The Effect Of Fish Cages On The Natural Environment In The Shallow Mediterranean Continental Shelf Of Israel - Preliminary Resaults

Due to the impoverishment of marine fisheries in the southeastern Mediterranean and the decrease in the yield of commercial fish species owing to over-fishing, a considerable emphasis has been placed on fish production through aquaculture. However, the increasing need for fresh water in Israel, for drinking and terrestrial uses, has diverted efforts to the establishment of marine aquaculture or mariculture.

Intensive culturing of marine fish may be accomplished either in ponds on the sea shore, or in cages submerged in the sea. Due to the high value of Mediterranean coastal real estate in Israel and the considerable investment involved in building ponds, pumping and aerating water, there is a clear advantage to cage culturing. Considering the hostile nature of the sea, cages for mariculture have so far been established mainly in protected areas: in the northern gulf of Eilat, in protected bays in the Aegean and Adriatic Seas and even behind breakwaters, as for example inside the Port of Ashdod.

It seems, however, that this method of intensive rearing of fish in dense, cage farms in protected marine areas, has a high environmental price. Uneaten fish food, together with the excretions of the fish, may accumulate on the seafloor under the cages, in conditions of low water exchange. This accumulation, especially in low productivity areas such as the coastal waters of Israel, may have a negative effect on the marine environment. The organic matter, as it accumulates, may undergo biochemical and microbial changes. Dangerously high levels of hydrogen sulfide may be produced in the marine substrates, followed by a deterioration in the oxygen level of this environment. These changes can cause the death of immovable organisms and repel mobile forms of life from the damaged habitat.

It has been postulated that mariculture of fish in cages in the open sea, despite the logistic and technical difficulties involved, will not damage the natural marine environment. It can, rather, contribute positively by adding nutritious material which is diluted and distributed by the currents.

In order to test this hypothesis, the natural population of fish and bottom dwelling invertebrates, existing under mariculture cages of gilthead sea breams in the open sea, 2 km off Mikhmoret, was compared to that existing in a control area of the same size, depth and substrate type.

Despite seasonal variations in species composition, the preliminary results point to a significant increase in species richness and quantity of both fish and invertebrates under the cages, as compared to the control. No negative phenomena associated with over enrichment by organic matter has been detected.

These findings must be approved in an extended, full-scale study. If this yields similar results, future mariculture cages could be anchored to artificial reefs in the open sea. The natural inhabitants of these man-made structures will enjoy a supply of organic matter from the cages to be recirculated by them without causing damage to the environment.

            Ehud Spanier

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