After the increase in productivity, the catch rate will temporarily increase as individual fishers try to take advantage of the corresponding increase in the catch rate to increase their profits. However, the increased catch rate now exceeds the growth rate, which causes the population to decrease. The competition for profits, open access to the fishery, and the constraints imposed by the biological growth function of the fishery combine to produce this seemingly counter-intuitive result. Once we understand this, it is easy to see why total revenue and total cost have decreased. Total revenue is lower because the catch is lower and price is assumed to be unaffected.
Total cost is lower because the total amount of effort is lower.
When open access is permitted, additional effort will be committed to a fishery up to the point at which total revenues equal total costs. Under typical revenue and cost conditions, the result is that more effort is expended to achieve the resulting equilibrium catch rate than is necessary. Given the relationships among effort, catch rates and population levels, the catch rate could be achieved with less effort at the higher population level than it can be at the lower population level. The primary intended effect of open-access regulations is to increase the cost of catching fish. This would reduce the equilibrium quantity of fish harvested.