Witteveen et al : Effect of prey removal by Megaplera novaeonglioe on fisfi abundance 



17 



in 2001 (n=96) and 2002 (72=147) were dominated by 

 Pacific sandlance (Murra et al., 2003). 



Ecological effects from humpback whale prey 

 consumption 



Although estimates of consumption are highly dependent 

 on estimates of population abundance and metabolic 

 rates, these values indicate that humpback whales were, 

 and still are. significant predators within the Kodiak 

 Island ecosystem. 



Historic commercial whaling reduced the population 

 in our study area to an estimated low of 27 animals 

 by 1938 (Witteveen, 2003). The removal of so many 

 large consumers likely had significant impacts on the 

 surrounding ecosystem. As modeled, reducing historic 

 consumption to that of current levels would release 

 nearly 10,000 tons of prey within the study area in a 

 single feeding season. Such a release could have caused 

 a trophic cascade effect. 



Cetacean removals in the Southern Ocean have dem- 

 onstrated how trophic cascades can affect marine eco- 

 systems through removal of large marine predators, 

 including whales (Laws, 1985). It has been hypothesized 

 that a similar reorganization of the marine community 

 may have occurred in the Bering Sea and Gulf of Alas- 

 ka, although the mechanisms of such a cascade are not 

 well understood (Merrick, 1997; Trites, 1997; Springer 

 et al. 2003). Removal of whales during commercial 

 harvest reduced predation on certain fish, cephalopod, 

 and zooplankton species, which were then available to 

 other consumers. This large number of unconsumed 

 prey, when combined with environmental factors such 

 as the 1977 regime shift, may have contributed to the 

 growth of sympatric marine predator populations from 

 the late 1940s to late 1970s. It is hypothesized that 

 whale stock resurgence, coupled with the 1977 regime 

 shift that favored the proliferation of groundfish species, 

 may have reduced prey availability to other piscivores 

 in the system and may have led to declines seen in har- 

 bor seal (Phoca vitulina), Steller sea lion, northern fur 

 seal iCallorhinits iirsinus), common murre (Ur-ia aalge), 

 thick-billed murre iU. lomvia). and red-legged kittiwake 

 {Rissa brevirostris) populations (Merrick, 1995, 1997; 

 NRC, 1996; Trites, 1997). The Gulf of Alaska and Ber- 

 ing Sea ecosystems may still be affected by changes 

 caused by baleen whale removals and their recovery 

 (NRC, 1996). 



Assuming that the Kodiak Island study area was 

 similarly affected by this trophic reorganization, an es- 

 timate of the current consumption by humpback whales 

 would help elucidate the role that a humpback whale 

 recovery is playing in ecosystem dynamics. If our diet 

 composition and subsequent consumption estimates are 

 accurate, our results indicate that the diet of hump- 

 back whales in Kodiak waters directly overlaps those 

 of sympatric piscivores and the biomass that is removed 

 may be substantial. The top species modeled in the 

 humpback whale diet represent important sources of 

 energy for multiple higher-trophic-level species and are 



known to be significant dietary species for Steller sea 

 lions (Wynne^), harbor seals (Jemison**). tufted puffins 

 (Fratercula cirrhata) (Piatt et al., 1997), blacklegged 

 kittiwakes (Murra et al., 2003), adult pollock. Pacific 

 halibut, and arrowtooth flounder (Livingston, 1993; 

 Yang, 1995; Merrick, 1997; Best and St. Pierre^). 



Our model indicates that humpback whales within 

 the study area may currently be consuming a signifi- 

 cant amount of fish, including over 3.26 x 10'^ kg of juve- 

 nile pollock and nearly 3.62 x 10'' kg of small forage fish, 

 such as capelin, eulachon and Pacific sandlance, during 

 a 152-day feeding season. In comparison, tufted puffins 

 consume less juvenile pollock (6.40x10'' kg) between 

 mid-July and mid-September, but this amount still 

 accounts for one-tenth of the age-0 pollock stock in the 

 Gulf of Alaska during early July (Hatch and Sanger, 

 1992). In addition, gadid removal by Steller sea lions 

 in 1998 was estimated to be 1.79 xlO» kg, or 12% of 

 the total gadid biomass that is removed by commercial 

 fisheries for that year (Winship and Trites, 2003). This 

 amount, although nearly 55 times the amount of pol- 

 lock removal due to consumption by humpback whales, 

 includes all gadid (not only pollock) species removals in 

 all Alaskan waters. More importantly, these fish are 

 likely larger (>60 cm vs. ^30 cm) than fish targeted by 

 humpback whales. 



Although humpback whales generally feed on smaller 

 age classes than are targeted by commercial fisheries or 

 Steller sea lions (Perez and McAlister'; Kenney et al., 

 1997), consumption of younger age classes may affect 

 future recruitment into the fishery. Barrett et al. (1990) 

 stated that consumption of young cod (Gadus morhua) 

 and saithe (PoUachius virens) by shags (Phalacrocorax 

 aristotelis) and cormorants (P. carbo) in the Northeast 

 Atlantic could be a limiting factor in recruitment in 

 years of low stock size, even if consumption of these 

 species was overestimated by an order of magnitude. 

 Thus, it is noteworthy that the removal by humpback 

 whales of an estimated 3.26x10'' kg of pollock (age 0-2) 

 equals 30% of the 2002 commercial pollock harvest of 

 1.09x10'' kg (ages 3 to 8) for the entire Kodiak Island 

 management area and 2.1% of the 2002 spawning bio- 

 mass of pollock for the entire Gulf of Alaska, which was 

 estimated at 1.58x108 kg (NMFSi"; NPFMCi'^^), 



These comparisons are based on mean estimates of 

 prey removal and do not take into account model uncer- 



' Wynne, K. M. 2002. Unpubl. data. Fishery Industrial 

 Technology Center, Univ. Alaska Fairbanks, Kodiak, AK 

 99615. 



* Jemison, L. A. 2001. Summary of harbor seal diet data 

 collected in Alaska from 1990-1999. In Harbor seal inves- 

 tigations in Alaska iR. J. Small, ed.), p. 314-22. Ann. Rep. 

 NOAA Grant NA 87Fx0300. Alaska Departmart of Fish 

 and Game, P.O. Box 240020, Douglas, AK 99824. 



^ Best, E. A., and G. St. Pierre. 1986. Pacific halibut as 

 predator and prey. International Pacific Halibut Commis- 

 sion Technical Report 21, 27 p. Website: http://www.iphc. 

 washington.edu/halcom/pubs/techrep/tech0021.pdf [Accessed 

 on 31 May 2003.1 



10. n. 12 ggg next page for footnote text. 



