692 



Fishery Bulletin 88(4), 1990 



1982 1983 1984 1986 1986 198? 1988 



RIGHT WHALES- 



1982 1983 1984 



1985 1986 



YEAR 



Figure 3 



Average number of baleen whales per unit effort for each year of 

 the study by species. Note: the scale for humpback and fin whales 

 (upper) is different from that of right and sei whales (lower). 



Discussion 



The data presented here strongly suggest that major 

 changes which occurred in the abundance and distribu- 

 tion of humpback, fin, right, and sei whales in the 

 Stellwagen Bank area, 1982-88, were not independent 

 of each other. During 1986 a dramatic decline in the 

 abundance of humpback and fin whales also co-occurred 

 with the unusual occurrence of right and sei whales dur- 

 ing the summer of the same year. During 1987 and 

 1988 the abundance of baleen whales returned to a pat- 

 tern similar to that observed in years prior to 1986 

 (right and sei whales absent, humpback and fin whales 

 abundant). Concurrent with changes in whale abun- 

 dance, an increase in copepod abundance on Stellwagen 

 Bank in 1986 correlated with a significant decline in 

 local sandlance abundance. 



These correlations likely reflect both the similarities 

 and dissimilarities in prey preferences of individual 

 whale species, and the predator-prey relationship be- 

 tween sandlance and copepods. Given the documented 

 preference of both right and sei whales for copepods 

 (Nemoto 1959, Mitchell 1975a, Nemoto and Kawamura 

 1977, Watkins and Schevill 1979), a strong positive cor- 

 relation between the two species should be expected. 

 A similar correspondence exists between humpback 



and fin whales, both of which show an apparent pref- 

 erence for schooling fish in this region (Watkins and 

 Schevill 1979, Mayo et al. 1988). Conversely, the strong 

 negative correlations reported here would be expected 

 between the planktiverous right and sei whales and the 

 pisciverous humpback and fin whales. 



Competition between whale species 



These results, in support of previous studies (Kenney 

 et al. 1981, Payne et al. 1986), suggest that the recent 

 distribution of humpback whales in the Gulf of Maine 

 has been dependent on the spatial distribution of 

 sandlance in that region. These results follow a trend 

 which began in the mid-1970s and, when taken with 

 the results of previous studies (Overholtz and Nicolas 

 1979, CeTAP 1982, Hain et al. 1982, Mayo 1982, Mayo 

 et al. 1988), indicate that sandlance has been the most 

 important prey item of humpback whales in the Gulf 

 of Maine since at least 1976. 



It is also of interest that, while fin whales also close- 

 ly followed the abundance patterns of sandlance, the 

 overall changes in fin whale abundance fluctuated much 

 less than those shown for the humpback whale. Al- 

 though both whale species prey on a variety of taxa 

 (Jonsgaard 1966) and have been characterized as gen- 

 eralists in their dietary preferences (Mitchell 1975a), 

 behavioral differences between the two species may 

 result in the ability of fin whales to exploit prey other 

 than sandlance in the study area. 



Watkins and Schevill (1979) noted clear differences 

 between the feeding behavior of humpback and fin 

 whales when exploiting the same prey (schooling fish); 

 fin whales lunge horizontally and humpbacks vertical- 

 ly. Humpbacks are also known to produce "bubble 

 clouds or nets" to potentially concentrate prey (Jurasz 

 and Jurasz 1979, Hain et al. 1982, Hays et al. 1985), 

 a feeding method not observed for fin whales. It has 

 also been suggested that the streamlined form and 

 greater speed of fin whales allow them to exploit more 

 widely separated patches of prey than humpbacks 

 (Brodie 1975, Whitehead and Carlson 1988). Thus fin 

 whales may be more independent of local fluctuations 

 in prey availability, explaining why they were located 

 in the study area in 1986, after humpbacks had aban- 

 doned it. 



Evidence clearly exists to support the belief that com- 

 petition exists between right and sei whales (Mitchell 

 1975a, Kawamura 1978). However, Kenney et al. 

 (1986) suggested that competition between right and 

 sei whales in the shelf waters of the northeastern 

 United States is unlikely given their present allopatric 

 distributions (excluding 1986). Perhaps the more im- 

 portant ecological question concerns the reason for the 

 present apparent allopatry. As detailed below, we 



