Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

Abstract

<em>Abstract.</em>—The use of a summer nursery ground by the blacktip shark <em>Carcharhinus limbatus </em>was examined to define the period of residency within the nursery and potential cues for emigration from the area. Newborn sharks were fitted with acoustic tags in each of four consecutive years and continuously monitored using an array of acoustic monitors. The duration of residency in the summer nursery was different between years. Individuals were born at the same time each year, but the last animals left between October and late November. Male sharks left the summer nursery on average a month earlier than females. It is unclear why this difference occurred. Two physical factors—day length and water temperature—were examined to determine if sharks used these cues to time their departure from the nursery. The day length at which sharks left the nursery was different between years and varied from 10.6 to 11.2 h. The water temperature at which sharks left the nursery was also different between years. However, departures of sharks were closely correlated with rapid drops in water temperature. These drops in temperature were caused by the passage of cold fronts, and resulted in drops of up to 5°C over 2 d. It was concluded that these drops in temperature were the primary cue that juvenile blacktip sharks used to time their emigration from the nursery area. The results of this study provide new insights into the utilization of essential habitat for young sharks and the cues that they use to leave these areas.