Behavioral and Morphological Development of Immature Hawaiian Freshwater Fishes.

Abstract

Studies of recent fish collections from 17 streams on four islands in Hawai'i provided key characteristics for species identification of immature forms of five native species of freshwater gobioids. Young Eleotris sandwicensis lack fused pelvic fins. A suborbital bar identifies Stenogobius hawaiiensis. Awaous guamensis can be distinguished by lack of notches in the upper lip, Lentipes concolor by a single median notch, and Sicyopterus stimpsoni by three notches. Recruitment from oceanic plankton occurred year round but peaked from February to April. Lentipes invaded streams mainly during the day with the wave surge; Sicyopterus entered mostly during the day regardless of tidal conditions. Awaous were transported onshore day or night. Stenogobius and Eleotris entered streams by passive transport, the former by day, the latter at night. Eleotris and Stenogobius larvae mature in the lower stream reaches while Lentipes, Sicyopterus, and Awaous usually move to upstream habitats. For these latter three fishes, ontogenetic changes in behavior, including predator avoidance during recruitment and aggressive interactions between immature fishes farther upstream, act in sequence to produce the instream distribution typical of adults. Schooling occurred in the estuary in Sicyopterus and Awaous postlarvae; singles and pairs also migrated into streams. Postlarvae remained in schools through the estuary where they were vulnerable to predation by adult Eleotris and Kuhlia sandvicensis. After climbing a waterfall that excluded most aquatic predators, Awaous and Sicyopterus developed coloration displayed during aggressive contests. Immature Sicyopterus that failed to develop bright colors lost agonistic encounters and usually fled upstream. Aggressive displays by postlarval Awaous usually displaced intruders. Lentipes moved quickly upstream; they did not school but exhibited aggressive behavior toward conspecifics that promoted upstream dispersal. Techniques and baseline data from this study were used in assessing recovery of three Kaua'i streams damaged by Hurricane Iniki (11 September, 1992).