Modeling biases in estimating production from copepod cohorts

Abstract

Production (P) by copepod cohorts between sample times 0 and t is estimated from cohort numbers (N 0 and N,) and biomasses (B 0 and B 1 ) as P = (1 + m/g)ΔB, where m = ln(N 0 /N 1 ), g = ln(B t /B 0 ), and AB = B t - B 0 . This equation assumes exponential changes in biomass and numbers during the interval. Individual-based simulation models show that variances in initial weights and growth rates do not bias estimates; only severe size (stage) dependence of growth rates introduces serious bias; extreme size dependence of mortality rates produces more bias, mitigated by more frequent sampling; negative size dependence of both growth and mortality rates tends to cancel biases; and underestimation of production when individuals accumulate in nongrowing stages is mitigated by sampling when such individuals first appear. With these qualifications, the above equation is adequate and detailed mortality and growth rates of individual stages are unnecessary when cohorts are distinguishable.