There are approximately the same number of fish at the start and end of each year, even though fishing activity has continued.
Fish can recover from being fished - the stock of fish is on a constant level
The fishing fleet annually harvests only as many fish as will be replenished in that year. 
The quantity of fish caught and the population of the fish is maintained such that there isn't a decline in the fish population and there is an adequate supply for the market.
It is managed sustainably when the maximum sustainable yield is maintained at a steady rate, essentially not taking more fish from the sea than can be replaced within the same time frame.
Fishing does not exceed the maximum sustainable yield of the baseline fish population.
They don't fish more than the allowed quota
That the size of the catch matches the amount that the fish population increases in any year.
If the fisheries are healthy, the fishing can be allowed at Maximum sustainable yield
The quantity of fish being caught is not larger than the rate at which the fish reproduce. The quantity of boats are smaller the amount of fish available to be fished.
Taking a fish catch that matches the growth in fish population for the size of population. There is scope to manage the fish population to an optimum size for reproduction providing that the key parameters for doing this are known.
The amount of fish that is taken is not more than the amount that can be regenerated in the same amount of time.
The same number of fish will remain in the fishery indefinitely, with a stable population
The growth rate of the fish population is at its highest and the boats are catching fish at the same rate the fish population is growing.
They don't catch any more fish than the replacement rate.
The fishery is managed in such a way that the fish population is maintained at a level which supports a fishing industry while protecting the long term survival of fish, the industry and the environment.
Fishing is done at a capacity less than maximum sustainable yield and the fish population doesn't decline significantly.
given that the needed renewal resources are maintained for the fish, fishing is done within the maximum sustainable yield 
No more than the maximum sustainable yield is caught each year. The population is not being over-fished.
fishing is kept within the maximum sustainable yield
The quotas for boats is lower than the maximum sustainable yield
This means the fish population does not decline over time.
The amount of fish that are harvested is equal to the annual growth in the numbers of fish, not more.
