Types of Crab Farms

The past 20 years has seen the evolution of several types of fish farming methods. Each of these methods has a particular set of advantages, drawbacks, and challenges that are for the most part directly applicable to crab farming practices. The first type of aquaculture is extensive—relying on photosynthetic processes and self-sustaining/self-regulating systems. The second type of aquaculture is intensive—a system of artificial habitats that rely on feeding and filtration systems in order to function. Extensive aquaculture has little to no environmental impact, but requires large tracts of land and significant labor for harvesting. For this reason, it is reasonable to assume that areas like the Eastern shore (where land comes at a premium and labor is comparatively expensive) are unsuited to extensive aquaculture. In contrast, intensive fish farming can be performed in smaller areas and at greater densities, minimizing land and labor, but the environmental impacts of these techniques can be significant (fig. 25). Intensive aquaculture projects use upwards of a million gallons of water per acre (about 1 m³ of water per m²) each year. If this water is not treated and reused onsite, it can lead to significant environmental degradation.
Figure 25 – Types of fish farming mapped against environmental impact. Author illustration.
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IRRIGATION DITCH/POND SYSTEMS
Irrigation ditch/pond systems are the oldest and most basic type of fish farming. These systems essentially duplicate a fish’s native habitat on an inland site. The pond contains a full predator/prey ecosystem, and is carefully balanced in terms of the flora and fauna present at any given time. The pond system must be quite large because it is designed to support a higher density of livestock than would be common in a naturally occurring biosystem. As global real estate goes up in value, and water resources are constrained, it is likely that the ditch/pond systems will over time give way to more intense forms of aquacultural production.

FRY FARMING SYSTEMS
Fry farming is a practice that has been used since the early 17th century. It essentially consists of culverts that channel water and fish from a nearby stream or body of water and allow these fish to grow up in a closed system supplied with an abundant artificial food source. These fish frys are typically concrete raceways, and can be cleaned by sluicing clean water into the system once the fish have grown to maturity and have been harvested. The great density of fish in these systems has proven a sticking point in the past. Disease and parasites are common, and entire crops can be lost without any warning.

CAGE SYSTEM
Cage farming is the most toxic of the three types of intensive fish farming. It consists of large cage or net system in which fish are kept in super high densities. The cage is located in open waters such that the system is guaranteed a constant supply of freshwater (fig. 26). Organic debris are allowed to exit the system along with the flow of the open water. While there has been much recent interest in cage fish farming, there is, as yet, no means for purifying the debris from the water before the effluent is sluiced into the open water (fig 27).


Figure 26 - Winning Entry—South Street Seaport by N.E.E.D. Architects, 2008.


Figure 27 - Environmental Risks of Marine Aquaculture. Image Courtesy of: http://www.motherjones.com/environment/2006/03/aquaculture-environmental-impact

INTEGRATED RECYCLING SYSTEMS
Within the aquaculture industry, integrated recycling systems are thought to offer the best way of minimizing the use of natural water resources while at the same time limiting the production of effluent. Integrated recycling systems consist of large plastic, metal, or concrete tanks in greenhouses. The marine livestock are supplied with nutrients and a food source (typically algae); the waste in the system is slowly circulated to hydroponic beds near the tanks, where carefully cultivated microorganisms convert (“fix”) the ammonia to nitrates which fertilize the plants along with the phosphates from the tanks. Other wastes are strained out by the hydroponic media, which can double as aerated pebble-bed biofilters (fig. 28).
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Figure 28 - Hydroponic Recirculation Diagram. Author illustration

This system, properly calibrated, produces more edible protein per unit area than any other form of aquaculture. Although a wide variety of plants can grow well in the hydroponic beds, most farms focus on herb production (e.g. parsley and basil), which command premium prices in small quantities year round. The most common customers are restaurant wholesalers. Since the system is greenhouse based, it adapts to almost all temperate climates, and may also be employed in tropical climates. The main environmental impact is the discharge of salt water. Currently, farmers use a variety of proprietary techniques to treat and reuse water, thereby reducing their expenses for salt and waste water discharge permits. In order to treat effluent and avoid expensive discharge permits, many farms are beginning to rely on ultraviolet and ozone disinfectant systems. These range in cost but are not unaffordable relative to the entire cost of the system.

After performing a cost/benefit analysis on the four types of fish farming (fig. 29), it became abundantly clear that an integrated recycling crab farm would provide the most edible mass with the smallest financial investment and least environmental threat. And while integrated recycling can be performed anywhere and is therefore not site specific, it can be made so by adapting its voluminous water needs to the local topography and regional plant ecosystems. For these reasons, an integrated recirculating crab farm was deemed the most sustainable, practical, and realistic for Maryland’s Eastern Shore.

Figure 29 - Cost Benefit Analysis of Various Crab Farms by Type. Author illustration.

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[19] McLarney, William. Freshwater Aquaculture: A Handbook for Small Scale Fish Culture in North America.
[20] Freshwater Aquaculture: A Handbook for Small Scale Fish Culture in North America, by William McLarney