As the world recognizes the limits of the oceans, many hope that aquaculture can take some pressure off wild populations. However, as aquaculture continues to expand, one of the challenges is making it more efficient and environmentally friendly.

One of the most effective ways to reduce pollution from aquaculture is through integrated multi-trophic aquaculture (IMTA), which, in the marine environment, is sometimes called integrated mariculture. In this process, several species of aquatic organisms with complementary feeding habits are raised in close proximity. The waste product of one species is food for the others, thereby reducing impacts on the environment. For example, in Sungo Bay, China, aquaculturists grow a combination of fish, abalone, seaweeds, and scallops in a sequence of cultures extending for 8 kilometers offshore. In this system, waste from the cage-cultured fish, both dissolved nutrients and particulate matter, is consumed by the other members of this integrated aquaculture system. Filter feeding species can directly consume wastes from the fish feeding operation, while primary producers benefit from dissolved nutrients (mainly nitrogen and phosphorus). The result is a substantial reduction in pollution and greater harvests of seafood (Figure 8.36).

Integrated multi-trophic aquaculture is being tested around the world. A system of integrated aquaculture involving Atlantic salmon, mussels, and kelp has also been developed for the Bay of Fundy, Canada. Again, the integrated system reduces pollution from the caged salmon and increases profits. Other systems involving multiple species are in place in Africa, South America, Australia, and Europe.

Biological approaches are also being used to make land-based aquaculture systems more sustainable. All intensive aquaculture systems have the potential to generate pollution. While IMTA can be used to reduce nutrient pollution in marine systems, other techniques are required for freshwater aquaculture in the terrestrial landscape. There are many possible engineering solutions to the problem of aquaculture wastewater treatment, but many are expensive both in terms of energy and money.

Increasingly, constructed wetlands, which are artificial wetland ecosystems constructed in areas where wetlands do not occur naturally, can be used to treat wastewater from freshwater aquaculture (Figure 8.37). Constructed wetlands foster the complex biological functions of nutrient removal by plants and microbes, and they can also provide habitat for wildlife. While they can be effective and self-sustaining if designed properly, they require long-term monitoring to ensure that they continue to function as designed. Because the problem of treating wastewater from aquaculture is slightly different from treating other forms of wastewater, we will defer a detailed discussion of constructed wetlands until Chapter 13.

There are several ways to reduce the impact of shrimp farming on mangrove forests. One problem is containment of the nutrient-rich water to minimize coastal pollution. Experts in the design of shrimp ponds point out that the soils where mangroves grow are generally too permeable to contain water. In those cases, the ponds may need to be lined with plastic or an impermeable clay so that waste water can be properly managed. Another problem associated with shrimp farming is that removing mangroves to build shrimp ponds increases the risk that the ponds, which are very expensive to build, will be damaged by storms.

Recognizing this danger, many shrimp ponds are now sited behind protective mangrove forests, and the necessary ocean water is delivered to the inland ponds through a canal or pipeline. Finally, because ponds dedicated to intensive shrimp culture need to be completely drained periodically, they are increasingly built above the high-tide level and therefore inland from mangrove forests. In these cases, reducing direct impact on mangrove forests allows them to continue to provide ecosystem services of fish nursery habitat and flood protection.

One of the most direct approaches to reducing the impact of sea lice on wild salmon is to monitor and treat lice infestations on captive salmon using pesticides. However, there are concerns that sea lice will evolve resistance to such treatments and that nontarget organisms will be killed—as has occurred with chemical control of agricultural pests (see Chapter 7). Another potential control measure includes restricting salmon farms to areas away from the migration paths of wild fish to minimize their contact. Canada and parts of Europe have already taken the precaution of excluding salmon aquaculture from some areas supporting valuable wild salmon stocks. Growing salmon in land-based tanks rather than sea pens and treating the effluent is another way to reduce infection of wild salmon.

Reducing the impact of aquaculture on wild forage fish populations hinges on finding substitutes for fish meal and fish oil. Much progress has been made in replacing fish meal with plant protein (e.g., soy). Fish oils have also been partially replaced by plant oils, including canola, soy, sunflower, or olive oil. Fish nutrition experts predict that three-fourths of fish oils currently used in aquaculture feeds could be replaced by plant oils with no loss in growth performance by fish or shellfish. Others have suggested that shifting aquaculture from large, carnivorous species, which require high-protein diets, down the food chain to omnivores or herbivores will also reduce the amount of fish meal used in aquaculture. For example, tilapia and catfish grow well on predominantly plant-based diets. Plant-based feeds are also growing in popularity among shrimp farmers as they have increasingly shifted from growing carnivorous tiger shrimp to growing omnivorous western white shrimp, Litopenaeus vannamei.