Effects of Degraded Optical Conditions on Behavioural Responses to Alarm Cues in a Freshwater Fish

Prey organisms often use multiple sensory cues to gain reliable information about imminent predation threat. In this study we test if a freshwater fish increases the reliance on supplementary cues when the reliability of the primary cue is reduced. Fish commonly use vision to evaluate predation threat, but may also use chemical cues from predators or injured conspecifics. Environmental changes, such as increasing turbidity or water colour, may compromise the use of vision through changes in the optical properties of water. In an experiment we tested if changes in optical conditions have any effects on how crucian carp respond to chemical predator cues. In turbidity treatments we added either clay or algae, and in a brown water colour treatment we added water with a high humic content. We found that carp reduced activity in response to predator cues, but only in the turbidity treatments (clay, algae), whereas the response in the brown water treatment was intermediate, and not significantly different from, clear and turbid water treatments. The increased reliance on chemical cues indicates that crucian carp can compensate for the reduced information content from vision in waters where optical conditions are degraded. The lower effect in brown water may be due to the reduction in light intensity, changes in the spectral composition (reduction of UV light) or to a change in chemical properties of the cue in humic waters

Piscivore-prey fish interactions - consequences of changing optical environment

Predator-prey interactions are a primary structuring force in aquatic systems. A change in the predator-prey interactions may cause a change in the strength of trophic cascades and even resulting in ecosystem shifts. However, individual properties of predators and prey, as well as environmental conditions, may affect the relative strength of predator-prey interactions. In our studies we observed two different behavioural defences against predators. Both crucian carp and perch decreased their activity when exposed to pike chemical or visual cue. However, the behavioural response elicited by the chemical cue was context dependent, i.e. crucian carp only reacted to the cue if information from visual senses was limited (turbid or brown water). Another behavioural response to predation threat is schooling as shown in roach, a common prey fish, that group into schools to limit predation risk. The interaction between the predator and prey fish was influenced by optical conditions. Reaction distance of pike showed to be negatively affected by reduced visual conditions. The reaction distance and attack distance of pike and escape distance of roach was also context dependent, where brown water increased the reaction and attack distance in pike and reduced the escape distance in roach. This resulted in an overlap in attack and escape distance, which may have positive effects on the capture success of pike. Pikeperch was less affected by changes of the visual conditions in the water. However, pikeperch was indirectly affected by changing optical conditions through a change in prey behaviour. Pikeperch showed a strong preference for perch under good optical conditions, but shifted their food preference to roach in poor optical conditions. The underlying mechanism to this pattern was observed in behavioural studies. Roach escaped long before the pikeperch initiated an attack, whereas perch was inactive which allowed pikeperch to approach and successfully attack. In poor optical conditions roach shoals were splitting up and roach escape distance was strongly reduced, which increased the foraging success of pikeperch resulting in that both roach and perch were included in the diet. These changes in the predator-prey interactions may explain some of the changes in fish growth rate observed in lakes along a gradient of brown colouration. Here, I found that the growth rate of pike and roach were reduced in browner lakes. Changes in environmental drivers, such as eutrophication and brownification, affecting the optical climate should thus have consequences for the strength of predator-prey interactions. This in turn may affect lake ecosystems through higher-order interactions

Foraging efficiency and prey selectivity in a visual predator: differential effects of turbid and humic water

Predators exert strong regulating forces on lower trophic levels through predation. As most fish are visual foragers, visual conditions in the water may alter the strength of this regulation. We evaluated effects of turbidity and humic water on foraging efficiency and prey-size selectivity in Northern pike (Esox lucius) feeding on roach (Rutilus rutilus). Encounter rates decreased in both turbid and humic water but were not counteracted by increased searching activity. Capture success was unaffected by turbidity but was nonlinearly affected by humic water by being high in clear and highly humic water but low in less humic water. In highly humic water, the visual range approached pike's strike distance and, together with its cryptic colours, pike may have initiated its attack before the prey detected it, limiting the possibility for prey evasive manoeuvres. Prey-size selectivity towards small prey in clear water disappeared in turbid water but was maintained in humic water. Owing to its optical properties, turbidity degrades the quality of the visual information more through scattering than humic water does through absorption. We show that the effect of visual degradation on foraging depends on the cause of visual degradation, which has not previously been acknowledged in the visual foraging literature

Effects of brown and turbid water on piscivore-prey fish interactions along a visibility gradient

