WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2015 65 may perform better in tranquil water conditions, which is not always the case here, where high winds and waves can prove very challenging and demand resilient floating systems. Under these situations, the best strategy was expanding the planting area to decrease border effects while gaining sturdiness and system integrity. Biotic Stressorfree Production As demonstrated previously for floating crops at sea (Radulovich 2010), these cropping systems are free of insect pests, pathogens and weeds, unless they are transferred from land, something that can be avoided with clean production of plantlets. This lack of biotic stressors allowed for organic and problem-free production requiring minimal maintenance, particularly for self-irrigated plants. Several experiments were conducted at La Virgen to compare insect damage on plants grown on the lake with those grown on land. There were large difference between floating and land cropping systems (Table 1). Visual comparison of the type of damage avoided by cropping on water is shown in Figure 9 for bean plants. In an 88-d trial in Lake Arenal, there was no insect pest presence and pathogen damage of beans, lettuce, maize and rice. Insect pollination was not a problem for the crops tested. However, two damaging events associated with birds occurred at the Lake Arenal site. Crop Yields Crop yields were consistently high for all crops grown on water at the three locations (Table 2). In La Virgen, in all cases, yields of crops grown on water were significantly greater than those of control crops grown on land. This was attributed mainly to fewer insect pests and other disorders affecting crops on water compared to those on land. In general, lettuce was a very easy crop to grow, yielding large healthy plants about 40 days after transplanting, attributable partly to short stature and fast growth. Beans were also very easy to grow and completed a cycle in less than 70 days after direct seeding, yielding on average 5.8 t/ha of mature beans. Other crops also grew well, including rice, bell pepper and tomato, the latter in one case in Arenal yielding 95 t/ha. Even maize plants grew adequately after some adjustments to stabilize pots to allow plants to grow erect, yielding 28 t/ha of sweet corn 112 days after direct seeding. Cultivating Aquatic Plants As a complement to experiments with vegetable crops, floating aquatic plants, normally referred to as ‘aquatic weeds,’ were cultivated in several trials. The basic premise was to initiate an effort to turn these plants into valuable crops, taking advantage of the fast and invasive growing habit of some species. Of the TABLE 1. Indicators of biostress on vegetable crops grown on floating rafts or traditional land plantings at La Virgen Lake, Nicaragua. Crop Whiteflies (no./plant) Leaf miner damage (no./plant) Floating On land Floating On land Bean 0 27.0 ** 1.3 32.3 * Bell pepper 0 13.5 * 0 0 Tomato 0 40.5 ** 1.9 27.9 ** *and ** significant differences at p < 0.05 and p < 0.01 using t-test, respectively FIGURE 7. Type of growth obtained using bottles as pots attached to floating trellis (top left), ‘aquatic farmer’ showing lettuce (bottom left), sweet corn and tomato yields (top right) and rice growth (bottom right). (CONTINUED ON PAGE 66)
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