Rainbow trout, Oncorhynchus mykiss, were reared in flowing deep seawater (DSW) pumped from a depth of 687 m in Suruga Bay. One hundred and eleven alive fishes (8.6 cm in fork length (FL) in average), cultured in freshwater (FW) for 9 months, were used for this experiment. These fishes were moved to 100% DSW on the 3rd day after acclimated in mixed waters (1/2 and 3/4 in DSW/FW ratios, 16.5°C) by adding DSW every day. They were reared at a temperature of 14.7°C for 330 days by feeding sufficient amounts of formula food daily. The survival rate of fish decreased from the 2nd to 6th day (particularly on the 4th day) but thereafter became stable (27.9% on the 7th day). The fishes were considered to be successfully acclimated to DSW after 7days because of their stability of survival, silver to white (smolting) body color and good appetite. The survival rates after initial depletion in three size classes (< 8 cm, 8-11 cm and ≥ 11 cm in FL) were 0, 39.7 and 100%, respectively. Of the 31 acclimated fishes, 9 fishes (29%) survived 330 (323) days after the start (completion) of acclimation. At the end of culture, alive fishes attained an average FL of 32.9 cm, which was larger than the commercial size in FW culture. Their body colour was silver to white, which was different from those obtained in FW culture. Although some techniques should be revised, these data and the estimated growth and survival patterns in the cases of acclimating larger juveniles (11 and 15 cm in FL) demonstrated the possibility of rainbow trout aquaculture using DSW. Because of its clean, cold and stable nature, use of DSW will support the health of cultured fish (i.e., decrease of pharmaceuticals against fish diseases) and in turn the safety as food.

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The authors previously reported the usefulness of deep seawater (DSW, known as clean, cold and nutrient-rich seawater pipelined from a depth of 397 m in Suruga Bay, central Pacific coast of Japan) in seed production of brown algae Eisenia arborea and Ecklonia cava and the effects of light intensity and water temperature on their juvenile growth. In the present study, juvenile sporophytes of the two species were cultured at four flow rates (0.5-9.4cm sec^-1) of DSW and surface seawater (SSW, pipelined from a depth of 24m) in a transparent pipe to determine the best flow rate condition. The culture was conducted under the natural light condition at a water temperature of 14°C for 24 days in E. arborea and at 18°C for 16 days in E. cava. As a result, no significant difference was found in the relative growth rates (in blade length) of the two species in the examined range of flow rate (but data was not obtained at 9.4cm sec^-1 in E. arborea) when cultured in DSW. However growth rates of E. arborea and E. cava increased with the flow rate in SSW [E. arborea: y = 0.40Ln (x) + 4.47 (r² = 0.99); E. cava: y = 0.84Ln (x) + 7.99 (r² = 0.99)], attaining each maximum, 5.4% day^-1 and 9.7% day^-1, respectively. These results indicate that juvenile sporophytes of E. arborea and E. cava can grow in DSW faster than in SSW regardless of the flow rate. Use of DSW may result in the decrease of unevenness of juvenile growth in the culture tank.

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Diel fluctuations in concentrations of nutrients (ammonia-N, nitrate & nitrite-N, phosphate-P) and their flow were determined in an integrated culture system (started on Nov. 18, 2003) using deep seawater (DSW) pipelined from a depth of 321 m in Toyama Bay, Sea of Japan. In the system, ezo-abalone, Nordotis discus hannai (260 shells, 6.19 kg on Aug. 26, 2004) were cultured in two indoor tanks (60l in water volume (w. v.)) by using fresh warmed DSW (18°C) and the wastes were supplied to an indoor tank (14°C, 400l in w. v.) in which barfin flounder, Verasper moseri (30 fishes, 21.0 kg), were cultured. The waste from the latter tank was used in another outdoor tank (10°C, 3600l in w. v.) in which makombu kelp, Saccharina japonica (600 plants, 55.8 kg) were cultured. For chilling, intact DSW (3°C) was supplemented to the barfin flounder and kelp tanks. Nutrient outputs from the abalone and barfin flounder tanks and uptake in the makombu kelp tank were determined every hour on Aug. 25, 2004. On the day, abalones and barfin flounders were fed 450 g/tank of cut-off blade of kelp cultured in the system at 16: 00 and 95 g/tank of artificial compounds at 9: 30, respectively. The two notable features of nutrient variation were (1) increase of ammonium-N level in barfin flounder tank and its uptake in makombu kelp tank and (2) increase of other nutrients in ezo-abalone tank just after feeding. The utilization of ammonia-N, nitrate & nitrite-N and phosphate-P by makombu kelp were calculated to be 38 %, 6 % and 12 % from the difference of nutrient concentrations between supply and drainage. The low utilization rates of nutrients suggest that enlargement of the kelp biomass can improve the nutrient utilization in this system.

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Some heterotrophic bacteria in surface seawater (SSW), living on natural organic compounds such as sugars and lipids, can decompose artificial chemical substances. The enriched nature of deep seawater (DSW) may facilitate the activities of these bacteria. In the present study, degradation of one, ten and one hundred mg l^-1 of phenol was compared in vitro among the four media, SSW, DSW, mixture of SSW and DSW (1: 1) and distilled freshwater. The DSW and SSW were pumped from depths of 440 and 0 m, respectively, in the Pacific Ocean off Owase, Mie Prefecture. Tested concentrations of phenol were degraded in SSW and DSW (but not in distilled freshwater) as follows: 1 mg l^-1 of phenol was almost completely degraded in SSW, DSW and the mixture in two days; 10 mg l^-1 of phenol was almost completely degraded in two days in DSW and the mixture, while degradation rate was less than 60 % in SSW; although 100 mg l^-1 of phenol was not perfectly degraded in any seawater after five days, the degraded amounts of phenols increased unlike with lower phenol concentrations. Initial numbers of bacteria in SSW, DSW and the mixture were 89000, 530 and 41000 CFU ml^-1, respectively. The number increased most in the mixture, attaining 10⁶ CFU ml^-1 during the period. These results suggested that the mixture of SSW (bacteria-rich, nutrientpoor) and DSW (bacteria-poor, nutrient-rich) was optimal and that the in situ release of DSW can be a good tool for enhancing bioremediation to degrade artificial chemical compounds.

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An application of environmentally degradable plastics for fishing nets may help solve the ghost fishing problem. In this study, biodegradation of aliphatic polyester, poly (ε-caprolac tone) (PCL) was studied in deep seawater under a low temperature of 0.6°C during the period of 8 months at atmospheric surface pressure. Processes of biodegradation were analyzed by monitoring the time-dependent changes in mechanical strength and scanning electron micrographs (SEM) of the surface of PCL monofilaments. The strength of 110 D fibers decreased to about 80 % of initial value after one month of soaking, about 60 % after three months and about 35 % after six months. After eight months of soaking, the filament did not keep its original shape and the strength reached zero. Thus it could not be subjected for the strength measurement any further. From SEM micrographs of PCL monofilament fibers soaked in deep seawater for 3 and 8 months, pinholes were observed on the surface of treated fibers. The number of these pinholes increased with the soaking period. This phenomenon strongly suggested that the degradation of PCL in deep seawater is caused at least partly by microbial degradation. From these results, the application of biodegradable plastics such as PCL for fishing gears is suggested to be effective in reducing ghost fishing problem caused by non-biodegradable fishing nets.

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