Theses and Dissertations

Defense Date

11-2-2009

Document Type

Thesis

Degree Name

M.S. Marine Biology

Department

Oceanographic Center

First Advisor

David W. Kerstetter

Second Advisor

Amy Hirons

Third Advisor

John F. Walter III

Abstract

A diet study of swordfish, Xiphias gladius, was conducted in the Straits of Florida from April 2007 to December 2008. The stomachs of 131 swordfish were analyzed. Thirteen species of teleosts, three species of cephalopods, and one species of crustacean were observed in the diet. Cephalopods dominated the swordfish diet by weight (73.38%), number (69.90%), and occurrence (80.91%) and ranked highest in importance in the diet when calculating the index of relative importance (IRI). Teleosts followed by weight (25.16%), number (26.34%), occurrence (68.18%), and IRI (3,510.97). The prey species with the greatest dietary importance was Illex sp followed by unidentifiable ommastrephids. Stomach fullness index (SFI) was calculated ranging from 0 (empty stomachs) to 3.57. Additionally, correlations for juvenile male swordfish length and prey length (r²=0.647) as well as female adult swordfish weight and prey weight (r²=-0.327) were found to be significant. Overall feeding seasonality showed that swordfish primarily fed on teleosts (58%) in the spring, cephalopods (70%) in the summer, and cephalopods (75%) in the winter. When considered individually, female, male, and adult swordfish followed this pattern; however juveniles did not follow the overall feeding trend according to season. They fed regularly on cephalopods during spring (57%) and summer (61%) consuming the highest abundance of cephalopods during the winter (85%). Overall, swordfish appeared to be opportunistic feeders altering its food choices when abundance of prey may have changed or when food items were present.

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