TABLE OF CONTENTS Pages Symposium 1 - the National Sea ...

Heat Budget Model for Predicting Water Temperature in
Outdoor Intensive Fish Tanks Under Greenhouse Cover
Introduction
Arik Ashkenazi 1* ; Noam Mozes 1 ; Ido Seginer 2
1 – National Center for Mariculture, IOLR, P.O. Box 1212, Eilat, 88112, Israel
2 – Department of Agricultural Engineering, Technion, Haifa 32000, Israel
Intensive fish culture in Israel is generally practised in covered out door tanks.
The transparent covers trap solar energy, resulting in water temperature that enables
fish to overwinter successfully in northern Israel and even to achieve some growth in
southern Israel. Common cover materials are semi-transparent white or green PVC and
PE sheets.
The objectives of this study were : (a) to examine experimentally the influence of
different cover materials on tank water temperature and (b) to develop energy budget
models to quantify energy fluxes in a covered tank and to predict tank water
temperature.
Data were collected between 1996-1999 at the experimental aquaculture
research station at Genosar (northern Israel) and at the National Center for Mariculture
(NCM) in Eilat (southern Israel). Two models were developed : (a) an extended
dynamic model (EM), which was used to estimate the relative importance of the
various energy fluxes, and (b) an equilibrium radiation model (RM), which was used to
predict mean water temperatures.
Experimental procedure
The effect of several cover types on water temperature was studied in ten 50 m 3
concrete tanks at Genosar. Materials used were semi-transparent white or green PVC
and PE sheets. The tanks were occupied by overwintering tilapia fish. All tanks had the
same water exchange rate (10-50%/day, depending on the year) and the same number
of paddlewheel aerators. Data were collected on an hourly basis for 3 months over
three winters (1996/97 to 1998/99). These included wind speed and direction, as well
as inside and outside solar radiation, air temperature and humidity. Also measured
were the temperatures of the tank water, inflow water, tank cover and sky.
At the NCM in Eilat, similar tests, utilizing three 100 m 3 sea water tanks for sea
bream culture, were conducted in the winters of 1996/97 and 1997/98. One tank was
covered with green semi-transparent PVC, another with white PVC and the third
remained cover-less. All three tanks had the same high water exchange rate
(240%/day) and the same number of paddlewheel aerators.
The air exchange rate under the covers was determined by the tracer (here CO2)
decay method (Nederhoff et al., 1985; Boulard et al., 1996). The measurements were
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