828 
Fishery Bulletin 95(4), 1997 
Table 1 
Study effort by both number of survey days and number of 
survey trips for both Stellwagen Bank and Jeffreys Ledge. 
“JLSN days” represent the total number of survey days 
represented by the Jeffreys Ledge Sighting Network 
(JLSN), established after the 1992 season (see text for fur- 
ther details). 
Year 
Stellwagen 
days 
Stellwagen 
trips 
Jeffreys 
days 
Jeffreys 
trips 
JLSN 
days 
1988 
145 
558 
16 
44 
0 
1989 
151 
550 
17 
20 
0 
1990 
166 
516 
32 
37 
0 
1991 
160 
460 
31 
36 
0 
1992 
171 
506 
34 
37 
69 
1993 
106 
364 
48 
79 
119 
1994 
86 
141 
86 
141 
138 
Ledge was the destination for just under half of the 
dedicated cruises from 1988 to 1992, all but four in 
1993, and all but two in 1994. 
Beginning in 1990, sighting and photo-identifica- 
tion data were also collected from a whale-watching 
boat operating out of Kennebunk, Maine, to obtain 
information from the northern end of Jeffreys Ledge. 
Observer coverage was for one trip per day, 3-5 days 
per week. Because of the unusually large number of 
whales first observed on our dedicated cruises to 
Jeffreys Ledge in 1992, a photo-identification net- 
work (consisting of three whale-watching boats work- 
ing on Jeffreys Ledge for one trip per day) was for- 
malized in fall 1992 (after the completion of field ef- 
forts), and existing 1992 data were obtained. Begin- 
ning in 1993, data collection from these vessels was 
standardized to be directly comparable with Stell- 
wagen Bank whale-watching data. Because 1993 
represented the first year in which Jeffreys Ledge data 
were collected in any kind of standardized fashion, oc- 
currence and occupancy (defined below) were not cal- 
culated for Jeffreys Ledge humpback whale sightings. 
Study areas 
Stellwagen Bank, now a National Marine Sanctu- 
ary, is a sandy glacial deposit approximately 32 km 
long with depths from 18 to 37 m (Fig. 1). It borders 
the eastern margin of Massachusetts Bay and is lo- 
cated approximately halfway between Cape Ann and 
Cape Cod, Massachusetts. Jeffreys Ledge is a more 
complex, winding, shallow ledge, with typical depths 
of 45 to 61 m and with a length of approximately 54 
km. Its substrate is a mixture of rocky and muddy 
bottoms. The southern edge of Jeffreys Ledge is 9 
km northeast of Rockport, Massachusetts, whereas 
the northern end lies 36 km east of York, Maine. 
Stellwagen Bank and Jeffreys Ledge are separated 
by 21.6 km at their closest point. 
Field methods 
Individual humpback whales were identified from 
photographs of distinctive pigment patterns on the 
ventral surface of their tail flukes or from the shape 
of and scarring on the dorsal fin (or by both features) 
(Katona and Whitehead, 1981). Two observers col- 
lected data on each whale or group of whales. One 
observer was responsible for photographing each 
whale, while the second recorded the whale’s loca- 
tion (by means of LORAN-C), group affiliations, and 
behavior. This observer also recorded which photo- 
graphs were taken of each whale, as dictated by the 
photographer. Each group of whales in an area was 
usually observed for 1-30 minutes; most, if not all, 
whales in a single location (3-5 km radius) were iden- 
tified during each observation period. Field methods 
were consistent on all vessels. 
Age class and sex determination 
Individuals were identified by comparing photo- 
graphs with those of a catalog of humpback whales 
maintained at the Cetacean Research Unit (CRU), 
Gloucester, MA. Details on cataloging methods and 
contents of the catalog were given in Weinrich ( 1991), 
Weinrich and Kuhlberg (1991), and Weinrich et al. 
(1992) and are based on procedures outlined by 
Katona and Whitehead (1981). Whales were sexed 
by photographing them while belly up at the surface 
(and by noting the presence or absence of a small 
lobe immediately posterior to the genital slit 
[Glockner, 1983]), by observing a female with calf, or 
by using molecular techniques (Baker et al., 1991). 
Individuals were assigned to age classes (juvenile or 
adult) based on known age (first observation as a calf) 
or based on the consensus among all experienced CRU 
observers of an animal’s relative size at first sighting. 
The accuracy of the latter technique was confirmed by 
estimating the age class of animals of unknown iden- 
tity in the field and by finding that these estimates 
matched (photographically) animals of known age. 
No incorrect classifications were made (n=51). For 
the purposes of this paper, an animal was classified as 
an adult if it was known to be at least five years old, an 
age at which 50% or more of the population is mature 
(Chittleborough, 1965; Clapham and Mayo, 1990). 
Prey density 
In 1990-92, a SITEX HE-358 50-kHz echo-sounder 
and chart recorder aboard a whale-watching vessel 
