Weinrich et al. : A shift in distribution of humpback whales, Megaptera novaeangliae, in reponse to prey 
831 
Table 3 
The mean occupancy (in days) of humpback whales on 
Stellwagen Bank, 1988-94. 
Year 
Adults 
Juveniles 
Combined total 
1988 
67.9 
47.1 
60.4 
1989 
56.5 
71.6 
61.8 
1990 
52.3 
55.0 
51.8 
1991 
47.5 
73.1 
54.6 
1992 
32.4 
53.3 
42.2 
1993 
17.2 
48.4 
25.7 
1994 
1.3 
36.8 
21.6 
Table 2 
The mean occurrence (in days) of humpback whales on 
Stellwagen Bank, 1988-94. 
Year 
Adults 
Juveniles 
Combined total 
1988 
13.5 
7.2 
11.2 
1989 
12.2 
15.2 
13.1 
1990 
7.9 
12.5 
9.6 
1991 
6.2 
17.0 
10.7 
1992 
7.2 
19.6 
12.0 
1993 
3.1 
15.7 
6.6 
1994 
1.3 
19.8 
11.9 
1988 1989 1990 
1991 1992 1993 1994 
Year 
| | Stellwagen adults 
1 Stellwagen juveniles 
CH Jeffreys whales 
Figure 4 
The mean number of whales identified per day in each age class and year on 
Stellwagen Bank and Jeffreys Ledge, 1988-94. Jeffreys Ledge juveniles were not 
included because of their low numbers. Note the rapid decrease among adults on 
Stellwagen Bank beginning after 1988, and the decrease among juveniles on 
Stellwagen beginning in 1992. Jeffreys Ledge values were highest in the final three 
years, after the general decrease on Stellwagen Bank. 
each of these variables portrays 
a trend more similar to that seen 
from adults. From 1992 through 
1994, prolonged residency of a 
few juveniles skewed occurrence 
and occupancy values. During 
1991-93, median occurrence of 
juveniles fell from seven days to 
three, whereas median occu- 
pancy periods fell sharply, from 
59.5 days to 15 days. In 1994, so 
few juveniles were seen (four) 
that the relatively high values of 
two individuals severely skewed 
the results for that year. Median 
values of adult occurrence and oc- 
cupancy showed the same trends 
as those portrayed from the re- 
gression analyses. 
Number of whales per day 
One of the clearest indicators of 
habitat use is the number of iden- 
tified humpback whales sighted on 
Stellwagen Bank each day. This 
measure incorporates two of the 
above components — the number 
of whales identified as well as 
how often they were sighted in 
the area. Throughout the study period, a mean of 
12.7 (SD=11.31, n=l,072) whales were identified per 
day, ranging from an annual high of 17.7 (SD= 15.30, 
n=153 days) in 1988 to a low of 0.9 (SD=0.76, n= 97 
days) in 1994 (Fig. 4). Adults and juveniles again 
showed different trends. Adults per day declined 
steadily from 14.4 in 1988 to 0.1 in 1994 (B=-2.21, 
P<0.001), whereas juveniles showed no clear trend, with 
a high of 8.8 in 1991 and a low of 0.8 in 1994 (B=-0.44, 
P=0.501). Juvenile values showed a clear peak in 1990- 
91 as compared with other years (Fig. 4). 
Vertical prey density Mean overall vertical prey 
density decreased from 19.1% with prey traces in 
1990 to 2.8% with prey traces in 1992 (B=-0.38, 
P<0.001) (Table 4). Similar significant decreases were 
seen in each vertical quarter of the water column 
(Table 4). 
Although it was impossible to determine prey type 
from traces alone, catches of groundfish (mainly At- 
lantic cod [ Gadus morhua] and haddock [ Melano - 
grammus aeglefinus ]), and bluefish ( Pomatomus 
saltatrix) in the immediate area of trace recordings 
