Abalone is one of the most valuable fishery resources in Japan, particularly large abalone species. There are three large species in warm current regions (Haliotis discus discus, H. gigantea and H. madaka) and one large species in cold current regions (H. discus hannai). The total catch of these species has drastically decreased since 1970. Although artificially produced abalone seeds are released in fishery grounds, the total catch has not recovered. Thus, effective management is needed for the recovery of large abalone populations. Abalone can disperse during their planktonic larval stages. The recolonization through their dispersal is important for the replenishment of depleted populations. One of the restoration measures is the establishment of harvest refugia, which provide recruits to the surrounding area. For the recovery of populations, the refugia should be established or assessed with quantitative methods. Modeling approaches allow simulations of larval dispersal and quantitative assessments of the refugia. In the present studies, larval dispersal processes and establishment of effective harvest refugia were investigated for fishery grounds on three coastal regions characterized by different topographic features, using hydrodynamic and particle-tracking models.

Larval dispersal and establishment of effective harvest refugia of large abalone in a large bay

Suitable locations for harvest refugia of three species (Haliotis discus discus, H. gigantea and H. madaka) in a large bay (Sagami Bay) were investigated. In the larval dispersal simulations, observed current data were assimilated into the hydrodynamic model, and particles were released at the time of hatching, which was calculated from the estimated time of fertilization/potential spawning and based upon the water temperature. The aims of the study in this chapter were to (1) clarify larval dispersal processes and settlement sites, and (2) evaluate the existing harvest refugium and estimate the suitability of other areas for larval sources in the large bay. The model results for two periods were considered. The results of larval dispersal simulations indicate two different dispersal patterns: (i) transport toward the coast after dispersal offshore, and (ii) gradual dispersal offshore. The estimated settlement sites were the area along the southern coast of Miura Peninsula in the dispersal pattern (i), the area from Otawa Bay westward in both patterns (i) and (ii), and the area located southwest of the existing refugium in the pattern (ii). The refugium was compared with three hypothetical reproductive sources located 1 km north, west, and south of the refugium. During settlement competency, transport success (transport to the area at the depth of adult habitat: 30 m or shallower) of particles released at the refugium was highest (27 to 75%), and suggested that the current location of the refugium is more suitable for a larval source than those of the hypothetical reproductive sources.

Larval supply and establishment of effective harvest refugia of large abalone in a small bay

Larval supply in a small bay (Oshoro Bay) was estimated by simulating larval dispersal inside and in the vicinity of the bay. Inside the refugium, located near the head of the bay, most juveniles of Haliotis discus hannai originated from wild individuals, although most adults were artificially produced individuals that had been released in this area. The objectives of the study in this chapter were to (1) estimate the larval supply from both artificially produced and wild individuals into the refugium, and (2) compare suitability of locations for larval sources in a small bay. The larval dispersal simulations were carried out for two periods in the spawning season. In the larval dispersal simulations, observed oceanographic and climatological data were incorporated into the hydrodynamic model, and the abundance of adult abalone was taken into account when estimating the larval supply. The simulation results indicate that the larval supply from the wild adults to the refugium was higher than that from the artificially produced adults. These results were consistent with the high abundance of wild juveniles in the refugium. The majority of larvae from the refugium were predicted to disperse out of the bay. The larval retention in the bay was estimated to be at least one order higher than that in the refugium. This indicates that the self-replenishment increases if the refugium is expanded to the scale of the bay. The suitability of locations in the head of the bay for larval sources showed little differences among compared sites, and thus the effectiveness of establishing new refugia in this area was expected to be at the same level as the current refugium.

Larval connectivity and establishment of effective harvest refugia of large abalone on an open ocean coast

Larval connectivity among fishery grounds of Haliotis discus hannai on an open ocean coast (Sanriku coast) was investigated. In the hydrodynamic model, oceanographic reanalysis data from JCOPE2 was used for forcing. In the particle-tracking model, particles were released at the timings determined on the basis of the spawning dates, which were estimated from the biological field data (i.e. shell lengths of newly settled abalone). The larval dispersal was simulated for two periods during the spawning season. The objectives of the study in this chapter were to (1) clarify the larval dispersal processes, (2) quantify the dispersal distance of larvae, and (3) estimate the larval connectivity among seven fishery grounds on the open ocean coast. The modeled hydrodynamics in the first period showed stormy conditions in which strong southward coastal currents due to the passage of low pressure were followed by a clockwise eddy causing northward coastal currents; the second simulation showed calm conditions in which relatively weak coastal currents ran northward. In the first period, the spawning of H. discus hannai appeared to be triggered by the low pressure, and the larval dispersal was estimated to be generally greater than the second period. The larval dispersal pattern in the first period can be expected to occur at times since the route of the low pressure was common for the cyclones that form and develop in the northwestern Pacific Ocean. The mean dispersal distances during settlement competency were less than 40 km in both simulation periods. The model results indicate that abalone in the fishery grounds on the mid- and southern coast exhibit two distinct larval dispersal patterns. The number of connected sites was 3-7 and 2-4 in the first and second periods, respectively. The extent of self-recruitment was generally higher in the second period. This indicates that calm hydrodynamic conditions were favorable for self-replenishment. The extent of self-recruitment was usually higher than that of connectivity with the other fishery grounds. However, poor self-recruitment occurred in some fishery grounds. This implies that the restocking and protection of local resource in a fishery ground does not always lead to an increase in local recruitment. Two fishery grounds on the northern coast were considered the most suitable locations for harvest refugia.

