Swimming ability in sea turtle hatchlings: a comparison between species

Abstract

After emerging from the nest, hatchling sea turtles leave their nests and head towards the surf. When reaching the water, they immediately engage in a frantic swim commonly known as the 'swimming frenzy', which lasts between 24 to 48 h. During this period, hatchlings rely solely on their yolk reserves as their energy supply to reach oceanic currents that will transport them to distant foraging grounds, where they remain until returning as immature sub- adults. Previous work has found that swimming effort of hatchling sea turtles during the swimming frenzy varies across species. However, detailed information about the changes in powerstroke and dogpaddling rates during the frenzy swim had only been documented for green turtle hatchlings. Loggerhead turtles have a similar life cycle to green turtles, with an open ocean long distance migration, therefore similarities in the energetics of swimming during the frenzy phase were expected. Flatback turtles, on the other hand, are considered endemic to the Australian Continental shelf and are assumed not to have a distinctive mid-oceanic phase. For this reason flatback turtle hatchlings were expected to exhibit a different pattern of swimming behaviour. The main focus of my thesis was to understand how sea turtles behave, from an energetic point of view, during the first crucial moments of their off-shore dispersal. I also established a cross-species comparison, in order to understand if differences in life history strategies contribute to the energetics of swimming of this phase. I found that aerobic metabolic rate and swim thrust in these three species decreased sharply during the first 2 hours of swimming, followed by a less sharp decrease until 12 hours. After 12h of swimming, swimming effort and oxygen consumption remained constant at lower levels until the end of the trial (18h). Overall oxygen consumption rates and thrust production was higher in green turtles, followed by loggerhead turtles with flatback turtles being the least vigorous swimmers. Although the same three-phase swimming pattern was identified in each species, flatback hatchling swimming effort decreased at a higher rate. Furthermore, the swimming efficiency of flatback hatchlings started decreasing after 6 hours of swimming, whereas it kept increasing in loggerhead and green turtle hatchlings. When analysing the factors that produce swim thrust in detail, green and loggerhead turtle hatchlings had similar power stroke rates and proportion of time spent power stroking, but the mean maximum thrust, and therefore overall thrust, was greater in green turtles. Flatback hatchlings, on the other hand, had overall lower values in all three components compared to green and loggerhead turtles, particularly in the proportion of time spent power stroking. These results suggest that flatback turtles have a less intense swimming frenzy, both in duration and distance of dispersal, which might be related to their restricted geographic distribution and to the absence of a pan-oceanic pelagic phase in their life cycle. The earliest stages of the frenzy swim are powered by both aerobic and anaerobic metabolic pathways. Analysis of changes in the concentration of blood lactate, glucose and corticosterone during the first four hours of swimming revealed a similar pattern in loggerhead and flatback turtle hatchlings. In these two species, blood lactate, glucose and corticosterone concentrations started high at crawling and progressively decreased over time. The pattern of green turtle hatchlings, however, appears to be different, with blood lactate and glucose concentrations peaking after 2 h of swimming and corticosterone peaking between 3 and 5 h of swimming. These results reinforce the idea that the highest energetic demand occurs at the beginning of the frenzy phase and that aerobic metabolism is the main energetic pathway used during the swimming frenzy. However, in flatback hatchlings, the aerobic contribution to swimming is lower than that in loggerhead turtle hatchlings, which supports the hypothesis that flatback turtle hatchlings have a less intense swimming frenzy. Powering off-shore swimming is not the only challenge hatchling sea turtles face during their off-shore dispersal. Predation rates are high after hatchlings emerge from the nest, particularly upon entering the sea. Social interaction between hatchlings while digging their way out of the nest and during beach crawl appears to increase their chances of surviving these phases of dispersal but the role, if any, of interactions between hatchling once they enter the water is still unknown. When investigating if loggerhead hatchlings interact with each other while swimming and if they react to non-moving and moving predator stimuli, I found that after 2.5 minutes of swimming hatchlings established a set swimming direction, which was maintained throughout the rest of the swimming trial. Although not socially interactive, loggerhead hatchlings adjust their initial swimming direction by swimming in a general direction towards that of a sibling when a sibling is present. In the presence of a model fish predator (moving and non-moving), most loggerhead hatchlings became inactive and usually adopted a tuck position while drifting away. This response lasts between 24-27 seconds before hatchlings resume their swimming. These results indicate that loggerhead sea turtle hatchlings are able to visually recognise predators and differentiate them from other visual stimuli, such as sibling hatchlings.