The Situation of Main Commercial Fishing in Lake Biwa

Transcription

1 The Situation of Main Commercial Fishing in Lake Biwa Then, I would like to explain the fisheries situation change in Lake Biwa for these 60 years by comparing with fluctuation of some fishing yields recorded on statistics. Before explanation of fishing statistics in the lake, I would like to brief on Japanese Fishing Statistics System. The start of fishing statistics as an institutional system in Japan was 1943 by the Japanese Ministry of Agriculture, Forestry and Fisheries. This year was just middle of the World War II, and Japan faced to sever food shortage. As one of counter measures to such situations, the national government decided to handle all foodstuffs under national food control system. Also fish were not excluded from such national control. In order to promote the national food control system smoothly, the national government introduced a statistics system on agriculture, forestry and fisheries, and all food productions in Japan started to be grasped under this statistics system since then. After the end of the War, the national food control system was repealed, but such statistics system has been continued because it is necessary as basic information for policy making on agriculture, forestry and fisheries. The fishing data in Lake Biwa too, therefore, have been recorded since And another characteristic of Lake Biwa fisheries in statistics system is a levy system for incomes from each fisherman's fishing activities. This system was introduced about 100 years ago for fisheries resource maintenance by fishermen themselves. They have paid 3% of their fishing incomes to their fisheries cooperative association. The statistics by this system, however, has not been exact because of self report system. Therefore, we officially use national statistics system. First on Ayu-fish fishing yield records; Basically, ayu-fish is a migratory species between sea and river. Ayu-fish in 1

2 Lake Biwa, however, does not migrate between sea and river. Instead of it, one part of them migrates between the lake and river and another keeps to stay in the lake throughout their lives. Migrating ayu-fish in spring to river grows in upstream area, whereas another one does not grow big. The reason of this difference is that river migrating ayu-fish can feed periphyte diatoms on rocks in upstream, which are indispensable to grow up, but lake staying one keeps to feed planktons in the lake. However, if once they are transported to river either naturally or artificially, they can grow up without fail. Using this character, many parts of ayu-fish caught as commercial fishing in the lake are transported to various parts of Japan in living style as seedlings for aquaculture or for releasing in rivers for game anglers. This figure shows the fluctuation of annual fishing yields of total fish and ayu-fish in the lake. We can read from this figure that about half of annual fishing yields of fish in the lake is occupied by ayu-fish, and the tendency of ayu-fish yields also controls fishing yield conditions. The reason why the total fishing yields as well as ayu-fish yields increase during from middle of 1960s to early 1990s is due that fishing tools were improved and fishing boats were powered up. After middle of 1990s, the impacts of such fishing activities began to appear on the result of annual fishing yields. On the other hands, the efforts for maintaining and increasing ayu-fish resources have been continued because of their importance for fisheries in the lake. The most famous program for those efforts is construction and promotion of two man-made rivers for spawning ground of ayu-fish. This program was planned as a compensation work for Lake Biwa Comprehensive Development Project because spawning grounds for ayu-fish around river mouth areas might be dried up by water level down on the project. The principle of structure on man-made river is to use spawning behavior of ayu-fish. The motivation for starting spawning in ayu-fish is water temperature in autumn, under 14 degree centigrade. If they can spawn in late summer before probable water shortage in autumn, ayu-fish resource might be maintained. Fortunately, Lake Biwa has thermo-cline in summer season. The man-made rivers were constructed for using this thermo-cline. Water to the man-made rivers are withdrawn through two pipe lines; one takes high temperature water, about 25 degree 2

3 centigrade, from epilimnion at 5m depth, and another takes low temperature water, about 10 degree centigrade, from hypolimnion at 25m depth. At the pumping site, these two different temperature waters are mixed for getting 14 degree centigrade water. Adult ayu-fish for spawning are cultured in pond by light culture for early growth. The ayu-fish resources have been maintained since the construction of two man-made rivers in Nowadays, however, a serious problem of this measures is that ayu-fish in the lake have faced sever fish disease by Flavobacterium psychrophilum, we call this disease as cold-water disease. This bacteria was introduced from trout aquaculture pond to ayu-fish aquaculture pond, and then introduced to hatched out fries in the man-made rivers, and has been expanded in all areas of the lake. The rapid decreasing since 1990 is also the result of mass decease by this fish disease. At this point of time, I would like to introduce an interesting phenomenon in the lake. From early summer to middle of September in 1994, the catchment area of the lake was visited by sever water shortage. In that season, rice paddy fields require large amount of water for satisfactory growth of planted seedlings. In that year, water from Lake Yogo, where is located on about 40m higher than Lake Biwa, was developed for irrigation in northern part of Shiga Prefecture and may be used in only necessary case, was ceaselessly irrigated to the paddy fields. For making up water lack of Lake Yogo, water from Lake Biwa was pumped up. Just in summer time, pond smelts are breeding season. And in that year, propagating pond smelts in Lake Yogo washed away to Lake Biwa from Lake Yogo with water supplied to Lake Yogo from Lake Biwa. In Lake Biwa, pond smelts were introduced several times some years ago. However, they were not able to grow in Lake Biwa because the lake was oligotrophic. Pond smelt is a fish species adapting to eutrophic water area. But in 1994, Lake Biwa was already changed to mesotrophic lake. Therefore, in this time, pond smelts flowed into Lake Biwa from Lake Yogo were able to propagate, and targeted for commercial fishing instead of ayu-fish decreased by fish disease. And the rapid decrease since 2000 is the result of over fishing. Then, we would like to observe impact from coastal line development on 3

