POPULATION-GROWTH OF NORTHERN ANCHOVY AND PACIFIC SARDINE USING STAGE-SPECIFIC MATRIX MODELS

We applied a stage-specific population matrix, the Lefkovitch matrix, to the northern anchovy Engraulis mordax and the Pacific sardine Sardi nops sagax to determine which vital rates in the life history have the most effect on population growth. Three vital rates considered are th e daily stage-specific instantaneous mortality rate, stage duration an d daily age-specific fecundity per female. The models incorporate vari ability in growth rates among individuals and link simpler stage-based and more complicated individual-based modeling approaches. The elasti city of population growth rate for anchovy percentage change from a 1% change of a vital rate was greatest for mortality in yolk-sac larvae, fecundity of 2 yr old fish (111 to 135 mm), and stage duration in lat e larvae (11 to 35 mm). For sardine, elasticity was greatest for morta lity in life stages of egg and yolk-sac larvae, fecundity of 5 and 6 y r old fish (211 to 250 mm), and stage duration in the early larvae (5 to 10 mm). When stage duration was variable, elasticity was 1/10 as la rge as when stage duration was constant, indicating that sensitivity o f populations to changes in vital rates is reduced by variation in gro wth rates. Population growth rates for both species increased with var iance in individual growth rates. Thus, a population with individuals that grow at varying rates is more likely to increase. Our models indi cate that growth rates are important determinants of population growth and recruitment and it is Likely that growth and mortality rates are linked. We suggest, therefore, that inexpensive estimates of larval gr owth rates could be used to help forecast future recruitment.