Buckel et al,: A comparison of biomass harvested by Pomatomus saltatrix with that harvested by fisheries 



781 



population (1995) consumed 0.9 million t per year 

 whereas the maximum-size population (1986) con- 

 sumed 2.5 million t of prey per year. The 1982 to 

 1995 average population biomass consumed 1.8 mil- 

 lion t of prey per year. 



Of the total biomass of prey consumed annually 

 by bluefish, the fraction attributed to the primary 

 resource species is an annual average of 10% butter- 



Oe+0 



020 

 c ^ 0,15 

 I" 00 0,10 



3 " ' 



60 



C 



o 

 U 



05 - 



1e+6 - 



M 6e+5 - 



3e+5 



Oe+0 



"I 1 r 



2 4 6 



Age (years) 



Figure 3 



Estimates of bluefish impact on prey by age class for the 

 U.S. east coast bluefish stock in 1995. Numbers of bluefish 

 (NEFSC3) and bluefish weight (kg; Wilk. 1977) at age (A) 

 were used to calculate bluefish biomass IB). Impact I kg l on 

 prey by each age class (D) was calculated by multiplying 

 bluefish biomass by maximum consumption rate (Hartman 

 and Brandt 1995a I for each age class (C). 



fish, 10% long-fmned squid, 5% boreal squid, and 5% 

 Atlantic menhaden. These are also averages across 

 bluefish body size; however, these averages may be 

 too simplistic given that bluefish diet changes with 

 increasing size. Much of the rest of the diet of blue- 

 fish is attributed to bay anchovy (described in Buckel 

 et al., 1999a) for which no fishery landings exist. The 

 average (minimum-maximum) annual consumption 

 of butterfish by bluefish is therefore 90,000 1 (48,000- 

 125,000 t); annual consumtion of long-finned squid, 

 is 90,000 t (48,000-125,000 t); and annual consump- 

 tion of boreal squid, is 45,000 t (24,000-63,000 t). 

 The annual consumption of these species by bluefish 

 is much higher than the annual fisheries landings 

 over the period 1984-92 (Fig. 4). The average con- 

 sumption of Atlantic menhaden by bluefish was 

 45,000 t (24,000-63,000 t). The menhaden fishery 

 takes seven times the biomass of menhaden that 

 bluefish consume (Fig. 4). 



Effects of pooling diet across ages 



For the simulated population, the daily consumption 

 of long-finned squid and butterfish calculated by 

 using age-specific data did not differ greatly from 

 daily consumption when diets were pooled across 

 ages (Fig. 5). However, the use of pooled diets over- 

 estimated daily consumption of boreal squid and At- 

 lantic menhaden in comparison to values obtained 

 with age-specific diets for both the simulated and 

 1995 bluefish population. For the 1995 population, 

 age-specific diets gave daily consumption values for 

 long-finned squid and butterfish that were higher 

 than those that were calculated when pooled percent 

 diet contributions were used (Fig. 5). Therefore, pool- 

 ing diets can lead to under- or over-estimates of the 

 predation pressure by bluefish; this bias can be affected 

 by the age structure of the bluefish population. 



Discussion 



The results suggest that for at least three of the prey 

 species of bluefish and humans, bluefish harvest a 

 much higher biomass than does the fishery. This 

 predatory loss will vary with fluctuations in blue- 

 fish abundance. We elaborate on management im- 

 plications below. 



Estimates of population consumption rate 



Our estimate of QIB (annual population consump- 

 tion) is likely robust for two reasons. First, the esti- 

 mate of bluefish annual QIB is similar to that of other 

 pelagic piscivores. Palomares and Pauly (1989) esti- 



