127 mm CL).
'This study was conducted in cooperation with the Department of Commerce,
National Marine Fisheries Service, under Public Law 88-309 as amended, Com-
mercial Fisheries Research and Development Act, Project 3-228-R.
2 Marine Resources Laboratory, Maine Department of Marine Resources,
West Boothbay Harbor, ME 04575.
'Harriman, D. M. 1952. Progress report on Monhegan tagging
1951-52. Unpubl. manuscr., 8 p. Maine Dep. Mar. Resour., W. Boothbay
Harbor, ME 04575.
Three tagging sites, Kennebunkport, Boothbay Harbor, and
Jonesport, representing the western, central, and eastern
sections of the Maine coast (Fig. 1), were selected on the basis of
geographical location and local availability of lobsters.
Well in advance of the scheduled dates for tagging, certain
lobster dealers were contacted at each tagging area, and
arrangements were made to purchase about 1 ,000 lobsters from
each area. It was specified that these lobsters be locally caught
and not sorted by size. These requirements would ensure that
the tagged lobsters were characteristic of the area studied in
terms of size, movement, and catchability.
To determine whether the tagged lobsters were representative
in size of those lobsters caught commercially, length- frequencies
were plotted by 1 mm increments for lobsters tagged at each tag-
ging site (Fig. 2). Because of the likeness between size composi-
tion data of this present study and data obtained from Maine's
Commercial Sampling Program (Thomas 1973), we are confi-
dent that the lobsters tagged were typical of the legal size range
of lobsters along the Maine coast.
The sphyrion tag developed by Scarratt and Elson (1965) and
later modified by Cooper (1970) was selected as the primary 1
mark as it can be retained through a molt. The model we used in
this study consisted of a supple yellow PVC (polyvinylchloride)
tube (2 mm diameter x 55 mm long) attached by a thin
polyethylene thread to a 7 mm long stainless steel anchor. Tags
were attached according to the technique described by Cooper
In order that the magnitude of tag loss could be evaluated a
secondary tag was used. The tag selected was the Floy cinch-up
which was secured to the pincer claw by either fastening it
around the proximal end of the propodus or around the carpus
of lobsters > 100 mm CL. Although this tag would be lost after
ecdysis, we anticipated that a sufficient number of lobsters
would be recaptured prior to molting, to enable estimation of
P LIBERATION SUES
O RECOVERY SUES
Figure 1. — Maine coasl showing the three lagging areas and recovery points of American lobsters that moved i 20 n.mi. (37.0 km). Lobsters released at Boothbay Har-
bor. Jonesport, and Kennebunkport, Maine, are denoted by B. J. and K in circles.
the rate of sphyrion tag loss. Experimentation with the nylon
cinch-up tag revealed that this material expands upon immersion
in water and consequently might slip off the claw. To minimize
stretching, the tags were soaked in tepid water prior to applica-
To ensure that fishermen and dealers would be informed of
the tagging program, posters advertising rewards for the return
of tagged lobsters were distributed to almost all lobster dealers
along the Maine coast. Cash rewards were S2.00 for return of
only the tag and $5.00 for lobster with tag(s) intact. Throughout
the study we strived to maintain the fishing community's interest
and cooperation through periodic press releases on the progress
of the tagging program and frequent contact with those dealers
most likely to receive tagged lobsters.
Tagging commenced in late April 1975, which was the earliest
that an adequate supply of lobsters could be guaranteed, yet
early enough for sphyrion tags to become firmly encysted in
advance of the peak molting period in August and September.
Before each lobster was tagged, carapace length, weight, and sex
were recorded along with the corresponding numbers of both
tags. Immediately after the tags were attached, the lobster was
placed in a partitioned fiber glass tray, where circulating
seawater hastened blood coagulation. Following a short
recovery period ('/i-lh) lobsters not displaying normal vigor
were discarded while all others were transferred to individual
sections of 10.2 cm diameter PVC pipe (23-28 cm long) con-
tained in rectangular wire cages. These cages were hung over the
side of the boat until all lobsters (about 1 ,000) for that area were
tagged and could be released simultaneously. The holding
period ranged from 1 to 5 d. This system of isolation eliminated
the loss and mutilation of sphyrion tags which occurs when tag-
S » ■
I i 1
CARAPACE LENGTH (mm)
Figure 2. — Length-frequencies of American lobsters tagged and released at
Kennebunkport (6 May 1975), Boothbay Harbor (17 May 197S), and Jonesport
(30 May 1975), Maine.
