midnight and sunrise (Period 2). Four whales had their shortest average dive from dusk to 

 midnight (Period 1). The longest (maximum) overall average dive duration for individual 

 animals varied from 102 s (PTT #831) to 342 s (PTT #833). 



A frequency histogram of average dive duration for all animals is shown in Figure 55. 

 Figures 56 - 62 demonstrate the distribution of average dive duration for the individual 

 animals. PTT #823 (Figure 56) and PTT #839 (Figure 61) represent 80% of the 299 

 samples for all PTTs. PTT #823' s overall average dive duration was approximately half that 

 for PTT #839 (54 s versus 100 s). Recall that the number of dives in a six hour period for 

 PTT #823 was precisely double that for PTT #839, affirming the reciprocal relationship of 

 these two characteristics. 



There were dramatic differences in diving strategies. "Willie"(PTT #823) had shorter 

 minimum average dive durations and less variation for all periods (14 s - 16 s) than any 

 other whale. These short average dive durations occurred in extremely warm water at the 

 southern extent of the whale's track. At the other extreme, "Stripe's" (PTT #840) lowest 

 average varied by 362% between periods from 42 s - 152 s. PTT #840 emphasized longer 

 dives with the highest overall average of 162 s. This was the pregnant female and also the 

 fastest moving animal. PTT #833, a juvenile which did not range far from its tagging site, 

 had the highest maximum average dive duration (342 s) for one period. 



Figures 63 and 64 show the relationship of average dive duration to calendar date. 

 For two of these seven animals (PTT #823 and PTT #833), the largest number of dives in 

 a 6-hour period occurred during the first day after tagging. While this could be a reaction 

 to tagging, it was not observed in other animals. The data show no obvious correlation with 

 weather. 



Dive durations are useful in examining diving capability, foraging strategies, 

 behavioral differences, possible harassment and estimating sighting factors for aerial and 

 boat surveys. The numbers reported here represent longer average dives (x = 86 s) than 

 those reported during Cetacean and Turtle Assessment Program (CeTAP) studies 

 (x = 52 s) conducted by ship (CeTAP, 1982). We cannot explain this difference with 

 assurance, however, it may be due to individual variations, area, time of year, behavior or 

 methodology. There may be some inadvertent harassment affect from the ship which must 

 stay within reception range during VHF monitoring. The difference is not attributable to 

 the definition of dives. If we had adjusted our definition of a dive to that used by CeTAP, 

 the differences in average dive duration would have been even larger. 



Long dive durations suggest that aerial and ship surveys at high speed may miss many 

 whales while they are submerged. For bowhead whales, another species in the right whale 

 family, Wursig, et al. (1984) observed dives averaging 47 s in 1980 and 1981, but 86 s in 1982 

 when there was less socialization. Wartzok, et al. (1990) found that dives of radio-tagged 

 bowheads averaged 50 s overall and 40 s when socializing. 



89 



