18

The University of the Virgin Islands (UVI) in St. Croix uses a variation on the multiple rearing unit system. They operate four fish tanks of the same size, with same-age fish in each, stocked in time increments. Fish grow from fingerling to market size in one tank, with no movement until harvest. In this scenario, there is always a tank that is either ready for or nearing harvest. While tank volume is not utilized efficiently, fish stress and labor costs are decreased, while knowledge of stock inventory is increased (Rackocy et al. 2006).

Plants

Stocking and harvesting strategies can also be implemented in the hydroponic portion of the system. The three most common strategies are staggered cropping, batch cropping, and intercropping (Rackocy et al. 2006). Their implementation and success depend on geographic location (tropical or temperate regions), crop variety (leafy vs. fruiting crops), and market demand.

Aquaponic producers typically grow leafy green crops, which have a lower value per unit value and high yield. Lettuce, Swiss chard, kale, basil, and other herbs are typically ready for harvest between 3-5 weeks from transplanting (6-8 weeks from seed), resulting in a steady income stream. Fruiting plants like tomatoes, cucumbers, and peppers take 10-16 weeks to harvest, resulting in longer growing periods and lower yields, but they have a higher individual value. Producers often grow a variety of crops to diversify their markets and reach a number of consumer groups.

It is critical to invest time in a production strategy that realistically evaluates inputs and product output. Market demands vary among countries, regions, and even among neighboring cities. Producers should calculate the real-estate value of their system, often in price per square foot. To illustrate this, a comparison between two types of lettuce can be used. Figure 12 shows two different types of lettuce grown at the University of Virgin Islands in St. Croix. Although Parris Island romaine has a higher individual value ($/head) than Boston bibb, when the planting density and growth period are considered, Boston bibb brings a higher value per square meter of growing area per week than the Parris Island romaine. The main takeaway here is that high density and frequent harvests may an increased value, even when individual value of the crop is low. Information presented here is just an example and calculations should be tailored to a specific crops, farm, market, and regional costs for production.

To understand if the crop is profitable, the cost of labor from seed to harvest, price of seed, propagation supplies, and retail packaging will need to be subtracted from the price/ m2/week. If the selling price is below that of its “real-estate value,” the hydroponic portion may be operating at a loss. In addition, producers may have multiple harvests from the same crop. Kale and Swiss chard are crops that can sustain multiple harvests without a decrease in quality of the produce, therefore increasing the value of that real estate. The strategies included here are not a comprehensive list but can be developed and adapted for individual plants.