Abstract
Numerous anthropogenic stressors impact rivers worldwide. Hypoxia, resulting from organic waste releases and eutrophication, occurs very commonly in Mediterranean rivers. Nonetheless, little is known about the effects of deoxygenation on the behavior of Mediterranean freshwater fish. To fill this knowledge gap, we assessed the impact of three different dissolved oxygen levels (normoxia, 48.4%, 16.5% saturation) on kinematics indicators (swimming velocity, acceleration, distance traveled) and shoaling cohesion of adult Iberian barbel, Luciobarbus bocagei, a widespread cyprinid species inhabiting a broad range of lotic and lentic habitats. We conducted flume experiments and video-tracked individual swimming movements of shoals of five fish. Our results reveal significant differences between the treatments regarding kinematics. Swimming velocity, acceleration, and total distance traveled decreased stepwise from the control to each of the two oxygen depletion treatments, whereby the difference between the control and both depletion levels was significant, respectively, but not between the depletion levels themselves. Shoaling cohesion showed dissimilarities between the treatments regarding the maximum distance between fish, as the high depletion treatment differed from each of the other two, indicating that under severe oxygen depletion some individuals move away from the shoal. Overall, our results show how oxygen depletion changes fish behavior, which may entail ecological responses, highlighting the need to maintain an unfragmented river network to ensure movement dispersal among habitats, thus providing conditions for species escapement from hypoxia.
Highlights
Mediterranean rivers are characterized by frequent hydrological disturbances, including seasonally predictable drought events, exposing biota to harsh environmental conditions [1].An extensive period of low summer flows naturally leads to an increase in temperature and primary production, and subsequently to a reduction of oxygen levels [2]
>80% [21]), (2) a mild depletion level of 48.4% DO saturation (4.4 ± 0.2 mg·L−1 ), and (3) a high depletion level of 16.5% DO saturation (1.4 ± 0.1 mg·L−1 ) (Table 1). These levels allowed for testing along a decreasing gradient, whereby the latter represents a level of extreme oxygen depletion that may induce stress or even death in fish [6,22]
Pairwise comparison found that fish in the control treatment (T0) swam faster than those in the mild depletion treatment (T1) (χ2 = 18.92, p < 0.01), as well as those in the high depletion treatment (T2) (χ2 = 26.96, p < 0.001), whereas the mild and high depletion treatment (T1 and T2) were not distinct from each other
Summary
An extensive period of low summer flows naturally leads to an increase in temperature and primary production, and subsequently to a reduction of oxygen levels [2]. Mediterranean fish populations are adapted to naturally occurring seasonal oxygen fluctuations [1]. Human activities, such as water abstraction, the discharge of organic wastes, and the diffusion of agricultural fertilizers into aquatic systems, are common problems in Mediterranean rivers, causing further diminishment of oxygen levels of already stressed freshwater systems and possibly leading to hypoxic conditions [3,4]. Hypoxic conditions can cause behavioral and physiological stress in fish [7], which can entail many ecological consequences, including impaired reproduction, feeding, and predator avoidance [8,9].
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