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Fishery Bulletin 88(2), 1990 



backs. Furthermore, while a humpback can often be 

 recognized from a single poor-quality photograph, iden- 

 tification of most individual fin whales requires a series 

 of high-quality photographs which must be taken from 

 approximately the same position relative to the whale. 

 Despite this, the frequency with which individual fin 

 whales were resighted is high. Roughly half of the fin 

 whales seen in one year were seen again the same 

 season, and about half were also resighted the follow- 

 ing year. Resighting intervals provide some evidence 

 for a degree of residency on the part of some whales, 

 and of bimodal occupancy by others. However, the 

 many gaps in the sighting histories of individuals are 

 difficult to interpret. Do the gaps represent whales that 

 were resident in the area for extended periods but were 

 not photographed, or do they indicate movement to 

 other areas between sightings? At this point, we can 



say only that, given the obvious bias in effort, it seems 

 likely that rates of within-season occurrence and an- 

 nual return of fin whales in Massachusetts Bay are con- 

 siderably under-represented in our data. When only 1 

 out of every 83 fin whales sighted is identified (as 

 occurred in 1981), it is unlikely that even an individual 

 which remained in the area for many weeks would be 

 recaptured more than a few times. This is supported 

 by the fact that the subset of individuals towards which 

 there was clear observer bias— those bearing large, 

 prominent scars— were among the most frequently 

 observed animals during this study. For example, 

 whale number 0081, named "Braid" (an animal with 

 very large propeller scars on its left side), was observed 

 in all 8 years, with a mean occupancy of 116 days, and 

 mean occurrence of 6 days. 



