Graduation Year

2005

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Biology

Major Professor

Philip J. Motta, Ph.D.

Committee Member

Robert E. Hueter, Ph.D.

Committee Member

Michelle R. Heupel, Ph.D.

Committee Member

Susan S. Bell, Ph.D.

Keywords

Batoid, Myliobatiformes, Feeding ecology, Charlotte harbor, Index of relative importance, Acoustic telemetry, Passive tracking, Home range

Abstract

Cownose rays are benthic, suction feeders whose foraging activities have been implicated in severe damage to commercial shellfish industries and seagrass habitat. With jaws highly modified for durophagy, it has been assumed that they are crushing specialists, feeding primarily upon hard molluscan prey. In addition, R. bonasus are believed to be highly migratory, transient residents of coastal inshore waters. However, minimal quantitative data exist regarding R. bonasus feeding or movement patterns in the Gulf of Mexico. Stomach contents from 50 cownose rays caught within the Charlotte Harbor estuary between July 2003 and July 2004 were analyzed using the index of relative importance (IRI). Crustaceans, polychaetes, and bivalves were the dominant groups present, with bivalves representing the smallest proportion of the three dominant groups. High dietary overlap was observed between sexes, size groups and seasons. Shoalmates exhibited significantly more similar diets to each other than to members of other shoals. Although currently believed to be a hard prey specialist, these results suggest the cownose ray may behave as an opportunistic generalist, consuming any readily available prey. Between July 2003 and November 2004, 21 cownose rays were tagged and tracked within Charlotte Harbor using passive acoustic telemetry. Residence time ranged between 1-102 days. No significant relationship was detected between activity patterns and tidal stage or time of day. Minimum convex polygons (MCP) and kernel utilization distributions (KUD) were calculated to demonstrate the extent of an animalís home range and core areas of use. Daily MCPs ranged between 0.01 and 25.8 km2 , and total MCPs ranged between 0.81 and 71.78 km2 . Total 95% KUDs ranged between 0.18 and 62.44 km2 , while total 50% KUDs were significantly smaller, ranging from 0.09 to 9.68 km2 . Both MCPs and KUDs exhibited a significant positive relationship with residence time and with disc width. As mobile, pelagic swimmers capable of traversing large distances, these data also demonstrated that cownose rays may remain within relatively small areas for extended periods. These diet and tracking results provide insight to R. bonasus use of a south Florida estuary and allow predictions regarding the impact of this species in similar environments.

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