Summary

The rockfishes (genus Sebastes) - important resource in recreational and commercial fisheries - comprise over 110 species worldwide distributed on both sides of the Northern Pacific Ocean. As many other species, rockfishes occupy different habitats during their development stages which can be used as an adaptive strategy: i.e. the ontongenic movement used to maximize settlement and avoid offshore dispersal. Catches of rockfishes in Japan decline as reported in species captured along coastal waters such as Sebastes inermis. Efficient restoration and protection strategies of these species require the identification of spawning grounds, juvenile nursery habitats as well as their population structure. The main objective of this thesis is to assess the life history traits, population structure and physiological condition of the rockfish S. inermis from different sites around Miura Peninsula, central Japan: two in the southwest coast, Aburatsubo Bay (Abt) and Sajima Bay (Saj) and one in the northeast coast inside Tokyo Bay (Tkb). Data of age and growth, physiological condition, reproductive traits, otolith microchemical composition and otolith shape were assessed to determine S. inermis ecological and biological features such as: larval and juvenile behaviour, settlement timings, quality of nursery areas, extension and shift of spawning season, the adult's health condition within the reproductive season and connectivity/separability of different sites with two different habitats: the inshore seagrass nursery areas and the adjacent nearshore/offshore deeper areas.

Age and growth

Age and growth structure of fish sampled from distinct sites around Miura Peninsula were investigated. Inshore areas in all sites were sampled in summer of 2010 and 2011 by set net and boat seine net, while nearshore/offshore areas were sampled from January 2011 to February 2012 by set net. Based on otolith microstructure, length frequency and age-at-length of S. inermis (N = 1296) were used to estimate the age and growth structure in the three sites. Growth function was fit to the observed length-at-age data (Cubic von Bertalanffy function; L∞ = 282.5, k = 0.31 year-1, t0 = 0.71) and the growth rate of fish in each site by gender was compared. Tkb had a higher mean age (4.6 years), while in Abt and Saj areas, younger fish were more frequent (3.4 and 2.9 respectively). Distinct growth patterns were identified shown by the different growth rates and maximum sizes found among sites. The geographic features (e.g. bottom composition, seagrass canopy), the depth profile, distance from shoreline from each sampling site and gear selectivity might be influencing the distinct age and growth structures found in Sebastes inermis in this study.

Early Life

Young-of-the-year (YOY) otolith microstructure was analysed to identify possible factors regulating larval extrusion, growth rates, and settlement patterns of S. inermis larvae and juvenile from Abt and Saj. In addition, physiological condition indexes (RNA:DNA ratio, total proteins, Fulton´s K) were examined for habitat quality assessment. Four monthly extrusion groups were identified by daily age backcalculation. Growth rate during the planktonic period was positively correlated with the duration of this stage and with water temperature. Flexible settlement timings of each group suggested an adaptive strategy to variability in environmental condition: larvae experiencing unfavourable temperature settle earlier at smaller lengths and grow faster after settlement. Clear differences in variables related to planktonic and post-settlement stages between sites imply the utilization of different habitats during early life, and shows that offshore mixing of larvae might not be occurring. Saj had higher growth and nutritional indicators (RNA:DNA ratio, total proteins and Fulton's K), indicating a higher habitat quality and importance as nursery for S. inermis juveniles.

Reproductive traits

To assess population reproductive structure and adult physiological condition of S. inermis, presence and duration of different maturity stages and GSI variability were analysed to describe and compare the spawning season in each site. Fecundity and size/age at maturity were also examined to compare the reproductive traits among sites (including a southern population, Seto Inland Sea, off Shikoku Island coast - Set, as a geographic control) and within gender. Adults physiological condition during reproductive period was examined throughout the spawning season from 2011 to early 2012 using nutritional indicators such as, RNA:DNA ratio and total proteins, reproductive indicator (GSI), fecundity indicator (RF) and the morphological index Fulton´s K. M:F ratio reduced strongly during the spawning peak. Shift in GSI and duration of maturity stages, indicated a spawning period from November to January with a peak in December in Saj and Tkb. Fish from Set spawned one month later than fish from Miura Peninsula. The distinct timing of maturation of young and older females and the decrease of length and age throughout the spawning period in both sites showed that older and bigger females spawn earlier than younger and smaller females. Macroscopic analysis of maturity stages and positive relationship between total length and relative fecundity confirmed the higher reproductive potential of bigger females. The age and length at 50% maturity estimated was 3.4 years and 177 mm for females and 3.2 years and 171 mm for males. Reproductive structure of S. inermis along Miura coast showed variability and the estimation for Tkb and Saj indicated that Tkb individuals mature earlier than Saj ones. Female's physiological condition increased within a spawning season achieving almost a simultaneous peak with GSI and Saj was considered the site where adult rockfish presented higher fitness confirmed by the higher biochemical and morphometric condition indicators. Saj also had the better reproductive performance (higher fecundity) when compared with the other sites due to the higher number of bigger females found in this site. Comparative analysis by gender showed distinct reproductive traits both in females and in males within site.

