Predation Effects on Age-0 Red Snapper Lutjanus campechanus on Artificial Reefs in the Northern Gulf of Mexico
Type of Degreethesis
Fisheries and Allied Aquacultures
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Small artificial reefs were built in July 2008 (n = 20), and 2009 (n = 20), 28 km south of Dauphin Island, Alabama, in the northern Gulf of Mexico. Each reef consisted of a polyethylene pallet (1.22 x 1.02 x 0.14 m), to which 10 concrete half-blocks (each 10 x 20 x 41 cm) and a plastic crate (65 x 35 x 28 cm) were attached with 79 kg cable ties. Also, larger steel cage artificial reefs were built in April 2008 (n =10) and 2009 (n =10, 1.2 x 2.4 x 2.4 m). All artificial reefs were secured to the substrate with a 1.2 m ground anchor. Each year, 10 small reefs were placed 15 m away from the larger cage reefs and 10 were placed 500 m away from the larger cage reefs. Each set of reefs (two small and one large) were placed 1.7 km apart. Reefs were surveyed in August 2008, and August and September 2009. During each survey, two SCUBA divers identified and counted all fish present on the reef and estimated 25 mm length categories. In 2009, all small reefs were videotaped and photographed. In the laboratory, fish in the photographs were identified to species and counted with Image-pro software. A similar procedure was used with a single frame from the Hi8 video. Significantly lower numbers of age-0 red snapper Lutjanus campechanus were detected on small reefs that were near (15 m) the large reefs compared to those that were far (500 m) from the large reefs. In August 2008, the mean ± SD density of age-0 red snapper on the 500 m reefs was 15.9 ± 12.3 m-3 and 0.0 m-3 on the 15 m reefs (ANOVA: F1, 36 = 19.54, P < 0.05), in August 2009 it was 23.6 ± 21.0 m-3 on the 500 m reefs and 0.1 ± 0.2 m-3 on the 15 m reefs (ANOVA: F1, 36 = 10.02, P < 0.05), and in September 2009 it was 77.3 ± 41.2 m-3 on the 500 m reefs and 8.6 ± 13.2 m-3 on the 15 m reefs (ANOVA: F1, 36 = 21.60, P < 0.05). In addition to red snapper, significantly higher densities of rock sea bass Centropristis philadelphica, and pygmy filefish Stephanolepis setifer were observed on the 500 m treatment in both August 2009 (rock sea bass t-test: t18 = 2.04, P = 0.056; pygmy filefish t-test: t18 = 2.24, P < 0.05) and September 2009 (rock sea bass t-test: t18 = 2.81, P < 0.05; pygmy filefish t-test: t 18= 2.23, P < 0.05). Mean species richness was also higher on the 500 m treatment in August 2009 (t-test: t18 = 2.37, P < 0.05). Differences in reef fish community structure were detected using nonparametric multidimensional scaling based on Czekanowski’s similarity index in all three surveys (ANOSIM P < 0.05). The larger reefs attracted larger fish (> 300 mm TL) of several species (e.g., red snapper, gag Mycteroperca microlepis, gray triggerfish Balistes capriscus, and greater amberjack Seriola dumerili). Subsequently, these larger fish probably reduced the recruitment of age-0 red snapper to the small reefs that were close (15 m) to these potential competitors or predators.