4 statistics records. The typical fish species, which propagate in coastal areas of Lake Biwa, are crucian carp and moroko-fish. The Up-ward trend of crucian carp until middle of 1960s shows the result of improvement on fishing tools, mainly gillnet. Especially, rapid up-ward in early 1960s is the result of introduction of triple gillnet, named hell-net. The reason why named hell-net is, when once fish touch the middle net and then turn back for wanting to escape, the fish are caught by outer-net and can never escape. The rapid decrease since 1965 is the impact of reclamation of satellite lakes around lake Biwa. Because satellite lakes around the lake had been main spawning grounds for crucian carp. The same trend of moroko-fish has the same meaning as crucian carp case. The improvement of fishing tool for moroko-fish was change from gillnet to dragnet using winch machine on boat. The stability of fishing yields from 1968 to 1986 in case of crucian carp and from 1976 to 1994 in case of moroko-fish is thought to have been balanced between fishing efforts and fish resources. The time lag for about 10 years in increasing case and decreasing case between crucian carp and moroko-fish may be attributed to the difference of their spawning behavior, crucian carp has to spawn in thick aquatic weed community, on the other hand, moroko-fish can spawn in thin aquatic weed community too and also on root of floating weeds. The both rapid decreasing trends from 1986 in case of crucian carp and from 1995 in case of moroko-fish are impacts of coastal line development by Lake Biwa Comprehensive Development Project and invasion of carnivorous fish, black bass. For counter measures to such impacts, Shiga Prefecture has promoted the crucian carp and moroko-fish fry producing and releasing project and black bass exterminating project collaborating with the fishing association in the 4

5 prefecture. Then, I would like to explain the rise and fall of freshwater pearl aquaculture in Lake Biwa. The technology for freshwater pearl aquaculture was introduced to Lake Biwa in early 1950s. The main aqua-culturing grounds were satellite lakes around Lake Biwa. And as pearl shellfish for freshwater pearl, an indigenous spices of Lake Biwa, Hyriopsis schlegeri, Japanese name "Ikechou-gaiz" means pond butterfly from its shell open shape, was used. At the same time when the pearl culturing started, fishing of pearl shellfish had suddenly occupied a main position in the lake fisheries. This position had been continued until middle of 1970s, but over fishing did immediately impact to pearl shellfish resource and it had been directly affected to the statistics. Being in inverse portion to decreasing of pearl shellfish fishing yields, cultural pearl amounts produced in the lake had rapidly increased until For dealing with this situation, Shiga Prefecture had developed a technology for culturing pearl shellfish. The stability for 15 years from 1970 to 1985 shows the effect of such technology. In this duration, pearl shellfish fishing has almost disappeared in the lake. The rapid decrease trend after 1985 is the result of technology transfer to China. In accordance with economic principle, cheap and large amounts of aquaculture pearl coming into the market from China have perfectly expelled the Biwa pearl since then. Nowadays, We can almost find no freshwater pearl aquaculture plant as well as no pearl shellfish fishing activity anywhere in and around the lake. By the way, it is very ironical that the technology for cultivating pearl shellfish bore a new ecological problem in the lake. Pearl shellfish can not grow without parasitic life on fish fin for 10 to 15 days in fry stage. All of indigenous fish species in the lake are too weak for cultivated host to pearl shellfish fry. Therefore, fisheries laboratory staff of Shiga Prefecture searched the most relevant exotic fish species for host. The searching work was resulted by introduction of bluegill. During the period when the cultivation of freshwater pearl had been sustained, introduced bluegill had 5

6 been cultured in enclosed circumstances. However, when it was not necessary to be cultured for pearl production, enclosed bluegill were released to the lake. Nowadays, bluegill as well as black bass is threatening the lake ecosystems. 6