Aquaculture International 3, 116-122 (1995)

Seasonal stratification in fish culture and irrigation reservoirs: potential dangers for fish culture A. Milstein’*, M. Zoran’ and H.J. Krambeck2

‘Fish and Aquaculture Research Station, Dor, M.P. Hof HaCarmel, 308290, Israel *Max Planck Institute of Limnology, P. 0. Box 165, D-2320 Ploen, Holstein, Germany

Dual-purpose reservoirs for field irrigation and fish culture are characterized by the simultaneous (a) increase in standing crop biomass and feeds during the fish culture period and (b) decrease of water level due to irrigation. This management practice affects stratification, which in turn affects water quality and may affect the fish stocked. To study the limnology of dual-purpose reservoirs, an extensive sampling programme including automatic continuous recording and manual water sampling in deep and shallow reservoirs was carried out. This paper describes limnological changes during the week in which a 3 month long seasonal stratification ended and fish kills occurred. Management implications of stratification are also discussed. The data presented are the first continuous records of this process in this particular type of water body.

KEYWORDS: Fish culture and irrigation reservoir, Stratification.

Dual-purpose reservoirs for field irrigation and fish culture are increasingly being used in Israel to take advantage of the scarce water resources available. These reservoirs are characterized by the simultaneous (a) increase in standing crop biomass and feed input during the fish culture period and (b) decrease of water level due to irrigation (Hepher, 1985), and have larger area and depth than conventional fish ponds. They are built by excavating the bottom and elevating the embankment, are generally rectangular, and are filled and emptied every year. Reservoirs 3-16 m deep are currently in use for fish culture in Israel.

Stratification in deep water bodies affects water quality. The epilimnion, subject to direct influence of the sun and meteorological forces, presents a strong daily and seasonal variability, while in the hypolimnion changes are gradual. If there is strong thermal stability, an anoxic hypolimnion is more likely. If thermal stability decreases and the water column mixes, the oxygen of the epilimnion is consumed throughout the water column. In fish culture reservoirs this may result in unacceptably low dissolved oxygen (DO) concentrations for the large fish biomass stocked, and hence fish kills might occur. This is more dangerous in deep (> 5 m) reservoirs where the seasonal stratification lasts for several months, than in shallow (34 m deep) ones in which the night turnover affects

* To whom correspondence should be addressed.

0967-6120 0 1995 Chapman & Hall

Seasonal stratification dangers 117

the entire water column and prevents the development of a large DO deficit in the hypolimnion.

To study the limnology of dual-purpose reservoirs, an extensive sampling programme including automatic continuous recording and manual water sampling in deep and shallow reservoirs was carried out (Krambeck et al., 1992, 1994; Milstein et al., 1992a,b, 1994a,b; Zoran et al., 1994). The general temperature and oxygen processes in these reservoirs are similar to those found in fish ponds and small lakes studied with continuous data records (e.g. Piedrahita et al., 1987). However, the management of dual- purpose reservoirs as practiced in Israel affects stratification, which in turn may affect the fish. Thus, in an 8 m deep reservoir, daily and seasonal stratification were found during spring and summer. Both stratification types persisted while the water level decreased, until water depth reached about 5 m. From then on, only daily stratification occurred (Milstein et al., 1994b). This paper concentrates on the transition between these two periods, describing limnological changes during the week in which the seasonal stratification ended and fish mortality occurred. Management implications of stratification are also discussed. The data presented are the first continuous records of this process in this particular type of water body.

MATERIALS AND METHODS

The study was carried out at the farm Tirat Zvi, located in the Jordan Valley. The reservoir, when full, has an area of 12 ha, a mean depth of 8 m and a volume of 1000 000 m3. Filling with spring water was started in December 1990. By early April 1991, when the reservoir was almost full and mean water depth was 7.5 m, seasonal stratification was already established. In May no more water entered the reservoir, and water was pumped out for irrigation from the hypolimnion near the bottom. In June the water column was 5.5 m deep, and in November the reservoir was empty. The reservoir was stocked in December 1990 with 80 000 common carp, Cyprinus carpio, of mean weight 38 g and 22 000 mullet, Mugil cephafus and Liza ramada, of 195 g. In early May 144 000 hybrid tilapia, Oreochromis niloticus x 0. aureus, of 65 g were added. Fish were fed through an automatic feeder at a daily rate of 3% of the fish biomass. Three paddle-wheel aerators, located in the feeding area, were activated at night.