ged lobsters are crowded together. Other advantages were: 1)
reduction of postrelease tag mortality (most deaths attributable
to this cause would occur prior to release); 2) opportunity for
the sphyrion tag to become firmly attached during the lobster's
quiescence in "solitary confinement"; and 3) considerable
savings in boat-running time by eliminating daily excursions to
On 6 May, 957 tagged lobsters were released 2 n.mi. (3.7 km)
seaward of the mouth of the Kennebunk River. Next on 17 May,
942 lobsters were released 10 n.mi. (18.5 km) south of Boothbay
Harbor. Finally on 30 May, 983 tagged lobsters were liberated
about 12 n.mi. (22.2 km) southwest of Jonesport. Although
immediate release points were virtually void of traps, substantial
numbers of traps were within 1-5 n.mi (1.9-9.3 km).
All recapture sites were identified and the latitude and
longitude determined and plotted. The straight line distance be-
tween release and recapture points was measured and the
number of days at liberty were calculated for each lobster. All
data were coded and key punched for subsequent tabulation
(Krouse 1978). 4
RESULTS AND DISCUSSION
Of 2,882 American lobsters tagged during the spring of 1975,
75.9% were recaptured through September 1977 (Table 1).
Returns by tagging area were 85.2% at Jonesport, 74.8% at
Kennebunkport, and 67.4% at Boothbay Harbor. These different
return rates may be explained, in part, by the proximity of
'Krouse, J. S. 1978. Listing of data for lobsters tagged and recaptured off
the Maine coast (1975-77). Research Reference Document 78/8, 37 p. Maine
Dep. Mar. Resour., W. Boothbay Harbor, ME 04575.
Table I.— Monthly tag recoveries of American lobsters by release area off Maine, 1975-77. Numbers in parentheses refer
to lobsters that moiled.
release sites to zones of moderate to high fishing intensity. For
instance, at Boothbay Harbor tagged lobsters were released
more seaward than at the other areas and were therefore more
removed from immediate fishing pressure. Also, based on our
sightings of boats towing their nets near the release area shortly
after liberating tagged lobsters and rumors of trawlers catching
tagged lobsters but not reporting them (unlawful for trawlermen
to land lobsters in Maine), there is reason to believe that perhaps
several of the Boothbay Harbor releases were removed from the
fishery by trawlers. In Jonesport, where returns were the
highest, even though releases were in an area with very few
traps, substantial concentrations of traps were only about 1
n.mi. (1.9 km) away in all directions; whereas, at
Kennebunkport, where returns were intermediate to the other
two areas, the proximity of the area's release site to the trap
fields would be ranked between that of Jonesport and Boothbay
Because differences in tag recoveries by area might be par-
tially due to any variations in the tagging adeptness of the two
biologists who applied the tags in this study, we evaluated this
possibility by comparing the proportions of the number of
lobsters returned with those tagged by biologists at each release
site (Table 2). As there were no significant differences (chi-
square test, P>0.05) between these proportions, it appears that
the biologists applied the sphyrion tags with nearly equal skill;
thus any major variations in returns from different areas could
not be related to differences in numbers of lobsters marked by
any one tagger.
Table 2. — Comparison of the proportions of American lobsters recaptured with
those tagged by two biologists at each release area, 1975-77. Chi-square values
indicating no significant difference (P>0.05) between proportions are denoted
Table 3. — Mean sizes (carapace length) of tagged American lobsters recaptured
along with those lobsters not recaptured, 1975-77.
Number la re- Number la re- Number °!a re- Number la re-
Tagger tagged turned tagged turned tagged turned tagged turned
Four months after release, 53-81 % (67% combined) of the tagged
lobsters had been returned in each area, and after 1 yr 66-85% (75%
combined) had been recaptured. These high rates of return,
which corroborate the lobster fishery's high exploitation rate,
have undoubtedly been reduced by tag loss, incomplete report-
ing of recaptures, and natural and tag induced mortality. Based
on our observations of lobsters following tagging until time of
release and our close familiarity with the fishing community, it
appears that only a negligible number of lobsters died as a result
of tagging or were captured and not reported (exclusive of
Boothbay Harbor). Thus, in this study, tag loss and natural
mortality (< 10% annually, Thomas 1973) were probably the most
important sources of error.
