Aspects of the potential impact of marine aquaculture on the environment

Abstract

Ecological changes due to coastal aquaculture depend on the pattern and levels of farming as well as the physical, chemical and biological characteristics of the site. In this study a preliminary base line survey of the more relevant environmental parameters of two fish farms, one a land based and the other an offshore cage farm, was first carried out. Water samples of different origins were then mixed in varying protocols, placed in clear plastic bags and monitored for the more relevant parameters in order to gain some insight into the potential effect of farm effluent water on the environment. At the offshore site, temperature, salinity, dissolved oxygen and pH were, as expected, not significantly affected by the presence of the fish farming activities. On the other hand, levels of nitrates were significantly higher at the cage site when compared with the control site. Phosphates were significantly higher at the cage site on two occasions but identical on the remaining three. The chlorophyll a levels at the cage site were also significantly higher on three occasions. This implies that the nutrients released from the fish farming activities have a long enough half life to allow for phytoplankton to take advantage of this localised enrichment. In tum, the nutrient and chlorophyll a values at the control site are noticeably higher than those normally encountered in Maltese open coastal waters. This is presumably due to the fact that, at the particular site under study, it takes more than 300 meters for the released nutrients to be diluted enough to fall below the threshold level for affecting phytoplankton growth. It is also possible that there are other nutrient inputs into the area. On the other hand, land based farms are normally much more susceptible to fluctuating environmental conditions than their open sea based counterparts as they do not enjoy the stabilising effect of a large body of water such as the open sea can offer. When open sea and fish pond water were mixed in a number of combinations. the mixture containing 60% fish pond water exhibited the highest peak in chlorophyll a but when the experiment was part repeated it was the bag containing only fish pond water that exhibited the highest chlorophyll a value. In all bags containing combinations of these two water sources however, the peak in chlorophyll a was, without exception, recorded four days from initiation of the experiments. This confirms that in open waters it is the diluting factor of the water movements that prevent enhanced primary productivity from increased nutrient levels from fish farm effluents. When borehole water was mixed with open sea water the increase in chlorophyll a concentrations was more gradual and continued throughout the duration of the experimental period. This possibly reflects the relatively lower starting concentration of both phosphate and phytoplankton cells and results in a longer latent period taking much longer for cell proliferation to take place. In contrast when effluent pond water was mixed with open sea water, the chlorophyll a peaked much quicker, that is, after two days possibly reflecting the higher starting concentration of phytoplankton cells. By and large, in all experiments, chlorophyll a peaks coincided with decreasing nutrient levels and increasing oxygen and pH values. In all cases it was the phosphates that became limiting and led to population declines, leaving relatively abundant concentrations of nitrates in the water. This is in line with all previous reports on Mediterranean seawater which, without exception, conclude that phosphate is the limiting factor. Studies on species composition show that, as expected, cell counts peaked concomitantly with chlorophyll a levels. Species composition did not change in the course of the experiments and three diatom species dominated, viz., Biddulphia aurita, Achnantes longipes and Navicula sp.