SURVIVAL AND REPRODUCTION OF BLUE TILAPIA (OREOCHROMIS AUREUS; CICHLIDAE) IN PONDS STOCKED WITH BOWFIN (AMIA CALVA; AMIIDAE) TO SERVE AS PREDATORS. (PISCES)

Trabajo recibido el 1º de diciembre de 1981 y aceptado para su publicación el 16 de junio de 1982.

RESUMEN

INTRODUCCIÓN

Tilapias are hardy, prolific cichlid fishes that have gained rapid acceptance throughout the world at both subsistence and commercial levels. Although temperatures below 10-15ºC, depending on species, kill them. The tilapias tolerate intense crowding and resume breeding soon after temperatures warm at temperate latitudes. Both the mouthbrooders, genus Oreochromis, and the nest guarders, genus Tilapia, are so prolific that the resultant stunting from overcrowding reduces potential yields substantially, Bardach et al. (1972) review the major approaches used to address this problem. These include periodic removal of unwanted fry, monosex culture with males obtained by inspection, hybridization, or hormone treatment of fry, high density culture in ponds, raceways, or cages, and stocking of predators to consume unwanted fry and fingerlings. The first and fourth options are the most appealing in subsistance or recreational systems as considerable care and technology are required to produce monosex tilapias or culture them intensively.

The purpose of our study was to determine the effects of the bowfin, Amia calva, on growth, survival, and reproduction at maturity of blue tilapia fingerlings, Oreochromis aureus, stocked in ponds supplied with small amounts of chicken manure as "food". The bowfin is a primitive, air breathing predator native to North America (Clay, 1975). It can estivate in dried pond bottoms and can exist indefinitely in anaerobic systems. The blue tilapia is one of the most cold tolerant tilapias and has found wide commercial acceptance as a food fish in Israel (Sarig, 1980) and the U.S.A. (Langfordet al., 1978). It is extremely tolerant of low oxygen conditions and has been cultured in ponds receiving massive amounts of animal manures (Stickney et al., 1977). Although we did not intend to use water depth as an experimental variable, we used both shallow (0.15-0.4 m) and conventional (0.4-1.1 m) ponds in order to increase replications and found major differences between the two types of ponds with respect to fish production.

Blue tilapia fingerlings (Oreochromis aureus) (roughly 25 mm in length-0.5 g and 30 days old) were stocked at a rate of 7500 per ha in 6 shallow (0.15-0.4 m) and 3 conventional (0.4-1.1 m) 0.04 ha ponds on 30 May,1979. Bowfin fingerlings, (Amia calva) (roughly 200 mm in length - 100 g and 90 days old) were stocked in 11 June 1979 at the following rates, 0, 75, and 150 per ha, in shallow (n = 2) and conventional (n = 1) ponds. Ponds were lo. cated near Baton Rouge in Southern Louisiana (U.S.A.).

Beginning on 7 June, fresh chicken manure from laying hens was added to each pond at the rate of 13.6 kg per ha, 5 days per week. This continued until 27 July when an equivalent amount of water soaked, 50:50 mixture of chicken manure from broilers and bagasse was added to each pond. Bagasse is the fiberous material remaining after sugar cane has been crushed. Application of organic matter ceased on 2 November 1979. By that time ponds (7 of the original 9) had received the equivalent of 1700 kg of wet chicken manure per ha. One shallow pond (C-1) was lost because of excessive flushing with deoxygenated, iron rich well water on 2 July. It was restocked on 6 July with tilapia fry and bowfin fingerlings (7500 and 75 per ha) but was lost again on 22 August for the same reason. Notes were kept on growth of tilapia in this pond for comparisons with other ponds. Another shallow pond (C-2) was harvested on 6 October (129 days) because of an uncontrollable leak. All other ponds were harvested on 12 or 14 November 1979 (166 and 168 days). Adult tilapia were counted and weighed. Subsamples of fry and adults were measured (total length) to the nearest millimeter.

Beginning in August, ponds were checked monthly for tilapia reproduction by seining. Stocked tilapia were rarely caught as they readily eluded the seine. Fish removed from pond C-1 in July and August and pond C-2 in October did provide growth rate data. At harvest, the relative abundance of bowfin prey (tadpoles, poecillid minnows, and small, native prawns) was noted.

Shallow ponds lost much water via evaporation compared to conventional ponds. They were frequently less than three-quarters full because of pump failures. Weekly addition of water would have been necessary to maintain maximum depths. Conventional ponds did develop thermal stratification but shallow ponds did not. No obvious water quality problems were encountered. Conventional ponds T-11 (no bowfin) and T-17 (150 bowfin per ha) developed extensive growths of rooted, vascular vegetation, Sagittaria sp., by late August.

Survival (92, 90%) and production (868, 846 kg per ha) of tilapia in shallow ponds were greatest in ponds with surviving bowfin (Table 1). Relatively speaking, these ponds had the fewest tilapia fry present (Table 2). Mean bowfin growth (100-140 mm) and survival (0,50, 100%) were poor in shallow ponds. Conversely, survival (43, 58%) and production (505, 715 kg per ha) of tilapia in conventional ponds were inversely related to the number of bowfin present. The relative abundance of tilapia fry also decreased with an increase in bowfin numbers in conventional ponds. Bowfin survival (100%) and growth (about 220 mm) were much greater in conventional ponds than in shallow ponds.