The effect of size on catchability was examined by compar-
ing the mean carapace length of lobsters recaptured at each
release site with those tagged lobsters not recaptured before
October 1977 (Table 3). The /-test (P>0.05) revealed no
significant difference between the mean sizes of those lobsters
caught with those still at large. Similarly, the chi-square test
indicated no statistical differences (P>0.05) between sex
ratios of lobsters returned to those liberated (Table 4).
Kennebunkport Boothbay Harbor
CL (mm) SE CL (mm) SE
CL (mm) SE
86.5 ±0.14 87.5 ±0.21
86.6 ±0.24 87.0 ±0.29
Table 4. — Comparison of the sex ratios of tagged American lobsters released
with those recaptured at each release area, 1975-77. Chi-square values indicating
no significant difference between sex ratios of lobsters released to those recap-
tured are denoted by NS.
6 9 (6:9)
rf 9 (rf:9)
rf 9 (6:9)
316 400 0.79:1
415 542 0.77:1
314 320 0.98:1
456 486 0.94:1
359 478 0.75:1
439 544 0.81:1
Although the above analysis indicates that there probably
was no difference in the catchability of legal-sized lobsters by
size and sex, plots of the percentages of lobsters not recaptured
against carapace length show that 4-9% fewer 81 than 82 mm CL
lobsters were returned (Fig. 3). This disparity might appear to
be due to gear selectivity, but is in fact unlikely since previous
studies (Krouse 1973; Krouse and Thomas 1975) show that
lobsters become fully vulnerable to conventional lobster pots
at about 75 mm CL. Actually this lower than expected catch of
small legal lobsters is due to the Maine fishermen's method of
measurement and interpretation of what lobsters are legal to
keep. The minimum legal size is 81 mm (3-3/16 in) CL in
Maine; but the minimum size retained in practice is closer to
82-83 mm. This conclusion is further supported by length fre-
quencies of Maine commercial lobster catches compiled by
Thomas (1973) which showed marked deficiencies of the 81
mm group; in fact, even the 82 through 84 mm sizes were less
numerous than expected.
O BOOTHBAY HARBOR
60 65 90 9S 100
CARAPACE LENGTH (mm)
Figure 3. — Size distributions of tagged American lobsters still at large after
September 1977 (about 28 mo since release) at each tagging area.
From July 1975 through May 1977, only 66 (3.0%) lobsters of
2,188 returns had molted prior to recapture. This extremely
low number of recaptured new-shell lobsters may be attributed
primarily to the high rate of return during the first 3 mo before
the peak of the molting period. Accordingly, a decidedly
higher proportion of those lobsters recovered after 4 mo had
molted (Table 1). In fact, of 46 lobsters recaptured after the
first season (1975) at all release areas, 28 (60.9%) had molted.
Molt increments in weight ranged from 21 .9 to 64.4% (40.9%
mean) at Boothbay Harbor, 21.3 to 52.8% (39.8% mean) at
Kennebunkport, and 27.3 to 67.5% (46.2% mean) at Jonesport
(these values exclude lobsters with missing chelipeds).
Increases in carapace length were 7.3-18.1% (12.7% mean) at
Boothbay Harbor, 11.5-16.0% (13.1% mean) at Kennebunkport,
and 10.6-18.5% (15.1% mean) at Jonesport. Variations between
area molt increments are reflected by the analysis of
covariance which indicated significant differences (P = 0.05)
between the coefficients of the linear regressions of postmolt
carapace length on premolt carapace length (Fig. 4). Despite
these differences in growth increments by area, which might be
resolved with additional data, the overall increase in carapace
length (areas combined) approximates Dow's (1964) estimate
of 14% for Maine lobsters.
Estimates of von Bertalanffy growth parameters (Gulland
1969) were not realistic (negative K and very low Z,„ values)
due to the highly variable growth increments, small sample
sizes, and the limited range of sizes and ages represented by the
r = 0.94
85 90 95 100
PREMOLT CARAPACE LENGTH (mm)
-Premolt-postmolt carapace length relations of recaptured tagged
American lobsters that molted at each release site.
