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i q ~l 'l ~\ tu~\ ~\tl~ ~\Cp Rethi~ng the Stock Concept: A Phyldgeographic Approach ANDREW E. DIZON Southwest Fishedes Science Center National Marine Fisheries Service La Jolla, CA 92038, U,S.A. CHRISTINA LOCKYER NERC, Sea Mammal Research Unit British Antarctic. Survey Cambridge CB3 OET, UK WILLIAM F.PERRlN DOUGLAS P. DEMASTER JOYCE SISSON Southwest Fisheries Science Center National Marine Fisheries Service La Jolla, CA 92038, U.S.A. Abstract: The "stock" is the fundamental population unit of legally mandated conservation efforts, yet its formal defini. tion in the scil/ntific literature and in two US, conservation acts Is varied and so general that attempts to apply it in practice are arbitrary, Because choice of stocks deserving management protection is sometimes politically conten. tious, improvement of the working definition is important. A key element should be tbe degree to which a population can be considered an evolutionarily significant unit. We propose that a hierarchial classification scheme be applied to stock designations. Category I populations, having the highest probability of being evolutionarily significant units, are characterized by a discontinuous genetic divergem:e pat- tern where loeally adapted and closely related genome as. semblages are separated from others geographically and by significant genetic distances. Category II populations are similarly characterized by significant genetic diversity, but with weak geographic partitioning, Category III populations are the converse of II, having little genetic differentiation between assemblages that are clearly separate and likely to be reproductively isolated Category N assemblages have the lowest probability of being evolutionarily significant units and are characterized by extensive gene flow and no subdi- vision by extrinsic ba1Tiers. In addition to pbylogeograpbic designation, the following information Is used in the clas- sification, as indicated by single-letter abbreviations: distri- bution (a), population response (b), phenotypic (c), and Paper submitted February 12, 1991; revised manuscript accepted August 16. 1991, Z4 CoaservatiOD. Biology Volume 6. N.i, 1. March 1992 Resumen: El stock es la unidad poblacional fundamental para los esfuerzos de conservacion requeridos por ley, sin embargo, su definici6n formal en la literatura cientifica y en dos aetas de conservaci6n de los Estados Unidos es diversa y tan general que cualquier intento para usario en la practica es arbitrario, Dado que la elecci6n de los stocks merecedores de protecci6n en su manejo es a veces un becbo politica. mente contencioso, es muy importante mejorar la definicion en uso, Un elemento ciave deberia ser el grado en que una pobla. ci6n puede ser considerada como una unidad evolutiva- mente significativa. proponemos que se aplique un esquema jerarquico de clasificacion a la desigam:i6n de stocks. La Categorla I, poblaciones con la mayor probabilidad de ser unidades evolutivamente significativas, son caracterizadas por un patrOn discontinuo de divergencia genetica donde agrupaeiones de genomas loealmente adaptados e tntimam. ente relacionados estan geograficamente aislados y separa- dos de otros por distancias geneticas significativas. La Cat- egoria II esta tambien caracterizada por una significativa diversidad genetica pero con una debil subdivisiOn geografica. La Categoria III es la inversa de II, poseyendo una pequetia diferenciaci6n genetica entre agrupactotU!S que es- tan claramente separadas y probablemente estan alsladas reproductivamente. Las agrupaciones dentro de la Categorla Iv, que tienen la menor probabilidad de ser unidadeS evol- uUvamente significativas, estdn caracterlzados por un flujo genico extensivo y por una ausencia de subdivisIOn a traves de barreras extrtnsecas. Ademas de desiganciones filo- geograficas, la' informacion usada para clasificar tambten ~