Warming ocean temperatures due to climate change are already having an observable impact on ecological processes. It is important to understand the altered distribution of species and their habitat preferences as a reaction to warming waters. Sea turtles are already impacted significantly by human activities, so how are increased oceanic temperatures impacting loggerhead turtles?
Loggerhead turtles (Caretta caretta) are named for their large heads and powerful jaws, which they use to feed on hard-shelled prey, like whelks and conch. This species is the most abundant of all turtle species in the waters of the United States. Juvenile loggerheads live in coastal U.S. waters, and many adult loggerheads use the U.S. beaches to nest migrated from neighboring countries, like Bahamas, Cuba and Mexico. These turtles are however found worldwide in subtropical and temperate regions of the Atlantic, Pacific, Indian Ocean, as well as the Mediterranean Sea. The loggerhead turtle is classified as Vulnerable on the IUCN Red List.
Threats to Loggerhead Turtles
Like other turtle species, loggerhead turtles are threatened by human activities. A primary threat to sea turtles is their unintended capture in fishing gear, known as “bycatch”. When entangled in lines or nets, or caught on a hook, the turtles can drown, or it can lead to injuries and death. Bycatch in the trawling, longline and gillnet industry remains the primary threat to loggerhead turtle populations.
Coastal development coupled with rising sea levels due to climate change is resulting in a loss of nesting beach habitat for loggerheads. Construction along the shoreline, such as seawalls, can result in a complete loss of dry sand used by the turtles for nesting. A seashore that is brightly lit can also deter females from coming on shore to nest, and may disorient turtle hatchlings trying to find the sea when they leave their nests.
Historically, loggerhead turtles were killed for their meat and leather, and although this practice was stopped around the 1970s, poachers still illegally harvest the eggs for consumption.
All sea turtles are also threatened by a warming climate, as it changes the morphology of beaches, resulting in higher nest temperatures which can be lethal to eggs, as well as altering the ratio of male and female hatchlings. Climate change is resulting in increased storm events, which may cause beach erosion or flooding, damaging nests. Changes in ocean temperatures are also impacting the distribution of food resources, which is causing a shift in the foraging range of loggerhead turtles.
Shifts in Loggerhead Habitat
To allow for proper conservation management, it is important to understand the distribution and habitat preferences of turtles. Loggerhead turtles are particularly susceptible to climate and ecosystem changes, due to their sensitivity to temperature in their environment. Most studies on the impact of climate change on turtles have documented the impacts on reproductive biology, as well as hatchling sex ratios in a warming climate. A new study published in Nature has now investigated the projected shifts in loggerhead sea turtle habitat as a result of warming waters.
In marine biology, species distribution modeling has been limited by the availability of species occurrence data and environmental data. Although satellite telemetry (using satellite tags) has been used to monitor marine animals over 35 years, it can be expensive to monitor a single population over many consecutive years. As a result, the relationship between sea turtle population distribution and changes in their marine environment has only been studied on small and short turn telemetry studies, or data collected from fisheries bycatch. Modern day technology, such as remote sensing tools (satellite relayed data loggers) as well as long-term environmental data now means that species distribution models can be more easily compared with oceanographic variables and changes in the turtle’s habitats.
The researchers fitted 196 loggerhead turtles in the north-west Atlantic Ocean with satellite tags from 2009 to 2018. The turtles were either late-stage juveniles or adults. The tags delivered 45 840 daily locations within the north-west Atlantic, of which 44 865 daily locations occurred on the continental shelf within the Middle Atlantic Bight. These tag locations were then combined with depth and remotely sensed sea surface temperature (SST) to determine the current thermal range of the species, as well as modeling future shifts in the species’ distribution. The study determined that most turtles preferred temperatures between 17.7 oC and 25.3 oC and depths between 26.1 and 74.2 m. More specifically, the turtles occupied regions with a surface temperature of 21.5 oC at depths of 50 m.
Using this data, and based on the current relationship between loggerhead occurrence and SST, the study projected that increases in SST as a result of climate change will likely result in an expansion of potentially suitable loggerhead habitat in the north-west Atlantic Ocean. Although the models used to predict future changes used a ~3 oC increase, realistically SST increases will not be uniform, and will differ across regions.
There are likely other shifts in loggerhead distribution that may occur as environmental conditions change, that could not be predicted by the model. Rising SST may result in an increase in hurricane intensity, which may impact sea turtle migrations, altering the seasonal movement patterns that have been projected by the study. Thermal changes may also result in shifts in prey densities and species compositions, which may alter the projected distribution of loggerhead turtles.
What Does The Results Of The Study Mean For Turtle Conservation?
Climate-based shifts in the distribution of sea turtles and commercially harvested marine species may alter future patterns of bycatch. Changes to the distribution range of loggerheads may cause these turtles to overlap with fishing industries that have not previously been prone to turtle bycatch. These fisheries can be promptly alerted to the possibility of turtle bycatch so that sea turtle conservation measures can be put in place.
It is most likely that sea turtle seasonal habitat use and distribution is linked to more than just SST and depth as investigated in this study. Other factors potentially include the availability of prey, reproductive cycles and life stages of the turtles. However, based on the available data and the knowledge of loggerhead ecology, the study presents a reasonable assessment of a potential future distribution of loggerhead turtles in the north-west Atlantic, and the results of this study can guide expectations for likely future distributions. This can be used to plan for climate change-resilient conservation measures to protect loggerhead turtles.