Before movement trends of recaptured tagged lobsters can
be thoroughly analyzed, it is necessary to consider the inten-
sity, distribution, and seasonality of fishing effort at each
release site. Unfortunately, sufficient data were not available
to quantify effort by area; however, in view of catch and effort
information of the Maine commercial lobster fishery collected
coastwise by DMR's Lobster Research Project personnel, it
was apparent that fishing pressure was extremely intense at all
tagging areas. Seasonal changes in fishing intensity and loca-
tion of lobster trap fields are well-known occurrences along
the Maine coast (Dow 1961; Thomas 1973; Cooper et al.
1975). During the summer-fall period when the most intense
fishing activity occurs, most traps are rather uniformly
distributed along the shores of the mainland, around islands
and ledge outcroppings where usually rough, rocky substrates
provide ideal lobster habitat. In winter and spring when fishing
effort is minimal, most traps are moved to deeper water (>30
m) (Cooper et al. 1975) where 1) traps are less apt to be
damaged or lost due to severe winter storms, 2) warmer water
temperatures cause lobsters to be more active and subse-
quently more catchable, and 3) lobsters are now more abun-
dant due to the fact that most traps are fished in shoaler water
(<30 m) during the warmer months.
Another factor which should be considered when assessing
movement trends of this study was the release of tagged
lobsters at locations differing from those of original capture.
Nevertheless, as mentioned previously, all lobsters tagged and
released at a certain site were caught within that general area.
Movement patterns were initially assessed by plotting the
points of recapture at each release site (Figs. 5-7). Of the Ken-
nebunkport releases (Fig. 5), most lobsters were recaptured in
close proximity to shore within a 5 n.mi. (9.3 km) radius of the
release site. Only 14 recaptures traveled >5 n.mi. and 10 of
these lobsters moved in a southerly direction. The most
notable movements were by a male (90 mm CL) which was at
large 369 d and traveled 63 n.mi. (116.7 km) to Boston and a
female (88 mm CL) which was allegedly caught near Tiverton,
R.I. (185 n.mi., 342.6 km), 199 d after release.
At Boothbay Harbor (Fig. 6), most lobsters were recovered
between the mouths of the Kennebec and Damariscotta Rivers.
Only one lobster was recaptured in the Damariscotta River,
while none was reported from the Kennebec River. By con-
trast, numerous tagged lobsters were returned from the
Sheepscot River estuary. Twelve lobsters traveled ^.10 n.mi.
(18.5 km) up this estuary. Significant easterly and southerly
movement was limited to a female (87 mm CL), at large 23 d,
that traveled 14 n.mi (25.9 km) to Monhegan Island; a male
(107 mm CL), at large 88 d, which moved 42 n.mi. (77.8 km)
to Cape Porpoise; and a female (99 mm CL) caught at Jeffreys
Ledge (61 n.mi., 113.0 km) after 197 d at liberty.
In comparison with other areas, directional movement of
Jonesport recaptures appeared to be less restricted (Fig. 7).
Although several lobsters were recaptured seaward of the
release locations, most were taken inshore. The greatest
movements (^.20 n.mi., 37.0 km) were by three lobsters that
traveled southwesterly. The farthest distance moved was 134
n.mi. (248.2 km) (to Kennebunkport) by an 89 mm CL male at
large 405 d, followed by a 29 n.mi. (53.7 km) trek to Great
Duck Island by a small male (81 mm CL) at large 49 d, and a
Figure 5. — Kennebunkport, Maine, region showing dispersal of recaptured tagged American lobsters. May 1975-September 1977. Number of recaptures given
at each recover, point. Shaded area represents percentage of recaptured lobsters that traveled in a given direction (30° bearing intervals).
20 n.mi. (37.0 km) movement to Schoodic Head by a 96 mm
CL female at large 327 d.
To evaluate directional movement more objectively, com-
pass bearings were assigned to all recapture coordinates and
grouped by 30° increments (Figs. 5-7). At Kennebunkport and
Boothbay Harbor most lobsters were recovered at bearings
1 °-30° and 270°-360° from the release areas while only 8.5<Vo of
the returns from both areas traveled in other directions. At
Jonesport there appears to have been more movement in an
easterly direction (60°-90°); however, this is somewhat
misleading because only 15 of the 184 lobsters that traveled
toward the east exceeded 1 n.mi. (1.9 km), the remaining 169
lobsters were caught about 1 n.mi. due east of the release site
near Nashes Island (Fig. 7).
