Japan Sea National Fisheries Research Institute
Suido-cho, Niigata 951, Japan
ABSTRACT The Japanese flounder, Paralichthys olivaceus, is one of the most important fishes in the coastal fisheries of Japan. But recently, overfishing has caused a reduction of the stock size. To enhance the stock, artificial seeds of Japanese flounder have been released. Interactions between released and wild flounder were examined to determine the success of the stocking program. We performed experimental releases of artificial seeds in the shallow waters off Igarashi-Hama on the northwestern coast of Japan, from 1990 to 1992. The growth rate of wild flounder varied annually depending on the abundance of mysids that are the most important food for the flounder on the nursery ground. When mysids were less abundant, released flounder dispersed rapidly from the release site, ingested small amounts of food, and grew slowly compared with other years.
The feeding habits of released flounder differed from those of wild flounder when mysids were less abundant. Flounder released then ingested less food and also consumed gammarids which the wild flounder never ate. It was assumed that an abundance of mysids was more critical for released than for wild flounder. Further investigations on the carrying capacity of the nursery ground and improvement of the quality of artificial seeds are needed to enhance the stock size of Japanese flounder efficiently.
INTRODUCTION The Japanese flounder, Paralichthys olivaceus, is a large flatfish with maximum total length of 1 m and weight of more than 10 kg. The fishery production of the flounder is about 7,000 metric tons a year; however, there have been increased fishing efforts suggesting that the stock of the flounder has been decreasing. Recently, artificial seeds of Japanese flounder have been released to enhance the stock size, and the number of released seeds has been increasing. In 1991, about 15 million artificial seeds were released in Japan (Fisheries Agency and Japan Sea-Farming Association 1993). The life cycle of Japanese flounder is shown in Figure 1. In Niigata, mature Japanese flounder inhabit offshore areas where the water depth is about 100 m. They spawn in shallower water of less than 50 m from April to June (Kobayashi 1974, Nashida 1984). Eggs and larvae undergo a planktonic life for 1 or 2 months (Minami 1984). Metamorphosing larvae are transported to near-shore by currents (Imabayashi 1980a, Minami 1985, Fujii et al. 1989, Goto et al. 1989) and settle on sandy nursery grounds of less than 10 m in depth (Koshiishi et al. 1985). They spend about 2 months in their nursery ground and then migrate offshore. This migration is associated with the shift of feeding habits from mysids to pisces in August when they reach 100 mm in body length. On the nursery ground, the most important food for Japanese flounder is mysid crustaceans (Imabayashi 1980b, Yasunaga and Koshiishi 1981, Koshiishi et al. 1985, Kato 1987).
The size of most artificial seed is less than 100 mm in body length when they are released. Seed are released into the natural nursery ground where the density of the flounder is normally the highest. In this stage, releasing artificial seeds may cause severe interactions between released and wild fish, such as competition for habitat or food; the latter results in cannibalism or starvation.
In this report, we focus on the ecological interactions between released and wild flounder.
MATERIALS AND METHODS
This study was carried out from 1989 through 1992 in the shallow waters off Igarashi-Hama on the northwestern coast of Japan (Fig. 2). Igarashi-Hama is part of a 60 km sandy beach.
Field surveys of wild flounder and food organisms were carried out from 1989 through 1992. Experimental releases of artificial seeds were carried out on June 28, 1990, July 2, 1991, and June 30, 1992, which is the suitable season for the release of Japanese flounder (Koshiishi et al. 1986, 1988, 1991). The artificial seeds were released in 6-m-deep water (release site located along line with a dot; see Fig. 2). Hence, this line will be called "the release line." The number of released flounder was 120,000 in 1990, 80,000 in 1991, and 58,000 in 1992. The mean body length was 43.4 mm in 1990, 52.5 mm in 1991, and 43.9 mm in 1992 (Table 1).
The flounder and food organisms were collected periodically along the lines in Figure 2 and around the research area. To collect the flounder, a small beam trawl of 2 m width and 0.4 m height was used. The net aperture was 2.1 mm in early season and 3.6 mm in later season. To collect food organisms, a sledge net of 0.6 m width, 0.4 m height, and 0.76 mm mesh aperture was used. The towing speed of both gears was about 1.5 knots. Some flounder collected by the small beam trawl were preserved in a 10% seawater formalin solution, and others were frozen on the research boat and stored at -80oC for otolith microstructure or RNA/DNA analysis. The organisms collected by the sledge net were preserved in 5% seawater formalin.