Otolith elemental and shape signatures

Otolith microchemistry was investigated and specific signatures of YOY, sub adults and adults were determined to define the population structure for S. inermis around Miura Peninsula with possible interaction between inshore and offshore habitats in each study site. It was also aimed to analyze otolith signatures in both coasts of Miura Peninsula to examine a possible influence of the environmental factors characteristic of an estuarine area -Tokyo Bay. Elemental composition (Sodium-Na; Estroncium-Sr; Barium-Ba; Manganese-Mn; Magnesium-Mg; Iron-Fe and Calcium-Ca) of otoliths of each size group (YOY, sub adults and adults) was analysed using inductively coupled plasma, atomic emission spectroscopy (ICP-AES). Differences among sites and size groups' signatures were detected by CDA with 74% and 85% of classification success respectively. Abt and Tkb were highly separated and Ba:Ca, Mn:Ca and Sr:Ca ratios were the main determinant which might be related with estuarine and nearshore water environments. Signatures of fish occurring in inshore habitat (YOY and sub adults) were separated among sites as well as fish found offshore (adults), with classifications of 75% and 97% of success respectively. Na:Ca and Sr:Ca were the determinant to separate adult signatures. The putative breeding populations were assessed by examining any proximity of inshore/offshore signatures in each site. There was a geographic separation of S. inermis signatures around Miura Peninsula and the similarity of signatures within all size groups in each site support a possible connectivity of habitats within small geographical range. The analysis of otolith elemental composition showed the relative importance of the distinct inshore nursery areas in supporting the respective adult population offshore.

Univariate shape indicators (e.g. aspect ratio, circularity) and Fourier elliptical descriptors were determined for each otolith and used to define the specific shape tag in each size group in the study site. Sexual differences in otolith shape tag were found for adults and univariate descriptors were the main determinant for such differences. Sites were well differentiated (77%) within small geographical range. The southwest coast sites - Saj and Abt - were separated from the one in the northeast coast - Tkb. Sub adult's otoliths - with broader shape - were more similar to adults rather than YOY. Differences in otolith shape amongst site showed distinct features corresponding to possible distinct S. inermis groups in Miura Peninsula and size group shape similarity found in each site might indicate inshore/offshore connectivity.

Conclusions

Age and growth data, reproductive traits patterns, physiological indicators and otolith signatures enabled to identify differences in S. inermis groups that inhabit Miura Peninsula coast waters and to define the population structure of this species in this area. Site in the southwest coast (Saj) seems to assure better spawning and nursery conditions for the rockfish S. inermis when compared with that in the northeast coast (Tkb), under the influence of Tokyo Bay (e.g. environmental, anthropogenic and geographic). Fish from both sides of Miura Peninsula were also differentiated based on otolith elemental and shape signatures indicating a possible division of this S. inermis population in: two closer groups from southwest coast (Saj and Abt) and one separated group in northeast coast (Tkb).

The discrimination of site and size-specific elemental and shape signatures and life traits was successful within a small geographic range. It is suggested that, habitat connectivity within each site enables the ontogenic movement of larvae from nearshore to inshore settlement areas - shown by distinct early life traits - and enables the permanence of sub adults and adults in nearshore/offshore habitat - shown by the significant differentiation of elemental signatures in both groups among different sites. It is believed that the information of the population structure together with the information of habitat and site connectivity is a good indicator of a sustainable S. inermis population in waters around Miura Peninsula.

This study increases the knowledge of the ecology and reproductive biology of this valuable resource and provides information regarding some possible sites for spawning and subsequent recruitment of this marine fish around Miura Peninsula. Such data is increasingly important given several pressures affecting rockfish populations: i.e. habitat loss, decline in endemic aquatic fauna or fishery overexploitation effects. Further studies in larger spatio temporal scale should be conducted to investigate deeper S. inermis population in Japan.

