830 
Fishery Bulletin 95(4), 1997 
200 
„ 150 
0 
03 
_C 
£ 
° 100 
50 
1988 1989 1990 1991 1992 1993 1994 
Year 
1.2 
1 
0.8 
0.6 
0.4 
0.2 
0 
I j Stellwagen adults O Stellwagen juveniles 
■ Jeffreys adults B Jeffreys juveniles 
Figure 2 
The number of individual humpback whale adults and juveniles identified per year 
on Stellwagen Bank and Jeffreys Ledge, 1988-94. Note the rapid annual decrease of 
adults on Stellwagen Bank starting in 1991, and the corresponding increase on Jeffreys 
Ledge beginning in 1992. Juveniles started a rapid decrease on Stellwagen Bank in 1992. 
Stellwagen 
Jeffreys 
Total 
cow-calf pairs 
Figure 3 
The number of cow-calf pairs on Stellwagen Bank, Jeffreys Ledge, and in the entire 
Gulf of Maine, 1988-94. The percentage of known mother-calf pairs that were sighted 
on Stellwagen began to decrease dramatically in 1992, the same year that the per- 
centage mother-calf pairs began to increase on Jeffreys Ledge. 
showed no significant trend in 
the absolute number seen on 
Stellwagen (B=-1.214, P= 
0.424). Numbers of cows and 
calves began in 1991 to decline 
sharply, especially when com- 
pared with the total number of 
cow-calf pairs in the Gulf of 
Maine. By the last year of the 
study no cow-calf pairs were 
seen (Fig. 3). 
Occurrence and occupancy 
Mean occurrence of humpback 
whales on Stellwagen Bank 
within a single season ranged 
from 13.1 days (1989, n= 147) 
to 6.6 days (1993, n= 69) (Table 
2; B=-0.30, P=0.501). Adults 
showed a within-year mean oc- 
currence of 6.4 days (SD=4.8, 
n=720), with a statistically sig- 
nificant declining trend through 
the study period (B=-1.98, 
P<0.001). Compared with 
adults, juveniles showed a 
higher mean within-year occur- 
rence (mean= 14.5 days, SD = 
4.2, n= 352), which signifi- 
cantly increased throughout 
the study period (B = 1.63, 
P=0.030). 
Occupancy of individual 
whales within years declined 
significantly from a mean of 
61.8 days (1989, n=147) to 21.6 
days (1994, n=l) (Table 3; B=- 
7.07, P=0.002). Again, age 
classes showed different trends. 
Adults had a mean occupancy 
period of 39.3 days (SD = 23.56, 
n=720) throughout the study 
period, with a significant declin- 
ing trend (B=-10.65, PcO.001). 
In contrast, juveniles had a 
mean occupancy period of 55.0 
days (SD=13.21, n=352), with 
no significant trend apparent 
(B=-2.82, P=0.296). 
Although juveniles showed 
no significant trend in occu- 
pancy and had occurrence val- 
ues that actually increased 
throughout the period, a com- 
parison of median values for 
