Zusammenfassung: Biases in estimating population sizes of the plus group are examined for bluefin tuna (Thunnus ) and swordfish (Xiphias gladius ). It is shown that the VPA estimates used in the 1990 ICCAT assessment are sensitive to the systematic ageing errors. The biases in the estimates of population sizes of the plus group might increase exponentially with tracing back to the earlier year.

Zusammenfassung: A recently introduced multivariate state-space time-series modeling algorithm, system-theoretic time-series, can be used to create a particularly robust state-space representation. Combining the model approximation methodology with time-series modeling of the residuals in accurate models of short-run dynamics. The approximate structural modeling methodology is illustrated with single species models of yellowfin sole (Limanda aspera ) biomass, and multispecies models of Pacific halibut (Hippoglossus stenolepis ), yellowfin sole, and red king crab (Paralithodes camtschatica ) biomasses in the eastern Bering Sea. A final application of the approximate structural/system-theoretic time-series model includes walleye pollock (Theragra chalcogramma ), Pacific halibut, Pacific cod (Gadus macrocephalus ), yellowfin sole, and red king crab. (Prepared in cooperation with Alaska Univ., Fairbanks.)