Date Awarded

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Roger Mann

Abstract

Aspects of the ecology of the cownose ray, Rhinoptera bonasus, in the lower Chesapeake Bay and its tributaries were studied using aerial surveys, biotelemetry, and examination of stomach contents. The Chesapeake Bay was surveyed by airplane during 1986-1989, to examine the distribution of cownose rays and estimate their abundance using line transect methods. Cownose rays resided in Chesapeake Bay throughout the summer months, entering in early June and emigrating in late September. They were usually absent, or nearly so, by late October. Mean monthly abundance ranged from none present in May and November, to a September average of 9.3 &\cdot& 10&\sp6& rays. Maximum estimated abundance was 3.8 &\cdot& 10&\sp7& cownose rays in September 1988, a year when cownose rays formed an exceptionally large pre-migratory school. Sonic and radio-frequency transmitters were attached to free-swimming cownose rays which were followed for periods ranging from 6-13 h to examine swimming behavior. Six adult cownose rays were tracked and all except one showed directed movement which differed significantly from random circular movement. All but one of those swam in the direction of the tidal current. These results were in concurrence with theory suggesting that negatively buoyant fishes should benefit by using tidal stream transport to minimize energy expenditure. Analysis of stomach contents to determine prey species was not possible because the bivalve prey could not be identified to species from the small shell fragments present. For this reason, an index of relative importance could not be calculated. The molluscan families Mytelidae (mussels) and Solenidae or Psammobiidae (razor clams) predominated in the stomachs and spiral valves of trawl-caught specimens from the Chesapeake Bay eastern shore. Gastrointestinal tracts from pound net-caught specimens were generally empty, and the one specimen harpooned in the York River contained a few shell fragments from hard clam. Assuming a daily ration of 3% of its body weight, an average weight of 7.13 kg, and using the mean September abundance estimate, it was estimated that in Chesapeake Bay cownose rays could consume an average of 1.95 &\cdot& 10&\sp3& metric tons of biomass daily.

DOI

https://dx.doi.org/doi:10.25773/v5-3by6-gq28

Rights

© The Author

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