Bioturbation as a possible means for increasing production and improving pond soil characteristics in shrimp-fish brackish water ponds

The objective of the present work was to assess the potential of brackish water finfish to improve bottom soil conditions and thereby increase the growth and production of Penaeus monodon (tiger prawns). Experimental tanks with a bottom area of 6 m 2 were stocked with shrimp (6 shrimp/m 2) and finfish (3 fish/m 2), and fed a commercial diet. Five triplicate treatments (four experimental and one control) were executed. The four finfish selected for the experiment were: Etroplus suratensis (pearlspot), Mugil cephalus (mullet), Chanos chanos (milk fish) and Oreochromis mossambicus (tilapia). Bottom soil properties indicative of mixing and redox conditions were evaluated. Bottom soil cores were taken from the experimental tanks and sectioned to 0–1, 1–2, 2–3, and 3–7 cm layers. Organic carbon (OC), easily oxidized matter (EOM), exchangeable ammonium (NH 4–N), total dissolved sulfides, soluble manganese and soluble iron were determined as the indicators of anaerobic metabolites. Fluorescent stained sand particles (luminophores) were added to the bottom soil as bioturbation tracers. Tracer analysis results indicated that particles were displaced down to 7 cm depth in tanks with tilapia. In Etroplus and shrimp combination tanks, particles were displaced to depths of 3 cm, suggesting that the stirring activity of tilapia and Etroplus was effective down to 7 cm and 3 cm, respectively. Evaluation of chemical parameters revealed a significant decrease in the concentrations of anaerobic metabolite indicators for all fish co-stocking, but particularly in Etroplus and shrimp combination tanks. In addition, co-stocking, especially with Etroplus, led to more intensive degradation of the organic matter and significant lowering of the EOM fraction. Assessment of growth parameters also revealed higher growth rate (P < 0.05) and survival (97 ± 4) in the Etroplus - shrimp combination treatment. These results confirmed the effect of finfish on bottom soil bioturbation, especially with Etroplus co-stocking. Two types of bioturbation were identified: 1) site specific digging and soil displacement in the case of tilapia, and 2) continuous stirring, re-suspension and resulting oxidation in the case of Etroplus. Technologies for the evaluation of the effects of finfish bioturbation are proposed.