Previous modeling studies on larval dispersal of abalone were conducted without considering detailed physical and biological field data. Based on the detailed field data, the present studies revealed that large abalone species have two potential patterns in the larval dispersal processes in the large bay and open ocean coast. The extent of larval dispersal was estimated to be greater on the open ocean coast than in the large bay. The simulated larval dispersal processes varied depending on hydrodynamic conditions and local topographic features. Even in the small bay, most larvae could disperse out before reaching competency for settlement. The present studies showed that larvae could be dispersed in kilometers to tens of kilometers. The results imply that local restocking can raise the self-replenishment but does not always result in improvement of the local stock. In the present studies, methods to simulate the larval dispersal processes of large abalone were developed, and the quantitative investigations for the establishment of effective harvest refugia were demonstrated. The findings provided through these studies can be utilized for the improvement of fishery management of depleted large abalone populations.

大型アワビ類は、我が国において最も高価な水産資源のひとつであり、3種(クロアワビ、メガイアワビ、マダカアワビ)が暖流域に生息し、1種(エゾアワビ)は寒流域に生息する。これらの種の総漁獲鼠は1970年から急激に減少し、人工種苗が漁場に放流されているものの資源の回復には至っていないのが現状である。この資源回復には、アワビ類は浮遊幼生として分散するため、幼生から稚貝に至る生活史を通じた効果的な管理、とくに幼生を成長・生残に適した海域へ供給する保護区の設定が重要と考えられる。そこで、本研究では、異なる地形的特徴を持った3海域の漁場を対象に、上述した保護区の有効性を定最評価することを目的に、アワビ幼生分散モデルを開発し、幼生分散シミュレーションに基づいて解析を行った。本論文が明らかにした内容の要点を以下にまとめる。

1.大規模湾における大型アワビ類の幼生分散

相模湾東部における暖流系大型アワビ類を対象に、(1)幼生分散過程と着底場所の解明、(2)現在設定されている保護区の評価、(3)幼生供給源に適切な海域の推定を目的に解析を行った。その結果、大規模産卵時には長距離分散(沖へ分散後、沿岸へ輸送)、小規模産卵時には短距離分散(ゆるやかに沖へ分散)する分散様式の相違が認められ、着底海域は、長距離分散では三浦半島南部、短距離分散では保護区南西部、また、分散様式に係わらず小田和湾から西の海域に分布することが分かった。着底期における輸送成功率を保護区とその北、西、南に1km離れた三つの仮想幼生供給源で比較したところ、保護区の輸送成功率が最も高く(27--75%)、現在の保護区の位置は、他の海域よりも適していることが分かった

2.小規模湾における大型アワビ類の幼生供給

北海道の小規模湾である忍路湾では、親貝の9割が放流されたエゾアワビ人工種苗であるにもかかわらず、放流貝由来の稚貝は1-2割と少ないことが分かっている。そこで、同湾における(1)保護区への放流・天然親貝由来の幼生供給の推定、(2)幼生供給源としての海域の適性比較を目的に解析を行った。その結果、保護区への幼生供給は、放流親貝由来よりも天然由来の割合が夏秋ともに高い(6-7割)ことが示された。この結果は、保護区に天然稚貝が多く存在していることと一致し、保護区で産まれた幼生の大部分は、保護区外へ分散することが分かった。湾内における幼生滞留率は、保護区における滞留率と比べて1オーダー以上高く、保護区を拡大した場合には再生産が増加し、湾奥の複数の海域は幼生供給源として保護区と同等の有効性を有するものとみられる。

3.外洋に面した沿岸における漁場の連結性

三陸沿岸におけるエゾアワビ漁場からの幼生分散を再現し、外洋に面した7漁場間の連結性を検討した。モデルにより再現された大規模産卵時の流動場は、低気圧通過に伴う強い南方向の沿岸流と時計回りの渦を示し、小規模産卵時には比較的弱い北方向の沿岸流を示した。大規模産卵時には産卵が低気圧により誘発され、幼生分散が小規模産卵時と比べて広範囲に及ぶことが分かった。シミュレーション結果からは、中・南部沿岸の漁場では大規模産卵時の長距離分散と小規模産卵時の短距離分散の二様式を持つことが示され、これは大規模湾における分散様式と一致した。また、他漁場に幼生が輸送される割合よりも、産卵漁場に幼生が回帰する割合が高いことも分かった。しかし、流況や漁場により、産卵漁場に幼生が回帰する割合が減少する場合があり、局所的な放流や保護がその漁場での加入増加につながらない可能性があることが示唆された。結果として、沿岸北部の2漁場は、連結性から保護区として最適な海域であると判断された。

本研究では、大型アワビ類が、大規模湾と外洋に面した沿岸において大規模産卵時に長距離、小規模産卵時に短距離の幼生分散過程を持つ可能性があることを明らかにした。外洋に面した沿岸では、幼生分散が大規模湾よりも広範囲であるものの、流況やその海域の地形的特徴により大きく変化し、小規模湾であっても、幼生の大部分が着底期前に湾外へ分散することがある。本研究の結果から、局所的な増殖がその海域の資源改善に繋がらない可能性も認められ、幼生期における定量的な評価手法に基づいた管理が重要であると考えられる。本研究では、大型アワビ類の幼生分散過程を再現する手法を開発し、有効な保護区設定のための定量的な検討方法を明示した。これは際立った成果といえ、また、得られた知見は、低迷している大型アワビ類資源の回復に大いに役立つものと考えられる。

上記の諸点を考慮し、審査委員一同は、三宅陽一氏は独立した研究者として研究を遂行していくのに必要とされる全ての能力、知識、経験、学問的実績を持っており、博士(農学)の学位を授与するのにふさわしいとの結論を得た。