Clay-eating as one of the forms of geophagy is well known and documented in many animals (for more data, see Dietary Clay Boilies. Natural product number one in the ethical angling world). Unfortunately, next to nothing is known about clay-eating in fish in the nature (in particular, Ms. Ulli Limpitlaw does not report such cases: personal communication).

Some our observations on indirect clay-eating in fish (in clayey localites of the Goryn river and other right affluents of the Pryp’yat river, Ukraine, beginning from the 1970s) are given below.

Among various clayey localities in the foregoing region, those localities that are colonized by the burrowing invertebrates, such as maylies Ephemeroptera (in particular, Ephemera vulgata), with the accompanying microfauna, are most attractive for fish. Localities of this type are easily detected on the above-water (dried) and underwater clayey or clayey-carbonate grounds dotted with the numerous burrows of mayfly larvae. Cyprinids (about 15 species, except limnophilic species), percids (such as perch, Perca fluviatilis, sander, Stizostedion luciopera, and Don ruffe, Gymnocephalus acerina) as well as juvenile pike, Esox lucius, wels, Silurus glanis, and burbot, Lota lota, constantly keep these places. Judging by the compostion of their intestines, larvae E. vulgata are the main component (up to 90-95 weight percent in benthivorous cyprinids) of their diets. Large benthivorous cyprinids, such as bream, Abramis brama, and others, can dig the clayey and (harder) clayey-carbonate grounds unaidedly: in our context, searching for the larvae, they can ingest the clay. Other fish explore actively and nibble the crumbling pieces of the clayey bank or feed in the windy weather, when the waves break up the clay ground (with the characteristic muddy trails) and wash the larvae.

Fresh chunks of the burrowing clay, placed purposely at the closely spaced sites of the sandy shallow (30-50 cm depth), attract juvenile cyprinids, like ever-present roach, Rutilus rutilus, and others, with an intensive nibbling.

Besides the general observations, interesting results have been obtained with the assistance of express pair comparisons of the olfactory preferences for the different clays. Of the two balls (4 cm diameter, equal color) made, one, of the fresh burrowing clay and, the other, of the same but old, sun-dried clay (collected at the shore) and placed at the sandy shallow at the 30 cm distance each from other fish have prefered the first (the total number of pair tests n = 12, fixing the first nibble, sign test, P < 0.05).

The olfactory attractivness of balls made of the fresh burrowing clay must be determined, in the first turn, by the exometabolites of live E. vulgata larvae. At high ... Read more »