Mechanisms enabling the self-recruitment of passive larvae in the Great Barrier Reef

Abstract

This paper evaluates the conditions experienced by water-born passive larvae of broadcast spawning coral and crown-of-thorn starfish and how they self-recruit to their natal reefs in the Great Barrier Reef. The hypothesis that passive larvae are trapped for extended periods around specific areas of their natal reef (100s of metres) was found to be generally invalid. However, at some sites long-term trapping may occur when flow separation at headlands and in reef passages creates recirculating flows in embayments and behind concave-shaped reefs. Linear reefs do not trap passive larvae. This was demonstrated using satellite images and oceanographic modeling. The degree of self-recruitment at locations depended on the details of the incident flow speed, the shape of the headlands and the reef passages, the orientation of the reef compared to that of the tidal currents, the aspect ratio of the embayment, the curvature of the reef, and the time that the developing mushroom tidal jets takes to pass in front of the embayment. Self-recruitment of passive larvae depends on the spatial scale; at scales of 100s of metres, it is a rare process in the Great Barrier Reef. An exception was in a high-density reef matrix where the sticky water effect prevailed and self-recruitment was higher. Further, at scales of whole reefs (kilometres) and clusters of reefs (kms to 10s of kilometres) the likelihood of self-recruitment was higher. The probability of self-recruitment for reef fish larvae swimming directionally to their natal reefs following auditory and chemical cues is predicted to be much higher.