Effects of fish aquaculture on inorganic nutrient levels in Gokasho Bay

Eutrophication derived from aquaculture effluent and consequent environmental deterioration are becoming increasingly problematic in many parts of the world. On the other hand, eutrophication has been reduced in coastal waters of Japan because of mitigation efforts over the past thirty years. While such efforts have positive effects on maintaining environmental integrity, excessive reduction of nutrient load to the coastal environment is thought to have reduced the productivity of many fishery resources and unfed aquaculture. Our hypothesis is that putting aquaculture effluent into practical use enhances the production of coastal fisheries and unfed aquaculture. To acquire basic information on the influential extent of aquaculture effluent on carrying capacity of coastal waters, environmental surveys were conducted to investigate the behavior of nutrients and primary production covering the whole bay area and more intensively around the red seabream, Pagrus major, aquaculture cages in Gokasho Bay, Mie, Japan. Seawater samples were collected monthly at 3 depths (surface, mid-layer and bottom) at 19 points in the bay, and seasonally at surface and depth of 5 m (i.e. midpoint of the cage depth) at 47 points approximately 20 m apart from one another around the red seabream aquaculture cages. The concentration of the following nutrients was analyzed: dissolved inorganic nitrogen (DIN: NH4-N, NO2-N, NO3-N), PO4-P and SiO2-Si. A CTD was used to obtain the vertical profile of temperature, salinity, dissolved oxygen (DO), and chlorophyll a from the surface to the bottom. Although the DIN-N and DO levels in December 2016 were comparable to those from the same month in 1980s where aquaculture was more prosperous, the PO4-P level was an order of magnitude lower probably due to diminished aquaculture production and improved feeding management, including the switch from raw bait to formulated feed. During a diatom bloom observed in November 2016, the depletion of SiO2-Si implied that the availability of Si, but not N or P, was the limiting factor for the diatom growth in Gokasho Bay. There were spots with higher NH4-N levels at the center and 20 m outside the red seabream cage area in January 2017. The chlorophyll a level was higher and NO3-N was lower around the cages than in the surrounding water. The observed lower NO3-N concentration may indicate that the enhanced primary production triggered by the NH4-N supply from the aquaculture effluent rapidly consumed not only NH4-N but the background NO3-N in the area, thereby indicating that DIN-N excreted from red seabream may not travel a long distance from the cages before absorbed by primary producers. In order to fertilize seaweed with red seabream aquaculture effluent, the seaweed should be cultured near the fish cages. The elevation of chlorophyll a indicates the possibility of productive co-culture of red seabream and bivalves.

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bibliographic_information

ja : 水産研究・教育機構研究報告
en : Bulletin of Japan Fisheries Research and Education Agency

巻 49, p. 121-127, ページ数 7, 発行日 2019-02

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水産研究・教育機構

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出版者

Japan Fisheries Research and Education Agency

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item_10002_source_id_9

収録物識別子タイプ

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収録物識別子

2432-2830

item_10002_source_id_11

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NCID

収録物識別子

AA12767505

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2024-10-03 01:30:19.722424

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