Buckel et a\: A comparison of blomass harvested by Pomatomus saltatnx with that haroested by fisheries 



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Effects of pooling diet across ages 



One potential shortcoming of this analysis was the 

 use of diet data pooled across age or size groups. This 

 procedure was unavoidable because of the lack of data 

 describing bluefish diet by age or size for different 

 seasons and geographical locations on the east coast 

 of the United States. The use of pooled diet data may 

 bias impact estimates given that prey such as squid, 

 butterfish, and squid are more important diet items 

 of older and larger bluefish. To address this poten- 

 tial bias, we performed a separate analysis to calcu- 

 late bluefish consumption of prey using size or spe- 

 cific diet data (where available, see Morris') as op- 

 posed to pooled data. The daily biomass of prey con- 

 sumed by each age or size class ( uncorrected for tem- 

 perature) was calculated for the simulated popula- 

 tion (a population with constant recruitment and 

 mortality, see "Estimates of population consumption 

 rate" section) and the 1995 bluefish population (a 

 population whose biomass was dominated by older 

 fish, see "Impact by age class" section and Table 1). 

 These consumption-by-age estimates were multiplied 

 by percent-contribution-by-weight data from Morris' 

 for the four prey species described in the main analy- 

 sis. The biomass of each prey type that was consumed 

 was summed over all age classes to obtain the popu- 



lation consumption of each prey type. These values 

 were then compared with values of population con- 

 sumption of each prey type that were obtained by 

 multiplying the total population consumption of all 

 prey by the pooled prey contribution from Morris. 



Results 



Estimates of population consumption rate 



The annual QIB estimate from Pauly's (1986) model 

 for the east coast bluefish population is 7.7. This 

 means that the east coast bluefish population con- 

 sumes a biomass of prey that is equivalent to ~8 times 

 its own biomass over a one year period. The estimate 

 of Q/B with Hartman and Brandt's ( 1995a) consump- 

 tion-rate-at-weight data, adjusted for temperature, 

 and estimated for the 1995 bluefish biomass on the 

 U.S. east coast, was 10.8 (Table 1). 



Impact by age class 



For the simulated population with constant recruit- 

 ment and mortality, the peak predatory impact of 

 the bluefish population occurs between approxi- 

 mately age 1 and age 3 (Fig. 1). In 1984, a year with 



