Theses and Dissertations

Title: The Feeding Habits of Gray Triggerfish, Balistes capriscus (Gemlin), From the Northeast Gulf of Mexico

Name: Blitch, K.M.

Degree: MS

Chair: Stephen T. Szedlmayer


University: Auburn University

Location: Auburn, Alabama

Date: 2000

Pages: 85

Keywords: gray triggerfish,artificial reef,diet,Gulf of Mexico


The size distribution and feeding habits of gray triggerfish, Balistes capriscus, were investigated for fish collected from artificial reefs deployed off coastal Alabama. Artificial reefs were sampled by two SCUBA divers using a drop net. Fish were measured, weighed, and the contents of the entire gastrointestinal tract removed for study. Percent composition by weight, number, frequency of occurrence, and the percent composition of an index of relative importance (%IRI) were used to describe the diet of gray triggerfish. Differences in the diet of gray triggerfish were also investigated among artificial reefs of different ages (3 month, 14-19 months, 29-31months), and fish of different sizes Three prey levels were examined based on taxonomic definitions (broad - above family and, narrow - family and genera) and prey habitat type. For the broad prey level, pelagic mollusks, crustacean larvae and decapod crustaceans were the dominant prey. Narrow prey level analysis (family and genera) identified important genera in the diet of gray triggerfish. Portunus, Hepatus, and Pinnixia, were the most common decapods consumed. Cavolina, Crepidula, and Atrina were important gastropods, and Balanus was the top barnacle genus. Ophiolepis and Astropecten represented the identifiable genera of echinoderms. At all prey levels, different reef ages, and fish sizes, pelagic animals were important in the diets of gray triggerfish. The primary pelagic prey items were crustacean larvae (mostly brachyuran crab zoea), cavolines, and pteropods (referred to as 'pelagic mollusks', along with squid, etc.). Decapod crustaceans, 'miscellaneous' species (primarily encrusting organisms), and 'other crustaceans' (amphipods, stomatopods, etc.) were also important diet items. In habitat comparisons, mixed habitat prey species, reef-associated species, and pelagic prey were all important in the diets of gray triggerfish. Gray triggerfish consumed more echinoderms and fish as reef age increased, based on chi-square and ANOVA analyses (P<0.05). Analysis on numeric data showed that gray triggerfish consumed more gastropods and bivalves and less 'other crustaceans' on 29-31 month old reefs compared to 3 month old reefs (X2: P <0.05). Analysis of prey weight in the diet showed increased feeding on decapod crustaceans and 'miscellaneous' prey on older reefs (ANOVA and Tukey: P<0.05). Additionally, gray triggerfish were found to feed more on reef -associated and mixed habitat species as reef age increased (ANOVA and Tukey: P<0.05). However, an expected decrease in feeding on sand-associated species did not occur (as has been suggested by other authors). In fact, gray triggerfish showed less feeding on sand-associated prey from the 3 month reefs compared to the 29-31 month old reefs. The dietary overlap index (Simplified Morisita) on percent IRI values further supported differences in gray triggerfish diets among reefs of varying age, and also identified 3 month old reefs as the cause (taxonomic prey level). However, a high level of dietary overlap (0.90 or greater) occurred when prey habitats types were compared. This was due to the fact that the 'mixed' and 'pelagic' prey types accounted for at least 80% of the total % IRI values for all reef ages. Lengths of gray triggerfish collected from artificial reefs were found to vary by season and by reef age. However, gray triggerfish showed few shifts in diet with increasing size. Diets of smaller fish (90-110 mm SL) showed higher %IRI for the echinoderm and fish taxon, while larger fish (>195 mm SL) showed greater %IRI for decapod crustaceans. Generally, the smallest size class differed from the largest for the overlap index. In conclusion, gray triggerfish diets in the northeast Gulf of Mexico comprised a variety of organisms from different habitats. Pelagic prey were consumed in high numbers, while reef-associated prey, decapod crustaceans, and encrusting organisms accounted for large percentages by weight. Independent of reef age or prey level analyzed, reef-associated prey were important diet components, suggesting that artificial reefs may be productive environments for the gray triggerfish.

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