Abstract.The strong relationship found between the yield of bream,Abramis brama L., the average chlorophyll‐a concentration of the water and the shore‐line:water area ratio of the various parts of Lake Balaton can be described mathematically. Such a descriptive model seems to be suitable for the estimation and rough prediction of the bream yield. The regional differences between the bream yields within the lake are caused to a large extent by morphometric features, and to a lesser extent by the cholorophyll‐a concentration. A 10‐fold increase in the chlorophyll‐a content results only in a twofold increase in bream yield expressed in catch per unit of effort (CPUE); however, the shore‐line:water area ratio has a much stro