Witteveen et al,: Effect of prey removal by Megaptera novaeangliae on fish abundance 



13 



Figure 2 



A close-up of the study area showing locations of humpback whale (Megaptera novaeangliae) sightings 

 and prey tows ( + ) for 2001 and 2002. Only mid-water trawl locations are shown. 



occurrence within the diet. Thus, diet B simulated a 

 weighted availability of prey species based on temporal 

 and spatial overlap between prey surveys and humpback 

 whale sightings within the study period (Fig. 2). 



Consumption rate 



A seasonal consumption rate was estimated for both 

 the current humpback whale population and the pre- 

 whaling humpback whale population. The prewhaling 

 consumption rate was estimated by using diet A only. 

 Diet B was used to estimate the consumption rate by 

 the current humpback whale population and to project 

 the consumption rate by a humpback whale population 

 at the prewhaling abundance level. 



The active metabolic rate (kcal/day) of feeding 

 humpback whales was estimated in this study as 

 £ = 192A/o '5 where Kleiber's (1961) model for basal 

 metabolic rate (BMR; E=70M"'^) was modified by us- 

 ing average oxygen consumption estimates for feeding 

 baleen whales, where M is average body weight (kg) 

 (Wahrenbrock et al., 1974: Sumich, 1983; Perez and 

 McAlister, 1993). 



Daily prey consumption was then estimated as 



1 



K 1,000 



where / = total prey consumption (kg/day); 



E = estimated daily energy requirements (kcal/ 



day); and 

 K = the estimated energy density (kcal/gram wet 



weight) of presumed prey. 



The average body mass for humpback whales (Mi was 

 set equal to 30,408 kg (Trites and Pauly. 1998). The 

 total energy density iK) of each diet was calculated by 

 multiplying the average seasonal energy density of each 

 prey species sampled in the study area by the percentage 

 of that species within each diet and summing across all 

 species. Values of A' for individual prey species came from 

 proximate compositions that were determined from prey 

 collected during 2002 trawl surveys for all months within 

 the study period (Foy*). For each month, energy density 

 was calculated by multiplying percent lipid by 9.4 kcal/g 

 and percent protein by 4.3 kcal/g, which are conversion 

 factors based on heat produced during metabolism of food 

 (Schmidt-Nielson, 1997). Carbohydrates were considered 

 to be bound and not available for nutrition (Gaskin, 



