A global renowned fisheries scientist presented one paper in the scientific journal “Nature” about 7 years ago stating that the Pacific tuna stocks were all endangered. His theory was based on the fact that the nominal catch per unit of effort (CPUE) of the Japanese longline fishery declined rapidly in the 1950s. The CPUEs he used were simple values of reported catches divided by reported efforts, while no scientific adjustments were made for various changes in the real fishery.
Many fishery scientists of the world were concerned with this paper and pointed out the errors and incorrect assumptions that the paper contained. Even a joint protest of various scientists was sent to the editorial of the journal, but that opinion was only posted in the column of the “comments from public”. Thus, only the original sensational report remained on record.
It is factual that the apparent catch rate (CPUE) of Japanese longline fishery dropped sharply in the 1950s. It is possible that this drop is a phenomena often observed at an initiation of the exploitation of populations which had never been fished. In 1952, when the Marine Activities Restriction Lines set after the World War II for the Japanese fisheries around Japan were lifted, the tuna fleet expanded very rapidly into the distant waters where tuna fishing had never occurred until that point. Naturally unfished tuna populations found in these waters would have been overpopulated and starving. Therefore, the initial catch rates were excessively high, not reflecting real abundance of the population. This situation is very often observed in many fish species.
Some scientists believe that the initial decline of catch rate partly may reflect the learning process of fish (i.e. fish learns danger of hooked bait by experience). This theory has not been proven but may be the case.
It is important to note the fact that the fishery changed very quickly during this period, as the longline fleet expanded to new areas and seasons. In addition the longline catches were first sold to tuna canneries but soon they were sold at the sashimi market with the development of super freezer. This means that their initial objective was to get maximum volume of tuna regardless the species (as price did not differ much in canning industry among species and quality), and later, to obtain maximum values by targeting high-priced fish. This shift would have resulted in apparent decline in overall hook-rate.
Current tuna catch in the Pacific Ocean is more than 20 folds of the catch in 1950s. If the paper in question was to be correct, there would have been no way to catch that much fish at present. Instead, with such amount of catches, tuna stocks should have been completely depleted. The fact is that most of the Pacific tuna stocks are still at proper sustainable levels. This fact alone would be a solid evidence proving the theory in that paper is wrong.
Unfortunately this paper is, like the Bible, still very frequently cited as scientific basis claiming that tuna stocks are endangered. Even some scientists who should be very well aware that the paper contains misconceptions cite it. It is a bit scary that some erroneous (regardless intentionally or through misconceptions) information in one paper, once published could mislead the public on status of tunas to such a great degree and extent.