The artificially produced flounder have melanism on the blind side and wild flounder dont. Released seed were distinguished from wild flounder easily by the melanism. The stomach contents of 979 wild flounder and 695 released flunder were examined to determine the feeding habits. The otolith microstructure was analyzed by Gotos method (Goto unpublished data) to estimate the growth rate of wild flounder. The modified Schmidt-Thaunhauser- Schneider (STS) method (Nakano 1988) was used to quantify RNA and DNA in the nucleic acids to evaluate the nutritional condition of released flounder.
RESULTS AND DISCUSSIONS
Population Size and Growth of the Wild Flounder in Relation to the Abundance of FoodThe water temperature at 4 m depth from June through August is shown in Figure 3. In mid-June, when Japanese flounder began settling, the temperature was between 18oC and 21oC. The highest temperature reached 26oC in late August. The mean water temperature in July and August was 23.0oC in 1989, 24.4oC in 1990, 23.1oC in 1991, and 23.0oC in 1992. The density of wild Japanese flounder in the research area increased in June and July, during the settling season, and decreased in August, during the offshore emigrating season (Fig. 4). The mean density of the wild flounder in the research area between mid-July, after the end of settling, and mid-August, before the beginning of the emigrating, was used as the population size index of that year. The population size index of the flounder was 2.4 in 1989, 5.4 in 1990, 7.2 in 1991, and 2.0 in 1992. The density of mysids, including about 20 species of which Acanthomysis robusta is dominant (Hirota et al. 1986), changes seasonally in the research area. It began to increase in May and June, peaked in July, and decreased rapidly in August (Fig. 5). The maximum abundance was found to be 339 individuals/m2 in July 1989. The mean density of mysids in July and August, when Japanese flounder stay in the nursery, was used as the population size index of mysids of that year. The population size index of mysids also fluctuated annually and was 121 in 1989, 44 in 1990, 77 in 1991, and 114 in 1992.
During the nursery residence, Japanese flounder grow lineally in terms of the body length (Koshiishi et al. 1988). The daily growth rate of the wild flounder estimated from the microstructure of otolith was 1.9 mm in 1989, between 0.8 mm and 1.3 mm in 1990, depending on whether settling occurred late or early in the settling season, respectively, between 1.6 mm (late settlers) and 1.8 mm (early settlers) in 1991, and 1.8 mm in 1992 (Fig. 6). Rearing experiments show that high temperatures between 17 and 26oC result in high growth rates for all body lengths (Fig. 7; Fujii unpublished data). However, field data showed an inverse relationship between water temperature and the growth rate (Fig. 8). The relationship between population size of mysids and daily growth rate of wild flounder is shown in Figure 9. The growth rate depended on the population size of mysids. Rearing experiments showed that maximum daily growth rate is about 2 mm (Fujii unpublished data). The observed daily growth rate of 1.8 to 1.9 mm is higher than any other nursery ground in Japan (Imabayashi 1980a, Goto unpublished data). The growth rate was not limited by food in 1989 and 1992. In 1990, when mysids were less abundant a decrease in the growth rate was observed. In 1991, the decrease in growth rate was observed only among the late settlers. Therefore, growth rate in 1990 and 1991 is considered limited by food.
It has been reported that the growth rate of plaice Pleuronectes platessa in the western Wadden Sea, the Netherlands, is not limited by food (Van Der Veer 1986). However, it has also been suggested that growth is limited by both water temperature and availability of food in some species of flatfish such as plaice (Berghahn 1987, Van Beek et al. 1989 Karakiri et al. 1989), winter flounder Pseudopleuronectes americanus (Sogard and Able 1992), and sole Solea solea (Marchand 1991). It appears that the abundance of food is more effective in influencing annual growth rate of Japanese flounder than the water temperature for the research area. If the supply of the food is sufficient, then the water temperature might determine the growth rate.
