In this paper, the authors examined the impact on coral growth and reproduction of fish farm effluent in the Gulf of Aqaba (Eilat) in the Red Sea. The commonly held belief is that high particulate and nutrient levels released by such operations are damaging the corals on the local reefs. Indeed, it is generally accepted that high nutrient levels can affect coral growth and reproductive fitness. What is less commonly known is that as the nature and concentrations of these nutrients vary, so do their impacts on corals.
Two study sites were selected, the Ardaq fish farm on the north shore of the Gulf of Eilat where gilthead seabream are cultured in sea pens and a more southern “pristine” reef off the Inter-University Institute (IUI) of Eilat. The levels of nitrite, nitrate and phosphate (micromolar) measured at the two sites are listed in the table 1.
Forty branches of Acropora eurystoma were removed from each of ten different colonies located at the IUI. The resulting 400 pieces were mounted on to PVC plates, 200 were placed at 6 m depth on the reef at the IUI, and 200 were placed at 6 m depth, 20-40 m from the fish cages within the middle of fish farm. All fragments were stained with Alizarin red so that skeletal growth could be later determined. After seven months, the fragments were collected and measured for weight, linear extension of branches and “ecological volume” (the water volume occupied between the branches of the coral) and compared.
In another experiment, 190 small pieces of Stylophora pistillata were glued onto PVC plates and placed at each location. Survival rates and lateral growth were then measured at 3, 4 and 13 month intervals.
In addition, 14 colonies of similar sized Stylophora pistillata growing at each of the two sites were sampled to assess gonadal development. Finally, the lipid content of both species of coral from both sites was examined.
The result showed that Acropora grown at the fish farm site had identical survival rates to those from the IUI site. However, those from the fish farm exhibited a significant 3-fold increase in weight and linear extension, and a significant 4-fold increase in ecological volume compared to those at the IUI site.
The fragments of Stylophora showed a similar trend with those at the fish farm site showing significantly greater sizes after 13 months. However, those at the IUI site showed greater lateral growth rates, presumably due to the higher degree of deposition and accumulation of particulate matter at the fish farm site. Whereas the average growth rate of the IUI fragments was high, those at the fish farm exhibited a gradual increase in growth rate over time and eventually (after 13 months), those colonies at the fish farm site were significantly larger than those at the IUI site.
The reproductive status of Stylophora was examined in 2001 at the beginning and peak of the reproductive season (January and May 2001) and at the beginning of the 2002 (December 2001) reproductive season. The number of polyps with female gonads was higher at the fish farm in January and May 2001, but in December, all colonies at both sites had ovaries. The size of the oocytes in colonies growing at the fish farm was larger but not significantly so. However, the average number of oocytes per polyp was significantly greater at the fish farm and there were significant differences between seasons too with December 2001 having significantly greater numbers than the previous season. The proportion of colonies with male gonads was also greater at the fish farm site.
The lipid content of naturally occurring colonies of Stylophora at the fish farm site was significantly greater than at the IUI site. However, there was no difference in the lipid content of the Acropora used in the study after 7 months.
This study tends to contradict earlier studies that showed detrimental effects of increased nutrient levels on not only coral growth but also coral reproductive state. Although various studies have shown that elevated nutrient levels are generally associated with detrimental effects on coral growth and reproduction it has been proposed that the chemical form and concentration of these nutrients may actually determine whether there is a positive or negative effect. It is well known that corals will absorb ammonium and use it to enhance growth. Nutrient types and concentrations may very well dramatically determine the type of response exhibited by reef organisms. While elevated nutrients are often cited as major contributors to the decline of reef health, it is often difficult to distinguish the effects of any one factor when dealing with multiple impacts on reef health such as anthropogenic pollution, human use (i.e. SCUBA), siltation, sand deposition etc. Of course, the short term
of this study may mask any long term detrimental effects of elevated nutrient levels on coral health as well.
For aquarists the lessons are clear, the obsession with nutrients such as ammonia, nitrate and phosphate may be slightly misplaced. While it is generally advised that these nutrients be as low as possible (0 is often quoted as the ideal for ammonia for example), they may not be detrimental at certain concentrations and it may actually be beneficial to have slightly elevated levels of certain nutrients such as nitrogen to aid coral growth. Of course, one must not loose sight of the fact that hobbyist test kits are a far cry from being able to measure the levels mentioned in this study and it is probably safe to assume that the levels for many nutrients are still several times that of oligotrophic reef waters.
At this point, I would like to mention a pet “theory” of mine. One of the widespread “fads” in reef keeping today is the addition of phytoplankton cultures and other “foods” to reef aquaria. Often these are fed to tanks containing predominantly LPS corals in the belief that these coral will feed on this food. Observations are often made that the corals look much better and colorful after several weeks of such feedings. I would like to propose the theory that the benefit may not be so much from the actual ingestion of the food but the decomposition of the food leading to the generation of nitrogenous wastes which are then absorbed by the corals resulting in better pigmentation and increased zooxanthellae populations.
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