Two rafts carrying a datalogger and sensors for measuring temperature at the surface, 1 m, 3 m and near the bottom (5 m in the data presented here), DO at the surface, global solar radiation, and wind velocity and direction, were anchored in April 1991 and remained in place until November. The rafts were placed in the leeward and windward sides of the reservoir, far from the banks, the aerators and the feeding area. The data were collected every 5 min. Further details on the system have been provided by Krambeck et al. (1992).

RESULTS

Through the culture season, light penetration in the reservoir studied was rather constant (3540 cm Secchi disc from May onwards), and stratification during daylight hours affected the upper 2-3 m of the water column. In addition, the reservoir presented seasonal stratification at about U m depth from late March until the end of June, when water depth reached 5.5 m (Milstein et al., 1994b).

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Seasonal stratification dangers 119

In late June, wind (Fig. lc) showed the daily pattern characteristic of the Israel summer: weak wind (up to 2 m s’) from the E-NE blowing during the night and morning; while in the afternoon the stronger sea breeze (4-8 m s-‘) blowing from the W-NW reaches the Jordan Valley. From 26 to 28 June, wind direction changed, the morning wind was from the south, and the afternoon western breeze blew longer and stronger. The sky, usually dusty during summer, was partially covered by developed clouds which reduced solar radiation during those days (Fig. lb).

Temperature of the air and at the water surface and 1 m depth followed daily cycles, while at 3 m and 5 m depth there was a gradual increase through the week studied (Fig. la). Temperature at the water surface increased sharply after sunrise, reducing vertical water circulation, decreased steeply in the afternoon with the arrival of the sea breeze, and continued to decrease gradually during the night. Temperature increase at 1 m lagged behind the surface water heating, so that stratification developed in the morning. After the wind mixed the water in the afternoon and during the night cooling, temperature in the first 1 m of the water column was homogeneous. At 3 m depth, heat transfer from the surface led to increased temperature throughout the week, which on the night 25-26 June allowed water at that depth to mix for the first time with the upper layers. This process reached 5 m depth 3 days later, after the southern wind was blowing. During that period there was a transition from seasonal stratification to stratification during the day and mixing during the night. The process described was similar in both recording stations, so only the results of one of them (leeward) are presented in Fig. 1.

DO at the surface (Fig. lb) followed a consistent pattern. DO increased after sunrise due to photosynthesis, exceeding saturation (7-8 mg 1-l at June temperatures) during the almost windless mornings. It decreased in the afternoon when the wind speed began to increase, more drastically with increasingly strong sea breezes. Two processes accounted for the wind-dependent surface DO decreased: one was the partial loss of oxygen to the atmosphere when surface waters were supersaturated, and the other was mixing with the deeper DO-poor waters. DO at the surface continued to decrease gradually throughout the night due to respiration. The lowest DO, generally of about 2 mg 1-l at the surface, occurred by the end of the night, before the onset of photosynthesis, and was sustained for 2-3 h. On the night of 25-26 June, when the mixed layer at night reached 3 m, DO was already affected, but it was near zero for a time short enough not to visibly affect fish. On the next night the anoxic layer mixed into the upper layers was sufficient to keep very low DO through the water column for 10 h. In the morning fish were found concentrated at the surface around the aerators, and many of them died (no records of the amounts were kept). During the following 3 days feeding was stopped to avoid extra organic loading. To increase DO, aerators were added and water was pumped into the reservoir. During those days there was complete mixing of the water column at night, a process which continued until the end of the culture season.

