by Ashley Hill,
Marine conservation student
Open ocean habitats are innately difficult to access. As a result, the majority of research on sea turtles is restricted to beach and coastal areas. However, there is a time span of several years from when hatchlings venture offshore to when the larger, juvenile turtles return to coastal waters. It is thought individuals of this life stage must live in the open ocean, but the lack of concrete, direct evidence has led to the term “the lost years” (Carr et al. 1978). The majority of the open ocean is desert like, with vast areas of minimal amounts of food or shelter. Oceanic processes push water together to form areas of convergence. These areas typically contain higher levels of plankton and therefore a higher abundance of other organisms that take advantage of the increased food source. In the Atlantic, Caribbean and Gulf of Mexico, convergence areas are often traced by lines of a branching, floating alga called Sargassum (Thiel and Gutow 2005, Butler et al 1983). Each individual clump of Sargassum is less than 80cm, but mats spanning hundreds of meters wide and tens of thousands of meters long can be formed in convergence areas (Butler et al 1983). In a way, these Sargassum drift communities can provide an oasis of nourishment and shelter for an assortment of organisms, including sea turtles.
Witherington et al. began collecting data in 1992 off the east coast of Florida between Port Canaveral and Fort Pierce in order to determine the importance of Sargassum drift communities to juvenile sea turtles. In 2005, the survey area was expanded to also include the west coast and panhandle of the state. Transects were conducted by boat within and outside Sargassum communities by heading perpendicular from the coast until they reached a line of Sargassum, at which point they changed course to run through the drift material. During transects, they recorded the behavior of 1884 juvenile sea turtle and their association with Sargassum. Dive behavior of four of the juvenile turtles was tracked with acoustic tags that logged time, temperature and depth. Diet items were collected from forty-two juvenile turtles by flushing the mouth for recently ingested items and fecal collections. Associations between observed turtles and remotely sensed data were also analyzed. This data included changes in sea surface temperature or amount of chlorophyll a, which can be used to indicate convergence zones. Finally, models were used to quantify the relative densities of sea turtles within and outside Sargassum communities.
The results of this study showed the majority (89%) of turtles were within 1 meter of Sargassum and were 100 times denser in Sargassum patches than the open ocean. The majority of observed turtles were also within 1 meter of the surface. Therefore, the researchers proposed the term “surface-pelagic” to describe this life stage, meaning they are within the upper water column of the open ocean. Sea turtles in the Atlantic were often observed near boundaries associated with the western wall of the Gulf Stream. While in the Gulf of Mexico, a relationship between sea turtles and sea surface temperature or chlorophyll a boundaries was not as consistent. Diet composition consisted mostly of species associated with Sargassum. However, 13% of the dry weight of all diet items was plastic material and 67% of all turtles had consumed synthetic material.
Current sea turtle conservation efforts have primarily focused on beach and coastal areas. However, Witherington et al (2012) have now provided strong evidence that “the lost year” turtles are highly associated with open water Sargassum drift communities. These habitats are extremely important for young sea turtles but are often polluted by plastics and oil, which are easily trapped within Sargassum. Every year, there is an estimated 20 million tons of plastic that finds its way into our oceans (Vannela 2012). Including sea turtles, 267 species have been documented as being affected by plastic (Laist 1997). During data collection, Witherington et al. (2012) reported observing tar balls, oiled plastics and liquid oil within the Sargassum. In addition, the National Oceanic and Atmospheric Administration (NOAA) reported that almost 500 surface-pelagic juvenile turtles were collected from oiled Sargassum following the BP Deepwater Horizon Oil Spill (NOAA 2012). If pollution is causing high rates of mortality in the crucial habitat for the surface-pelagic life stage, an increase in hatchlings won’t necessarily translate to an increase in the adult breeding population. The surface-pelagic life stage and drift communities are currently the “weak link” in overall sea turtle conservation. Conservation plans must incorporate protection for each of the various habitats and life stages to create the greatest chance to restore sea turtle population sizes to historic numbers. Finally, if we are to be successful in reducing pollution of the Sargassum habitat and our oceans in general, the “thinking globally and acting locally” attitude (Derraik 2002) must be adopted by legislators, the scientific community and the general public.
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Witherington B, Hirama S, Hardy R (2012) Young sea turtles of the pelagic Sargassum-dominated drift community: habitat use, population density, and threats. Mar Ecol Prog Ser 463:1-22. 10.3354/meps09970