A long-term feeding experiment with tilapia, Oreochromis niloticus, in a fish-rearing closed tank (air-tight tank) with an O₂/CO₂ exchange unit was conducted to establish a closed ecological recirculating aquaculture system (CERAS). In this experiment, with the exception of evaporation/condensation to/from the air, and of use for water analysis, the water was not changed. The experiment was conducted for 189 days at 28°C with two other open tank systems having different experimental conditions respectively. One was exchanged rearing water (control tank) and the other minimized water exchanges (open tank). Fish density was adjusted every three weeks and fish fed a commercial diet daily. The water qualities, NH₄-N, NO₂-N and NO₃-N were periodically monitored. Dissolved oxygen (DO), flow rate and pH were continuously recorded by a personal computer. The biofilter holding the bacterial community from an established tilapia aquarium (cultured for one month before the experiment) was effective to nitrify ammonia to nitrate. Thus, toxic ammonia and nitrite in the water were negligible, while nitrate increased by the end of experiment (over 450mg NO₃-N/1). Replacement of the O₂/CO₂ exchange unit was required four times because of decreasing DO and water flow during this experiment.
In this closed system, tilapia showed normal growth and high feed efficiency compared with the recirculating open tank under a high concentration of NO₃-N over the 189-day period. After the experiment, nitrogen and phosphorus budgets in the closed system were calculated; 38.2% of N and 50.7% of P in the diet supplied were retained as fish growth; 3.4% of N and 38.5% of P were removed from the system as solid waste; and 49.3% of N and 1.4% of P were accumulated in the rearing-water.

抄録全体を表示

循環濾過飼育におけるティラピアの成長と水質変動との関係を調べた。循環濾過において, サンゴ砂を用いなければ, pHが急速に6を下回り, 亜硝酸化成細菌の働きの低下に伴い, アンモニア態窒素が漸増し, その状態が持続されることにより, 硝酸化成細菌の活動も鈍り, 亜硝酸態窒素も蓄積すること, 水質の悪化に伴い, 飼育水の着色が目立つことが明らかになった。一方, サンゴ砂を用いることは, pHの低下を防止でき, 硝化細菌全般の活動の活発化に極めて有効に作用することが判明した。なお, 水質悪化に伴い助長される着色成分 (黄色色素) は, 8時間程度の活性炭による処理により速やかに吸着・除去できた。
濾材を比較検討した結果, バイオアルファ, シポラックス, ゼオライトとも, 魚体に影響を及ぼすほどの大きな差は認められなかった。
また, 本実験においてアンモニア態窒素の増加による, ティラピアの増重率や日間成長率の低下は認められなかった。なお, 成長と総窒素濃度との間に相関がみられたが, 今後更なる検討が必要である。今回の結果から, ティラピアを循環濾過式水槽で飼育する場合には, アンモニア態窒素濃度81mg/l, または硝酸態窒素濃度616mg/lまでに達する飼育水中 (初期値はそれぞれ0) で70日間ほぼ正常に飼育できることが明らかとなった。

抄録全体を表示

Torquigener albomaculosus has been found at depths of 10–30 m on the southern coast of Amami-Oshima Island, Kogoshima Prefecture, and at 100 m off Hamahika-jima Island, Okinawa Prefecture. However, there have been no reports on the morphology and pigmentation of the larvae and juveniles. Eggs of To. albomaculosus were collected from the southern coast of Kakeroma-jima Island, Kagoshima Prefecture, at a depth of 32 m, and development of eggs, larvae, and juveniles were observed in captivity. Comparisons were made with previous reports on the development of pufferfish species inhabiting Japan and adjacent seas. Eggs were 0.96 ± 0.02 mm (n = 20) in size, spherical in shape, colorless, transparent, demersal, and adhesive. Immediately after hatching, larvae were 2.43 ± 0.08 mm (n = 11) in total length and the number of myomeres was 8 + 11 = 19. Dendritic melanophores were present on the dorsal surfaces of the head and body, dorsal and ventral sides of the abdominal cavity, but were absent from the caudal region. The mouth and anus had already opened but the yolk still remained. The yolk was absorbed within 2 days of hatching. At 19 days after hatching, ossification of teeth began in the upper and lower jaws. Simple small spinules appeared on the gill covers and abdomen. They were pointed at the distal end and not divided. Black melanophores were distributed from the snout to the region between dorsal- and anal-fin bases, absent from the caudal region. The larvae became juveniles 36 days after hatching. At 61 days after hatching, the sides and dorsal surface of the body were silver, the ventral surface was yellow, and the area covered by small spinules extended posteriorly beyond the dorsal and anal fins. The larvae and juveniles of To. albomaculosus can be distinguished from those of other pufferfishes inhabiting the waters around Japan by the absence of black melanophores on the caudal region, the distribution and development process of small spinules, and body color.

抄録全体を表示