Payne et al : Abundance fluctuations of baleen whales in the southern Gulf of Maine 



693 



believe that this recent distributional change has 

 resulted from differing degrees of interspecific com- 

 petition between each of the two whale species and 

 sandlance, moreso than from any competition between 

 the whales themselves. 



Competition between sandlance and 

 pianktiverous whiale species 



Historically, and in recent times, right whales have 

 been recorded in the Stellwagen Bank region in late 

 winter and early spring (Allen 1908, Allen 1916, Wat- 

 kins and Schevill 1982, Schevill et al. 1986, Hamilton 

 and Mayo 1990). The species has been virtually absent 

 from the area in summer, when the population's 

 distribution is centered in the Bay of Fundy and on the 

 Scotian Shelf (Kraus et al. 1982, 1986; Mitchell et al. 

 1986; Winn et al. 1986; Stone et al. 1988; Murison and 

 Gaskin 1989). Therefore, the prolonged residency 

 by a group of right whales in the Stellwagen region 

 throughout summer 1986 may represent the most sig- 

 nificant departure from their usual seasonal pattern of 

 occurrence recorded this century. 



Is the annual movement of right whales out of the 

 southern Gulf of Maine and into the Bay of Fundy and 

 Scotian shelf areas in early summer related to in- 

 creased activity and competition from pianktiverous 

 fish for calanoid copepods? The question of why right 

 whales generally undertake this northward migration 

 has yet to be resolved. The occurrence of right whales 

 in the Stellwagen Bank area in the summer of 1986 im- 

 plies that, if certain conditions are met, the normal 

 northward movement is not inevitable for at least part 

 of the population. The occurrence of pianktiverous fish 

 (either sandlance, herring or mackerel)— and conse- 

 quently of increased competition through predation on, 

 or disruption of, copepod patches in spring— may be a 

 major factor in directing the seasonal movement of 

 right whales out of the area to seek more acceptable 

 patches elsewhere in the Gulf of Maine. 



Not only are the size and density of copepod patches 

 important to the feeding energetics of right whales, but 

 also the relative proportions of adult copepods within 

 each patch (Wishner et al. 1988). Wishner et al. (1988) 

 described a copepod surface patch (with associated 

 skim-feeding right whales) which contained a regional 

 maximum of copepodite IVs and Vs, the older, ener- 

 getically richest, developmental stages of copepods 

 (Comita et al. 1966). Although the feeding ecology of 

 right whales is likely more complex than previously 

 thought (Mayo and Marx 1990), these dense aggrega- 

 tions of older, caloric-rich copepods seem to be the 

 required characteristics for energetically successful 

 foraging by right whales (Kenney et al. 1986, Wishner 

 et al. 1988). 



The principal diet of sandlance >21 mm is also the 

 larger, older developmental stages of C. finmarchicus 

 (Norcross et al. 1961, Scott 1973, Monteleone and 

 Peterson 1986). If the superabundant sandlance are 

 selecting the caloric-rich stages of the copepod popula- 

 tion, then there may be insufficient prey available in 

 the remaining developmental stages (independent of 

 abundance) to provide right whales with the required 

 energy densities (as described by Kenney et al. 1986) 

 to meet the metabolic and reproductive demands of the 

 right whale population. Therefore, during those years 

 when sandlance (or ecologically similar pianktiverous 

 fish species) are abundant, intraspecific competition 

 may be so intense that right whales may only be achiev- 

 ing basal metabolic needs in the southern Gulf of Maine, 

 precipitating the northward movement of right whales 

 immediately following the period of increased sand- 

 lance abundance and activity. 



The occurrence of consistent numbers of sei whales 

 in 1986 followed many years in which the species had 

 not been recorded from the study area during any 

 season (Mayo et al. 1988). Jonsgaard and Darling (1977) 

 describe "invasion years" where unusually large num- 

 bers of sei whales occur in an area. These years are 

 generally followed by an equally sharp decrease in the 

 number of whales in the same area. Albeit simple, an 

 "invasion year" in response to increased prey abun- 

 dance accurately describes the occurrence of sei whales 

 on Stellwagen Bank during 1986. 



While sei whales seem to prefer planktonic prey 

 (Watkins and Schevill 1979), they have the ability to 

 forage on fish (Nemoto 1959, Nemoto and Kawamura 

 1977). Despite this, available distributional evidence 

 suggests that sei whales have not exploited the abun- 

 dant sandlance in the Gulf of Maine to a significant ex- 

 tent. Scant data suggest that sei whales in this region 

 instead subsist primarily on euphausiids and copepods 

 which occur in the waters south of Georges Bank (Ken- 

 ney and Winn 1987). By remaining on the shelf edge 

 and exploiting a wider range of planktonic (and possibly 

 fish) prey, they may be able to minimize the effect of 

 competition from sandlance for copepods, separating 

 them from the more stenophagic right whales. 



Since the early 1950s sandlance populations have 

 fluctuated on remarkably few occasions. Monteleone 

 et al. (1987) described relatively high sandlance den- 

 sities during 1965-66 (following the depletion of her- 

 ring and mackerel stocks) and 1978-79 (following the 

 larval sandlance explosion in 1975), and exceptionally 

 low densities of sandlance in 1971-74. Meyer et al. 

 (1979) also reported that adult sandlance were not 

 reported on Stellwagen Bank between 1967-76 and 

 that the mean sandlance catch/10 m- in spring 1977 

 was nine times greater than in spring 1974. Therefore, 

 the two recorded time intervals when sei whales and 



