April 16, 2015
Over 20 million tons of fish per year are farmed in marine environments to meet a rapidly increasing global demand for seafood. Americans eat around 3 ounces of seafood per week, and the 2010 USDA Dietary Guidelines suggest that we more than double consumption. But how will the production of all that fish affect the world’s oceans? Can we meet growing demand in a way that is environmentally sustainable?
In a recent review article published in Aquaculture Environment Interactions, Carol Price and James Morris from the Center for Coastal Fisheries and Habitat Research at NOAA and colleagues from the UK and Canada tried to address this question by summarizing current literature on the environmental effects of marine cage culture—marine fish farming in enclosed pens—with a focus on water quality impacts due to the “loading” of nutrients in water. One aim of the review was to inform a better management framework for developing and sustaining fish farms that will maximize production and minimize harm to oceans.
The paper outlines evidence that fish farming impacts water quality via two main modes of action: 1) direct effects, for example, the release of excess nitrogen into water from fish waste; and 2) secondary effects, which first impact primary producers in the marine ecosystem, such as algae that form harmful algal blooms and suffocate life below the water’s surface—a symptom of the nutrient loading process called eutrophication.
The authors found some evidence that fish farming practices have improved over time with regard to impacts on water quality from nutrients. According to current literature, most individual fish farms have a limited direct negative effect on marine water quality due to nutrient loading. This is most likely a result of practices like siting farms in deep waters where nutrients are easily dissipated by the current, and more efficient feeding that prevents uneaten particles from creating a buildup of excess nutrients.
Importantly, this review does not cover the effects of fish escapes, fish diseases moving from farms to wild fish, or veterinary drug or pesticide use that may impact the environment. The article also fails to address potential occupational health and safety associated with offshore marine aquaculture, the topic of a recent publication authored by faculty members at the Center for a Livable Future. If a commercial marine aquaculture industry develops in the U.S., the authors strongly urge that scientists “continue to examine the potential risks and benefits of aquaculture in the marine environment.”
Another practice described in the review is integrated multi-trophic aquaculture (IMTA), which is similar to the concept of integrated pest management in land-based systems. To practice IMTA, a farmer raises her fish alongside other species from different trophic levels—in marine environments this is typically mussels and kelp—that help to break down waste and absorb excess nutrients. This provides the added benefit of diversifying the variety of products a farmer can sell. The authors note, however, that the literature suggests nutrient loading in water is not a problem in some areas—especially at deep-sited fish farms—because the nutrients are naturally flushed away by ocean currents. In some regions, such as the Mediterranean Sea, nutrient levels may be too low to support mussels and the water too warm for kelp. Thus, the use of IMTA may not be appropriate in all situations.
The evidence presented is not conclusive on the issue of secondary effects on primary producers, and whether waste effluent from multiple fish farms in the same area negatively impacts water quality. In some areas, primary producers in the marine ecosystem take up extra nutrients and cause eutrophication, to the detriment of many nearby aquatic species, including the farmed fish. In other areas, the effects are negligible. This will be an important area for further research. We need better models to predict the cumulative effects of aquaculture operations on ecosystems since it is possible that fish farms will be more densely clustered in the future in order to meet growing global demand. We encourage aquaculture stakeholders to take a wider view of potential impacts of various production methods, and to include public health professionals in conversations about expanding aquaculture.
Image: Gordon Hatton [CC BY-SA 2.0], via Wikimedia Commons