Salmon, Michael

Hatching rhythms of intertidal crabs vary in timing with species, burrow position, tidal regimes and larval vulnerability. Most fiddler crabs time larval release with the nocturnal high fide. This is possible in semidiurnal tidal regimes because there are nocturnal high tides each lunar day. However, mixed tidal regimes have a semidiurnal and a diurnal period. During the diurnal phase high fide occurs in the afternoon. This variation in tidal inundation patterns may influence the timing of larval release. Comparisons between two populations of fiddler crab (Uca thayeri ), exposed to different tidal regimes, indicated they time larval release differently. The population exposed to semidiurnal tides timed larval release with the nocturnal high tide prior to the maximum amplitude tide. The population exposed to mixed tides timed larval release with the diurnal maximum amplitude tide. The differences between the populations can be related to their ecology but other variables may contribute.

We understand very little about the relationships between eye anatomy and visual ecology in sea turtles. Sea turtles use visual information in important contexts, such as selecting habitats, detecting predators, or locating mates or food. This study represents an effort to clarify the form/function relationship between retinal morphology and the behavioral ecology of sea turtle hatchlings. Thus, it is an important first step in relating sea turtle eye anatomy with visual ecology and relating the two to sea turtle natural history. Some organisms possess retinas that contain morphologically specialized cellular areas. The "visual streak," is one such area; receptor cells and associated interneurons are concentrated in a horizontal band in the retina. Three species of sea turtles (Chelonia mydas, Caretta caretta, and Dermochelys coriacea) possess a visual streak located along the horizontal mid-line of the retina, although they differed in streak development. The differences in streak development can be related to their ecology.

The beaches of Boca Raton, Florida serve as a rookery site for three species of sea turtles, all of whom are considered either federally threatened (loggerhead) or endangered (green turtle, leatherback). At this beach, nest security was though to be compromised by both human visitors and increasing mammalian predators populations. Since 1988, the City has employed the use of square wire cages to protect the nests from both factors. Cage effectiveness was questioned due to high predation rates, despite caging. Evaluations to determine the effectiveness of this method were conducted. Pairs of caged (control) and uncaged (experimental) nests were established in zones reflecting high/low traffic and high/low predator attacks and monitored on a daily basis. Following emergence, nest fate comparisons were made. Decoy cages were used to determine predator efficiency. Data suggest that cages are not needed in areas of low traffic or predator attacks. Additional data suggest that cages actually attract, rather than deter, mammalian predators.

Caging of sea turtle nests has used by City of Boca Raton's sea turtle conservation program to deter terrestrial predators and reduce human disturbance. The juxtaposition of nest cages and artificial lighting may pose serious threats to hatchlings. I addressed possible negative effects of cages on sea turtle hatchlings' seafinding abilities and potential "cage-trapping" under natural and artificially illuminated conditions. Cages did not effect hatchling orientation on artificially illuminated or dark beaches. Hatchlings did not experience any delay in cage escape on dark beaches, but a significant number were trapped inside the cages on artificially illuminated beaches. The highest incidence of trapping occurred on nights surrounding a new moon. Disoriented hatchlings exited cages either after city lighting was reduced (past midnight) or as natural levels of illumination increased shortly before sunrise. When cages were darkened all turtles escaped but many still exhibited signs of disorientation.

Jupiter Island is a barrier Island on the central East Coast of Florida whose beaches are subject to severe erosion. At intervals of several years, lost sand is replaced by the addition of sand ("renourishment") from other locations. In this study, I determined the effect of sand replacement on sea turtle nesting activity, and on the survival of nests placed on renourished and an adjacent natural beach. Renourishment caused a reduction in nesting activity by the turtles, which lasted about two years. Thereafter, turtle nesting on renourished and control beaches was similar. There were no differences in nest survival between the two sites. Renourishment prevents the loss of beach and shoreline property, but is not necessarily beneficial to sea turtles.

Hatchling loggerhead sea turtles emerge from their nests on oceanic beaches, crawl to the surf zone, and swim out to sea. How do turtles maintain oriented headings once they lose contact with land? I tested the hypothesis that by swimming into surface waves hatchlings establish an offshore heading (directional preference), and that once out to sea this heading is transferred to, and maintained by, a magnetic compass. This hypothesis was supported by laboratory and field experiments, described herein. A directional preference can also be established by oriented crawling (from the nest to the surf zone). Thus hatchlings possess two mechanisms (crawling and swimming) for the establishment of an offshore heading. The use of these alternative mechanisms probably assures that turtles escape from shore under the broad range of conditions which they naturally encounter after emerging from their nests.

Hatchling sea turtles emerge at night from underground nests, crawl to the ocean, and swim out to sea. In this study, I determined how offshore orientation and shallow-water predation rates varied under natural (sand bottom and patch reef) and modified (submerged breakwater and open-beach hatchery) ecological circumstances. Hatchling offshore orientation in the sea was normal under all conditions; there were no significant differences in either scatter or direction among groups. However, predators (tarpon, snapper, barracuda, jacks, and grouper) took more hatchlings as they swam over submerged reefs, and after they entered the water in front of hatcheries. Predators were concentrated at both of these sites probably because prey (small fishes and invertebrates at patch reefs and turtles entering the water where nests were concentrated in hatcheries) occur in greater abundance.

The hearing abilities of two adult manatees were tested using a forced two-choice paradigm and an up/down staircase psychometric technique. Prior to this research, no hearing tests or audiograms had been measured for any Sirenian species. This test was also the first controlled underwater infrasonic hearing test conducted on any marine mammal. The audiogram demonstrated a wider range of hearing (0.5 to 38 kHz) and greater sensitivity (50 to 102 dB re:1muPa) throughout this range than had previously been suggested by averaged evoked potential and anatomical studies. The audiogram was a typical U-shaped mammalian hearing curve and was compared with other marine mammal underwater audiograms. Both manatees were most sensitive to frequencies between 12-18 kHz at 50-54 dB. The manatees' hearing sensitivity dropped more than 20 dB per octave below 1600 Hz. This higher frequency sensitivity may have evolved as an adaptation to a shallow water existence where low frequency sound propagation is limited. Limited hearing sensitivity at lower frequencies may be a contributing factor to the manatees' vulnerability to boat collisions.

Recent studies have shown that hatchling loggerhead sea turtles possess the ability to orient to the earth's magnetic field. These experiments did not identify the specific component of the field used by turtles to determine direction. One of the field's most important characteristics, inclination, has been implicated as the specific property used by birds to orient. This study investigated the possibility that sea turtles use the inclination of the earth's field in a similar manner. Results show that turtles determine direction with the use of an inclination compass similar to the one used by birds to orient. This study has important implications regarding the mechanisms used by animals to orient and navigate.

The fate of loggerhead (Caretta caretta, L.) clutches at four physically different beaches, including a renourished site, was investigated in the Boca Raton area. I compared the four sites with regard to their thermal environment, moisture content, sand grain size, and pore spacing at depths of a typical nest. Significantly more sea turtle hatchlings emerged from nests deposited on the renourished beach than at the other sites. At the renourished beach moisture content and pore spacing were highest. Pore spacing and moisture content correlated negatively with the number of hatchlings which died in the nest, and correlated positively with emergence success. At the natural beach, hatchling weight was lowest and straight-line carapace length (SLC) was shortest. I conclude that at the renourished beach, conditions favored hatchling production. However, that may not be the case in all instances of renourishment, as effects probably vary with the nature of the sands used to replenish the beach.