Weinrich et al.: A shift in distribution of humpback whales, Megaptera novaeangliae, in reponse to prey 
833 
habitat for the species. The vertical distribution of 
prey has also been reported to be different between 
concentration areas of the two age classes. Adults 
are found where prey is concentrated in the upper 
reaches of the water column (Belt et al. 5 ) where a 
humpback whale’s bubble and cooperative feeding 
strategies are most effective (Hain et al., 1982; 
D’Vincent et al., 1985; Weinrich et al., 1992; Weinrich 
et al. 6 ) or where foraging is most efficient because 
energy expenditures associated with diving are low- 
est (Dolphin, 1987). Juveniles appear to concentrate 
more often in areas where prey are predominantly 
subsurface, often feeding on or near the sea floor 
(Swingle et al., 1993; Hain et al., 1995; Belt et al 5 ; 
Weinrich et al. 6 ). In the years where juvenile use in- 
creased while adult use decreased (1990-91), echo- 
sounder data showed that prey were most concen- 
trated in the bottom 25% of the water column. Even 
within the year 1990, prey traces were found to be 
more common in the upper portions of the water col- 
umn on days when more adult whales than juveniles 
were present (Belt et al. 5 ). 
These findings suggest that there are multiple 
ways of assessing habitat quality for whales. Past 
reports of population trends have included only the 
number of whales sighted per unit of effort as a guide 
to habitat quality (Payne et al., 1986, 1990; Piatt et 
al., 1989). However, indicators such as independent 
trends in occurrence and occupancy of individual 
whales, the number of individuals identified over a 
given time period, and even the age class of individu- 
als, may also be important indicators of habitat qual- 
ity. Although all of these measures (except the last) 
are factors of sightings per unit of effort, these indi- 
vidual components may be illuminating in detailed 
studies of a particular area. Prey type, for instance, 
could influence factors such as occurrence or occu- 
pancy (or both). In this case, a relatively nonmi- 
gratory prey species, such as sand lance (which are 
tied to areas of particular bottom substrate and to- 
pography) could lead to residency extremes (with 
whales staying in an area for prolonged periods or 
avoiding the area altogether), while a less habitat- 
restricted prey (such as herring) could lead to highly 
variable intraseason distribution patterns. 
5 Belt, C. R., M. T. Weinrich, and M. R. Schilling. 1991. Effects 
of prey density on humpback whale (Megaptera novaeangliae) 
distribution in the Southern Gulf of Maine. P. 6 in Abstracts 
of the 9th biennial conference on the biology of marine 
mammals. Society for Marine Mammalogy, Chicago, IL. 
6 Weinrich, M. T., C. R. Belt, M. R. Schilling, and M. E. 
Cappellino. 1985. Habitat use patterns as a function of age 
and reproductive status in humpback whales. Abstract in 
Abstracts of the 6th biennial conference on the biology of ma- 
rine mammals. Society for Marine Mammalogy, Lawrence, KS. 
Although the number of whales on Stellwagen 
Bank showed a dramatic decrease, the number of 
whales photographed on Jeffreys Ledge more than 
doubled in the last three years of the study. The cor- 
responding increase in observer effort during the 
same period no doubt had some effect on the dra- 
matic increase in both the number of identified indi- 
viduals and the mean number of whales identified 
per day. However, existing opportunistic data were 
collected following the 1992 season because of the 
increased use of the area suggested from our dedi- 
cated vessel surveys, where methods remained stan- 
dard across years. Further, captains of whale watch- 
ing boats and naturalists who had worked on Jeffreys 
Ledge since the mid-1980’s unanimously agreed that 
there was a sudden, dramatic increase in daily whale 
sightings beginning in 1992. Therefore, we fully be- 
lieve that an increase in effort is not the sole, or even 
the primary, cause for any increase in humpback 
whale numbers reported beginning in 1992. 
Our data show that the sudden increase in hump- 
back whale abundance on Jeffreys Ledge was pri- 
marily the result of whales seen on Stellwagen Bank 
earlier in the study relocating for much or all of their 
summer feeding season. What is perhaps more sur- 
prising is the relatively small number of whales that 
appeared in both areas during 1992 and 1993, de- 
spite the relative nearness of these areas to each 
other. Most of those whales photographed in both 
areas were seen on Stellwagen Bank for a brief pe- 
riod in October 1993, when herring stocks are known 
to migrate through the area (Fogarty and Clark 7 ). 
The consistent timing of whale aggregations on 
Jeffreys Ledge in each year (starting in early summer) 
corresponds with both the major influx of herring onto 
the Ledge and the start of their spawning season 
(USDC, 1991; Fogarty and Clark 7 ). The biomass of the 
Georges Bank herring population (of which this is a 
segment — Stephenson and Komfeld, 1990; Fogarty and 
Clark 7 ) has increased dramatically over the past de- 
cade and, by 1991, was comparable to that of its pre- 
exploitation size (Stephenson and Kornfeld, 1990; 
Sherman, 1992; NMFS 8 ). Echo-sounder data, obser- 
vation of surface prey, and catches of local fishing boats 
all indicated that herring were common on Jeffreys 
Ledge at the same time and location as aggregations of 
7 Fogarty, M. J., and S. H. Clark. 1983. Status of herring stocks 
in the Gulf of Maine region for 1983. Woods Hole Laboratory 
Reference Document 83-46, NMFS, NOAA, 33 p. [Available from 
Northeast Fisheries Center, Woods Hole, MA.] 
8 NMFS (National Marine Fisheries Service). 1992. Report of 
the thirteenth Northeast regional stock assessment workshop 
(13th SAW). Northeast Fisheries Science Center Document 92- 
02, Northeast Fisheries Center, NMFS/NOAA, Woods Hole MA. 
71 p. 
