Warming increases the number of apparent prey in reaction field volume of zooplanktivorous fish

WOS: 000427077300003Improved vision due to the cranial endothermy known in large-bodied marine fishes suggests that increased water temperature alone might also increase the speed with which visual information can be processed and therefore improve the vision of small planktivorous fish without invoking endothermic heating. We check this for two freshwater species by testing whether a temperature increase results in an increase in the reaction distance (RD), the distance from which a foraging fish can spot its tiny zooplankton prey. We demonstrate that at a given light intensity, with a temperature increase of 10 degrees C, both the reaction field volume and prey encounter rate of planktivorous fish are doubled due to a 21-23% increase in RD. This was found for each of the two small-bodied freshwater planktivorous fishes: rudd from the temperate zone (foraging at 16 degrees C and 26 degrees C), and Malabar danio from the tropics (foraging at 21 degrees C and 31 degrees C), and may be expected to be important for other fishes as well. An increase in RD at a higher temperature could translate into a higher "apparent prey density" or number of prey within the reaction field volume of a foraging fish (a vertically flattened hemisphere or cone with a horizontal radius equal to the RD). For fish in the wild, this information may compel them to continue foraging despite a low actual density of prey in the habitat. This is particularly important while feeding under the risk of predation, when attention is partially allocated to risk assessment.Polish National Science Centre [2011/03/B/NZ8/02093, 2014/15/B/NZ8/00245]; NAMASTE Consortium (Networking and Mobility Actions for Sustainable Technology and Environment in India) within the framework of the Erasmus Mundus Program; EU European Regional Development FundEuropean Union (EU)We thank J. Wolnicki, R. Kaminski, and M. Kaminski for producing the synchronic cohorts of rudd; P. Maszczyk, E. Rutkowska, K. Rutkowska, A. Hankiewicz, P. Dynak, and M. Kamaszewski for assistance with the experimental work; D. Murzynowska for the photos of the experimental fish; A. Dell and anonymous reviewers for their very instructive suggestions; S. Sarma, S. Nandini, and C. Espinosa Rodriguez for preliminary efforts to evaluate the effect of temperature on the vision and foraging activity of planktivorous fish at Iztacala Campus of the Universidad National Autonoma de Mexico. This research was supported by grants 2011/03/B/NZ8/02093 and 2014/15/B/NZ8/00245 from the Polish National Science Centre to Z.M.G., and two 6-month Postdoc Fellow grants to R. Kumar and S. Kunjiappan from the NAMASTE Consortium (Networking and Mobility Actions for Sustainable Technology and Environment in India) within the framework of the Erasmus Mundus Program. The study was performed at the Biological and Chemical Research Centre, University of Warsaw, established as part of a project cofinanced by the EU European Regional Development Fund under the 2007-2013 Innovative Economy Operational Programme