1. Environmental changes such as eutrophication and increasing inputs of humic matter (brownification) may have strong effects on predatorprey interactions in lakes through a reduction in the visual conditions affecting foraging behaviour of visually oriented predators. 2. In this experiment, we studied the effects of visual range (25200 cm) in combination with optically deteriorating treatments (algae, clay or brown humic water) on predatorprey interactions between pike (Esox lucius) and roach (Rutilus rutilus). We measured effects on reaction distance and strike distance for pike and escape distance for roach, when pike individuals were exposed to free-swimming roach as well as to roach held in a glass cylinder. 3. We found that reaction distance decreased with decreasing visual range caused by increasing levels of algae, clay or humic matter. The effect of reaction distance was stronger in turbid water (clay, algae) than in the brown water treatment. 4. Strike distance was neither affected by visual range nor by optical treatment, but we found shorter strike distances when pike attacked roach using visual cues only (roach held in a cylinder) compared to when pike could use multiple senses (free-swimming roach). Escape distance for roach was longer in turbid than in brown water treatments. 5. Changes in environmental drivers, such as eutrophication and brownification, affecting the optical climate should thus have consequences for the strength of predatorprey interactions through changes in piscivore foraging efficiency and prey escape behaviour. This in turn may affect lake ecosystems through higher-order interactions

Prey-type-dependent foraging of young-of-the-year fish in turbid and humic environments

Fish, which are generally visual foragers, experiences reduced reaction distance in visually degraded environments, which has consequences for encounter rates with prey. Small prey is detected at shorter distances than larger prey, and piscivores are therefore predicted to be more strongly affected by visual degradation. In experiments, roach (Rutilus rutilus) were fed two plankton prey types and pike (Esox lucius) were fed Daphnia and larval roach, in clear water, algal turbid water and water coloured brown by dissolved organic matter (DOM). Planktivorous foraging in roach was not affected by visual degradation, while pike foraging on both Daphnia and larval roach was. Pike showed increased reaction distance to Daphnia in visually degraded water, while it was severely reduced with roach as prey even if the visual range was not reduced below pike reaction distances in clear water. Pike foraging on Daphnia was not affected, but when foraging on roach, the reduced search efficiency was counteracted by increased attack rates. However, there was no increase in movement and no difference between turbid and DOM treatments. Effects on piscivores will likely become more pronounced at later life stages as prey size and the reliance on long-distance detection increases at the same time as changing climatic conditions may further deteriorate the visual conditions in future

Spectral distribution of downwelling light in clear, algae, clay and humic brown water at 5.5 cm depth.

<p>Spectral distribution of downwelling light in clear, algae, clay and humic brown water at 5.5 cm depth.</p

Piscivore-Prey Fish Interactions: Mechanisms behind Diurnal Patterns in Prey Selectivity in Brown and Clear Water.

Environmental change may affect predator-prey interactions in lakes through deterioration of visual conditions affecting foraging success of visually oriented predators. Environmental change in lakes includes an increase in humic matter causing browner water and reduced visibility, affecting the behavioural performance of both piscivores and prey. We studied diurnal patterns of prey selection in piscivorous pikeperch (Sander lucioperca) in both field and laboratory investigations. In the field we estimated prey selectivity and prey availability during day and night in a clear and a brown water lake. Further, prey selectivity during day and night conditions was studied in the laboratory where we manipulated optical conditions (humic matter content) of the water. Here, we also studied the behaviours of piscivores and prey, focusing on foraging-cycle stages such as number of interests and attacks by the pikeperch as well as the escape distance of the prey fish species. Analyses of gut contents from the field study showed that pikeperch selected perch (Perca fluviatilis) over roach (Rutilus rutilus) prey in both lakes during the day, but changed selectivity towards roach in both lakes at night. These results were corroborated in the selectivity experiments along a brown-water gradient in day and night light conditions. However, a change in selectivity from perch to roach was observed when the optical condition was heavily degraded, from either brown-stained water or light intensity. At longer visual ranges, roach initiated escape at distances greater than pikeperch attack distances, whereas perch stayed inactive making pikeperch approach and attack at the closest range possible. Roach anti-predatory behaviour decreased in deteriorated visual conditions, altering selectivity patterns. Our results highlight the importance of investigating both predator and prey responses to visibility conditions in order to understand the effects of degrading optical conditions on piscivore-prey interaction strength and thereby ecosystem responses to brownification of waters

Foraging success of juvenile pike Esox lucius depends on visual conditions and prey pigmentation

Young-of-the-year pike Esox lucius foraging on copepods experienced different foraging success depending on prey pigmentation in water visually degraded by brown colouration or algae. Both attack rate and prey consumption rate were higher for E. lucius foraging on transparent prey in brown water, whereas the opposite was true in algal turbid water. Pigments in copepod prey may have a cryptic function in brown water instead of a photo-protective function even if prey-size selectivity was stronger than selection based on pigmentation in juvenile E. lucius. (C) 2011 The Authors Journal of Fish Biology (C) 2011 The Fisheries Society of the British Isle

Relative change in swimming activity (mean ± standard error) in crucian carp upon experiencing chemical cues from a pike.

<p>Clay, algae and brown water treatments represent a visual range of 40 cm created by the different substances, whereas clear water is a control with no visual deterioration.</p

Behavioural parameters of pikeperch foraging on perch (white bars) and roach (grey bars) at visual ranges of 0.5 and 2 m.

<p>Behavioural parameters include number of interests (a) and strikes (b), as well as capture success (c). In the analyses of attack distances (d), data for the two prey species are pooled. Error bar denote 1 SE.</p