Effects of a cestode (Schistocephalus sp.) on the response of ninespine sticklebacks (Pungitius pungitius) to aquatic hypoxia

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Abstract
Sticklebacks (Pisces, Gasterosteidae) serve as one of the intermediate hosts for cestodes of the genus Schistocephalus. Previous authors have suggested that, by increasing oxygen demand, these parasites induce sticklebacks to swim nearer the surface, where the fish are more vulnerable to piscivorous birds, which are the definitive hosts. We compared surface use and survival of parasitized and nonparasitized ninespine sticklebacks (Pungitius pungitius) at different levels of dissolved oxygen. Parasitized Pungitius spent more time in contact with the surface performing aquatic surface respiration under hypoxic conditions (0.75–2.0 mg O2∙L−1) than nonparasitized individuals did. When frightened by a model simulating an aerial predator, parasitized sticklebacks left the surface for a shorter time than nonparasitized fish. Parasitized sticklebacks also had a higher lethal oxygen level than nonparasitized sticklebacks, suggesting that the differences in surface use are due, at least in part, to differences in oxygen demand.