A dynamic fisheries model is developed to simultaneously optimize investment in the resource stock (the fish) and investment in the capital stock (the fleet). Each of these investment problems faces a major complication; investment in the resource is constrained by the natural population dynamics, while investment in the physical capital stock tends to be irreversible because capital used in natural resource industries is often nonmalleable. The model assumes a seasonal fishery in which annual escapement and capital investment levels can be controlled. A dynamic programming approach is used to analyze the model heuristically and numerically. The comparative dynamics of optimal investment strategies are studied, with regard to (i) delays in investment, (ii) population dynamics parameters, (iii) fish price, (iv) capital cost, (v) depreciation rate, and (vi) discount rate. In particular, the depreciation rate and the ratio of unit capital costs to unit operating costs play interesting and complex roles in determining optimal investment levels.