NOTE Murphy: Occurrence and group characteristics of Balaenoptera acutorostrata 



579 



reported. All whalewatching cruises were nonrandom 

 and search tracks for these cruises were generally 

 decided by the captain of the vessel. 



Search effort was based on four-hour trips. It was 

 not possible to quantify precisely observer effort; time 

 spent searching inevitably varied somewhat between 

 trips. However, because at least one observer was 

 searching for whales constantly for the duration of 

 each cruise (both on the whalewatching cruises and 

 dedicated surveys), it is unlikely that significant dif- 

 ferences existed in effort between months or years. 

 Furthermore, because all trips were primarily fo- 

 cused on larger mysticetes, virtually all sightings of 

 minke whales were opportunistic; consequently, 

 search effort towards this species was effectively even 

 throughout the study period. To minimize the possi- 

 bility that minke whales were present but not sighted 

 owing to bad weather conditions, all cruises con- 

 ducted in fog or in sea states above Beaufort 4 were 

 excluded from analysis. The total number of cruises 

 for each month of all years is summarized in Table 1. 



The following information was routinely collected: 

 date, time, location (by using LORAN-C) and, where 

 determinable, group size and behavior. Although 

 photographs were occasionally taken during this 

 study, the resulting data are not discussed here. 



The following terms are used in this paper: single- 

 ton refers to a lone animal, group refers to two or 

 more animals that were considered associated if they 

 were swimming side by side and were generally co- 

 ordinating their speed and direction of movement 

 during their surfacing and diving behavior. Animals 

 that were farther apart and did not show such coor- 

 dination of movement were not considered associated. 

 It is possible that two or more minke whales that 

 were not side by side were in acoustic contact and 

 therefore associating; however, such associations are 

 impossible to identify in the field and were not con- 

 sidered in this study. Feeding refers to a whale ob- 

 served with its mouth open or lunging at the surface 

 where prey was visible in the water. It is highly likely 

 that there were other instances in this study when 



feeding occurred below the surface but could not be 

 observed; consequently, the feeding rates reported 

 here should be considered minimum values. Calf 

 refers to an animal considered to be a first-year calf 

 if it was observed in close association with a large 

 whale and was not more than half the apparent 

 length of the latter (presumed to be the mother). 



Results 



Temporal distribution 



There was a significant difference in the sighting rate 

 of minke whales (number of whales observed per trip) 

 from year to year (x 2 13 = 2188.7, P<0.001); the maxi- 

 mum sighting rate was recorded in 1989 and the 

 minimum in 1982 and 1986 (Fig. 2). Minke whales 

 were observed in all months except January and 

 February and showed significant differences in abun- 

 dance by month (pooled overyears) (Fig. 3). Pairwise 

 comparisons of mean monthly values for the period 

 of greatest effort (March through October) were con- 

 ducted by using a one-way analysis of variance and 

 are revealed in Table 2. There were no significant 

 differences in the number of sightings between the 

 months of March, April, May, and June (hereafter col- 

 lectively termed "spring"), nor between July, August, 

 September, and October (hereafter referred to as "sum- 

 mer-autumn") (Table 2). However, three of the four 

 months of spring differed significantly from those of 

 summer-autumn in all pairwise comparisons. The ex- 

 ception was March, for which sighting rates were not 

 significantly different from those of any other month. 



Calves 



Only three calves were observed during the entire 

 study period. Each sighting occurred at a different 

 time of year: 8 May 1981, 3 October 1989, and 28 

 August 1991. Two of the calves were part of a pair, 

 and the third was part of a group of three. 