In view of the information presented herein, it can be seen
that the majority of recaptured lobsters moved inshore at all
Figure 6. — Boothbay Harbor, Maine, region showing dispersal of recaptured tagged American lobsters, May 1975-September 1977. Number of recaptures given at
each recovery point. Shaded area represents percentage of recaptured lobsters that traveled in a given direction (30° bearing intervals).
release areas. Of course, it should be remembered that this
shoreward movement may have been influenced by the reloca-
tion of tagged animals from where they were originally caught.
Limited movement toward the east, which was particularly evi-
dent at Boothbay Harbor, might be the result of the
counterclockwise current along the Maine coast. Accordingly,
all long distance migrants (_^_20 n.mi., 37.0 km) of this study
appeared to travel in the direction of the prevailing south to
southwesterly coastal currents (Fig. 1). Likewise, the major
migrants of Dow's (1974) tagging study followed a south by
southwesterly course as they moved from Maine coastal waters
toward New Hampshire and Massachusetts. Moreover, recent
returns of several tagged Canadian lobsters (released off
Grand Manan Island, N.B. (Fig. 1)) from various locations in
Maine and as far south as Cape Cod (Groom 1978, pers. com-
mun. 5 ) further substantiate this southwesterly movement
undertaken by some lobsters (usually the larger mature
Another factor likely to be related to movement, particu-
larly in view of this study's high return rate, is the time lobsters
were at large prior to being recaptured. Mean times (days) at
large varied markedly from long (86. 1 ± 3 .4) ( ± 1 SE) at Boothbay
Harbor, to medium (70.5 ±2.6) at Kennebunkport, and short
(51.8 ± 1.4) at Jonesport. Considering that recaptured lobsters
tagged and released at Boothbay Harbor were at large the
longest and also traveled the farthest (mean = 4.6 n.mi., 8.5
km) (Table 5), the degree of movement seems to be dependent
Table 5. — Average distances moved by recaptured American lobsters at each
tagging area. Sexes were combined since there were no statistical differences
between the distances moved by males and females (r = 0.594, 0.301, and 0.677
for Kennebunkport, Boothbay Harbor, and Jonesport, respectively; />>0.05).
upon time at large. However, an examination of the plots of
average distances traveled (nautical miles) against time at large
(weeks) indicates that after an 8-10 wk postrelease period, dur-
ing which time lobsters apparently dispersed from the point of
release, there was little if any association between the time
lobsters were at large and the extent of movement (Fig. 8). For
example, tagged lobsters recaptured near Boothbay Harbor
that had been free 6 mo to 1 yr had moved no farther than
those lobsters caught after only 2 mo of liberty. Furthermore,
8 of 30 (27%) lobsters recaptured after being at large at least 1 yr
were caught within 1 n.mi. (1.9 km) of the three release areas.
Similarly, Fogarty et al. (1981) reported that lobsters tagged
and recaptured along the coast of Rhode Island moved greater
distances as the time at large increased to 90 d, after which
movement appeared to level off.
Average distances traveled by recaptured lobsters were
calculated for each tagging area (Table 5). Lobsters at
Boothbay Harbor moved the farthest (mean = 4.6 n.mi., SE =
±0.14), followed by Jonesport (mean = 2.9 n.mi., SE = ±0.18),
and then by Kennebunkport (mean = 2.5 n.mi., SE = ±0.28).
These variations in distances moved at each tagging area
appear to be associated with the proximity of the release site to
neighboring trap fields, the configuration of the immediate
coastline, and, possibly, to where the lobsters were originally
caught. For instance, at Boothbay Harbor where lobster
movement was the most extensive, the liberation area was not
only farther from shore relative to the other areas, but also
more removed from zones of moderate to intense fishing
pressure. Of course these factors, particularly the latter, also
20 30 tO 50 60 70
o o OO
o o O
o OO o
TIME AT LARGE < *••*■)
5 W. Groom, Fishery Biologist, New Brunswick Department of Fisheries,
Fredericton, New Brunswick, Canada, pers. commun. March 1980.
