There are obvious barriers for marine scientists interested in characterizing the foraging success of endangered marine predators. Issues ranging from decreasing animal numbers, to vast migratory patterns and sheer oceanographic scale must be considered. Prior to technological advances, researchers seeking to discern behavioral and nutritional aspects of the food chain – in the very deepest reaches of the marine ecosystem – had to rely on analyses of data collected over sporadic observations. However, in a recent Canadian collaborative study published in PLoS ONE, Susan Heaslip, Sara Iverson, Don Bowen and Michael James, from Dalhousie University and the Bedford Institute of Oceanography, describe how they managed to surmount each of these obstacles to precisely quantify the foraging behavior of a threatened species: the leatherback turtle.
Before this study, direct and sustained observation of the leatherbacks' foraging strategy had not been documented because of the turtles' immense east–west migration pattern – up to 18,000 km round trip from tropical nesting beaches to foraging grounds – and the enormous depths that they plumb. In order to understand the immediate survival challenges faced by the turtles, the team set out to document the animals' foraging behavior and to observe how they capture and handle prey, using recent technological advances. The authors tracked the turtles' progress with GPS while recording their foraging behavior with 19 turtle-borne cameras.
Using continuous video, the authors documented 601 prey captures. They found that the turtles' diet was very restricted – the lion's mane jellyfish was the turtles' most common victim – and they showed that the jellyfish were attacked and consumed completely in each hunt. The researchers were able to estimate the approximate size of each jellyfish prior to ingestion, which they found ranged from 3.1 to 22.7 cm in diameter – although the authors add that these prey-size calculations are likely to underestimate the actual size, as measurements were made based on images taken prior to capture, when jellyfish were positioned at a greater distance from the camera. Also, when calculating the rates at which the turtles encountered prey, the team noted foraging hotspots – patchily dispersed zones with high numbers of jellyfish – that reflect crucial foraging areas for leatherback turtles.
In order to better understand the turtles' daily energy intake in relation to their energy output, the researchers measured their capture success rates and migratory patterns, as well as the size of the captured prey. Based on their observations, the authors calculated that turtles consume an average of 73% of their body mass, approximately 261 lion's mane jellyfish per day, equating to an average energy intake of 3–7 times their daily metabolic requirements.
This study intimately documents the highly efficient (albeit highly selective) feeding behavior of the leatherback turtle. The researchers provide insight into how the animal sustains an extended migration and their data support the importance of conservation strategies that consider the entire food chain in the context of the animal's ecosystem.