THE RISK OF HATCHLING LOSS TO NEARSHORE PREDATORS AT A HIGH- DENSITY LOGGERHEAD NESTING BEACH IN SOUTHEAST FLORIDA. by Kelly R. Stewart A Thesis Submitted to the Faculty of The Charles E. Schmidt College of Science In Partial Fulfillment of the Requirements for the Degree of Master of Science Florida Atlantic University Boca Raton, Florida August 2001 i Copyright by Kelly R. Stewart ii iii ABSTRACT Author: Kelly R. Stewart Title: The risk of hatchling loss to nearshore predators at a high-density loggerhead nesting beach in southeast Florida. Institution: Florida Atlantic University Thesis Advisor: Dr. J. Wyneken Degree: Master of Science Year: 2001 It has been recognized that mortality is high for juvenile stages of long- lived vertebrates such as sea turtles, however few studies have quantified mortality rates. The objective of this study was to assess the relative risk that hatchlings face in their first few minutes in the water, at the commencement of their offshore migration from a natural high-density nesting beach (Juno/Jupiter, FL). I followed 217 hatchlings at night by kayak, as they left the beach and documented the proportion surviving the initial 15 minutes in the water. Of these, 206 survived for an empirical survival rate of 95%. Tarpon were the most common predator observed. This survival rate is much higher than that previously observed at a hatchery (72%); this may be due to temporal and spatial variation in nest location at the natural beach. Juno and Jupiter beaches are therefore highly productive sea turtle rookeries. iv TABLE OF CONTENTS List of Tables…………………………………………………….. vi List of Figures …………………………………………………… vii Introduction …………………………………………………… 1 Materials and Methods ……………………………………… 5 Results ……………………………………………………….. 12 Discussion ……………………………………………………. 17 Appendix 1 ……………………………………………………… 29 Appendix 2 …………………………………………………….. 31 Appendix 3 ……………………………………………………… 35 Literature Cited …………………………………………………. 36 v LIST OF TABLES Table 1. A summary of site descriptions. Bottom topography, distance from the Juno pier, Jupiter Inlet, and abundance of fish species is indicated for each site …………………………………………………………………. 8 Table 2. A summary of predatory fish caught, dates, site type, and stomach contents ………………………………………………………………………. 11 Table 3. A summary of predation events on hatchling loggerhead turtles. Water depth, and the hatchling’s last known compass heading were recorded upon being taken by fish. Hatchlings were taken at reef sites, but when they were taken, they had already crossed the reef and were over sand bottom ………………………………………………………………………. 12 Table 4. Orientation of hatchlings migrating from sites along Juno/Jupiter beaches during the summer of 2000. Sample size (n) for each site is given. R-vector is a measure of dispersion………………………………………….. 13 Table 5. A summary of catch-per-unit-effort (for predatory fish only) for each month of the study. Catch-per-unit-effort is in units of fish per hour ……….. 22 Table 6. Comparison of hatchery sites (Hillsboro hatchery) and natural high-density nesting beach sites (Juno and Jupiter beaches). * The effective density was ~6000 nests/km, however the actual length of the hatchery was only 0.1 km, and represented a nest density an order of magnitude higher than the natural site………………………………………………… 24 vi LIST OF FIGURES Figure 1. Location of the study site in Palm Beach County, Florida, USA. Six sampling sites are indicated by labels at right of the shoreline map of Palm Beach County. Three site-types were identified; sand, reef, and transitional categories were based on bottom substrate ………………………….. 6 Figure 2. A tracing of hatchling tracks as they left the beach and migrated to deeper water. The line at left represents the shoreline, and the terminal point of each hatchling track is the point where they were released from their tether. This particular set of tracks was taken from data that were recorded at a transitional site-type Other site-types had similar track lines. The average heading that hatchlings took on this offshore migration was 77.8°……… 14 Figure 3. Percentage of hatchling turtles surviving the first 15 minutes of offshore migration, at each site-type during the hatchling season of 2000……….. 