The three major harvesters of North-East Atlantic mackerel are modelled as exercising alternative coalitional strategies. Incentives for coalition formation are analysed by finding the maximum rents that harvesters can obtain under all possible coalition arrangements. The main result is that for a wide range of parameter values, the all-player coalition is unstable if joint rents cannot be redistributed amongst the players. Without transfers, the outcome is the type of non-cooperative fishing currently observed. However, if transfers are possible, side payments can be found for the mackerel fishery to sustain the efficiency gains of joint maximization. It is shown how they can be set to enhance the likelihood of their being accepted by players operating under a regional fisheries management organisation, or to be self-enforcing and protect against defection by sub-coalitions.