Similarities in Behavioral Ecology among Amphidromous and Catadromous Fishes on the Oceanic Islands of Hawai'i and Guam

About seven families of fishes occur routinely in fresh water on oceanic high islands of the tropical Pacific; others (sharks, jacks, bonefish, etc.) are occasional visitors. However, amphidromous fishes (freshwater adults, marine larvae) of the families Gobiidae and Eleotridae are predominant in island streams. Hawai'i, representing the northernmost extent of Polynesia, has five species of gobioid fishes whose adults are limited to fresh water, but Guam, in the Mariana Islands of the far Western Pacific, has more than four times that number. Hawaiian stream fishes are strikingly similar to their Guamanian relatives in their distribution, ecology, and behavior. At both localities, these fishes typically exhibit strong species specificity in the section of stream inhabited by adults, in the microhabitat selected, and in their food and feeding. Although incompletely understood, aspects of the life cycles of amphidromous island fishes (spawning, migrations into and from the sea, and others) are cued by seasonal and short-term changes in stream flow. In the Hawaiian Islands, water-use decisions based on the imperatives of allowing no net loss of habitat for aquatic animals and maintaining stream-ocean pathways for migrating animals have facilitated both management and conservation of diversity in island streams.