FIG. 1. Continuous records (5 min frequency) at Tirat Zvi deep reservoir, 2230 June 1991. Results of water profiles are from the leeward station. Vertical grid lines indicate midnight of each day. (a) Air temperature (“C) and water temperature (‘C) at the surface and at 1 m, 3 m and 5 m depth. (b) Global solar radiation (W m-*) and dissolved oxygen in surface water (mg 1-l). (c) Wind direction (degrees from North) and velocity (m s-‘) values lower than 1 m 5’ set to zero, because the instrument was not sensitive to that range).

120 A. Milstein et al.

During this second part of the culture season, the complete vertical mixing of the water column every night avoided the development of strong differences between epi- and hypolimnion, and allowed the increase of DO during the night in the hypolimnion.

DISCUSSION

The end of the seasonal stratification does not necessarily imply fish kills as described above, but it probably will if other factors which reduce DO are affecting the system. Such factors may be the eutrophic state of the water, the fish biomass, the organic matter in the sediment and water, or the duration of the seasonal stratification. An end to seasonal stratification without fish kills was recorded in the same reservoir in 1992 and again in 1994. In both years the seasonal stratification was also already established in April, with the anoxic hypolimnion < 4 m depth. However, in 1992 fish stocking rate was 25% lower than in 1991 (45 000 carp, 117 000 tilapia and 24 000 mullet) and feeding was reduced accordingly, thus no fish kills occurred when the 3 months’ seasonal stratification ended in mid-June. In 1994 the seasonal stratification was interrupted by a storm after only 1 month, it restarted a week later and ended in late June. The interruption in stratification did not allow the 3 months’ continuous isolation of the hypolimnion and consequent worsening of water quality, and no fish kills occurred when stratification ended in June although fish stocking rate was 22% higher than in 1991 (17 000 carp, 260 000 tilapia and 24 000 mullet).

Natural water bodies for the Israeli geographical location are of the subtropical monomictic type, in which the water depth does not change and the transition from seasonal to daily stratification patterns is expected to occur in autumn when solar radiation decreases and surface water cools. In the dual-purpose reservoir studied, this transition occurred in the middle of the summer, related not to solar radiation or light penetration into the water, but to the change in water depth due to irrigation. The decrease in water depth reduced thermal stability because less energy was needed to mix a shallower water column. Hence, when the water level was 5.5 m and a rather stronger breeze blew during several consecutive days, it was enough to end the seasonal stratification. Also, in 1992 and 1994, the water level was 5.5 m when the seasonal stratification ended. In other fish culture and irrigation reservoirs studied by the authors in which maximum depths were from 3.5 to 5.5 m, seasonal stratification did not develop at all (Milstein et al., 1992b; Krambeck et al., 1994). In deeper reservoirs, seasonal stratification developed while the water depth was > 5 m and wind affected only the upper 3 m of the water column. This occurred unless the velocity of the regular sea breeze was increased by the topography of the area, so that in large-area reservoirs, waves of > 0.5 m height developed during the afternoon (no such waves in the other reservoirs), and seasonal stratification did not occur (Milstein et a/., 1992b). For the same geographical location but in more hypertrophic impoundments such as wastewater reservoirs (Secchi disc 5-10 cm as compared with the 35-40 cm in fish culture reservoirs), stratification depends not only on density gradients, wind, and reservoir depth and surface area, but also on parameters related to the chemical composition of hypertrophic waters (solar radiation trapped in the upper turbid layers, biogenic chemical gradients, water viscosity, and reduced wind/water friction coefficient), which

Seasonal stratification dangers 121

may lead to the presence of more than one permanent cline in a reservoir of only 5.5 m depth (Juanico, 1994).

In fish culture reservoirs, the anoxic hypolimnion is a water volume not available for fish, and represents a daily danger for fish in cages. The risk for fish in cages is related not only to the breaking of the seasonal stratification, but also to the daily tilting of the thermocline due to the wind (Zoran et al., 1994). This may cause the anoxic layer to enter through the bottom of the cage, and because the fish cannot escape they are killed. Due to this problem, most farms in Israel have stopped growing fish in cages within reservoirs, including the one studied here. However, this practice might be continued in reservoirs in which seasonal stratification does not develop, provided that (a) the depth of the fish cages is reduced from the previously used 4 m to only 2 m, and (b) the cages are located in the leeward side of the reservoir in relation to the dominant winds in the area.

