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Foraging by estuarine juveniles of two paralichthyid flounders: experimental analyses of the effects of light level, turbidity, and prey type

Ursula A. Howson*, Timothy E. Targett, Paul A. Grecay, Patrick M. Gaffney

*Corresponding author:

ABSTRACT: Juvenile summer flounder Paralichthys dentatus and southern flounder Paralichthys lethostigma inhabit turbid salt marsh estuaries. Predation rates by juveniles (50-90 mm) were examined at five daytime light levels (6 x 1011 to 2 x 1014 quanta sec-1 cm-2) and darkness; and four turbidity levels (clear (≤1), 11, 20 and 40 NTU) at an intermediate light level. Both species fed equally well on benthopelagic mysid shrimp and benthic spionid polychaetes at all daytime light levels tested. However, predation on mysids was significantly reduced in the dark. Consumption of polychaetes was not reduced in the dark by either species, illustrating the effectiveness of non-visual foraging methods on benthic prey. Turbidity levels tested did not affect predation on either prey type by either flounder species. Locomotor behavior was examined at the same turbidity levels. P. lethostigma spent more time swimming in the water column than P. dentatus in lower turbidity (clear – 20 NTU), and both species reduced swimming at 40 NTU. It appears that both species use primarily a benthic-oriented ambush foraging strategy under high turbidity conditions. This is a particularly pronounced switch in foraging style for P. lethostigma. Estuarine turbidity is increasing due to impacts of climate change. When turbidity is elevated enough to eliminate light sufficient for visual feeding on mysids (between darkness and lowest light level tested) feeding on this motile prey is negatively impacted for both species. Turbidity can thus alter foraging modes and types of prey consumed, affecting nursery habitat quality and the prey base supporting these young fishes.