By-catch reduction devices increase the equilibrium population of the by-catch species at every level of effort directed at the target species of fish. Also, cost per unit effort is increased and this reduces effort and profit. It is shown that for effective bycatch reduction devices, the fall in effort makes an unimportant contribution to the increase in the equilibrium population of the by-catch species. Thus, it is concluded that mandatory by-catch reduction devices be introduced at minimum cost. That is, without additional regulations which mainly reduce effort and profit.