|Title||Effect of effluents from a new fish farming site on the benthic environment|
|Authers||Takashi TANIGAWA, Azumi YAMASHITA and Yoshitsugu KOIZUMI|
|Keywords||New fish farming site, Organic matter lord, Benthic environment, AVS-S, TP|
|Citation||Bull. Fish. Res. Agen. supplement No. 19,69-77, 2007|
In relation to improvement of the environment around fish (red seabream and yellowtail) farms in Shitaba Bay, Uwajima, Ehime Prefecture (Japan), about half of fish cages were shifted from the inside (ca. 20-50 m depth) to the outside (60 m depth) of the bay during May-November 2001. The spatio-temporal variability of the benthic environment at the newly established fish farm site was investigated, by analyzing the sediments at both the new (outside) and the old (inside) farm sites before and after the shift of fish cages. For example, the organic matter load from the new farm was measured to determine the assimilative capacity of the benthic ecosystem.
Acid volatile sulfides (AVS-S) of sediment at the center of the new farm were 0.03 mg/g dry weight in February 2001 without fish cages. As soon as the fish cages were set, AVS-S drastically increased (0.11 mg/g dry weight in July) and reached 0.26 mg/g dry weight in November when the fish cage shift had been completed. Moreover, AVS-S continued to increase and reached its maximum (0.46 mg/g dry weight) in April 2002. However, it remained at ca. 0.3-0.4 mg/g dry weight thereafter. Although AVS-S increased also at the edges and surrounding areas of the new site, concentrations were lower than those at its center. AVS-S increased slightly at areas over 50 m distance from the edges. Total phosphorus (TP) was about 2 times higher in the old farm site than that of the new farm site, while no marked differences of total organic carbon (TOC) and total nitrogen (TN) were detected between the two sites. TN and TP of sediment at the new farm site were slightly higher than concentrations observed in a pearl farm and a non-farming site (as reference sites).
From data on the amount of feed sold, production and feeding amount to cultured fish for some cages, about 50% (5,000 ton in dry weight) of the total amount of feeds sold by the fisheries cooperative association was consumed in the new farm site in 2002. These feeds may be reflected in the TOC, TN and TP flux caught using sediment traps set at 5 m depth above the bottom, which were equivalent to or more than the concentrations at the old farm site.
Thus, it was suggested that the AVS-S increase was largely due to effluent of organic matter (uneaten food and fecal) from fish farming activities closely linked with the shift of fish cages, and that the effect of the loaded organic matter on the benthic ecosystem was low in areas more than 50 m away from fish farming site.