Figure 8. — Average distances (nautical miles, 1.9 km) traveled by tagged
American lobsters during weekly time intervals prior to recapture. Mean
distances were calculated by dividing total miles moved by tagged lobsters recap-
tured during a given week by the number of recaptures that week.
explain why Boothbay Harbor recaptures were at large the
Most tagged lobsters remained in the vicinity of the release
areas as indicated by the fact that 74, 92, and 98°/o of the returns
at Boothbay Harbor, Jonesport, and Kennebunkport, respec-
tively, were caught within a 5 n.mi. (9.3 km) radius of the
release site and only about 1 % of the recaptures wandered > 10
n.mi. (18.5 km) (Table 6). Even more restricted movement pat-
terns were observed by Harriman (see footnote 3) and Cooper
(1970) who reported that most lobsters tagged near Monhegan
Island were recaptured within 2 n.mi. (3.7 km) of the island.
Similarly, based on observations made by scuba divers and
from research submersibles on lobsters near Boothbay Har-
bor, Cooper et al. (1975) concluded that large-scale seasonal
movements on and off the shallow (<24 m) inshore fishing
grounds (notion of many fishermen) do not occur. More
recently, Fogarty et al. (1981) noted that the majority of tag-
ged lobsters released along the Rhode Island coast were
recovered within 3.2 n.mi. (6 km) of the release site.
The association of lobster size with movement was evaluated
by averaging the miles moved by lobsters in 5 mm CL
increments and then plotting these values against carapace
length (Fig. 9). Although there appears to be no relationship
between size and movement, it should be noted that only 2.2%
of the lobsters tagged in this study were > 100 mm CL, and
according to the studies of Dow (1974) and Groom (see foot-
note 5) the majority of major migrants along the Maine coast
exceeded 100 mm CL. Aside from the fact that relatively few
large lobsters (>100 mm CL) were tagged in this study, it
should also be mentioned that only 19(34.5%) of 55 recaptures
> 100 mm CL were at large more than 3 mo and only 4 (7.3%)
Table 6. — Summary of the distances traveled by recaptured tagged American lobsters at
each tagging area, 1975-77.
'These values are less than total number of recaptures reported in Table 1 because location
of recapture was not known for all returns.
91-95 96- 100 IOI-I05
CARAPACE LENGTH (mm)
Figure 9. — Distances moved by recaptured American lobsters of various sizes
tagged and released at Kennebunkport, Boothbay Harbor, and Jonesport,
lobsters were free longer than 1 yr prior to being caught.
Perhaps, at least for the larger lobsters, reductions in times at
large may have curtailed movement.
Mortality rates were estimated from a linear regression of
the number of tagged lobsters recaptured on the time at large.
Regression coefficients were substituted into Gulland's (1969)
log e n r = -(F+M)rT + log £
intercept (a) = log
( 1 _ e-V+Afl,
slope (b) = -(F+M) r T
n f = number of recaptures during interval r,
where r = 0, 1, 2, 3 ... weekly, biweekly, or monthly
period following release
T = length of interval of time (r)
N = number of tagged lobsters released.
Because an estimate of total mortality, derived with tagging data
along, is the sum of fishing mortality (/) plus not only natural
mortality (M), but also all other causes of reductions in the
number of tagged animals, the value ' 'A" ' (all sources of tag loss
plus natural mortality) should replace M in the equations.
The number of recaptures plotted over time indicated that
return rates increased during the first 4-8 wk, then leveled off
for a brief period and eventually began to decrease (Fig. 10).
10 12 14 16 18 20 22 24 26 28 30 32 34 36
W E E K LY
a ° • o
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
_i i l i i ' '
_| — i l_l i i i i i
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
TIME AT LARGE (Wtekl)
Figure 10. — Recaptures (logg scale) of tagged American lobsters as related to
time at large at each lagging area.
Increases in the number of recaptures during the first few suc-
cessive weeks following release may be attributed to slow mix-
ing of tagged animals with the fishable population in associa-
tion with spatial variations in fishing intensity. Accordingly,
mortality estimates were calculated from return data exclusive
of those initial recovery intervals (2-4 wk) when mixing of tag-
ged and untagged individuals was considered to be incomplete.
