It’s getting hot in here: how ocean acidification and warming affect shark hunting and behavior

Background

Sharks have an advanced sensory system that allows for efficient foraging using various senses such as sight, hearing, smelling, and touch.  They can even detect movement, and differences in pressure! Because of this, some sharks are high in the food web. So, any changes in their physiology and behavior due to the changing climate is likely to trickle down into lower trophic levels and alter the structure of the marine food web. What causes such changes, though?  When we burn fossil fuels such as coal, oil and gas for energy or transportation, excess carbon dioxide (CO2) gets released into the atmosphere. This excess CO2 acts like a heat trapping blanket and the ocean absorbs much of this heat. The ocean also absorbs about quarter of this excess CO2 where it reacts with seawater and making the ocean more acidic. This is called ocean acidification.

Studies have looked at how ocean warming and ocean acidification affects sharks separately. For example, a rise in ocean temperature increases the hunting activity of sharks. However, studies observing the combined result of these ocean changes are far and few. Thus, Pistevos et al. set out to investigate what the combined effect was on shark behavior and physiological responses. To do so, they studied the Port Jackson shark (Heterodontus portusjacksoni) which is an egg-laying shark found only in the mild or temperate waters of Southern Australia. It forages primarily on bottom-dwelling animals like mollusks, sea urchins, crustaceans, and fish.

Methods & Results

Recently hatched sharks are more sensitive to environmental changes, which is why the authors wanted to focus on studying their responses. As many as 95 eggs were collected from the Gulf of St. Vincent in Adelaide, Australia. The eggs were brought to the lab to study the effects of ocean warming and acidification on the baby sharks’ foraging ability.

The sharks were put to the test within 24 hours of when they hatched by placing them in an arena with two metal containers on either side of the tank, one containing an odor cue (urchin) and the other the same size as the urchin but without an odor cue. The urchins were collected from the same site as the shark eggs, possibly to test odor cues they might encounter in their habitat. The sharks were observed for the time spent and the distance traveled between the control zone to the urchin. After the odor tracking test, the motivational drive to forage or accept prey was studied by placing prawn meat and/or mussel meat on opposite sides of the tank, thus, containing both visual and odor cues. Sharks either ate any of the food introduced within 2 min or were hand fed the food with steel forceps for 2 min. While undergoing these hunting tests, the juvenile sharks were exposed to different temperatures and CO2 levels separately and in combination.

Conclusion

Temperature and increased CO2 levels did not affect foraging behavior in sharks based on odor cues. Surprised? Increase in temperature alone: (1) increased the amount of time sharks spent near chemical cues of prey by 55% (2) increased swimming activity during hunting, and (3) reduced the time sharks spent accepting prey. The positive effect of temperature was canceled out in combination with higher CO2 resulting in minimal response towards prey odor cues and more time spent accepting prey. Interestingly enough, there was no effect on swimming speed due to acidification.

Sharks face a dodgy future because of the rapidly changing ocean. Elevated temperatures raise the energetic demands of sharks and acidification can hinder the ability of sharks to meet those demands by preventing successful prey encounters and adversely affecting growth. With some sharks being at the top of the food web, reduced hunting by sharks could result in a major shift in the food web and distorted predator-prey interactions, and ultimately, extinction. Therefore, it is important to look at different interactions of stressors on sharks. Further studies focusing on the possibility of sharks not meeting their exceeding energy requirement could shed more light on their future as some of the top predators of the marine ecosystem as well.

Aditi Tripathy, Ocean Bites, 27 May 2017. Article.

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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

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