Poster Abstract
21. ECOLOGICAL INTERACTIONS BETWEEN HATCHERY AND WILD FISH: A CASE STUDY OF THE STRONGLY PISCIVOROUS JAPANESE SPANISH MACKEREL
Kaori Nakajima, Yasuhiro Obata, Katsuyuki Hamasaki and Shuichi Kitada
Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato, Tokyo 108-8477,Japan
A surplus carrying capacity is indispensable to augmenting fisheries production by release of hatchery-reared animals. If juveniles are released beyond the carrying capacity, a reduction in the growth rate of wild fish could occur, or in extreme cases, hatchery fish might replace wild fish due to competition for prey between hatchery and wild fish or cannibalism. However, evidence supporting such ecological impacts has so far been sparse. Top predator fish species would clearly show the impacts if this actually occurred under a limited carrying capacity. Here, we investigated the ecological interaction between hatchery and wild fish using data for Japanese Spanish mackerel (JSM), Scomberomorus niphonius, in the Seto Inland Sea (SIS).
JSM is a large piscivorous fish, mainly distributed off the western coast of Japan. This species is an important fishery resource, especially in the SIS. Larval and juvenile fish are piscivorous and grow very rapidly. Figure 1 shows the annual catch and release statistics of JSM in the SIS. To recover the decreased commercial catch of this species, the National Center for Stock Enhancement initiated a stocking programme in 1998. A total of 1,080,146 and 371,715 hatchery-reared juveniles have been released in the eastern SIS from 1998 and the western SIS from 2002, respectively. The contribution of hatchery fish to the total catch in the eastern SIS was estimated at 6.1% and 30.8% for the 2002 and 2003 releases, respectively.
Large variations in the catch of prey fish, Engraulis japonicus and Ammodytes personatus, explained well the catch history of JSM in the eastern SIS, suggesting that the population dynamics of JSM depends on their prey dynamics, which should define the carrying capacity of JSM. The annual catch of newly recruited JSM (0+ age) was negatively correlated with its average weight. The average weight increased until 1995 but turned to decrease in 1996 with variation. In the eastern SIS, the number of released fish was negatively correlated with the average weight of 0+ age wild fish, suggesting that JSM used the limited carrying capacity to the full in this area and the releases affected the growth of wild fish. The size distribution of the 0+ age fish sampled showed that released fish were larger than wild fish. We also estimated the size and individual ages (days) of JSM cannibalized by released fish on the basis of standard length, mouth diameter of fish at release. The possibility of replacement of wild fish by hatchery fish due to cannibalism is discussed on the basis of catch and release statistics and the result of the simulation given in the companion paper by Obata et al., which will be presented in the poster session.
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