Annual instantaneous rates of fishing mortality (F) and
apparent total mortality (Z') which ranged from 4.14 to 7.31
and 5.89 to 8.73 (Table 7), respectively, were extremely high as
Table I. — Annual instanlaneous rales of nppfent total (Z') and
f ish!ng (/") mortality on American lobsters estimated from returns
grouped by different time intervals. Annual mortality rates expressed as
- j s are in parentheses.
the result of the actual return rates which were, I believe, not
fully representative of general conditions; the fact that annual
mortality rates were calculated from tag return data collected
when the catches of the seasonal lobster fishery were highest,
and systematic errors inherent in most tagging studies. Gulland
(1969) has classified these errors according to their effects on
the various estimates. Types A and B errors result from tag
loss and systematically bias mortality rates causing an
underestimate of fishing mortality and an overestimate of the
true total mortality (Z), respectively. Type A error, which is
caused by death of fish shortly after tagging and incomplete
reporting of recaptures, affects F but not Z. Type A errors
appeared minimal except at Boothbay Harbor where trawlers
were suspected of unreported catches of tagged lobsters. In
fact, the relatively lower estimates of F at Boothbay Harbor
may be attributable, in part, to this error. Of the Type B
errors, which include natural mortality, emigration, and tag
detachment, only the latter was of significant magnitude in
this study to warrant consideration.
Quantitative estimates of tag loss were obtained by follow-
ing Gulland's (1963) methodology for estimating tag retention
rates with data from double tagging experiments. Due to
problems that we encountered initially with this procedure,
Russell (1980) analyzed this method and corrected some of
Gulland's basic equations.
In all cases, estimated losses of the sphyrion tag were higher
than those of the cinch tag (Table 8). Considering differences
in modes of attachment, higher losses of sphyrion tags were
expected; however, cinch tag losses were greater than antici-
pated. Evidently some of the cinch tags became loose and
subsequently slipped off the chela (claw). In retrospect, this
type of loss would have been minimized had the tag been
secured around the carpus (section proximal to the propodus)
of the pincer claw.
A comparison of the relatively high annual loss rates of indi-
vidual tags (range of 39.4-51.5%) with those of both tags (range
of 15.0-24.0%) clearly indicates how tag returns would have been
reduced if only one tag rather than two had been used. Never-
theless, in view of these estimates, we feel that tag loss was of
sufficient magnitude to bias mortality estimates. This error,
termed Type B, is an additional cause of mortality ("X") and
results in an overestimate of Z but has no effect on F. Unfor-
tunately, if we convert the highest annual tag loss rates
(39.4-51.5%) (Table 8) to instantaneous rates (0.50-0.72) and
then subtract these values from estimates of Z' (5.89-8.73)
(Table 7), this only results in an insignificant reduction in Z'.
Thus it is apparent that other factors besides tag loss have
caused overestimates of Z. When these errors are operative
only F is estimated from tagging data; thus Z is derived from
some independent estimate and M is the difference between F
Undoubtedly, the most meaningful mortality estimates
derived from data of this study are those of F and even these
values as well as estimates of Z are inflated as the result of
incomplete mixing of tagged lobsters with the untagged popu-
lation [Gulland's (1969) Type C error] along with other factors
previously stated. Despite this bias, estimates of F do indeed
reflect the Maine lobster fishery's extremely high rate of
1 . Of 2,882 lobsters tagged in the spring of 1975, 2,188 (75.9%)
were recaptured through September 1977. Lobsters
released at Jonesport had the highest return (85.2%) follow-
ed by Kennebunkport (74.8%) and Boothbay Harbor
2. Catchability of legal-sized lobsters did not vary by sex nor
3. Twenty-four ovigerous females ranging from 82 to 109
mm CL were recaptured.
4. Sixty-six (3.0%) of the lobsters recaptured had molted while
at large. Percentage of increases in carapace length varied
from 7.3 to 18.1% (12.7% mean) at Boothbay Harbor, 1 1.5 to
16.0% (13.1% mean) at Kennebunkport, and 10.6 to 18.5%
(15.1% mean) at Jonesport.
5. The majority of returns from Kennebunkport (98.0%),
Boothbay Harbor (73.6%), and Jonesport (91.5%) were
caught within a 5 n.mi. (9.3 km) radius of the release sites.
Recaptured lobsters moved on the average more at
Boothbay Harbor (4.45 n.mi., 8.2 km) and less at
Kennebunkport (2. 16 n.mi., 4.0 km). Only about 1% of the
returns wandered >10 n.mi. (18.5 km).
6. Most movement was shoreward with a westerly drift from
the point of release. Few lobsters traveled in an easterly
Table 8. — Estimated percentage of tag loss after various time intervals for American lobsters
released at Kennebunkport, Boothbay Harbor, and Jonesport, Maine.
direction. All long distance migrants (_s_20 n.mi., 37.0
km) followed a south to southwesterly course.
