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Sharks vs turtles – A landscape of fear (model)

Published on: 29th July, 2015 | Marine Science
Basic instinct suggests that where there are more predators, prey make more effort to avoid being eaten. For example, most people (non-divers) avoid swimming where great white sharks aggregate. However, turtles apparently don’t care if they are in shark infested waters!

Prey species usually adopt risk avoidance tactics when in regions with high predator numbers. It appears this isn’t the case when the home ranges of tiger sharks and loggerhead sea turtles overlap during migration periods, according to new research published in the leading journal Ecology. This study of predator-prey interactions in the northwestern Atlantic Ocean explored the ‘landscape of fear model’, which predicts that an increased risk of predation in a particular area causes an animal to change it’s behaviour to avoid the added risks.

Turtles need to reach the surface to breathe, creating perfect opportunities for ambush predators such as tiger sharks to strike from below. The authors behind this breaking research expected to find that where tiger shark numbers were greater, turtles spent less time at the surface.

Bizarrely, when turtles were recorded in shark infested waters (so to speak), they did not change the way they surfaced to avoid spending time in the risky surface zone. However, in those same locations, where there were high numbers of both species, sharks were observed to change their surfacing behaviours, presumably to increase their chances of scoring a meal.

The conclusion seems to be that sharks modify their actions when prey is plentiful, but loggerhead turtles don’t. This may be due to the turtles having other things on their mind, such as avoiding boat strike, or attempting to locate their own prey, which could have more influence over their actions than the abundance of sharks. The science isn’t yet clear. (Although I wonder whether turtles are just less cunning?!)

This research builds on the back of an increasing understanding about the interactions between loggerheads and tiger sharks. For example, other scientists have found that tiger sharks use cognitive maps to align their migration paths with locations of abundant prey species, such as nesting grounds.

How do you study interactions between sharks and turtles you might ask? In this case, satellite tags were attached to 31 tiger sharks and 68 female loggerhead turtles. The scientists first followed the movements of each animal to establish their home ranges and then collected data on when, where and for how long each animal surfaced.

This is the clever part, the scientists recognized that the satellite tags only transmit a signal to the satellite receivers when a tag’s salt water switch circuit breaks at the water surface. Neat huh?!

Interestingly the pattern observed in this study is opposite to what happens when (great) white sharks forage near Cape fur seal rookeries. The sharks are known to hunt more stealthily, staying low to see the outline of the seals above, and correspondingly, the seals increase their vigilance, swimming in groups, reducing splashing on the surface and switching up the times they go foraging.

So what is it about the loggerheads vs tiger sharks? It is possible that a history of overfishing and disturbance has changed the way the two species interact. Populations of both species have dramatically declined, which may have led to these unusual behaviours - they are simply less likely to see each other.


The original article by Neil Hammerschlag and his co-authors in the August 2015 issue of Ecology is available here: http://www.esajournals.org/doi/abs/10.1890/14-2113.1 

AUTHOR

Emma McIntosh

TDI Adv. Nitrox & Deco. Procedures. IANTD Cavern Certified. PhD Candidate in Ecology