תליא ץרפמב םיבולכב םיגד לודיג לש תירשפאה העפשהה
By Dan M. Popper*
National Center for Mariculture P.O.Box 1212, Eilat 88000,Israel
A slightly irrelevant introduction to this lecture is required. This is because the lecturer is part of a group of Eilat residents who initiated a campaign against the flow of municipal sewage into the Gulf. The campaign has been blamed for being a camouflage for the damage caused by the fish cage culture. The technology for this culture was partially developed at the national center for mariculture (NCM).
The facts are that NCM has developed technology for mariculture in Eilat over the last two decades with concern for the environment. Major effort is given to the development of an integrated land based pond system for the culture of fin fish, bivalves and algae. This system is environmentally finely in the sense that it results in effluents that are nearly pure sea water. This can be done only in land based facilities where the control over water flow is nearly complete. Unfortunately, the development of such a system is complex and therefore cage culture was temporarily recommended (by declaration) as means for fish production until the development of the "clean" system is ready for commercial use.
Currently the cages are operated by two commercial entrepreneurs. The involvement of NCM in cage related studies is aimed at (1) minimizing damage to the environment by improving food conversion rate so as to reduce nutrient release into the surroundings (2) better understanding of the actual impact of cage culture on the environment.
In this lecture I will try to review some of the present understanding on the expected effect of the cage culture in the northern Gulf of Aqaba on the precious coral reef and on the environment in general.
No conclusive information is available in the literature, on actual short term or chronic levels of dissolved nutrients that would kill or eliminate particular species of coral reef communities. However it is well known that tropical coral reefs survive only in clear oligotrophic waters (Fishelson, 1977; Loya and Rinkevich, 1980). The Gulf of Eilat was once famous worldwide for its crystal clear water and spectacularly diverse coral community (Fishelson, 1971 ).
Unfortunately no hard core information exists, which would prove or indicate the deterioration of water quality in the gulf. Also it would take years of input of nutrients before the level of N (=Nitrogen) and P (=Phosphate) would be detectable. This is because the oligitrophic nature of this water, which would take up any amount of N and P to be consumed by the starved environment. This information will interfere with the environment. Recent figures provided by Gordon et al., 1994 show that the nitrate levels in surface layers of the Northern Gulf are lower and in agreement with levels expected for oligotrophic waters i.e. nearly 0 for the upper layer. An exceptionally high level (0.028 mg/L) is presented for March 1992. This however can be attributed to intense mixing at this time of the year.
A study carried out in Hawaii (Bell et al., 1989) suggested that the background values for dissolved nitrogen should be 0.014 mg/L and for Phosphate 0.007 mg/L and should not be permitted to increase beyond 10% of these values. In a study from the great barrier reef in Australia (Baldwin, 1989) similar values were suggested. It is important to note that the two locations are exposed to open oceanic waters of the largest water body on our globe whereas the Gulf of Aqaba is limited and confined. However theoretical calculations based on simulation model offered in a survey of environmental impact of cage culture in Eilat (Angel et al., 1993) suggested an increase of up to 0.0028 mg/L in dissolved nitrogen as a result of rearing 150 ton of fish at the head of the Gulf. The calculation is based on the assumption of a complete water exchange in 534 days. This figure means an increase of at least 10% over the baseline highest level (0.028 mg/L at highest surface measurements according to Gordon et al., 1992) and is alarming in view of the 1000 ton cage culture planned for Eilat alone.
The coral reef of Eilat was unique on earth with by far the largest species diversity of hermatypic corals recorded anywhere (Loya and Slobodkin, 1972). Today the coral reef environment is ruined beyond recognition (personal observation at the "Japanese Gardens", Jan. 1995). Over 50% of the reef wall coverage is dead (Gundermann, personal communication, 1995). Many of the fragile coral species depending on depth are completely absent (Meroz, A., personal communication, 1995) and the fish population is dwindled in terms of numbers of individuals and species, down to 50 m (personal observation).
If nothing is done immediately, I am afraid that this process will become irreversible. Even if there is only a doubt that the cage culture is part of the reason for the catastrophy, it is our responsibility to draw conclusions and try save this unique natural phenomenon: the most northerly and diverse coral reef on earth. It is not too late yet.
* Lecture held in the conference: The Ecosystem of the Gulf of Aqaba in Relation to the Enhanced Economical Development and the Peace Process - II. Jan 30-Feb 2, 1995 (pp.72-74)