The level of catch and associated level of fishing effort that maximises profits in the industry over time and on a sustainable basis.; The theoretical greatest difference between total revenues and total costs of exploiting a fish stock under existing environmental conditions and where inputs are valued at their social opportunity costs. MEY is equal to the maximum resource rent and is obtained where the marginal product of effort is equal to the marginal cost of effort. MEY is realized at a level of fishing effort which is below that one which produces maximum sustainable yield.