Dispersion, Feeding, and Growth of Released Flounder
An example of dispersion is shown in Figure 10 for 1992. Released flounder dispersed gradually both offshore and parallel to the coast. Offshore movements were limited to water depth less than 8 m. During the nursery residence, the farthest distance where released flounder were recaptured was 2.5 km from the release site, and most released flounder were recaptured less than 1 km from the release site. The number of flounder recaptured in the first 5 days after the release along the "release line" was greater in 1992 than in 1990 and 1991 (Fig. 11, upper column) although the number of released flounder was the smallest in 1992 (Table 1). The density of mysids along the release line decreased rapidly just after the release but began to recover 10 days later. The minimum density of mysids along the release line was 20 individuals/m2 in 1990, 73 in 1991, and 108 in 1992. In 1992, the minimum density of mysids was five times as much as 1990, and it recovered more rapidly than in 1990 and 1991 (Fig. 11, lower column). Perhaps a shortage of food hastened the dispersion of flounder in 1990.
The incidence of feeding (percentage of fish found with food in their stomach) and feeding rate (stomach content weight of all fish/body weight of all fish) of released flounder recaptured in the research area is shown in Figure 12. The incidence of feeding of released flounder was greater in 1991 and 1992, and feeding rate was also high except for a few sampling data. It appears that in 1990, when the growth of wild flounder was limited by food, the food shortage caused both the reduction of the feeding incidence and feeding rate of released flounder. The RNA/DNA ratio, an indicator of nutritional condition of the flounder (Fujii 1990), decreased for 3 days after the release in 1990, while it didnt in 1991 and 1992 and, in fact, was maintained at a higher level than in 1990 (Fig. 13). It is suggested that the nutritional condition of released flounder in 1990 was poor compared with other years.
Released flounder did not grow for 4 or 5 days after release. This phenomenon has also been observed in the rearing experiments (Fujii unpublished data). Since feeding of the artificial seed is stopped 2 or 3 days before the release to prepare for the transportation, it is likely that it takes 4 or 5 days for released flounder to recover physiologically prior t the beginning of growth. After that, they grew lineally and the daily growth rate was 1.0 mm in 1990, 1.2 mm in 1991, and 1.4 mm in 1992 (Fig. 14). The mean water temperature during the first month after release was 23.6oC in 1990, 22.1oC in 1991, and 21.6oC in 1992. Rearing experiments show that higher temperatures in this range led to higher growth rate (Fig. 7; Fujii unpublished data). It is suggested that the abundance of mysids resulted in the highest growth rate in 1992 in spite of the low water temperature.
Comparisons of Distribution, Feeding Habit, and Growth Between Released and Wild Flounder
The number of captured wild flounder along the release line was low or decreased rapidly after the release and increased as the the density of released flounder decreased except for 1992, when the population size of wild flounder was relatively small (Fig. 11, middle column). At other stations in the research area, population movements of wild and released flounder appeared to be more independent.
The comparison of incidence of feeding, feeding rate, and stomach contents of wild and released flounder in 1990, when wild flounder were abundant and mysids were less abundant, is shown in Figure 15. Released flounder were inferior to wild ones in both incidence of feeding and feeding rate. Food composition was slightly different between released and wild flounder. Released flounder recaptured on days 1, 2, and 4 had ingested gammarids, which the wild flounder had not. In 1992, when mysids were abundant, more than 85% of the released flounder ingested food on every sampling date. There is no significant difference in the feeding rate between wild and released flounder (Fig. 16). Released flounder did not ingest gammarids. It is suggested that when the density of mysids is low, released flounder do not ingest food efficiently compared with wild flounder.
The Japanese flounder is cannibalistic (Minami 1986). It has been reported that if the difference in body length is more than three times, then cannibalism can occur (Tanaka et al. 1989). From 1990 through 1992, the difference in body length between released flounder and the "late settlers" was enough to cause cannibalism. However, none was found among 0-year-old Japanese flounder. Some of the 1-year-old Japanese flounder collected in the research area ingested released flounder (Table 2). Released flounder may be inferior to wild flounder in their ability to escape from predators.
The growth rate of released flounder was always less than wild flounder. For example, reduced feeding ability of artificial seed (Furuta 1988) may cause a reduction in the growth rate.
CONCLUSION
An abundance of mysids is needed for the success of artificial seeds on their nursery grounds. In the absence of food, release of artificial seeds will not be successful. Furter investigations on the carrying capacity and improvement of the quality of the artificial seeds are needed to enhance the stock size of Japanese flounder efficiently.
ACKNOWLEDGMENTS
We would like to thank the fishermen of Igarashi-Hama; without their support and cooperation, our research could not have been accomplished. We are also grateful to N. Naganuma and K. Kubota for their helpful assistance.
This study was supported by Bio-Cosmos Program from the Ministry of Agriculture, Forestry, and Fisheries of Japan.
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