Kentucky State University

Polyculture of Nile Tilapia (Oreochromis Niloticus) either Confined in Cages or Unconfined in Freshwater Prawn (Macrobrachium Rosenbergii) Ponds

Institution

Kentucky State University

Abstract

Previous work has shown that tilapia (Oreochromis niloticus) confined in cages suspended in prawn (Macrobrachium rosenbergii) ponds were able to reduce phytoplankton populations and pH levels. However, tilapia might be even more effective if allowed to freely access all portions of the water column. This study was designed to compare the effects of confined and unconfined tilapia, in polyculture with prawns, on prawn growth, tilapia growth, algae populations and water quality. Juvenile prawn (Macrobrachium rosenbergii) were stocked into each of nine 0.04 ha ponds as 60 day nursed juveniles (0.8 ± 0.3 g) at 62,000/ha. Monosex (male) Nile tilapia (Oreochromis niloticus) (89 ± 24 g) were stocked unconfined into three of the ponds at 4400/ha. In three other ponds the same number of tilapia were confined in two cages (1-m3 with 100 fish/cage). There were 3 replicate ponds per treatment. Prawns were fed a sinking pellet (28% protein) twice daily at a standardized rate based on percent of body weight per day. Tilapia were fed a floating pellet (32% protein) to apparent satiation (2x/day). PrawnsUnconfined tilapia significantly decreased (P < 0.05) average prawn weight (26 ± 4g) and production (1,625 ± 211 kg/ha). These variables did not differ significantly (P>0.05) between the prawn monoculture and confined tilapia treatments (38 ± 1 g, and 2,465 ± 44 kg/ha, respectively). Prawn stocked with unconfined tilapia had significantly higher (P < 0.05) feed conversion ratio (FCR) (3.0 ± 0.4) than prawn in monoculture or stocked with tilapia in cages, which were again not different (1.9 ± 0.0). Tilapia-Tilapia harvest weight and production were not different significantly different in the confined and unconfined treatments averaging 454 g and 2,293 kg/ha, respectively. Tilapia stocked in cages had a significantly higher (P < 0.05) survival rate (99.7 ±1.4) and FCR (1.5 ±0.1) than unconfined tilapia (90.3 ±3.8 and 0.8 ± 0.0, respectively). There were no consistent trends in treatment differences among water quality variables and phytoplankton populations. In ponds with unconfined tilapia, prawn production and size were decreased and FCR was increased compared to ponds where they were confined. This likely indicates competition for food. This is supported by the decreased FCR in unconfined tilapia. This competition was made worse by tilapia reproduction in the unconfined ponds. There was no successful tilapia reproduction in the confined tilapia treatment. In summary, confinement of tilapia in cages appears preferable when polycultured with prawn. Under these conditions the two species appear to operate independently and additively. Also, the problem of unintended tilapia reproduction is avoided.

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Polyculture of Nile Tilapia (Oreochromis Niloticus) either Confined in Cages or Unconfined in Freshwater Prawn (Macrobrachium Rosenbergii) Ponds

Previous work has shown that tilapia (Oreochromis niloticus) confined in cages suspended in prawn (Macrobrachium rosenbergii) ponds were able to reduce phytoplankton populations and pH levels. However, tilapia might be even more effective if allowed to freely access all portions of the water column. This study was designed to compare the effects of confined and unconfined tilapia, in polyculture with prawns, on prawn growth, tilapia growth, algae populations and water quality. Juvenile prawn (Macrobrachium rosenbergii) were stocked into each of nine 0.04 ha ponds as 60 day nursed juveniles (0.8 ± 0.3 g) at 62,000/ha. Monosex (male) Nile tilapia (Oreochromis niloticus) (89 ± 24 g) were stocked unconfined into three of the ponds at 4400/ha. In three other ponds the same number of tilapia were confined in two cages (1-m3 with 100 fish/cage). There were 3 replicate ponds per treatment. Prawns were fed a sinking pellet (28% protein) twice daily at a standardized rate based on percent of body weight per day. Tilapia were fed a floating pellet (32% protein) to apparent satiation (2x/day). PrawnsUnconfined tilapia significantly decreased (P < 0.05) average prawn weight (26 ± 4g) and production (1,625 ± 211 kg/ha). These variables did not differ significantly (P>0.05) between the prawn monoculture and confined tilapia treatments (38 ± 1 g, and 2,465 ± 44 kg/ha, respectively). Prawn stocked with unconfined tilapia had significantly higher (P < 0.05) feed conversion ratio (FCR) (3.0 ± 0.4) than prawn in monoculture or stocked with tilapia in cages, which were again not different (1.9 ± 0.0). Tilapia-Tilapia harvest weight and production were not different significantly different in the confined and unconfined treatments averaging 454 g and 2,293 kg/ha, respectively. Tilapia stocked in cages had a significantly higher (P < 0.05) survival rate (99.7 ±1.4) and FCR (1.5 ±0.1) than unconfined tilapia (90.3 ±3.8 and 0.8 ± 0.0, respectively). There were no consistent trends in treatment differences among water quality variables and phytoplankton populations. In ponds with unconfined tilapia, prawn production and size were decreased and FCR was increased compared to ponds where they were confined. This likely indicates competition for food. This is supported by the decreased FCR in unconfined tilapia. This competition was made worse by tilapia reproduction in the unconfined ponds. There was no successful tilapia reproduction in the confined tilapia treatment. In summary, confinement of tilapia in cages appears preferable when polycultured with prawn. Under these conditions the two species appear to operate independently and additively. Also, the problem of unintended tilapia reproduction is avoided.