Abstract

The Nassau grouper was an economically and ecologically important species of Caribbean coral reefs but is now listed as Endangered by the IUCN. In Puerto Rico (PR) and the US Virgin Islands (USVI) stocks have declined dramatically and several spawning aggregations were fished to extinction. However, two sites within the US EEZ have recently been found with Nassau grouper aggregating during the main reproductive season, the Grammanik Bank (GB) in the USVI and Bajo de Sico (BDS) off western PR. For the 2012 - 2013 spawning season, a joint study was conducted for both sites to characterize and assess these spawning aggregations simultaneously. Underwater visual census and passive acoustic monitoring techniques were used to estimate spawning population stock, size distributions, and spawning periodicity to compare the characteristics of the Nassau grouper at these spawning aggregations. At BDS data were also collected with ultrasonic acoustic transmitters implanted in 10 Nassau grouper at depth and a grid of passive acoustic recorders was deployed around the aggregation site and the shallow bank areas. Preliminary results documented the maximum number of Nassau grouper observed at BDS was approximately 100 individuals, roughly half of that estimated at GB. Passive acoustic data provided high-resolution data to show slightly different temporal patterns between sites, where BDS had two major monthly peaks in reproductive activity followed by a third minor one, and at GB it was one minor peak followed by two larger ones. Nassau grouper with transmitters at BDS provide confirmation of the presence patterns detected with passive acoustics with remarkable resolution. The variability of reproductive timing of Nassau grouper at these two spawning aggregation sites suggest short seasonal fishing closures may not be enough to recover this endangered species.

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