RECOMMENDATIONS

(1) In deep fish culture impoundments, mainly, in those in which the water level drops during the fish culture period, the vertical temperature structure of the water column should be checked to determine whether seasonal stratification develops.

(2) If it does develop, its depth and the depth of the daily thermocline should be monitored. The sampling station should be as far from the embankment as possible. Automatic continuous measurements at several depths are the best. If this is not possible, then manual temperature (and DO) profiles should be measured at least three times a day: when maximum daily stratification is expected (around noon, depending on winds in the area), and in the early evening and at sunrise, to detect the beginning and end of long-duration vertical mixing episodes near the deep thermocline (like the temperature increase at 3 m on the night of 25-26 June in Fig. la).

(3) As summer progresses and the shallow (daily) and deep (seasonal) thermoclines approach, one should be prepared for an emergency situation, especially if the fish biomass and related feed inputs are high.

(4) Emergency measures before the water column mixes might include (a) partial harvest to reduce fish biomass, (b) the use of aerators during the night to provide for a larger SOS area for the fish, and/or (c) stopping feeding to reduce organic loading. The latter two measures have to be continued for several days, until the transition from seasonal stratification to daily stratification is over and the DO cycle comes back to normal.

(5) If fish cages are used in reservoirs with seasonal stratification, they should be harvested before vertical mixing occurs. Cages are more appropriate for reservoirs which do not develop seasonal stratification. In any case their depth should be lower than the epilimnion depth.

ACKNOWLEDGEMENTS

This research was supported by grant I-92-134, 8/88 from the GIF, the German-Israeli Foundation for Scientific Research and Development. Without the logistic support and encouragement of Akiva Eiger and the aquaculture staff of the farm Tirat Zvi, the work could not have been done.

122 A. Milstein et al.

REFERENCES

Hepher, B. (1985) Aquaculture intensification under land and water limitations. Gedournal 10, 253-259.

Juanico, M. (1994) Limnology of a warm hypertrophic wastewater reservoir in Israel. I. The physical environment. Internationale Revue der gesamten Hydrobiologie 79, 42-36.

Krambeck, H.J., Milstein, A. and Zoran, M. (1992) Physical aspects of the ecosystem structure of dual purpose reservoirs in the Israeli coastal area: results of a solar driven data acquisition system. Limnologica 22, 129-135.

Krambeck, HJ., Milstein, A. and Zoran, M. (1994) Physical limnology of reservoirs in Israel used for crop irrigation and fish farming. Verhandlungen International vereinigung fuer Limnologie 25, 1373-1378.

Milstein, A., Zoran, M. and Krambeck, HJ. (1992a) Fish performance and oxygen dynamics in a dual purpose reservoir (fish farming and crop irrigation) in the Israeli coastal area. Limnologica 22, 43-50.

Milstein, A., Krambeck, H.J. and Zoran, M. (1992b) Effects of wind and depth on stratification in reservoirs for fish culture and field irrigation. Limnologica 22, 375-384.

Milstein, A., Zoran, M. and Krambeck, H.J. (1994a) Water quality variability in a shallow (4 m) reservoir for simultaneous fish farming and field irrigation. Limnologica 24, 71-81.

Milstein, A., Zoran, M., Barsadschi, D. and Krambeck, H.J. (1994b) Water quality variability in a deep (8 m) reservoir for simultaneous fish farming and field irrigation. Limnologica 24, 82-92.

Piedrahita, R.H., Ebeling JM. and Losordo, T.M. (1987) Use of data acquisition systems in aquaculture. In: Automation and Data Processing in Aquaculture (ed. J.C. Balchen) Pergamon Press: New York, pp. 259-262.

Zoran, M., Milstein, A. and Krambeck, H.J. (1994) Limnological aspects of dual purpose reservoirs for irrigation and fish culture in the Coastal Area and the Jordan Valley. Israeli Journal of Aquaculture (Bamidgeh) 46, 6475.