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Adaptive color change in flatfish has long been of interest to scientists, yet rarely studied from an ecological perspective. Because color change can take a day or so in some species, movement between sediments with differing color or texture may render fish more conspicuous to predators. We conducted laboratory experiments to test the following hypotheses related to adaptive color change in flatfish: 1) fish which do not cryptically match sediment will be more vulnerable to predation, 2) fish will reduce activity and bury to minimize conspicuousness when on a sediment they mismatch, and 3) fish will choose a sediment they match when given a choice. Experiments were conducted using three co-occurring north Pacific juvenile flatfishes: English sole Parophrys vetulus, northern rock sole Lepidopsetta polyxystra and Pacific halibut Hippoglossus stenolepis. As per expectations, juvenile flatfish were more vulnerable to visual predators when they mismatched sediment. Mismatched fish tended to behave differently than fish which matched the sediment. Rather than burying and becoming inactive, they became more active and less likely to bury, perhaps contributing to their predation vulnerability. This increased activity may have represented search for better matching sediment, a stress response, or conspicuousness-related density dependent behavior. Fish which had acclimated to light colored sediment preferred light over dark sediment in choice trials. In contrast, fish acclimated to dark sediment demonstrated no preference. These experiments demonstrate that adaptive coloration is an integral part of the flatfish detection minimization strategy and that movement between habitats can increase risk of predation.

Pleuronectid flatfish are generally thought to utilize stereotypical anti-predator behavior to reduce encounters with potential predators. In this study, we tested the relative vul- nerability of 3 juvenile (Age-0) flatfish species to predation under controlled laboratory conditions. Predators consumed significantly more English sole Parophrys vetulus than either Pacific halibut Hippoglossus stenolepis or northern rock sole Lepidopsetta polyxystra in clear water (0 nephelo- metric turbidity units, NTU), suggesting differing anti-predator strategies and/or capabilities. In con- trast to halibut and rock sole, which remained buried and inactive, English sole were often observed actively moving about. As a result, English sole were encountered more frequently by predators than the other 2 species, suggesting that they were more conspicuous to the predators. This was further evidenced by a higher frequency of stalk encounters for English sole, whereby the predators detected and approached them prior to prey flushing. However, as an estuarine-dependent species, English sole are generally exposed to lower densities of predators in an environment that tends to be highly turbid. When similar trials were conducted at higher turbidity (18 NTU), predation rates on English sole decreased to rates comparable to that for halibut and rock sole in clear water (0 NTU). Although English sole appeared more vulnerable to predation, their behavior may have evolved in response to, and be mitigated by, the turbid nature of the estuarine nursery environments to which they recruit, where predation pressure is significantly reduced in comparison to open coastal habitats.

Pleuronectid flatfish are generally thought to use stereotypical anti-predator behavior to reduce encounters with potential predators, including burial, maintaining a low profile on the bottom, cryptic coloration, and reduced activity. However, a series of laboratory experiments demonstrate significantly different predation rates on juvenile (Age-0) English sole ( Parophrys vetulus ), northern rock sole ( Lepidopsetta polyxystra ), and Pacific halibut ( Hippoglossus stenolepis ) by Age-2 Pacific halibut predators, suggesting differing anti-predator strategies and/or capabilities among species. In this study, behavioral attributes that control how conspicuous juvenile flatfish are to their predators, such as burial, body posture, and activity levels, were examined both in the presence and absence of perceived predation risk. English sole exhibited risk-prone behavior; they tended to bury less, exhibit an arched body posture (with head elevated off the bottom), and were more active in both the absence and presence of predators when compared to the other two species. Conversely, rock sole exhibited risk-averse behavior, being buried, inactive, with a flat body posture regardless of predation risk. Halibut demonstrated risk-sensitive behavior; behaving like English sole in the absence of predation risk, and shifted to behavior similar to that of rock sole in the face of predation risk. As an estuarine dependent species, English sole recruit to an environment that tends to be highly turbid reducing encounters with predators. As a result English sole, may have “relaxed” anti-predator behaviors in comparison to northern rock sole and Pacific halibut which recruit to less turbid coastal nurseries.