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



777 



(1986). Previously published data on diet and recent 

 estimates of bluefish population size along the U.S. 

 Atlantic coast were used to determine the "harvest" 

 of prey by bluefish. We then compared "harvesting" 

 of prey by bluefish to the biomass harvested by the 

 fishery for the east coast as a whole. Prey consump- 

 tion by age class was calculated to determine at which 

 age the largest impact on prey occurs. We present 

 these analyses not as management tools but merely 

 as gross estimates of the probable magnitude of the 

 bluefish harvest of fishery species so as to stimulate 

 more detailed studies. 



Methods 



Estimates of population consumption rate 



The total U.S. east coast bluefish population con- 

 sumption of a given prey was calculated from esti- 

 mates of diet, the biomass of the bluefish popula- 

 tion, and food consumption rate. Bluefish diet infor- 

 mation was available from the literature for various 

 seasons and regions along the east coast (Lassiter, 

 1962; Richards, 1976; Buckel et al.. 1999a: Morris^: 

 Naughton and Saloman-). Bluefish biomass esti- 

 mates were available for 1982 to 1995 from a virtual 

 population analysis (VPA) performed by the North- 

 east Fisheries Science Center, National Marine Fish- 

 eries Service, Woods Hole, MA (NEFSC). However, 

 estimates of bluefish consumption rate were avail- 

 able only for young-of-the-year (YOY) (Juanes and 

 Conover, 1994; Buckel et al., 1995; Buckel and 

 Conover, 1997) or for age-0 to age-2 bluefish in Chesa- 

 peake Bay (Hartman and Brandt, 1995a). For this 

 reason, we used the approach described below. 



The quantity of food consumed annually by the 

 bluefish population was estimated by using Pauly's 

 (1986) age-structured consumption rate model. The 

 model is based on a growth function (e.g. von 

 Bertalanffy growth function, VBGF), the instanta- 

 neous total mortality rate (Z), and the relationship 

 of gross growth efficiency to weight. Pauly's (1986) 

 model in simplified form is 



T {dw/dt)xN, 



1 Morris, T. L. 1984. Food of bluefish. Woods Hole Ref. Doc. 

 84-22, 13 p. [Available from Woods Hole Laboratory, North- 

 east Fisheries Science Center, Natl. Mar. Fish. Serv., NOAA, 

 Woods Hole, MA 02543.] 



2 Naughton, S. P., and C. H. Saloman. 1984. Food of bluefish 

 {Pomatomus saltatrix) from the U.S. south Atlantic and Gulf of 

 Mexico. U.S. Dep. Commer, NOAA Tech. Memo. NMFS-SEFC- 

 150, 37 p. 



3 NEFSC (Northeast Fisheries Science Center). 1997. Report 

 of the 23rd Northeast Regional Stock Assessment Workshop 

 (23rd SAW) Stock Assessment Review Committee (SARC) con- 

 sensus summary of assessments. Northeast Fisheries Sci. 

 Cent. Ref. Doc. 97-05, 105 p. 



Q 

 B 



1 



K,. 



dt 



\ W,xN,dt 



where Q / B is the food consumption per unit popula- 

 tion biomass (see descriptions in Pauly ( 1986), Pauly 

 and Palomares (1987), and Palomares and Pauly 

 (1989)). The numerator of the model represents the 

 food consumption of the population (Q), which is de- 

 termined from the growth increment (dw /dt), num- 

 bers offish at age t (N^, determined from an initial 

 arbitrary number of recruits), and the growth effi- 

 ciency (/lj) for a given weight at age t. Values are 

 integrated over age classes from ^,. to ^,„^,j (age at re- 

 cruitment to a maximum age). The denominator is 

 the biomass term (B) which is the weight of individual 

 fish at age / ( W,) multiplied by the number of fish 

 ( A^, ) at age t. This value is integrated over all cohorts 

 from t^ to <„Qj- An annual Q/B estimate represents 

 the number of times the population consumes its own 

 weight per year (Pauly, 1986). 



Pauly and Palomares ( 1987) have provided a BA- 

 SIC progi'am to calculate Q/B from VBGF param- 

 eters, a parameter from the relationship describing 

 K^ as a function of weight, and an estimate of total 

 mortality (Z). Growth for the Q/B model is based on 

 the VBGF model expressed in weight form: 



W,=W,.x[l-e-' ■"-'"']' 



/here t = time in years; 



weight of bluefish at age t; 

 asymptotic weight; 

 growth coefficient; 

 age at weight zero; and 

 exponent of the length-weight relation- 

 ship (i.e. W=aL^). 



W, = 



k = 

 tn = 



For bluefish, Wilk's (1977) VBGF parameters were 

 used where k = 0.226, t^ = -0.123, W. = 8725g, and b = 

 3 (Wilk's estimate of 6 was 2.89 but 6 = 3 was used 

 owing to ease of use in Pauly's ( 1986) model). 



The Q/B model assumes that gross growth effi- 

 ciency (ifj ) data depends on weight ( W) according to 



A', = l-(W/W )/* 



To estimate (5 for use in the calculation of Q/B, blue- 

 fish gross growth efficiency and weight data from 

 experiments on YOY fish described in Buckel et al. 

 (1995) were used. The linearized form of the above 

 model described in Pauly ( 1986 ) was fitted to the data 



