생태계 모델링에 의한 바다목장의 자원평가 연구

Abstract

The structure and functioning of the Tongyeong marine ranching ecosystem, which is the target of the first case study in the Korean marine ranching program, was assessed by ecosystem-based models, and variations in biomass of target fishery resources in the ecosystem were evaluated by population-based models. Changes in ecosystem structures before and after marine ranching activities were compared using ecosystem-based models. The ecosystem structure model(i.e. Ecopath) was used to estimate : (1) the trophic level by each organism group, (2) the relative contribution of target species or groups to the total flow of energy(throughput), (3) niche overlaps among target species or groups, and (4) impacts of competition between target species or groups. The target species in the Tongyeong marine ranching ecosystem were jacopever rockfish and black rockfish, which belonged to the trophic level 3. After marine ranching activities, the biomass of adult and juvenile jacopever rockfish became 30.295 t/km2 and 1.038 t/km2 in 2006, which increased by 459% and 879%, respectively. The biomass of black rockfish was 3.820 t/km2 in 2006, which increased by 862%. While biomasses of target species increased, those of other species or species groups decreased after marine ranching activities. The increase in the biomass of two species accounted for the effects of stock enhancement activities. When the relative contributions of species groups to the total flow of energy were calculated for trophic levels 3～4, at which energy flows are most active, 82% and 86% of the total flow of energy were occupied by jacopever rockfish before and after marine ranching activities, respectively. Two approaches were used to calculate niche overlaps between species groups : (1) prey overlap which determined what extent any two species or species groups seek the same prey, and (2) predator overlap which determined what extent they are subject to predation by the same predators. In prey overlaps, adult jacopever rockfish had big overlaps with skates, other rockfishes, Moronidae, Scorpaenidae, and large pelagics. Prey species for juvenile jacopever rockfish overlapped with black rockfish, Hexagrammidae, other demersals, and Cephalopoda. Black rockfish shared prey with juvenile jacopever rockfish, Hexagrammidae, other demersals, small pelagics, and Cephalopoda. In predator overlaps, adult jacopever rockfish had big overlaps with juvenile jacopever rockfish, black rockfish, and grey mullet. Predator species for juvenile jacopever rockfish overlapped with most fishes, and those for black rockfish with grey mullet, and infauna. The combined direct and indirect impacts of competition and predation on groups over mixed trophic levels were explored by the Leontief matrix, which could show how the increased biomass of one species or group affected that of another. According to the mixed trophic impact graph, the biomass of adult jacopever rockfish increased when the biomasses of small pelagics, benthic feeders and plankton increased. The biomass of juvenile jacopever rockfish was positively influenced by the increase in the biomasses of adult jacopever rockfish and plankton. The biomass of black rockfish increased with the increase in the biomasses of Gastropoda and plankton. Using the ecosystem dynamic model(i.e. Ecosim), the mechanism of dynamic changes in the quantity of organisms was investigated for effects of stock enhancement and fishing intensity. The ecosystem dynamic model was used to estimate the carrying capacity of jacopever rockfish in the Tongyeong ecosystem. Effects by stock enhancement were elucidated in terms of the effect of releasing rockfish fry and the effect of building artificial reefs. Biomass estimates of jacopever rockfish and black rockfish showed a similar trend with observed biomasses when the effect of building artificial reefs was coupled with that of releasing rockfish fry in the Tongyeong ecosystem. Ecological effects on fishing intensity were examined for the two scenarios of fishing mortality level. The half-reduced fishing mortality caused an increase in the biomass of most groups, although it caused a small change in the biomass of jacopever rockfish. The doubled fishing mortality, however, caused the decrease in the biomass of black rockfish and most fish groups, except for the jacopever rockfish, which maintained the current stock level due to the excessive stock enhancement. Using the ecosystem dynamic model, the carrying capacity of jacopever rockfish was estimated to be 40.0 t/km2, which was compared with 65.9 t/km2 estimated from a population dynamic model. The difference of the carrying capacity might come from ecological interactions or density-dependent effects with other marine living organisms in the marine ecosystem. The acceptable biological catch(ABC) of jacopever rockfish was estimated to be 136～154 t under the conservative target fishing mortality of F40%. Optimum numbers of juvenile releasing were calculated and variations in the biomass and catch of jacopever rockfish by various fishing levels were projected by a Leslie matrix.