15 Figure 4. Percentage of hatchling turtles surviving the first 15 minutes of offshore migration, across the hatchling season of 2000…………………………… 16 vii Introduction Sea turtles, like other long-lived vertebrates are iteroparous (Heppell et al. 1999). Females with large body size produce numerous small offspring in several clutches. These characteristics indicate that, over time, evolutionary pressures have selected for a life history strategy in which investment in individual offspring is minimal and survival of young is generally very low. Many authors have recognized that mortality prior to maturation is probably extremely high in sea turtles (Stancyk 1982; Richardson and Richardson 1982; Heppell et al. 1996). Egg loss and nest predation are well documented (Stancyk 1982; Witzell and Banner 1980; Gyuris 1994). Since sea turtles have high fecundity, some laying approximately 200-600 eggs per season (Hirth 1980; Van Buskirk and Crowder 1994; Miller 1997), it is estimated that perhaps one in 10,000 hatchlings will survive to maturity (Frazer 1986). Marine turtles are logistically difficult to study in the ocean and, as a result, very few baseline data exist to describe vital rates of the aquatic stages. While much important information has been gathered on the nesting beach about the reproductive cycles, fecundity, and parameters that impact nest success of sea turtles, disproportionately little is known about life in the water. Life for hatchling sea turtles is inherently risky. Besides being very small, hatchlings have few defense mechanisms against predators. Some of the threats to reaching adulthood include predation during the earliest life stages, 1 e.g. as hatchlings crawling from the nest to the ocean, swimming in nearshore waters, and as pelagic-stage juveniles. Other sources of mortality include: incidental capture in fishing gear; ingestion of foreign materials such as tar and plastics; collisions with boats; and in some places, harvesting of eggs, subadults and adults (Lutcavage et al. 1997). Baseline population sizes (e.g. hatchling, juvenile, adult), recruitment levels, and mortality/survival values are described by estimates or arithmetically derived guesses and limited empirical data (Crouse et al. 1987; Heppell 1998; Heppell et al. 1999). In addition, very few studies have attempted to quantify and partition mortality at various life stages. To date, no demographic baselines have been established for the pelagic stage. The pelagic stage of loggerheads may last for 6.5-11.5 years (Bjorndal et al. 2000), and begins when hatchlings enter the water after having traversed the beach from the nest. Hatchlings typically emerge at night. Along Florida’s east coast, most hatchlings enter the water between the hours of 1930 and 0630, with peak emergences occurring from 2300 - 0000 h (Witherington et al. 1990). Several investigators suggested that this pattern of nocturnal hatchling emergence is both a predator avoidance mechanism as well as a behavior reflecting the hatchlings’ limits in thermal tolerance (Mrosovsky 1968; Lohmann et al. 1997). Hatchlings usually emerge en masse, orientate seaward, crawl vigorously down the beach and then proceed offshore, distancing themselves from the beach and its perils. Offshore 2 migration (after Dingle 1996) is characterized by hyperactive, oriented swimming (frenzy), which lasts about 24 hours (Salmon and Wyneken 1987), followed by less vigorous oriented swimming (post-frenzy) until turtles reach cover provided by flotsam such as Sargassum (Witham 1980; Carr 1986). The frenzy is believed to rapidly distance hatchlings from predator-rich nearshore waters (Salmon and Wyneken 1987). In one study of hatchling survival focusing on the early pelagic stage, Gyuris (1994) estimated that most of the first year mortality of green turtles could be attributed to aquatic predation within the first hour after entering the ocean off Australian beaches at the Great Barrier Reef. Gyuris (1994) followed 1740 hatchlings offshore over 3 seasons and found that predation rates in the water ranged between 0-85%, with a mean predation rate of 31%. Witherington and Salmon (1992) found that aquatic predators took 6.8% (5 of 74) of loggerhead hatchlings during daytime and nighttime swimming trials in turbid waters off the east-central coast of Florida. Most other studies simply reported predation or documented predators. Caldwell (1959), Witham (1974) and Fletemeyer (1978) reported seeing predation on loggerhead and green turtle hatchlings in Florida waters. Booth and Peters (1972) documented green turtle hatchlings in Australian waters being taken by crabs, black-tipped sharks and other species of fish. None of these studies attempted a quantification of the hatchling survival rate in the water. 3 It is well documented that coastal land use in Florida has degraded nesting habitats in many areas, and may be responsible for the spatial distribution of turtle nesting beaches that we now see (Salmon et al. 2000). For example in southeast Florida, habitat alteration has perhaps artificially concentrated nesting in areas that are physically most acceptable to nesting sea turtles – those beaches adjacent to appropriate oceanographic features (currents for dispersal, sufficient water temperature, etc.), having sufficient sand, being relatively