Green Sea Turtles: coastal foraging puts endangered turtle at risk

Perusing the other ocean conservation blogs, I came across an interesting post written by Jesse Senko, marine researcher and seafood consultant for the Blue Ocean Institute - an ocean conservation non-profit headed by Dr. Carl Safina. Jesse writes of the challenges faced by green sea turtles as they forage through coastal areas. While tagging of sea turtles has produced important data regarding their long distance movements, not much is known about their shorter coastal movements, which actually constitutes the greatest part of their activity.

Although protected by law in Baja, Mexico, the green sea turtles are subjected to lax enforcement and protection and many turtles are lost to poachers or drowning in fishing nets. The turtles' dilemma in Baja reminded me of the supposedly protected white sharks, the juveniles of which can often be found in local Baja fish markets.

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.

Above: 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.

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.

Above: Tracking a turtle

The full study in Journal of Experimental Marine Biology and Ecology can be found online at: http://wallacejnichols.org/wallacejnichols/Research/Entries/2010/9/2_JEMBE__Movements_of_green_turtles_in_Baja_files/*Senko_JEMBE_2010.pdf