FISHERY BULLETIN: VOL 85, NO. 3 



ing the biomass in landings in proportion to num- 

 bers of fish caught when compared to earher 

 years. 



Catchability coefficients ((/p^p) for the popula- 

 tion were estimated directly by dividing the Fpop 

 estimates by nominal effort (vessel weeks) for 

 years 1955-79. A plot of these estimates on popu- 

 lation size indicates a pronounced inverse rela- 

 tionship similar to that shown by Schaaf (1975b), 

 who estimated q differently (Fig. 15). Addition- 

 ally, there is a pronounced historical trend in the 

 data. There appear to be at least two families of 

 points and thus two functional curves in the fig- 

 ure, 1955-69 and 1970-79. Beginning in 1959, the 

 catchability coefficient progressively increased, 

 the stock size was decreasing, and the fleet was 

 becoming more efficient due to modernization and 

 increased vessel size, coupled with technological 

 innovations in the fishery operations themselves. 

 This trend in efficiency probably made the ascent 

 from the earlier, lower ^pop series of years excep- 

 tionally rapid. As population size began to in- 

 crease after 1971, the catchability coefficient re- 

 flected a steady decline in magnitude, but it was 

 at a level almost twice as great (hence a doubling 

 in killing power or efficiency") as from the late 

 1950's and early 1960's. Therefore, the reduction 



■^Given modern day work weeks, real time .spent fishing, and 

 intervessel competition, the killing power has probably more 

 than doubled. 



in the number of vessels from 1955 to the present 

 did not represent a proportional reduction in po- 

 tential effective fishing effort. In spite of the com- 

 pounding effect of a true increase in efficiency 

 (fishing technology), q, and thus F for given lev- 

 els of effort, appear to be responding in inverse 

 fashion to population size. 



The computed F^^p values are derived inde- 

 pendently of nominal effort. Hence this set of val- 

 ues was used to determine if effort would be use- 

 ful to verify trends in F at age or provide 

 supportive information for improving the VPA 

 estimates by employing more sophisticated 

 models (see Deriso et al. 1985). A scatter diagram 

 of the differences between estimates of Fp^p and 

 their mean on nominal effort demonstrated that 

 no useful information is available from nominal 

 effort when considering the entire time span in- 

 volved (Fig. 16). The progi-essive increase in effi- 

 ciency of the purse seine fleet, plus problems asso- 

 ciated with estimating abundance with CPUE 

 data from purse seine fisheries for schooling 

 fishes (see Clark and Mangel 1979) makes nomi- 

 nal effort useful only for relatively short-time 

 span comparisons between adjacent years, and 

 then only when changes are pronounced. 



Yield Per Recruit 



Yield-per-recruit calculations for the 1970-78 

 fishing seasons were performed using the com- 



o 



X 



I- 

 z 



UJ 



o 



u. 

 u. 

 tu 



o 

 o 



m 

 < 



I 

 o 



I- 

 < 

 o 



0.8 



07 



0.6 



0.5 



0.4 

 3- 

 0.2 

 0.1 



.73 



72 



■84 

 .65 



•77 

 .78 76 



I I L. 



1 2 3 



5 6 7 8 9 



10 1 1 ^"^8 



POPULATION SIZE (ages 1 to 8* ) IN BILLIONS 



Fl<;URE 15.— Estimates of the Atlantic menhaden population catchability coefficient 

 ((/p„pl for years 19.5.'S-79, plotted against the estimated population size (excluding 

 age-0 fish). 



588 



