Lesser sandeel (Ammodytes tobianus) is abundant in near-shore areas where it is a key prey. It exhibits the behaviour of alternating between swimming in schools and lying buried in the sediment. We first determined the species’ standard metabolic rate (SMR), critical partial pressure of oxygen
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\begin{document}$$(P_{{\rm O}_{2{\rm crit}}})$$\end{document} and maximal oxygen uptake
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\begin{document}$$(M_{{\rm O}_{2{\rm max}}}).$$\end{document} The sandeel were then exposed to an acute stepwise decline in water oxygen pressure (18.4, 13.8, 9.8, 7.5, 5.8, 4.0, and 3.1 kPa
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\begin{document}$$P_{{\rm O}_{2}}$$\end{document}). Swimming speed and routine- and post-experimental blood lactate levels were measured, in addition to levels associated with strenuous exercise. The SMR was 69.0 ± 8.4 mg O2 kg−1 h−1 and the
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\begin{document}$$M_{{\rm O}_{2{\rm max}}}$$\end{document} about seven times as high. The
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\begin{document}$$P_{{\rm O}_{2{\rm crit}}}$$\end{document} was found to be 4.1 kPa. A rapid decrease (within 1 h) in
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\begin{document}$$P_{{\rm O}_{2}}$$\end{document} from 18.4 to 3.1 kPa had no significant effect on routine swimming speed (0.9 ± 0.06 bl s−1), but steady levels at the lowest
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\begin{document}$$P_{{\rm O}_{2}}$$\end{document} (3.1 kPa) gradually reduced the swimming speed by 95% after 40 min. The routine blood lactate levels were 2.2 ± 0.6 mmol l−1, while the levels in the strenuously exercised groups were significantly higher with 5.4 ± 1.6 and 5.8 ± 1.3 mmol l−1. The highest levels were observed in post-experimental fish with 7.5 ± 2.7 mmol l−1. We argue that, as sandeel showed no decrease in swimming speed (to offset stress) nor an increased speed to escape the hypoxia, the fish either rely on a low SMR and being a reasonable strong oxygen regulator
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\begin{document}$$(\hbox{low}\;P_{{\rm O}_{2{\rm crit}}})$$\end{document} as a mean to cope when exposed to acute hypoxia, or that the hypoxia simply developed too fast for the fish to decide on an appropriate strategy. Not showing a behavioural response may in the present case be maladaptive, as the consequence was major physiological stress which the fish however appears tolerant towards. The high routine blood lactate levels suggest that anaerobic metabolism is associated with swimming in sandeel, which may be related to the specific lifestyle of the fish where they regularly bury in the sediment.
