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実験池において，イワナとヤマメの体内に残留させた釣り鈎の動向を調査した。両種ともに，口腔に残留させた餌釣り用と毛鈎釣り用の鈎はいずれも 21 日以内にその多く（70.0～100%）が脱落した。口腔より奥に残留させた餌釣り用の鈎の体外への排出率は 21 日後に 0～16.7% であり，81 日後でも 15.0～50.0% であった。口腔より奥に残留させた鈎の多くは 81 日後には錆びていたが，崩壊したものは少なかった。以上の結果から，口腔に残留させた鈎は比較的短期間で脱落するが，口腔より奥に残留させた鈎は体外に排出されにくいことが明らかになった。

一次元の養殖場分布モデルを用いてシミュレーションにより養殖場における伝染病の伝播を再現しリスク評価を行った。養殖場間の流通を (1) 隣接養殖場間の相互流通, (2) 隣接養殖場間の一方向的な流通, (3) ランダムな流通の3つのタイプに分け, 防疫策として3つの流通停止の方法を設定した。数値結果から流通 (3) を避ける事, および病死魚発見後は早期に流通を停止する事が有効であることが示された。流通の早期停止が不可能な場合は全養殖場間の流通停止, 未感染の養殖場間流通を先に停止することで伝播を抑制できることが明らかになった。

イワナ・ヤマメ養殖魚の小型魚と大型魚の釣獲放流後の死亡率と成長を実験池において調査した。餌釣り, 毛鈎釣りともに, 口腔にかかった鈎を除去した場合, いずれの魚種においても死亡率は低かった。餌釣りで口腔より奥にかかった鈎を除去した場合, イワナ小型魚とヤマメ大型魚では死亡率は高かった。口腔にかかった毛鈎を残留させた場合, イワナ大型魚では死亡率は高かった。死亡のほとんどが釣獲放流後14日以内に観察された。釣獲方法, 鈎がかりの部位, 鈎の処理方法は成長と肥満度に影響しなかった。

The fish propagation program that produces fish to be released into natural bodies of water is dramatically increasing. The swimming crab Portunus trituberculatus is one of the most valuable species for fisheries. The simulation model was constructed to evaluate the optimal stocking strategy for the swimming crab. The simulation process was formed by the six components, the swimming crab, the prey of the crab Musculus senhousia, two competitors (Portunus sanguinolentus and Charybdis japonica), and two predators (the gobies and the hermit crabs). The population growth of the mussel was assumed to obey the logistic equation. The crabs decreased by natural mortality (competition, predation, and cannibalism) and fishing, but increased by reproduction and releasing juveniles. The results of our simulation showed that the exists an optimal day to release the crabs in order to get the maximum catch in weight. The crabs grew larger when they were released early in the year. In combination with the releasing day and number, we could get the optimal day for the target catch amount.

The growth and reproduction of Palaemon pacificus and P. serrifer were studied in a rearing experiment and materials collected almost bi-weekly during a period from May 1987 to November 1989 in Tateyama Bay, Chiba Prefecture. The life span of P. pacificus and P. serrifer is estimated at 12 to 15 months and 13 to 16 months, respectively. The body size is larger in P. pacificus than P. serrifer. Breeding season of P. pacificus and P. serrifer lasted from May through November and from April through October, respectively. Incubation period of P. pacificus and P. serrifer is 10 to 20 days and 11 to 19 days, respectively. Juvenile P. pacificus and P. serrifer appeared from May and July, respectively. Spawning is carried out by yearling and hibernated individuals in both species. In P. serrifer, however, most off-spring are produced by the hibernated spawning group. P. serrifer carries more numerous but smaller eggs than in P. pacificus. Thus, the difference on reproductive ecology between the two species was recognized.

From a viewpoint of life history of fish, the natural mortality can be divided into three phases: initial, stable death, and death by senescence, and the respective phases correspond to the three growth phases: early, stable growth, and senescence. According to the relation between natural motality and the growth, a functional formula of the natural mortality coefficient is given in this study. When the von Bertalanffy's growth parameters are known, the natural mortality coefficient can be calculated for each age. As a function of age, the natural mortality coefficient has some general characters throughout the life.

A mathematical model is constructed with which stocking effects on a system that includes threshold densities for two competing species may be determined. The nullcline of the system changes its form with increase of stocking. The combination of the nullclines produces compli-cated phases. If stocking occurs for only one species, this species has a chance to eliminate the other depending on the initial conditions and stocking levels. The stocking is effective to control natural populations.

The restocking effects on a discrete growth population are considered in relation to two types (quadratic and Ricker) of reproduction system. Local stability analysis and numerical calcula-tions are carried out to clarify the dynamics of the population. The stability depends on all pa-rameters employed in the model. For the quadratic model, the restocking makes the equilibrium unstable, while for the Ricker model, the restocking produces stability. When increasing rate is high, and restocking level is low, then either population destruction occurs or crash cycles emerge. Restocking produces critical effects on population growth in some cases.

The life history and morphological characteristics of a hippolytid shrimp Hippolyte ventricosa were investigated during 1983 and 1984 in Kominato Bay, Chiba prefecture. The two or more generations in the year could be observed according to the distributions of carapace length and number of eggs carried. Two rostrum types were observed; Sword type and Blade type. In the Sword type, the lower border of rostrum was straight from base to distal edge, while in the Blade type, it was expanded roundly. The number of lower border teeth of rostrum was significantly different between two types. 99% of the male was Sword type. In the female, the proportion of the Blade type reached only 78%. The ovigerous ratio of the Blade type was higher than that of the Sword type. These results indicate the sexual dimorphism of Hipplyte ventricosa.

As described above in these results of the measurements it is considered that the radiance distribution in the near-surface layer of the sea depends considerably on the radiance distribution of the sky. Accordingly the authors attempt now to improve the instrument capable of measuring the radiance distribution in the underwater and the sky at the same time automatically and if this new instrument be accomplished the authors will study these detailed phehomena described above.