778 



Fishery Bulletin 97(4), 1999 



to estimate i3(/3 =-0.0445, r2=0.64, F=45.44, df=l,25, 

 P<0.0001). 



The current stock assessment of bluefish assumes 

 a natural mortality of M=0.25. The highest fishing 

 mortality (F) measured on the east coast bluefish 

 population was in 1992 at 0.51 and recent estimates 

 of Fare around 0.4-0.5 (NEFSC^). Therefore, an es- 

 timate of M + i^=0.75 was used as a total mortality 

 (Z) input into the Pauly ( 1986) model to estimate Q/B. 

 The value of Z does not affect the estimate of Q/B 

 dramatically because of its presence in both the nu- 

 merator and denominator of the equation (see above). 

 Annual consumption (Q/B) was estimated for blue- 

 fish from age-0 to age- 10. 



To determine if our estimate was robust, Hartman 

 and Brandt's ( 1995a) equation iC^^^ = 0.520 x W-o -88) 

 describing laboratory (20-25°C) estimates of maxi- 

 mum consumption rate iC^^^, g/(g • d)) as a function 

 of bluefish weight (W, g) was also used to calculate 

 the population consumption for 1995. First, bluefish 

 biomass by age class was calculated by multiplying 

 bluefish numbers at age (NEFSC^) by the mean blue- 

 fish weight at age ( W, calculated from Wilk's ( 1977 1 

 VBGF function and length:weight conversion equa- 

 tion; W^,^,^, Q was estimated at ^=0.5 ). Second, the maxi- 

 mum consumption rate was calculated for a mean 

 bluefish weight at age from the relationship between 

 ^max ^"^ bluefish weight provided by Hartman and 

 Brandt ( 1995a). To do so, we extrapolated out beyond 

 the range of fish sizes used in their laboratory ex- 

 periment, assuming that values obtained in this way 

 were accurate. Lastly, these age-specific consump- 

 tion rates were then multiplied by the biomass of 

 each age class. Summing across age classes provided 

 us with a maximum biomass of prey consumed by 

 the bluefish population at 20-25°C; this value was 

 divided by the biomass of the population to obtain a 

 daily Q/B value. To calculate an annual Q/B value, 

 the daily Q/B value was multiplied by 365 days. 

 Additionally, this Q/B value was temperature ad- 

 justed from 22.5°C to 17.5°C. This was done by mul- 

 tiplying by the ratio of bluefish consumption rate 

 estimated at 17.5°C (average temperature for coastal 

 bluefish [Munch, 1997] ) to consumption rate esti- 

 mated at 22.5°C (temperature of Hartman and 

 Brandt's [1995a] study). This ratio was estimated 

 from both Buckel et al. (1995) and Hartman and 

 Brandt ( 1995a) data and in both cases was found to 

 be -0.60. 



Impact by age class 



From the above analysis, the age class where total 

 biomass consumption peaks can be determined. This 

 calculation was first made for a simulated bluefish 



population having constant recruitment (N=l x IC) 

 and total mortality (Z=0.75, see above). However, 

 bluefish exhibit highly variable recruitment across 

 years. To illustrate the impact of variable age struc- 

 ture, we also calculated biomass consumed by age 

 class on the basis of actual abundances in 1984 (a 

 year when preceding recruitments had been fairly 

 constant and age structure was typical) and in 1995 

 (a year when preceding recruitments were highly 

 variable and age structure was unstable). 



Comparison of biomass consumed by bluefish with 

 that harvested by fisheries 



The estimate of the annual amount of prey consumed 

 by the western Atlantic bluefish population is the 

 annual Q/B estimate (described above) multiplied 

 by stock biomass. The prey consumption for three 

 different population biomass sizes was estimated 

 based on VPA estimates; these were the minimum, 

 maximum, and average population sizes from 1982 

 to 1995 (NEFSC^). The total annual consumption at 

 these three stock sizes was estimated for Atlantic 

 butterfish (Pepriliis triacanthits), long-finned squid 

 (Loligo pealei). boreal squid (Illex iUecebrosus). and 

 Atlantic menhaden (Brevoortia tyrannus). These are 

 primary species in the diet of bluefish that are also 

 landed by fishermen. 



Several studies were used to attribute the total 

 biomass consumption by bluefish to different geo- 

 graphic regions, seasons, and prey species. To simu- 

 late the migratory patterns of bluefish, it was as- 

 sumed that 100% of the population was north of Cape 

 Hatteras during summer and autumn and 100% of 

 the population was south of Cape Hatteras during 

 spring and winter. Diet data from Richards (1976), 

 Morris^ and Buckel et al. (1999b) were used to de- 

 scribe bluefish diet in the summer and autumn north 

 of Cape Hatteras. The diet studies of Naughton and 

 Saloman- and Lassiter (1962) were used to describe 

 bluefish diet south of Cape Hatteras (Carolinas and 

 southeast Florida) during winter and spring. Details 

 regarding these diet studies can be found in Buckel 

 et al. (1999b I. 



To calibrate the relative importance of bluefish 

 predation on squid, butterfish, and menhaden, the 

 annual bluefish consumption of these prey was plot- 

 ted along with the average, minimum, and maximum 

 fisheries landings ( 1984-92 ) for these species as com- 

 piled by the National Marine Fisheries Service 

 (Anonymous''). 



■•Anonymous. 1993. Fisheries ofthe United States, 1992. U.S. 

 Dep Commer., National Oceanic and Atmospheric Association. 

 National Marine Fisheries Service. Silver Spring, MD, 115 p. 



