Theses and Dissertations

Defense Date

2006

Document Type

Thesis

Degree Name

M.S. Marine Biology

Department

Oceanographic Center

First Advisor

Mahmood Shivji

Second Advisor

Richard E. Spieler

Third Advisor

Bradley Wetherbee

Abstract

There are currently over 300 sites in nearly 40 countries where a variety of marine animals are provided supplemental food by humans. The influence of this supplemental feeding on the behavior, physiology, growth, reproduction and movements of the animals involved is seldom known. Intentional supplemental feeding of the southern stingray, Dasyatis americana, has occurred at Stingray City (SC) and Stingray City Sandbar (SCS) at Grand Cayman since 1986. There are no specific regulations governing the feeding of D. americana at Grand Cayman, and neither the species nor the feeding sites are afforded any official protective status. This study investigated how supplemental feeding influences the movement patterns of D. americana at Grand Cayman, including activity spaces, rates of movement, site fidelity and diel patterns. This research is the first detailed investigation into the influence of supplemental feeding on the movement patterns of a marine animal. The objectives of this study were to investigate and compare the movement patterns of D. americana at supplemental feeding sites and non-feeding ‘wild’ control sites. Passive Integrative Transponder (PIT) tags were implanted in 327 stingrays, 183 of which were recaptured; 100% of recaptured stingrays retained their tags over the duration of the study, based on tissue sample scarring. External tags were attached to 35 stingrays. Tagging data indicate that a spatially isolated community of approximately 160 D. americana utilize SCS. Seven wild and seven provisioned stingrays were tracked manually from five to 72 h, and five mature females at SCS were tracked automatically using an array of two bottom monitors. Provisioned female stingrays at SCS utilized significantly smaller 24 h activity spaces (0.132±0.079 km2) than wild female stingrays (0.876±0.171km2). Both groups utilized significantly larger activity spaces at night than during the day. However, there was a marked difference in the diel activity levels between provisioned and wild stingrays: provisioned stingrays were active over a small area during daytime supplemental feeding, whereas wild stingrays were more active and foraged during the night (nocturnal). Average rates of movement did not significantly differ between the two groups. Tidal phase had no effect on activity space size or rate of movement for either group. The core areas of provisioned stingrays showed significantly more overlap than those of wild stingrays, indicating that supplemental feeding has disrupted the spatial distribution of the community at SCS and increased the local density of D. americana to atypical levels. Provisioned female stingrays consistently frequented SCS during periods of supplemental feeding and exhibited long term (at least up to one year) site fidelity to this site. These findings suggest that provisioned stingrays are highly conditioned to the supplemental food resources provided at SCS. Provisioned stingrays exhibited optimal foraging and have reduced and centralized their core areas and activity spaces at SCS in order to maximize their accrual of food resources. The availability of food resources is a significant factor regulating the size and location of core areas and activity spaces, population density and the diel activities (i.e. the spatial and temporal distribution) of D. americana at Grand Cayman.

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