Design and Field-test of Solar-passive Thermal Refugia for Overwintering Mozambique Tilapia (Oreochromis mossambicus) in Central Texas

The aquacultural importance of the Mozambique tilapia (Oreochromis mossambicus) as a forage species for largemouth bass (Micropterus salmoides) has motivated many private pond owners in the U.S. to try propagating Mozambique tilapia for themselves; however, this species’ intolerance to cold poses a challenge for anyone attempting to overwinter these fish outdoors above the Tropic of Cancer. Thus, there is a need for appropriate thermal refuge technology, which still remains unfulfilled. An experimental, solar-passive thermal refuge was designed, produced, and tested in triplicate trials for each of two insulation treatments. The refuge design envisaged a floorless, floating greenhouse with translucent roof and walls. Each unit was made buoyant by a 4” PVC pipe frame attached exteriorly to allow for partial submersion. Walls of three refugia were lined with foam board (FoamularÔ) to provide additional resistance to heat-flux; whereas, the other three refugia were without foam board. One refuge was installed in each of six similar-sized ponds on the Triple JJJ Ranch of Somerville, Texas, during the winter of 2006-07. Chlorophyll a concentrations were generally higher within the refugia than in the ambient water outside, but temperature was not. Although no tilapia survived the winter in this field test, the thermodynamic characteristics of the six floating refugia were monitored and evaluated. A simulation-based analysis indicated that all six refugia afforded substantial thermal inertia, with exponential “decay”-rate coefficients (k) ranging from 0.34 to 0.88 ^(o)C·day^(-1)·^(o)C^(-1). The refugia with supplemental insulation actually had larger values of k – i.e., less resistance to heat transfer – presumably because they floated higher in the water column, exposing more surface area to the air above and allowing more heat flux beneath due to the increased opportunity for water movement between refuge and pond