Sillaginidae is a commercially and recreationally important fish family in many Indo-West Pacific countries. In Thailand, however, the degradation of sillaginid fish resources has become a serious problem in recent years.

Two sillaginid species, Sillago sihama and S. aeolus, are widely distributed throughout tropical and subtropical coastal waters in the west-central Pacific and Indian oceans. In Sikao Bay, Trang Province, Thailand, preliminary observations indicated that these two species were the most dominant sillaginids, coexisting in shallow sandy areas throughout the year, although S. sihama was more abundant than S.aeclus.

Despite their abundance and popularity as food, little is known about the life histories of these fish, especially in tropical countries including Thailand. The objective of this study was to provide basic information on the biological traits of S. sihama and S. aeclus at Sikao Bay, by examining reproduction, age, growth, feeding habits, and juvenile seasonal occurrence.

Sikao Bay （7°30′N, 99°13′E） is a large bay, approximately 40 km long with a mouth width of 30 km, opening to the Andaman Sea. The bay has relatively short dry（January to April） and long rainy （May to December） seasons. Water temperatures at the sandy Rajamangala Beach in the bay ranged from 27.0 to 30.90℃, but no seasonal trends were apparent. Adult and large juvenile specimens of S. sihama and S. aeolus were collected monthly from gill net fishery landings operated within Sikao Bay between May 2003 and April 2004. Gill nets （500 m wide, 1 m deep, and 25 mm x 25 mm square mesh） were set on the sandy bottom in the central area of the bay （water depth about 15 m） between 05:00 and 07:00 hours, and retrieved between 09:00 and 10:00 hours. Both species were captured during the same gill net operation. Small juvenile specimens of the two species were collected in the surf zone of Rajamangala Beach using a small seine net（10 m wide, 1 m deep, and 1 mm x 1 mm square mesh）. This sampling was conducted monthly at flood tide （water depth about 1 m） between 6:00 and 9:00 hours from May 2003 to April 2004.

Reproduction

A total of 2,926 and 2,077 specimens of S. sihama and S. aeolus, respectively, were used to determine sex ratio, gonad development, spawning season, size at maturity, and lunar-related spawning patterns. The annual sex ratio (male : female) was 1 : 1.50 in S. sihama and 1: 0.88 in S. aeolus, each being significantly different from the expected 1: 1. Histological examinations of gonads showed that the smallest mature females and males in S. sihama were 117 and 106 mm in standard length (SL), respectively, and 113 and 109 mm, respectively, in S. aeolus. Body size at which 50% of individuals reached maturity (i.e., size defined as adults) was estimated to be 130 mm SL for both species and sexes. The lowest spawnable gonadosomatic indices for female and male S. sihama were 0.53 and 0.04, respectively, and 0.47 and 0.03, respectively, in S. aeolus. Spawning occurred continuously throughout the year for both species, with peaks in activity between August and November in S. sihama, and between July and December in S. aeolus. Oocytes at various developmental stages, together with postovulatory follicles, were found in the mature ovaries of the two species during their spawning periods, demonstrating that they are multiple spawners. Between June and August, both species at Sikao Bay showed a tendency for semilunar spawning cycles, with peak activity around the half moon, although this study was not conducted in other months.

Age and growth Age and growth parameters of S. sihama and S. aeolus were estimated from sagital otoliths of 285 and 317 fish, respectively. Whole sagittal otoliths were suitable for determining the ages of both species. The boundaries between translucent and opaque zones were clearly defined, 86 and 90% of the examined otoliths of S. sihama and S. aeolus, respectively, being readable. The observations of otolith edges showed that one annulus was deposited each year, with opaque zone formation occurring mainly in September and October for both species. The oldest S. sihama specimens collected in this study were 1.99 （157 mm SL） and 1. 97 years （162 mm SL） for females and males, respectively, and 1.99 （200 mm SL） and 1.99 years （168 mm SL）, respectively, for S. aeolus. The von Bertalanffy growth parameters of the two species were estimated for females and males separately, using length-at-age data. No significant difference was found in the growth curve between sexes in either species. Thus, the growth curve combined for both sexes of each species was as follows:

S. sihama　Lt=207.7[1-exp{-0.53（t+1.09）}]

S. aeolus　 Lt=190.6[1-exp{-0.81（t+0.54）}],

where Lt is SL in mm at age t in years. These two growth curves were significantly different from each other.

Feeding habits

The feeding habits of juveniles （< 130 mm SL） and adults of S. sihama and S. aeolus were examined on the basis of 892 （127 juveniles and 765 adults） and 734 （159 and 575）specimens, respectively. The diets of juveniles of both species changed progressively with increasing body size, with a shift from capturing small zooplankton, such as calanoid copepods, to larger benthic prey, such as polychaetes, shrimps, and crabs. The latter three items were also the most important prey of adults of the two species, together constituting more than 70% of stomach contents by volume. Pronounced seasonal changes in adult diet were not detected in either fish species. In addition, considerable overlaps of juvenile and adult diets between the two coexisting Sillago species were found during the study period, indicating that there may be little or no competition between them at Sikao Bay.

Seasonal occurrence of juveniles

All specimens of S. sihama and S. aeolus captured in the surf zone of Rajamangala Beach were juveniles smaller than 130 mm SL. Monthly changes in the mean number of individuals per haul （150 m2） indicated that juveniles of both species occurred at the beach throughout the year, with a peak abundance from November to January in S. sihama, and from January to February in S. aeolus. Moreover, the size frequency distribution of the two species revealed that most collected juveniles were small, less than 45 mm SL. The lengths of S. aeolus never reached 70 mm SL, and those of S. sihama rarely exceeded 70 mm SL. These results suggest that the two species display an ontogenetic habitat shift, from surf zones to offshore and deeper waters. Shallow nearshore habitats, including surf zones, may function as nursery areas for S. sihama and S. aeolus.