7. Male and female lobsters exhibited no differences in
movement. There was no apparent relationship between a
lobster's size (98% of the tagged lobsters ranged from 81 to
100 mm CL) and the distance moved.
8. Except for an initial period of about 8 wk, which we con-
sider unrepresentative, there was no association between
the time lobsters were at large and the distance traveled.
Therefore, even if the recovery rate had been lower there
is no reason to believe that the movement patterns would
have deviated from those observed.
9. Annual instantaneous fishing mortality rates, which were
calculated from return data grouped at weekly, biweekly,
and monthly intervals, were 4. 14 (98.4%) to 4.93 (99.3%) at
Kennebunkport, 3.98 (98.1%) to4.13 (98.4%) at Boothbay
Harbor, and 7.22 (99.9%) to 7.31 (99.9%) at Jonesport.
Although the accuracy of these values has been biased by
errors associated with tagging, the magnitude of these F s
still reveals the lobster fishery's precariously high level of
I thank David Libby for his assistance with all phases of this
study; Paul DeRocher and Donald Card (crew of the RV
Duchess) for their invaluable help during the entire tagging
operation. Thanks are also extended to Clarence Burke, David
Parkhurst, and Gary Robinson who provided assistance with
tagging. I am appreciative for the guidance offered by James
Thomas. I thank those lobster dealers and fishermen who,
often at an inconvenience to themselves, supported our pro-
gram by reporting returns and furnishing pertinent informa-
tion. An anonymous reviewer critically reviewed the
manuscript and offered many valuable suggestions.
COOPER, R. A.
1970. Retention of marks and their effects on growth, behavior, and
migrations of the American lobster, Homarus americanus. Trans. Am.
Fish. Soc. 99:409-417.
COOPER, R. A., R. A. CLIFFORD, and C. D. NEWELL.
1975. Seasonal abundance of the American lobster, Homarus
americanus, in the Boothbay region of Maine. Trans. Am. Fish. Soc.
DOW, R. L.
1961. Some factors influencing Maine lobster landings. Commer.
Fish. Rev. 23(9):1-11.
1964. Supply, sustained yield, and management of the Maine lobster
resource. Commer. Fish. Rev. 26(Ua):19-26.
1974. American lobsters tagged by Maine commercial fishermen,
1957-59. Fish. Bull., U.S. 72:622-623.
FOGARTY, M. J., D. V. D. BORDEN, and H. J. RUSSELL.
1981. Movements of American lobster, Homarus americanus, off
Rhode Island. Fish. Bull., U.S. 78:771-778.
1978. Interim investigation of lobster stock, size, and migration system
of lobster population in the Grand Manan region. New Brunswick Dep.
Fish., Fredericton, 69 p.
GULLAND, J. A.
1963. On the analysis of double-tagging experiments. Int. Comm.
Northwest Atl. Fish. Spec. Publ. 4:228-229.
1969. Manual of methods for fish stock assessment. Part I. Fish
population analysis. FAO Man. Fish. Sci. 4, 154 p.
KROUSE, J. S.
1973. Maturity, sex ratio, and size composition of the natural popula-
tion of American lobsters, Homarus americanus, along the Maine
coast. Fish. Bull., U.S. 71:165-173.
1978. Effectiveness of escape vent shape in traps for catching legal-sized
lobster, Homarus americanus, and harvestable-sized crabs, Cancer
borealis and Cancer irroratus. Fish. Bull., U.S. 76:425-432.
KROUSE, J. S., and J. C. THOMAS.
1975. Effects of trap selectivity and some population parameters on size
composition of the American lobster, Homarus americanus, catch along
the Maine coast. Fish. Bull.. U.S. 73:862-871.
RUSSELL, H. J., Jr.
1980. Analysis of double-tagging experiments: an update. Can. J.
Fish. Aquat. Sci. 37:114-116.
SCARRATT, D. J., and P. F. ELSON.
1965. Preliminary trials of a tag for salmon and lobsters. J. Fish. Res.
Board Can. 22:421-423.
THOMAS, J. C.
1973. An analysis of the commercial lobster (Homarus americanus)
fishery along the coast of Maine, August 1966 through December
1970. U.S. Dep. Commer., NOAA Tech. Rep. NMFS SSRF-667, 57 p.
NOAA TECHNICAL REPORTS
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