20 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG This seems to be related to some substance from the guano sticks that negatively affected the fish. When removing sticks to sample periphyton, a foul-smelling, yellow-brown substance was observed flowing out of them. Under the same conditions of inorganic fertilization without fish feed inputs, and with plastic bottles as substrate, tilapia performance was similar in the presence or absence of substrates. This rules out the use of guano sticks and would indicate that inducing periphyton growth, even on appropriate substrates, would not be advantageous for tilapia culture, calling into question our a priori assumption. However, in most experiments, tilapia performance was poor, growing from 25 to only 50 g in 12-14 weeks, in a range of conditions: with and without substrates, with inorganic fertilization and with commercial feeds. This led us to search for more general phenomena that might be affecting tilapia growth, such as chemical and biological processes related to the quality of source water (Fig. 4a). Yucatán has a karstic terrain and thus the subterranean waters have high alkalinity (>300 mg/L) and hardness (>380 mg/L), are rich in calcium (>100 mg/L), a slightly acid pH from a high partial pressure of CO2. TOP, FIGURE 2. Substrate types tested for periphyton attachment. (Photo: Martha Hernandez) BOTTOM, FIGURE 3. The experimental tanks. In the background, tanks for the fish culture experiments; in the foreground, tanks for the substrate experiment photographed three days after installation of substrates. (Photo: Martha Hernandez) BELOW LEFT AND RIGHT, FIGURE 4. Conceptual representation of the chemical and biological processes in the high-alkalinity waters of Yucatán (a) from groundwater to the first steps after filling fish tanks and (b) during the culture period.
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