Mean tilapia survival in shallow ponds stocked originally with bowfin was 82% (91% with surviving bowfin) versus 73% where no bowfin were stocked. By way of comparison, tilapia survival in the one conventional pond without bowfin was 71% while the mean for the two conventional ponds with bowfin was 52%. Factors that may have accounted for this contradictory relationship include relative bowfin growth and survival, water depth, relative abundance of bowfin forage, and other, unidentifiable factors. Certainly, the trend in conventional ponds to reduced survival with increasing bowfin numbers suggests that the presence of 75 bowfin per ha (the number present in shallow ponds with surviving bowfin) will not cause as dramatic a reduction in adult tilapia numbers as 150 bowfin per ha might. Thus, it may not be wise to stock bowfin at a rate greater than 75 per ha in polyculture with blue tilapia. However, this cannot be taken to be a definitive conclusion because of the lack of replication.

Shallow ponds were apparently more stressing to tilapia than conventional ponds. In general, density depedent considerations taken into account, adult and young tilapia were smaller in shallow ponds than deep ponds (Tables 1 and 2). Bowfin from shallow ponds were much smaller than those from conventional ponds (Table 1).

The first tilapia fry and nests were not observed until early September. Most ponds, shallow and conventional, apparently had two distinct spawns, as seen in the length frequency data presented in Table 2. At harvest estimated biomass of tilapia fry and fingerlings approached 100 kg per ha in the ponds with the greatest relative numbers of fry and fingerlings present.

The presence of bowfin appears to have had a negative affect on tilapia spawning success regardless of pond depth (Table 2), but the phenomenon was not as dramatic in conventional ponds as it was in shallow ponds. It might well be attributed to low numbers of adult tilapia than to bowfin predation on tilapia fry and fingerlings.

LENGHT FREQUENCY (%) AND PRODUCTION DATA FOR OREOCHROMIS AUREUS (7500/ha) AND AMTA CALVA STROVED TOGETHER

Tilapia stocked in pond C-1 grew roughly 1.8 mm per day from 30 May until 2 July and roughly 2.2 mm per day from 6 July until 22 August. Those recovered on 2 July averaged 87 mm (SD = 12.5; n = 29). Those recovered on 22 August averaged 121.0 mm (SD = 23.7; n = 25). Fish harvested from pond C-2 after 129 days averaged 190.4 mm (SD = 8.23; n = 51). Therefore, using an approximate growth rate of 2.0 mm per day, most tilapia probably reached the mean lengths recorded in this study after about 80 days in mid-August, roughly one half its duration of 166-168 days. We did not have a physical sample of fish at that time but fish in the 170-200 mm range were frequently observed in the ponds.

TABLA 2 LENGHT FREQUWNCY (%) DATA FOR OREOCHROMIS AUREUS FINGERLINGS PRODUCEND IN PONDS STOCKERD WIHT AND WITHOUT THE PREDATOS AMA CALVA

Although the tilapia from the conventional ponds were only slightly larger than those from the shallow ponds, they were 10-30 g heavier. Overall mean weights were around 130 g for shallow ponds and 150 g for conventional ponds. These were small but harvestable (Allison et al., 1976; Dunseth and Bayne, 1978). They were, however, much smaller than the 250-450 g monosex tilapias grown in an equivalent growing period in Israel (Sarig and Arieli, 1980) and Alabama (U.S.A.) (Cousins and Smitherman, 1978), but such fish are stocked at initial sizes well in excess of 50 g.

Observed tilapia growth rates were comparable to those summarized by Swingle (1966) for O. aureus (then identified a., Tilapia nilotica) in Alabama ponds. Our growth rates were also similar to those reported by Stickney and Hesby (1978) in Texas (same latitude and adjacent to Louisiana). They stocked similar sized but sex reversed O. aureus at densities of 10,000 per ha in 1.5 m deep ponds. The fish were harvested after about 114 days from ponds receiving swine wastes well over an order of magnitude greater than the chicken manure added to our ponds. In tropical El Salvador, Dunseth and Bayne (1978) and Bayne et al. (1976) stocked 5-15 g 0. aureus at rates of 6,000, 9,000, and 12,000 per ha with Cichlasoma managuense, a small cichlid predator. Tilapia were fed. Survival was not affected by the predator and the tilapia achieved sizes of 100150 g in 150 days.

There is no doubt that our manure applicate rate was very low when compared to quantities used in Israel where manure based aquaculture is practiced on a highly technical level (see Schroeder, 1978, for a thorough discussion of the subject). It is conceivable that our fish would have grown larger if more nutrient material were available as the stocking rate was low. However, the stocking sizes and rates of Stickney and Hesby (1978) were similar to ours and input of nutrients was an order of magnitude greater. Therefore, additional research must be conducted in order to make a definitive statement on this matter.

At the stocking densities used in our study, out data suggest that two crops can be grown in very shallow ponds in a warm, temperate growing season. Projected production could reach 1700 kg per ha. Such fish would be relatively small but harvestable. Input of organic matter would be minimal. Applications in subsistence aquaculture in tropical regions are clear. Problems from unwanted reproduction would be avoided as fish would be too young to spawn when harvested, a strategy recommended for tilapias by Swingle (1968).

We wish to emphasize that recently hatched youngof-the-years O. aureus fingerlings were stocked in this study. Tilapias are notorious for stunting and spawning at an age of 2-4 months. This probably reflects nutritional deficiencies (Bowen, 1979). Therefore, use of tilapia fingerlings of small size and unknown age could make any projections from this study valueless. Those fish could very well be sexually mature and respond to the improved environment not by growing but by spawning.

We wish to acknowledge the field assistance rendered by Mr. jay Stovall. Mr. Jose Collazo, translated the abstract. This project was funded, in part, by the Louisiana Agricultural Experiment Station.

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