GEOGRAPHIC VARIATION IN THE MILKFISH CHANOS CHANOS. I. BIOCHEMICAL EVIDENCE.

Since the first use of protein gel electrophoresis for the assessment of genic variation in natural populations (Lewontin and Hubby, 1966), a large number of plants and animals across many phylogenetic lines has been surveyed for electrophoretic variation. Results of these surveys and synopses of the related evolutionary hypotheses have been summarized in Lewontin (1974), Ayala (1976), and Nevo (1978). In reviewing these reports, it is apparent that one group of organisms has been generally overlooked: the marine fishes. For instance, many workers have examined the population structure of freshwater fishes (Avise and Selander, 1972; Avise and Smith, 1974; Echelle et al., 1975; Allendorf et al., 1976; Vrijenhoek et al., 1977; and others), yet there are virtually no data available for widely distributed marine fishes. In comparable studies of marine fishes, the work has dealt with diadromous fishes (Koehn and Williams, 1978; Utter et al., 1980), estuarine fishes (Johnson, 1975), or fishes from restricted geographic areas (Gorman et al., 1976; Gorman and Kim, 1977; Ward and Beardmore, 1977). In only one report were genetic differences assessed for a marine fish found in separate localities across the tropics, but this study remains unpublished (Soule, reference in Ehrlich, 1975). It is generally assumed that because of a planktonic larval stage, marine fish have a high potential for gene flow and hence may show little interlocality differentiation, but this topic remains unexplored. It would be interesting, therefore, to assess patterns of geographic differentiation of fish populations within and among archipelagos across an ocean basin. It has long been realized that small and/ or isolated populations may undergo considerable genetic changes. Semi-isolated populations of cavefish Astyanax mexicanus had reduced levels of electrophoretic variation relative to conspecific surface populations (Avise and Selander, 1972), and stream populations of Etheostoma radiosum, separated in different drainages of the same tributary, exhibited high levels of population differentiation (Echelle et al., 1975). In a marine habitat the epitome of isolation of an ichthyofauna is the Hawaiian inshore fish community. Randall (1976) estimates the level of endemism for the inshore fishes is 29%-the highest value for any oceanic island. Yet, there have been no electrophoretic estimates of the degree of isolation of the fishes in Hawaii. Thus, using electrophoretic data, the following questions were examined for the milkfish Chanos chanos: 1) What is the level of genic variation in this species and how does it change over large geographic distances? 2) What is the genetic population structure for a marine fish, such as Chanos, that has a planktonic larval stage? 3) What factors affect the observed pattern of geographic variation and how does the level of geographic differentiation compare to that of other species-terrestrial and marine? 4) How genetically isolated is the Hawaiian population of milkfish relative to other milkfish populations? Life history of milkfish. -Milkfish, Chanos chanos (Forskal), are tropical marine fish distributed throughout the Pacific 1 Hawaii Institute of Marine Biology Contribution No. 586.