PATTERNS OF POPULATION VARIABILITY IN MARINE FISH STOCKS

Exploited marine fish and invertebrate stocks fluctuate in a myriad of complex patterns, exhibiting variability on interannual, decadal, and longer time scales. To characterize various patterns of variation, ti me series of catch, catch per unit effort, or biomass from 30 stocks w ere examined with a variety of statistical methods including autocorre lation analysis and Lowess smoothing. A hierarchical cluster analysis classified the stocks into six identifiable groups: steady-state; low- variation, low-frequency; cyclic; irregular; high-variation, high-freq uency; and spasmodic. The observed patterns are consistent with life h istory traits; for example, stocks with high variability are generally small, pelagic species whereas low-variability stocks are generally s low-growing, demersal fish. Each of the six general patterns of variab ility can be produced from a simple multiple-equilibrium population mo del by varying the intrinsic rate of population growth, and the time s cale and amplitude of environmental variability. Suitable management p olicies depend on the type of variation observed, and the vast majorit y of stocks examined do not correspond to the steady-state assumptions of classical fisheries models. For example, management of spasmodic s tocks may alternate between periods of active exploitation and periods of rebuilding, a process enhanced by the existence of alternative fis heries.