Abstract

Quantifying fish abundance at spawning aggregation sites is critical for assessing stock status, but methods must overcome limitations posed by weather and labor-intensive diver-based surveys. This study attempts to quantify red hind (Epinephelus guttatus) utilizing hydroacoustic techniques. Here, we summarize the preliminary results for hydroacoustic surveys conducted at spawning aggregations at Mona Island (MI) and Abrir La Sierra (ALS), PR during January 7 and February 3 - 4, 2010, where we targeted fish within 1 m off the bottom only. A total of 14.519 km at MI and 3.465 km at ALS of hydroacoustic transects were collected. Fish in the 30 - 35 cm size class and larger were absent during January, but their abundance increased during February at both MI and ALS. This suggests the arrival of larger fish, which correlates with the data obtained by divers, except that the hydroacoustic data show an influx of fish sizes > 45 cm whereas divers observed an increase in fish below this size. The proportion of fish in the size classes 35 - 40 and 40 - 45 cm observed by divers seems to be skewed right when compared to the hydroacoustic data. The number of fish (n) per hydroacoustic survey is very similar for the month of February both for MI and ALS (n = 92, 114, 98, and 132, respectively). At ALS, fish target strengths (TS) values that coincide with calibration TS values for E. guttatus were detected within 1 m off the bottom, showing that the hydroacoustic system used can detect TS of fish close to the bottom. Fish density estimates of red hind at Mona Island were made for “IN” versus “OUT” the known spawning aggregation area. These data also show that spawning aggregation sites for red hind can be detected with active hydroacoustic methods within 1 m off the bottom.

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