832 
Fishery Bulletin 95(4), 1997 
Tab!e 4 
Percentage of the water column with echo-sounder prey 
traces by year in each quarter. Mean depth was 38.4 meters. 
Quarter of the water column 
Year 
Top 25% 
2nd 25% 
3rd 25% 
4th 25% 
Total 
1990 
17.2% 
15.0% 
16.8% 
24.3% 
19.1% 
1991 
3.1% 
4.5% 
7.3% 
12.7% 
7.9% 
1992 
1.4% 
0.3% 
0.8% 
1.1% 
2.8% 
by party-fishing boats indicated that sand lance were 
the predominant fish prey in stomach contents of hump- 
back whales; some small mackerel ( Scomber scombrus ) 
and herring were also observed in stomachs in much 
lower frequencies. Herring were more prominent in Oc- 
tober during each field season, when only a small num- 
ber of echo-sounder data points were recorded. 
Jeffreys Ledge 
Total number of humpback whales identified The 
number of humpback whales we identified on Jeffreys 
Ledge increased from a low of 35 (in 1988) to a high 
of 138 (in 1992) (B=19.57, P=0.004; Fig. 2). Although 
there was a generally increasing trend, there was a 
sudden increase from 58 in 1991 to 138 in 1992. 
The increase among adult whales also showed a 
significant increase across years (B=17.25, P=0.003). 
Although juveniles increased steadily throughout the 
period, and suddenly from 1991 to 1992, they did not 
do so at a significant rate (B=1.357, P=0.201). (The 
same analysis without 1992 data, where there were 
an unusually high number of juveniles, does show a 
statistically significant increasing trend among ju- 
veniles LB=0.914, P=0.006]). Cow-calf pairs also 
showed a significantly increasing trend (B= 1.429, 
P=0.049). 
In each year, identified humpback whales on 
Jeffreys Ledge were biased toward adults. No more 
than 17 juveniles were photographed on Jeffreys Ledge 
in any year, and the number of juveniles photographed 
exceeded 10 in only a single season ( 1992). The ratio of 
adult to juvenile whales ranged from a high of 34.0:1 
in 1988 to 7.1:1 in 1992, higher in all cases than the 
adult:juvenile ratios on Stellwagen Bank. 
Number of whales per day The mean number of 
whales per day ranged from a low of 2.9 (SD=1.9, 
n= 22) in 1989 to a high of 9.2 (SD=7.7, n=138) in 
1994 (Fig. 4; B=0.98, P=0.022). In 1993 and 1994, 
the only years with coverage comparable to Stell- 
wagen Bank levels, means of 6.2 (SD=6.9, n- 116) and 
9.2 (SD=7.7, n=138) whales were identified on each 
day of coverage, respectively. 
The pattern of humpback abundance on Jeffreys 
Ledge showed surprising seasonal consistency 
throughout the study. Sightings were sporadic dur- 
ing May, June, and early July, with few, if any, con- 
centrations of whales observed. In all years, concen- 
trations increased from late July through Septem- 
ber, with whales still abundant in three of the seven 
Octobers observed (1988, 1989, 1993). 
Identification comparison To determine whether 
the whales using Jeffreys Ledge were the same as 
those previously inhabiting Stellwagen Bank, we 
examined how many of the 210 humpback whales 
identified on Jeffreys Ledge in 1992-94 had been 
previously sighted on Stellwagen Bank. Of this group, 
123 (58.5%) were photographed on Stellwagen Bank 
during 1988-89. When the 17 animals that had 
not yet been born in 1988-89 were also discounted 
from the Jeffreys population, 63.7% of all animals 
were found to have been seen previously on Stell- 
wagen. By comparison, only 35 ( 16.6%) of the Jeffreys 
Ledge whales were also seen on Stellwagen Bank 
during the 1992-93 period, or 16.6% of the total 
Jeffreys Ledge population. 
Discussion 
Humpback whales, especially adult and cow-calf 
pairs, decreased their use of Stellwagen Bank dras- 
tically between 1988 and 1994. The decreased use is 
reflected in decreased numbers of whales identified, 
decreased numbers of whales (regardless of age class) 
per day, and decreased adult occurrence and occu- 
pancy. The decline led to a virtual abandonment in 
1994, when only seven humpback whales were seen 
on Stellwagen, and only two of those had occupancy 
periods longer than ten days. The decline in whale 
use corresponds with a decline in the amount of echo- 
sounder prey traces at the sites on Stellwagen Bank 
where whales were found over three years during 
the study. Although adults showed a clear decreas- 
ing trend on Stellwagen Bank, juvenile whales 
showed a less clear pattern. However, even juveniles 
showed a rapid decrease in use from 1991 to 1994. 
The increase in juvenile whales on Stellwagen 
Bank during 1990-91 while adult use decreased may 
also be a more subtle indicator of a shift in habitat 
quality. Previous work has shown that juvenile 
humpback whales are often found in areas where 
prey density is lower than in areas where adults 
predominate (Weinrich and Kuhlberg, 1991; Belt et 
al. 5 ), and may, therefore, be considered suboptimal 
