Late pelagic-stage goatfishes: distribution patterns and inferences on schooling behaviour

Abstract

Spatial and temporal abundance patterns of pelagic-stage goatfish (Family Mullidae) across the northern Great Barrier Reef were examined (1986–1991). From these data inferences were made on the behaviour and schooling dynamics of this largely unstudied life-stage. Sampling was conducted using both a plankton-mesh purse-seine deployed around small aggregation rafts (1 sx 1 m), and automated light traps. A comparison of the two techniques showed no difference in the size composition of fishes caught. Neither the total number of mullids caught at a sampling station nor their minimum size was related to the current speed as measured by drogues (0 and 5 m depth). A comparison of the species composition of raft and light trap samples with that of fishes settling onto the reef below the rafts showed that, of the 11 species known to occur in these waters, only five were caught by the two devices. Size frequencies of catches around rafts suggested that at times multiple schools were present within a 90 m sampling area (i.e. one site). Schools were composed of a number of mullid species, each having a broad size range [e.g. Upeneus moluccensis (Bleeker), 20–56 mm Standard length (SL)]. The largest individuals were competent to settle. Samples from light traps deployed off the reef around Lizard Island for 280 days over the summers (1986/87 to 1988/89) found that late pelagic-stage mullids occurred close to the reef in irregular pulses. Pelagic mullids collected in purse-seine samples were consistently common in inter-reefal waters. A cross-shelf sampling programme involving six stations stretching from the inshore Turtle Group, past mid-shelf Lizard Island to 3 km seaward of an Outer Barrier reef (Carter Reef) was conducted. Catch composition and abundance was found to vary among locations over 3-monthly samples. Upeneus tragula (Richardson) and U. moluccensis dominated the catch at all but the Outer Barrier station, where the genus Parupeneus was most abundant. Findings stress the complex interaction between behaviour, ontogeny and abundance patterns, and the potential for late pelagic-stage fishes to influence their recruitment patterns.