Over exploited marine populations:
If you pull too many fish out of the water there must eventually be none left. That’s obvious enough. If you fish at a level that does not result in a change in the number encountered in an area, then all is well. It has been found that if harvesting the population produces a mild decease in numbers the population will recover when you stop fishing. If you fish until the population is moderately over-fished and then stop recovery will be faster. This is greeted with surprise. (Phillip Neubauer et al. Resilience and Recovery of Overexploited Marine Populations SCIENCE vol. 340 no. 6130 April 19, 2013 page 347) Actually, the situation is not that simple. The history of the fishing of a population – the intensity of the fishing and the time before collapse – has an effect. Either high rates of harvesting or a period of time during which over-fishing occurred correlates with faster recovery but the combination makes things work. I cannot claim to understand why. But I do think I understand why a moderately depressed fish stock might recover faster than a mildly depressed stock.
If the population is only mildly reduced then nothing much has changed. But at a moderate level, evidently, the average relatedness of the fish has after a generation increased. Increase kinship and you increase fertility. It is not quite so simple. Inbreeding reduces fertility of course. And the effect seems to saturate out – further lack of kinship after a point has no further effect in the first generation but continues and probably gets worse in later generations of insufficient relatedness. All that seems to me to be well established by data I pulled out of the literature and posted on nobabies.net last December.
But now consider a fish in a badly depleted population. That fish must now travel a very long distance to find a mate at all. In that case what mate a fish can find is probably going to be very unrelated. The fertility depression continues and probably gets worse. Recovery must wait until at least some fish strike a fortunate match and a local community can develop that are suitably related. The time it takes for this to happen will be very unpredictable since one is waiting for unlikely events to happen to small numbers. It may never happen.
This as another example of how the principle can be seen at work in a large number of contexts and since the principle is poorly appreciated leave results that are not easily explained.
The UN is attempting to have legislation passed that will permit the many depleted fish stocks to be spared until they recover and then return to fishing to the point of maximum sustainable yield.
There is another strategy that has worked in the past. I refer to a book called The Creek by J. T. Glisson. Among other things the book describes fishing by local people in Florida in Orange Lake. Glisson’s father was evidently an astute man who talked with the locals and listened to their fishing experience. Then he had an idea and drove up to Georgia to see whether there was a market for the fish in the lake. He decided that people in Georgia would buy fish in effectively unlimited amounts. That done, he collected the local folk and persuaded them to try his idea. It was a matter of judicious over-fishing.
They divided the lake into zones, the rights to the fish in each zone being assigned to one family. There they fished. Their way of fishing was to use some kind of trap box they would lower into the water repeatedly. It was hard work, but it did bring up fish. The experience of any one family was that at first they caught many fish to sell. Then the stock would be exhausted and the marketable fish would be replaced by “trash fish,” of no market value. This was the moment of truth. Some families gave it up and did little more fishing. Other families industriously continued catching the trash fish. For them eventually the good fish returned and they were making money again.
The lake became very productive. In living memory it was a fisherman’s paradise, a legend. The locals ignored the sport fishermen; their take was relatively small. So the lake became a patchwork of over harvested and neglected regions. The recovery of the game fish is easily explained. A few would migrate in from unfished areas. Their degree of relatedness would be such that they would have a population explosion. In any one year there were areas untouched and areas with only the trash fish. But the overall productivity of the lake was extraordinary.
Alas the government got involved. They decided that trapping the fish wasn’t right; apparently the fish should be left for the rich sport fishermen. By hiring local young men as game wardens they were able to shut down the fishery. The local people of course were saddened. When it became clear that it was over Glisson’s father accurately predicted when, give or take a year, the lake would revert to type and the game fishing would no longer be spectacular. Everyone lost; the sport fishermen, the locals and the fish hungry folk of Georgia.
The ocean is not a lake. But the same principle might work. Leave zones of sanctuary but for the most part over fish for some years. It usually takes thirty to fifty years or more for an area of the ocean to be depleted. Then put it off limits for the roughly ten years it takes for recovery. Whether that would produce more fish than the maximum sustainable yield I do not know, but it would be a matter of arithmetic. Does the recovery bonus exceed the loss during the years the sea is fallow? You don’t even have to buy into the idea that relatedness and fertility are the cause although the evidence is there, for instance in the paper I have referred to being involved in about turning the concept against malaria; obviously there the purpose was to decrease mosquitoes rather than increasing fish. Other causes of the behavior of fish stocks have been considered such as a change in the environment, or genetic or behavioral changes in the fish have been considered. If empirically it works, then go for it. The down side of course is this requires having policies with decades to centuries as the time scale.
There have been 91 visitors in the past month.