- The North Pacific humpback whale (Megaptera novaeangliae) population has been increasing at an average annual rate of ~6% since the early 1990s. In northern Southeast Alaska alone, there are now more whales than estimated for the entire North Pacific several decades ago. An understanding of how this growing population is repopulating traditional foraging grounds will benefit from detailed investigations of their prey preferences and trends in whale abundance and distribution relative to those prey. This dissertation examines these issues from late May until early September 2008 in Frederick Sound and Stephens Passage, a Southeast Alaskan feeding area historically used by humpback whales. The foundation for the study is an analysis of the life histories and abundance patterns of euphausiids, the principal prey of humpbacks in the area, during late spring and summer. Four species, Thysanoessa raschii, T. longipes, T. spinifera, and Euphausia pacifica, were identified in plankton net samples collected at random locations throughout the study site (n = 49) and in locations where a strong scattering layer was observed on a 120 kHz echosounder (n = 48). Both sample types varied in euphausiid species composition. Abundance patterns of immature euphausiids coupled with observations of females carrying spermatophores indicated differences between species in spawning schedules. Thysanoessa spp. began spawning in early April with the spring phytoplankton bloom and continued until late June, whereas E. pacifica began spawning in early June and continued until late August. This protracted recruitment of immature euphausiids was geographically widespread throughout the summer in contrast to adults, which, although present all summer, were found primarily in slope and shallow (< 100 m) areas.
To determine if humpback whales preferred one euphausiid species or life-stage over another, net sample and hydroacoustic data collected in the vicinity of whales were compared to similar data collected in random locations throughout the study site. This revealed that whales targeted dense aggregations of adult euphausiids, but did not discriminate between the various species, which was surprising because of presumed differences in the energy density linked to their different spawning schedules. Additionally, whales did not spend time in areas with concentrations of immature euphausiids, which were likely not large enough during the study period to be suitable prey.
With this preference for adult euphausiids, the abundance and distribution patterns of humpbacks were examined in relation to prey availability. Whale abundance was lowest at the beginning of the study in late May at ca. 68 whales and peaked in late July at ca. 228 animals – approximately 12% of the region’s estimated abundance for the study year. This study did not detect a concomitant increase in the availability of adult euphausiids, which is unsurprising since immature euphausiids would not recruit into the adult population until after the end of the study, and post-spawning mortality and predation pressure is presumably high during this time. Instead, whales clustered increasingly around comparatively fewer prey as the summer progressed. These observations, combined with a plateau in whale abundance after July, suggest that their abundance in the area was limited by euphausiid availability.
Estimates of whales using the study site during the summer have remained similar over several decades despite a dramatic increase in humpback numbers in Southeast Alaska and elsewhere in the North Pacific. The results from this study suggest that, although the study site remains important seasonally to some whales, it is not a significant source of prey responsible for regional population growth in general. More likely, it is part of a network of feeding areas that has influenced the population trend. Further insight into these and the other issues raised in this dissertation could come from several additional analyses. An extended sampling season that captures the recruitment of immature euphausiids into the adult population would reveal whether a given year's prey cohort represents an important resource to whales in that same year, which has potential implications for interpreting mid-late season whale abundance patterns. As well, a photo-identification study
would be useful in characterizing whale residency patterns and determining whether the abundance trends reflect a relatively small subset of the regional population using the area for most of the season or a continuous flow of a larger portion of the population. Finally, similar analyses as those outlined here but conducted in other areas within the region would provide additional insight into the network’s capacity to support the recovering whale population.