Great Cormorant (Phalacrocorax carbo) numbers in Japan have increased in recent years, and currently (2013) there are estimated to be more than 100,000 birds. With the growing cormorant population, concerns about the impacts on fish stocks and fisheries, as well as on local forests, have grown.
The main cormorant-fishery conflicts occur at inland recreational fisheries, and the quarry/prey species is often the ayu (or ayu sweetfish Plecoglossus altivelis) which is popular both commercially and recreationally. Cormorants are deemed a serious threat to fish stocks, although the level of damage varies between regions. A variety of approaches has been employed to deter cormorants from selected areas.
The Japanese Ministry of the Environment has developed a Wildlife Management Plan which places cormorants under the responsibility of regional governors. In addition, a number of regional consultative groups have been established to discuss cormorant issues.
Examples of cormorant management include that at Lake Biwa (in Shiga Prefecture), where damage to fisheries and forests prompted the Prefectural Government to initiate intensive culling, starting in 2009, and in Yamanashi Prefecture where a scaring method as well as egg control has been employed since 2005/2006.
Despite the current conflicts, cormorants have historically been popular in Japan. They have been used by people for fishing, and local villagers have traditionally collected guano (droppings) to use for agricultural fertilization.
The population development, conflicts and management related to cormorants in Japan is described in more detail in a separate document.
Cast netting for the ayu sweetfish in Fuji River (Photo: A. Ashizawa).
The rise in cormorant numbers in Japan is ascribed to factors such as protection of colonies, simplification of river structures and an increase in food resources partly due to the release of fish. These factors are also thought to have contributed to the increase in cormorant numbers in Europe.
The figures for the decline in Japanese recreational fishermen, which fell from 13.4 to 9.6 million over a decade (1993 - 2003), appear to have been one of the prime motivations for the cormorant control actions. The conflicts caused by cormorant predation on farmed fish, which is one of the principal concerns in many European countries, is of far less significance in Japan. The main reason is that fish ponds in Japan are rather small and efficiently protected by nets, hindering larger birds from getting access to the fish.
The use of adaptive resource management of the cormorant population in Japan is mirrored by the approach used in some European countries. The examples of cormorant population management undertaken in Japan rely on techniques which have been used in other countries, albeit some of them (such as vibrating tapes) are not widely adopted in Europe.
In the Shiga Prefecture of Japan, the shooting of cormorants inside the breeding colonies by sharpshooters' was possible because (i) it appears to have been acceptable on ethical grounds (which is not the case in many European countries because of the issue of orphaned young), and (ii) the Government also paid 'sports hunters' to undertake shooting. This approach is not likely to be acceptable in Europe, although shooting of several thousand young was permitted in one area in Germany in recent years.
In the Yamanashi Prefecture of Japan, vibrating tapes appear to have been highly successful, although their efficacy has not been critically assessed. Attempts have been made in some European countries to employ similar methods, using 'Mylar' and other types of tape, but these have proved to be largely impractical: They are prone to breakage in windy conditions and are therefore not deployed widely.
The replacement of eggs with fake eggs is an established cormorant control method in Europe, but its success varies. It is almost only practical with ground-nesting birds, and it is very manpower intensive and potentially costly.
The use of 'dry ice' to render eggs infertile has not been used in European countries to a similar extent as in Japan; egg oiling is the most widely used technique in the few European countries where egg culling is taking place in selected colonies.