Abstract

Spawning aggregations of reef fish, extremely vulnerable to over fishing, are becoming depleted or extirpated throughout the Caribbean and the world. A relatively large spawning aggregation of yellowfin grouper (Mycteroperca venenosa) still exists on the edge of the Puerto Rican shelf south of St Thomas, and has been monitored by University of the Virgin Islands scientists since 2003 using visual surveys, Antillian fish traps and hydro-acoustic telemetry. Yellowfin aggregate to spawn by the hundreds on a narrow coral reef beginning on the full moon of February, March and April annually. Size histograms indicate the fish are protogynous hermaphrodites, with males ranging from 62.8 to 90.6 cm TL and females from 53.2 to 88.9 cm TL. Average sex ratio on the aggregation site is 1.5:1, male to female. Gonosomatic indices double over the 10 days after the full moon each month, and from day six to 12, during the afternoon hours, grouper move approximately 100m west of the reef to the shelf edge. There, groups of 50 to 600 fish ascend and descend slowly and cyclically up to 15m above the bottom, exhibiting distinct reproductive color phases and behaviors. These rises culminate in multiple spawning rushes of groups of 5 to 15 fish within the larger aggregation, all fish in a dramatic dark body/white head color phase. The daily commencement of spawning rushes appears dependent on ocean current. Rushes have been seen up to 30 minutes past sunset, when low light made further observation no longer possible.

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