30 JUNE 2023 • WORLD AQUACULTURE • WWW.WAS.ORG and “low” density is relative. In this study, the difference between traditional ponds stocked at low density (3.9/m2) and high density (5.5/m2) is not that great, and both can be considered high stocking density in the context of semi-intensive pond aquaculture, but is quite a bit less than the stocking density used in the IPRS (13.7/m2). In general, the results from this commercial farm indicate the well-known effects of density on production performance and highlight the interaction between stocking density, culture period and production performance (Table 3). Tilapia growth rate, calculated as weight gain per d, was ranked LD > IPRS > HD. Although the difference in growth rate between LD and HD ponds is expected, a stocking density in IPRS that was 2.5× that of HD ponds did not suppress growth, indicating the advantage of IPRS over HD ponds with respect to growth rate. Periodic biometric sampling of fish in each treatment provided intermediate data points that were used to generate growth curves (Fig. 4). Analysis of those growth curves indicates the superior performance of tilapia grown in IPRS, followed by tilapia grown at low density and then at high density in open ponds. Harvest yield was ranked IPRS > LD > HD. The harvest yield in IPRS was 1.4× the yield from LD ponds and 2.5× the yield from HD ponds. This effect was achieved in a culture period for IPRS of 147 d, compared to 231-237 d for the traditional open ponds. Survival of tilapia in IPRS (61.8 percent) was substantially less than tilapia in open ponds (73.5-76.6 percent). Presumably the high density of tilapia in IPRS and their confinement at high densities in raceway channels represents a stress factor not present in open ponds that led to lower survival. The final weight of tilapia stocked in HD ponds and IPRS were in the range for suitability for the domestic market (~0.5 kg), whereas only tilapia from LD ponds reached the size suitable for the export market. However, to reach a similar size, 84 fewer days was required for tilapia in IPRS compared to HD open ponds. Presumably, extending the culture period for tilapia grown in IPRS to about 230 d would allow fish to grow to export size. The economics of the production of tilapia in open ponds and in IPRS to final weights suitable for each market segment needs further evaluation. Feed conversion ratios were fairly similar among treatments, FIGURE 3. Morning and afternoon dissolved oxygen in two tilapia ponds for each treatment stocked at low density (top), high density (middle) and in IPRS raceway cells (bottom).