Background: Like all other animals, as fish metabolize food they produce wastes–soluble nitrogenous compounds and settable solids (feces). A priority has been the development of protocols to help farmers locate their farms so to avoid the build-up of sediment on the seabed or organic changes in the water column. There are now modeling tools that evaluate the suitability of prospective farm sites for their ability to provide for the adequate dispersal and prevent assimilation of wastes.
Usually this means locating mariculture facilities in areas with medium to strong tidal currents and water depths greater than 60 feet. A criticism of such an approach is that dilution is not the solution to pollution. But nutrients, managed properly, are beneficial wastes. On land, animal wastes are routinely used as fertilizer. In fact, under appropriately controlled conditions, this is a prerequisite of organic farming.
It is entirely possible that, if all the interacting factors can be understood, mariculture wastes can result in a beneficial, localized enhancement of marine flora and fauna, including natural fisheries.
- The digestive wastes from fish in mariculture facilities are the same as those generated by schools of wild fish. In mariculture facilities, there is often an additional amount of organic material (small proportion of uneaten feed) passing through the net meshes and into the water column. These wastes do not contain toxic substances, unlike many industrial, land- based agricultural and domestic wastes, and they are biodegradable.
- Ammonia, in the form of urea, is excreted by fish and is quickly oxidized by marine bacteria to become nitrate, which is a basic plant nutrient.
- Solid wastes are also biodegraded by bacteria or in some part consumed by marine invertebrates. But this process is slower and, therefore, these wastes can accumulate on the seabed if they are not adequately dispersed.
- Uneaten feed is, in well-managed farms, a very minor proportion of solid waste. Since it is not economical to use expensive raw materials inefficiently, fish farmers use a variety of monitoring devices, such as underwater cameras and acoustic Doppler systems, to ensure that the fish are adequately fed with minimal feed waste.
- Adverse effects on the cultured fish themselves are almost always the first indicators of excessive soluble wastes. It is in a fish farmer’s best interest to prevent this, so examples of soluble wastes causing problems in mariculture are extremely rare.
- Accumulation of solid wastes under a mariculture facility can now be predicted by powerful new computer models and can be monitored by regular sediment sampling. In properly managed mariculture facilities, these changes in the sediment are minimal. In some cases, the changes fall within the range of natural variability (i.e., changes observed following storms or seasonal fluctuations in water temperature). In other cases, deposition of organic material in the sediment leads to an increase in sea floor productivity and increased aggregations of fish and marine invertebrates, changes that may be considered beneficial.