Theses and Dissertations

Defense Date

2004

Document Type

Thesis

Degree Name

M.S. Marine Biology

Department

Oceanographic Center

First Advisor

Richard E. Spieler

Second Advisor

Mahmood S. Shivji

Third Advisor

David S. Gilliam

Abstract

The yellow stingray, Urobatis jamaicensis is the most common elasmobranch in the coastal waters of Southeast Florida. Despite their common occurrence the ecology of yellow stingrays remains poorly understood. In particular, yellow stingray daily movements, space utilization, seasonal distribution and population structure have not been described. This study was conducted to address the lack of knowledge of these fundamental life history parameters and to provide further information on the ecology of U. jamaicensis in coastal waters of Broward County, Florida.

The activity patterns and space utilization of U. jamaicensis were assessed by manual tracking with ultrasonic telemetry. Telemetry tracking of 17 stingrays was conducted from January 1998 to September 2001 with data presented on eight individuals tracked for a full diel cycle (24 h). Tracking data was analyzed with the Animal Movement Analysis Extension (AMAE) in Arcview® GIS to provide graphical representation of observed movements within the complex series of reef terraces and hardbottom communities of Broward County. Bottom topography had considerable influence on the space utilization of stingrays and observed movements varied with location in relation to proximity from the reef edge/sand interface. Movement was intermittent throughout the day, but displayed a highly significant increase during the nocturnal and crepuscular phases in comparison to diurnal movements. Nearly all stingrays demonstrated confined movements and indicated strong site fixity, which may imply the existence of home ranging behavior. The 95% (total 24h activity space) and the 50% (core area) Kernel Utilization Distributions (KUD) were constructed to visually display the shape and size of activity spaces. The data was pooled together for the eight individuals tracked for a full diel cycle and divided into four 6-h shifts. Statistically significant larger activity spaces for both the 95% KUD and the 50% KUD were observed during the nocturnal activity phase.

Seasonal distribution was assessed to determine animal residency within the study site and ascertain the occurrence and temporal patterns of onshore/offshore movements. Stationary visual fish census techniques (point counts) from several studies conducted in Broward County from January 1998 to December 2003 were combined to determine the level of abundance across three reef tracts, throughout the entire length of the county. Data was tested for monthly and seasonal differences and for variation between reefs. Analysis of seasonal distribution established population residency is year-round with no indication of offshore emigration associated with a temperature preference.

Population structure analyses were conducted to determine the sex ratio and size distribution of U. jamaicensis to examine any potential gender segregation or ontogenetic partitioning. The sex ratio was compared for differences monthly, seasonally and between reefs for expected vs. observed frequencies. Only spring observations (March, April, May) evidenced a statistically significant difference from a 1:1 ratio, where females dominated the inshore observations 20F:8M. Average size of both genders was 333mm TL, however, females dominated the larger size classes (>350mm TL). Few neonates were observed during this study with most observations occurring in shallow inshore water (depth), suggesting a nearshore nursery. Increased abundance and presence on the offshore reef among intermediate size classes (250-299mm to 300-349mm) suggests a potential ontogenetic shift to deeper water. Observations on the seasonal patterns of the reproductive condition of female yellow stingrays are also provided.

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