THE RELATIONSHIP BETWEEN FORAGING AND SHOAL POSITION IN A MIXED SHOAL OF ROACH (RUTILUS-RUTILUS) AND CHUB (LEUCISCUS-CEPHALUS) - A FIELD-STUDY

Feeding rates of mixed shoals of juvenile roach and chub were observed in a shallow stream near Cambridge (UK). Roach at the front of the shoal had significantly higher feeding rates than roach at the back and than chub in either front or back positions. Position in the shoal also had a significant effect on the kind of food consumed, with front roach feeding more on plankton and back roach more on bottom food. Altogether 36 fish from the stream were caught and marked. Half of these were deprived of food and the other half well-fed for 3 days in captivity. After release 36 % of them joined their old shoal again. Individuals from the starved group occupied front positions significantly more often than well-fed fish, but after 2 days this difference disappeared.