General

Small fish are forced to live in schools, in which the large number of potential prey hampers, in addition to other benefits, the hunting of predatory fish and other predators for an individual prey. This effect of predator desorientation is called confusion effect (Neill & Cullen, 1974; Gillett et al., 1979; Milinski, 1979; Landeau & Terborgh, 1986; Smith & Warburton, 1992; including theory Jeschke & Tollrian, 2005; Tosh et al., 2006; Ruxton et al., 2007; Ioannou et al., 2008). To avoid this effect, predatory fish use several tactics to hunt for schooling prey. One of them is eliminating in schools the so called odd prey that differ on their species belonging, shape, size, armour, color and behaviour from the school mates (Major, 1978; Ohguchi, 1981; Theodorakis, 1989; Ranta & Lindström, 1990; Krause & Godin, 1994; Peuhkuri, 1997; Mathis & Chivers, 2003; Almany et al., 2007; Jones et al., 2010; Rodgers et al. 2011).

Largemouth bass, Micropterus salmoides (powered by Joseph Tomelleri)

Landeau & Terborgh (1986) have conducted some experiments to clarify the roles of confusion effect and prey oddity as they interact to influence the hunting success of predators. Largemouth bass (Micropterus salmoides) quickly captured solitary silvery minnows (Hybognathus nuchalis), but performed many unsuccessful attacks as prey school size was increased. At school sizes of 8 and above, predators were effectively stymied demonstrating the confusion effect. An inclusion of 1 or 2 odd (blue dyed) minnows in the school of 8 members greatly increased the ability of bass to capture both normal and odd prey, but this effect disappeared at the school size of 15. The implications of the foregoing results for understanding the adaptive basis of mixed species flocks, herds and schools in various animals is discussed (Landeau & Terborgh, 1986).

The number of prey, rather than the density or area occupied by the group, has the greatest effect on the predator confusion (Tosh et al., 2006; Ruxton et al., 2007; Ioannou et al., 2008). In addition, it is shown (Tosh et al., 2006) that the confusion effect is U shaped with the strongest changes for 5-15 members of model groups.

Fishing practice

Umbrella rigs allow to imitate small groups of fish and other animals, with the number of lures ranged usually from 3-6 for casting rigs to several tens for trolling rigs. With this number of lures, the confusion effect may occur. To verify the occurrence of this effect, we have tested in the field commercial radially symmetric casting umbrellas equiped with 5 matched and 1 odd lures.

An umbrella pattern with 5 matched and 1 odd lures is demonstrated schematically in Fg.1 (six arms and hooks are not shown). Commercial Curly Tail Grubs (1″) of Green Body & Lime Tail (LT) and White Body & Red Tail (RT) colors were used.

Abundant common perch, Perca fluviatilis, were selected as an usable fish model.

If the confusion and prey oddity effects do not occur, the total number of caught perch must be distributed on average with an equal probability between six lures, 1/6. During 2 test days, 71 perch were landed: 50 for LT and 21 for RT. The mean value is 11,8 perch per one lure, both matched and odd one. Because an odd lure attracted 21 perch instead of mean 11,8, it means that in the foregoing special conditions of multiple choice perch prefer to select lures on color oddity.

Basic References

Almany G.R., Peacock L.F., Syms C., McCormick M.I., Jones G.P. 2007. Predators target rare prey in coral reef fish assemblages. Oecologia 152, 751-761

Gillett S. D., Hogarth P.J., Noble F.E.J. 1979. The response of predators to varying densities of Gregaria locust nymphs. Animal Behaviour 27, 592-596

Ioannou C.C., Tosh C.R., Neville L., Krause J. 2008. The confusion effect  from neural networks to reduced predation risk. Behavioral Ecology 19, 126-130

Jeschke J.M., Tollrian R. 2005. Effects of predator confusion on functional responses. Oikos 111, 547-555

Jones K.A., Croft D.P., Ramnarine I.W., Godin J-G.J. 2010. Size-assortative shoaling in the guppy (Poecilia reticulata): The role of active choice. Ethology 116, 147-154

Krause J., Godin J-G.J. 1994. Shoal choice in the banded killifish (Fundulus diaphanus, Teleostei, Cyprinodontidae): Effects of predation risk, fish size, species composition and size of shoals. Ethology 98, 128-136

Landeau L., Terborgh J. 1986. Oddity and the confusion effect in predation. Animal Behaviour 34, 1372-1380

Major P.F. 1978. Predator-prey interactions in two schooling fishes, Caranx ignobilis and Stolephorus purpureus. Animal Brhaviour 26, 760-777

Mathis A., Chivers D.P. 2003. Overriding the oddity effect in mixed-species aggregations: group choice by armored and nonarmored prey. Behavioral Ecology 14, 334-339

Milinski M. 1979. Can an experienced predator overcome the confusion of swarming prey more easily? Animal behaviour 27, 1122-1126

Neill S. R. S. J., Cullen J.M. 1974. Experiments on whether schooling of prey affects hunting behaviour of cephalopods and fish predators. Journal of Zoology (London) 172, 549-569

Ohguchi O. 1981. Prey density and selection against oddity by threespined sticklebacks. Advances in Ethology 23, 1-79

Peuhkuri N. 1997. Size-assortative shoaling in fish: the effect of oddity on foraging behaviour. Animal Behaviour 54, 271-278

Ranta E., Lindström K. 1990. Assortative schooling in three-spined sticklebacks. Annales Zoologici Fennici 27, 67-75

Rodgers G.M, Ward J.R., Askwith B., Morrell L.J. 2011. Balancing the dilution and oddity effects: decisions depend on body size. PLoS ONE 6(7): e14819

Ruxton G.D., Jackson A.L. Tosh C.R. 2007. Confusion of predators does not rely on specialist coordinated behavior. Behavioral Ecology 18, 590-596

Smith M.F.L., Warburton K. 1992. Predator shoaling moderates the confusion effect in blue-green chromis, Chromis viridis. Behavioral Ecology and Socibiology 30, 103-107

Theodorakis C.W. 1989. Size segregation and the effects of oddity on predation risk in minnow schools. Animal Behaviour 38, 496-502

Tosh C.R., Jackson A.L., Ruxton G.D. 2006. The confusion effect in predatory neural networks. American Naturalist 167, E52-E65