Short-term and long-term influences of the immigration of postlarval banana prawns Penaeus merguiensis, into a mangrove estuary of the Gulf of Carpentaria, Australia

Abstract

Postlarval P. merguiensis de Man were sampled regularly at the mouth of the Norman River estuary in the southeastern Gulf of Carpentaria, Australia, from 1975 to 1979. A grid of stations up to 10 km offshore from the estuary mouth was also occupied over a 24 h period during Nov. 1978. In these offshore stations, postlarvae migrated vertically in the water column in response to changes in tidal height, rising to near the surface just before low tide. Postlarvae entered the estuary throughout the flood tide with a peak in abundance occurring at the time of maximum current, regardless of time of day or night. During the monsoonal wet season (Dec.-March), when estuary salinities were lower, postlarvae occurred deeper in the water column. Over the 4 yr of sampling, immigration on a day-to-day basis was extremely variable but a consistent spring-neap tidal cycle could be demonstrated. More postlarvae immigrated during the spring-tide period than during the neaps, although the pattern was complicated by a larger peak associated with every 2nd spring tide (27.32 days). Although difficult to separate from tidal phase changes, lunar changes per se appeared to be less important than the relation between tidal phase changes and timing of moonset and moonrise. Immigration was restricted to the warmer months between Oct. and April each year, with a large pulse of recruits occurring usually in spring (Oct.-Nov.) and a less consistent pulse occurring in autumn (March-May). Inter-annual variations were also large; these year-to-year differences may be related to the amount of rainfall recorded during the previous wet season (9 mo. earlier) and the associated changes in salinity, temperature, nutrients and abundance of adult prawns. Over larger time scales, both size and developmental stage (number rostral spines) were directly related to postlarval abundance. Postlarvae tended to be larger and more well developed during spring and autumn of each year, and were also larger during years of higher immigration rates. Increased growth as well as survival in years following good rainfall and enrichment is suggested.