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Fishery Bulletin 90(3), 1992 



Bay to Frederick Sound and back between 16 July and 

 14 August. Animal #616 was last identified in Fred- 

 erick Sound on 30 August. 



Regional occupancy 



The interval between the first and last identification 

 of an individual whale provided a minimum estimate 

 of its occupancy in southeastern Alaska (Baker et al. 

 1985). Although it was not possible to document con- 

 tinuous residency of individual whales in either of the 

 primary study areas (i.e., Glacier Bay or Frederick 

 Sound), there was no evidence that individuals mi- 

 grated to other known feeding regions between surveys 

 (see 'Abundance and regional fidelity'). The longest 

 documented regional occupancy was 192 days for 

 animal #587. This individual was first identified on 1 

 June in Glacier Bay and last identified on 9 December 

 in Frederick Sound. Animal #587's identification rec- 

 ord, discussed by Baker et al. (:!987), showed that she 

 lost a calf sometime during the summer of 1986. Three 

 other adults and one calf had documented occupancies 

 of nearly equivalent length: 191 days for #616, an 

 animal of unknown age-sex class (see also Table 6); 183 

 days for #350, an animal of unknown age-sex class; and 

 186 days for #161 and her calf. 



Foraging behavior 



During summer surveys, whales in Frederick Sound 

 tended to occur in aggregations of 20 to 80 animals 

 often clustered along submerged ridges and mounts, 

 as determined by reference to fathometer recordings 

 and navigational charts. Observations of whale feces 

 and fathometer recordings of dense scattering layers 

 below feeding whales indicated that euphausiids were 

 the primary prey for these aggregations. During the 

 late-fall survey, we were unable to collect fathometer 

 recordings or to observe whale feces in order to con- 

 firm the primary prey species. However, the surface- 

 movement and diving patterns of whales and the loca- 

 tion of feeding aggregations were similar to that 

 observed during summer surveys, suggesting that 

 euphausiids were again the primary prey. 



The predominant prey of humpback whales in Glacier 

 Bay was schooling fish, as evidenced by fathometer 

 recordings and observations of schooling fish at the sur- 

 face. Within the Bay, whales fed singly or in pairs on 

 dense schools of capelin and sandlance. Outside the 

 Bay, in the adjacent waters of Icy Strait, the predomi- 

 nant prey of humpback whales appeared to be herring 

 as demonstrated in previous years using hydroacoustic 

 techniques and net tows (Wing and Krieger 1983, 

 Krieger and Wing 1984 and 1986). As in previous years 

 (Baker 1985a), whales near Icy Strait formed a social- 



ly cohesive pod of 7 to 9 individuals that appeared to 

 cooperate in foraging on schools of herring. 



Discussion 



Population characteristics 



The number of individual whales photographically iden- 

 tified during the 1986 surveys, 238 adults and 19 

 calves, can be considered a minimum estimate of abun- 

 dance for the southeastern Alaska feeding herd. 

 Capture-recapture analyses of across-year identifica- 

 tion records, however, provide estimates of this 

 regional population that are two or three times larger 

 than that based on the 1986 census alone. Although 

 these analyses are more likely than simple counts to 

 provide realistic estimates of regional abundance, they 

 should be interpreted with caution since the behavior 

 of whales seldom conforms strictly to the theoretical 

 assumptions underlying these models (e.g., Hammond 

 1986, Perry et al. 1990). Violation of the assumption 

 of equal catchability among southeastern Alaska 

 whales, for example, is indicated by the analysis of 

 reidentification frequencies across the 8-year study 

 period. Births, deaths and permanent emigration ob- 

 viously contribute to this unequal catchability (i.e., 

 reidentification inequality). Another probable source of 

 unequal catchability is heterogeneity due to local 

 habitat preference by individual whales and the vari- 

 able and limited geographic coverage of the surveys. 

 While births and deaths cause a positive bias in the 

 Petersen estimate of abundance, reidentification het- 

 erogeneity causes a negative bias (Hammond 1986 and 

 1990). 



Assuming, however, that adult mortality among 

 humpback whales is low (e.g., Buckland 1990) and that 

 permanent emigration to other feeding regions is in- 

 frequent (e.g.. Perry et al. 1990), the weighted Peter- 

 sen estimate of 547 whales (95% CL: 503-590) may be 

 our most robust for the southeastern Alaska subpop- 

 ulation in 1986. By using the cumulative reidentifica- 

 tion records of individuals across years and weighting 

 the final estimate by the largest sample year, the 

 weighted Petersen should be less biased than the 

 between-year Petersen estimates by heterogeneity due 

 to local habitat preferences or variation in survey ef- 

 fort. Births during the study period are included in the 

 cumulative population estimate when the calves mature 

 sufficiently to become available for individual identifica- 

 tion. The weighted Petersen is also consistent with 

 other estimates derived from the individual identifica- 

 tion records. The upper confidence interval of this 

 estimate overlaps with the total count of 579 whales 

 identified during the 8-year study and agrees closely 

 with the unbiased Petersen estimates from the pair- 