The present study showed that the overall biological aspects are very similar between S. sihama and S. aeolus at Sikao Bay. Both species typically had a high growth rate and short life span. The vast majority （>80%） of individuals attained maturity at the end of their first year, by which time they were generally 130 mm SL or more, and all individuals died before reaching two years of age. However, this is far less than those of conspecifics and other Sillago species in different locations. This phenomenon, in addition to the continuous decline in the annual catch of Sillago in southwestern Thailand in recent years, suggests that S. sihama and S. aeolus at Sikao Bay have been overfished. Therefore, effective management should be implemented to reduce some of the pressure from the harvests of both species, before it is too late to save the stocks. Further investigation is needed to establish an appropriate management plan for Sillago fisheries at Sikao Bay.

本論文は、タイ王国南部、トラン県シカオ湾（7° 30' N, 99° 13' E）におけるキス科、2種Sillago sihama （アオギス）および S. aeolus,（モトギス）の生態について調査研究したものである。キス科魚は近年、輸出用の水産物としてタイ王国で漁獲量が増大している。その一方、漁獲圧の増大による資源の枯渇が懸念されているが、生態・漁獲実態を含めて、タイ国を含み熱帯域においてはキス科魚に関する集中的な研究はなされていない。本研究はそのような社会的背景のもとになされた。

論文は、第1章：序論、第2章：再生産、第3章：年齢と成長、第4章：食性、第5章：潮間帯への加入、および、第6章：総合考察の6章から構成される。

序論に引き続いて、第2章においては、2003年5月から2004年4月の期間に毎月、刺網によって採捕された総計2,926（S. sihama）および2,077（S.aeolus）の個体について、体長、体重、卵巣重量を調べるとともに生殖腺の組織学的観察を行い、性比、成熟サイズ、産卵機関について検討した。その結果、性比は1:1.50 （S. sihama）および1:0.88 （Ｓ．aeolus）で、雌雄の最小成熟サイズ（標準体長）はS sihamaで117mｍおよび1.06、S. aeolusで113および1.09であった。いずれの種も標準体長130mm前後で雌雄ともにその大半が成熟するものと推測された。また、産卵可能となる雌雄の生殖腺指数（GSI）はS. sihamaにおいて0.53 および 0.04, S. aeolus. において0.47および0.03であった。いずれの魚種も多回産卵で、周年産卵していたが、平均卵巣重量の変化よりその産卵ピークは 6月から8月と推定された。また産卵ピークには、産卵は月令と同調している傾向が見られた。

第3章においては、曳き網によって漁獲した235 （S.sihama）および317（S.aeolus）個体について、耳石の観察を行った。耳石の観察を行った個体のうち86%（S.sihama）および90%（S aeolus）の個体において、明瞭な透明帯と不透明帯が観察され、不透明帯は9月から10月に形成されていた。この不透明帯は年輪として用いることが可能であった。これを用いて、魚の年齢推定を行ったところ、最も高齢の個体はいずれの魚種でも1.99歳と推定された。また、いずれの魚種も雌雄で成長速度に大きな違いはなく、その成長は次式に近似できた。

S. sihama Lt = 207.7 [1 − exp {−0.53 （t + 1.09）}]

S. aeolus Lt = 190.6 [1 − exp {−0.81 （t + 0.54）}],

Lt ：標準体長、

T：年齢.

第4章においては、食性の成長に伴う変化を調べた。2種の間で食性に大きな違いは認められず、いずれの魚種も、稚魚は初期にカラヌス属などのカイアシ類を主に摂餌していたが、成長に伴って、多毛類、エビ、カニなどの底生動物食へと変化した。また、成体においては、食性の季節的な変動は見られなかった。

第5章では、潮間帯への稚魚の加入について調べた。いずれの魚種も、潮間帯への加入は周年観察され、その多くは45ｍｍ以下の個体であった、このことから、潮間帯は両種の稚魚の生育場として機能しているものと考えられた。

最終章（第6章）では、温帯地域で行われた既往の研究成果との比較によって、熱帯に位置するシカオ湾におけるキス科魚の成体の特徴について論じた。その結果、生態的に大きく違わない近縁種が同所的に生息していること、産卵が周年的に行われていることなど、シカオ湾におけるキス2種の生態は、温帯地方におけるキス科魚の生態とはいくつかの点で異なっていると結論した。さらに、その寿命の短さが、高い漁獲圧による可能性を指摘した。

以上のように本論文は、熱帯地域におけるキス科魚の生活史を、初めて明らかにしたものであり、特にその産卵が周年行われていることを明らかにしたことの生物学意的意義は大きい。また、生態的にほぼ異ならない2種の近縁種が同所的に存在しているという事実の指摘は、高い漁獲圧の影響も含めて、それが可能となっているメカニズムの解明という、興味ある今後の課題を投げかけている。さらに、推定された寿命の短さから、その原因が高い漁獲圧に起因する可能性をしてきするなど、タイ国沿岸の資産資源の管理について警鐘を鳴らしており、社会的にも大いに意義のある研究論文であると評価できる。

論文審査委員会は、本論文の学問的・社会的意義を高く評価し、Prasert Tongnunui君が今後さらに研鑽を重ね、タイ国における科学の発展と日タイの相互理解の推進に寄与することを期待して、本論文を農学博士の学位に値するものと評価した。