Publication Abstract

Diving behaviour of green turtles:dive depth, dive duration and activity levels

G.C. Hays, S. Hochschied, A.C. Broderick, B.J. Godley and J.D. Metcalfe

Hochscheid et al. (1999) used time-depth recorders (TDRs)and activity recorders (Hochscheid & Wilson 1999) to examine diving behaviour andactivity of green turtles during their internesting period at Cyprus. Turtles were foundto spend a large propor-tion of time undertaking dives with a characteristic 'U-shaped'profile, i.e. dives where individuals re-mained at a fixed depth for a long period,surfaced briefly and then descended to the same depth again. Information from the activitysensor suggested that some of these U-dives were associated with resting, others withbenthic activity.

While TDRs have been extensively deployed on marine mammalsand birds, their deployment on sea turtles is still in its infancy. We are only nowstarting to appreciate the wealth of information that these instru-ments can provide aboutfree-living turtles. Unlike many marine mammals, shallow diving marine tur-tles-such asthe green (Chelonia mydas) and logger-head (Caretta caretta) turtles-usetheir lungs both to regulate their buoyancy and as a major O₂ store (Milsom1935, Lutcavage & Lutz 1997). An individual clearly cannot be positively buoyant if itis to passively stay on the sea-bed, either resting or foraging. Since the lung volumewill be reduced with increasing depth (due to gas compression in accordance with Boyle's law), a turtle is able to dive with a larger lung volume when diving deeper, and still attain negative buoy-ancy. Hence it might be predicted that there will be a general tendency for dive duration to increase with depth, due to this larger lung store; indeed this has been found in several studies (Minamikawa et al. 1996, Hochscheid et al. 1999, Hays et al. 2000). However, in addition to the total O₂ store, dive duration may also be influenced by metabolic rate during the dive. In simplest terms, as the metabolic rate increases, dive duration might be expected to decrease. Hence we predict that dive duration maybe linked to both dive depth and metabolic rate.

Reference:

G.C. Hays, S. Hochschied, A.C. Broderick, B.J. Godley and J.D. Metcalfe, 2000. Diving behaviour of green turtles: dive depth, dive duration and activity levels. Marine Ecology Progress Series, 208: 297-298.