Daily movements of green sea turtles

Large marine vertebrates, such as sea turtles, are particularly vulnerable to human impacts due to their long lifespans, late maturity, slow reproductive rates, and extended migrations. Like most large marine vertebrates, sea turtles play key ecological roles in their environment when they are abundant. Green sea turtles are especially important in coastal areas because their grazing behavior significantly reduces nutrient cycling times in seagrass pastures.

In Baja California, Mexico, green sea turtles are protected by law, but lack of enforcement, coupled with drowning in fishing nets and illegal poaching has led these turtles to the brink of extinction. The majority of green turtles that are killed in Baja California are juveniles inhabiting coastal foraging areas; thus, understanding their movements and habitat use in this environment is a priority for conservation efforts. Nevertheless, while researchers have tracked the long-term movements of mainly nesting sea turtles, there is very little known about the short-term movements of green turtles in coastal foraging areas. Understanding this aspect of their biology is particularly important because green turtles spend the majority of their lives in these environments where they come in direct contact with fishing nets and poachers who often sell their meat on the black market.


Jesse Senko (author) holding a green turtle before it is released


Recently, a team of biological scientists set out to better understand green sea turtle fine scale daily movements in a coastal foraging area along the Pacific Ocean in Baja California, Mexico. They developed a novel tracking device to conduct their study. The tracking tag consisted of a buoy that housed a GPS logger to record turtle movements and a VHF transmitter to locate the tracking tag. The researchers tethered the buoy to six green sea turtles. They found that green turtles were active throughout 24-hour periods while moving large distances over surprisingly short time periods. “We were surprised to see how far some of the turtles moved over temporal scales as short as one or two days. We had some turtles that moved total distances as far as 29 kilometers (18 miles) and occupied areas as large as 1,575 hectares (6 square miles) in a single 24-hour period”, said Senko, the study’s lead author.


Releasing a tagged turtle


The researchers also found that turtles were active throughout day, night, and crepuscular (dawn and dusk) periods of activity. “These results indicate that turtles were active throughout 24-hour periods, and did not show preferences for certain periods of the diel cycle (one 24-hour period). Given our findings that turtles moved large distances over short time periods and were active throughout 24-hour periods, conservation strategies intended to protect this endangered species may ideally need to encompass the entire coastal foraging area rather than focus on a few high use zones”, added Senko.


Tracking a turtle

Blog written by Jesse Senko, BOI seafood consultant

2 Comments on “Daily movements of green sea turtles

    • That is a really good question. While I can’t definitively answer it with the movement data I collected, I can say that we know sea turtles either “sleep” or “rest” (defining both can be tricky!). Some studies have shown them to rest at night, whereas other studies have found them to rest during the day or show no preference. This suggests that the habitat and the type of predators that may be there likely play a big role in where and when turtles sleep. For example, if an area has sharks they probably will rest at the bottom or try and hide in structure, and will likely rest when the sharks are not active. In our study, turtles avoided deeper water at night, probably because our habitat lacks deep water structure (i.e. rocks and reefs), but other studies conducted in areas w/ lots of structure have found turtles to like to rest in these areas. Given our movement data, my best guess would be that the turtles may alternate b/w periods of activity and inactivity (as opposed to a “set” time to rest, e.g. day or night), if that makes any sense? Also, our habitat contained strong tides, and the turtles likely need to use the tides to help them move around to find different feeding areas, which is also is probably why the turtles did not show distinct resting periods, as the tidal cycle is constantly changing. But in short, yes, they are probably sleeping or resting. Likely they are doing one of two things – moving and the resting, then moving again, sort of on a continuous pattern, or moving while they rest, in other words bouncing along the bottom or mid water as they rest. My guess would be the former and not the latter, but its hard to say for sure.
      – Jesse Senko

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