ONTOGENETIC AND SITE-RELATED TRENDS IN THE DIET OF THE ATLANTIC SHARPNOSE SHARK RHIZOPRIONODON TERRAENOVAE FROM THE NORTHEAST GULF OF MEXICO

摘要

Atlantic sharpnose shark Rhizoprionodon terraenovae (Richardson, 1836) diet is described from Crooked Island Sound, an embayment of the northeast Gulf of Mexico. Diet was assessed by life-stage and quantified using six indices: percent by number, percent by weight, frequency of occurrence, the index of relative importance (IRI), IRI expressed on a percent basis (%IRI), and %IRI based on prey category (%IRI_(PC)). Young-of-the-year (YOY) sharks (n = 56 examined, 37 non-empty stomachs) fed on a mix of teleosts (mostly clupeids, 44.6 %IRI_(PC)) and invertebrates (combined, 25.1%IRI_(PC)), juveniles (n = 185 examined, 118 non-empty stomachs) on sciaenids (40.7%IRI_(PC)) and clupeids (37.8%IRI_(PC)), and mature sharks (n = 105 examined, 67 non-empty stomachs) on sciaenids (71.4%IRI_(PC)). Differences in diet by site and ontogeny were tested by comparing the diet from Crooked Island Sound with previous published data from areas off St. Vincent Island in Apalachicola Bay, an adjacent estuary. Stomach contents were also used to expand on published prey size-predator size information. Spearman correlation analysis, Pianka's overlap values, null-model simulations, and simple correspondence analysis showed that life stage diet differed within and between sites. Three of four size-selectivity tests showed negative size selection. Absolute prey size and the range in absolute prey size increased with increasing shark size. Atlantic sharpnose shark diet was dominated by prey that were < 40% of shark length; however, 69% of prey items were 21%-40% of shark length while 26% were ≤ 20% of shark length. Variations in diet composition within and between the two sites are likely due to differences in shark size, overall habitat structure, and availability of potential prey species.