申請者は,三浦半島周辺のアカメバルSebastes inermisの生活史特性と資源構造を明らかにするため,同半島周辺の3地域(佐島湾,油壺湾,東京湾)で漁獲されたアカメバルのサンプルについて,以下の調査分析を実施した。

1.年齢と成長

漁獲物の体長組成,耳石日周輪・年輪の解析を行った。いずれの湾でも湾内(沿岸)のサンプルでは当歳魚および未成魚が,湾外(沖合)のサンプルでは成魚が主体であった。海岸からの距離が遠いほどサンプルの体長が大きい傾向が認められ,成長に伴って浅海から沖合へ向けて生息場を移動していくことが示唆された。検討した成長式のうち,von Bertalanffyの3乗式の妥当性が最も高く,成長の地域差が認められた。

2.初期生活史の特性

仔魚の孵出,成長速度,着底パターンに影響を与える要因を特定するため,佐島湾と油壺湾で採集した稚魚・幼魚サンプルの耳石日周輪の解析を行った。また,生息場所の質を評価するために,魚体栄養状態の指標(RNA:DNA 比,総タンパク質,Fultonの肥満度指数K)の分析を行った。日齢を用いた逆算推定により,月ごとの4つの孵出群が認められた。浮遊仔魚期の成長速度は水温および浮遊期の期間の長さと正の相関があった。各群の着底時期が可変的であったことから,好適でない水温を経験した仔魚は小さな体長で早期に着底を行って着底後に速く成長するという,環境変動に対する適応戦略の存在が示唆された。浮遊期および着底後の種々の形質に関する湾間の違いにより,浮遊期仔魚の沖合での混合は生じていないことが示唆された。佐島湾の稚魚は速い成長速度と高い栄養指標値を示したことから,当湾は生息場所としての質が高いと考えられた。

3.繁殖特性

各地域における繁殖特性を明らかにするため,月別の成熟段階,生殖腺指数(GSI)の調査を行うとともに,魚体栄養状態の分析を行った。成熟段階とGSIの月別推移から,佐島湾と東京湾での産卵期は11月から1月で,12月が盛期であり,瀬戸内海よりも約1ヶ月早かった。産卵期における性比が雌に偏ったことから,繁殖に伴う雌雄別の移動が示唆された。50%成熟年齢と体長は,雌では3.4歳,177 mm,オスでは3.2歳,171 mmで,メスのほうが成熟体長が大きかった。また,高齢の個体のほうが若齢の個体よりも早く産卵期を迎えることが示唆された。雌の栄養状態は,いずれの指標でも産卵期に向けてピークを迎えた。佐島湾のサンプルの栄養状態の指標は他地域と比べて高い値を示した。佐島湾では同一体長での産仔数が他地域よりも多く,同湾はアカメバルの繁殖にとって優れた条件を備えていると考えられた。

4.耳石微量元素組成

当歳魚,未成魚,成魚の耳石微量元素組成をICP-AESで分析し,成長段階別,地域別の組成の差異を調べた。正準判別分析(CDA)の結果,成長段階間,地域間のいずれにおいても明瞭な差異が認められ,判別成功率はそれぞれ74%,85%であった。地域間ではBa:Ca比,Mn:Ca比,Sr:Ca比の違いが判別に大きく寄与していた。湾内のサンプル(当歳魚と未成魚)は地域間で明瞭な差異が認められ,さらに湾外のサンプル(成魚)とも明瞭に区別された。判別成功率はそれぞれ75%,97%であった。成長段階の判別にはNa:Ca比とSr:Ca比の違いが大きく寄与していた。以上により,三浦半島周辺には耳石微量元素組成の異なる複数の地域グループが存在することが示唆された。

5.耳石の形態

当歳魚,未成魚,成魚の耳石形態の指標(Circularity,Roundness,Solidity,Aspect ratio,elliptic Fourier descriptors)を解析し,成長段階別,地域別の耳石形態の差異を調べた。CDAによる解析の結果,成魚では耳石形態の雌雄差が認められた。地域間にも明瞭な差異が認められ,判別成功率は77%であった。成長段階間でも明瞭な差異が認められた。以上により,三浦半島周辺には耳石形態の異なる複数の地域グループが存在することが示唆された。

6.総合考察

以上の研究により,三浦半島周辺のアカメバルの生活史特性と資源構造に関する有用な知見を得ることができた。同半島周辺の沿岸・湾内に着底した稚魚は,成長に伴って浅海から沖合へ向けて徐々に移動していくが,他地域の個体との混合の度合いは低く,同半島周辺には複数の地域グループが存在することが示唆された。

以上,本論文は,アカメバルの個体群に関して,地理的に相当近接した地域間でも交流の少ない,互いに異なる複数の地域グループが存在することを明瞭に示したものであり,学術上の貢献は大きく,審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認めた。