Bioremediation by the mullet Mugil cephalus feeding on organic deposits produced by intensive shrimp mariculture

Abstract

Intensive pond shrimp aquaculture produces a large quantity of organic deposits due to uneaten food and fecal matter. In this study, a laboratory feeding trial was conducted to assess the potential of using bottom-feeding mullets (Mugil cephalus) to reduce the negative impacts of marine shrimp farming. First, ingestion and absorption by M. cephalus of organic deposits collected from the pond culture of white shrimp (Litopenaeus vannamei) were quantified for various sizes of mullets. Twenty-four 80 L flow-through aquariums were stocked with five mullets each for the feeding trial. Subsequently, a polyculture experiment with L. vannamei and M. cephalus was conducted with six different densities of medium sized (18.0 ± 4.1 g ind−1) mullets cultured with shrimp (ML-0, ML-1, ML-2, ML-3, ML-4, and ML-5 corresponding to ratios of fish to shrimp of 0:300, 1:300, 2:300, 3:300, 4:300, and 5:300, respectively). The results showed that, in the feeding trial, the specific growth rate (SGR) of small and medium mullets was significantly higher than that of large mullets, and SGR was negatively correlated with mullet body weight. Medium mullets had the highest ingestion rate, whereas small mullets had the highest feces production rate. The organic matter and N assimilation efficiency values of medium mullets were highest (60.7% and 82.2%, respectively). The fish removed 0.62 g ind−1 d−1 of organic matter and 0.043 g ind−1 d−1 of organic N from the enriched sediment. In the polyculture experiment, the survival rate and yield of white shrimp in treatment ML-4 (96.4% ± 0.8% and 5.2 ± 0.2 kg, respectively) were higher than those in the other treatments, and the survival rate was significantly higher than those of the ML-0 and ML-2 groups (P < 0.05). During the polyculture experiment, mullet density significantly affected the water environment. Overall, the best survival rate and yield of shrimp were achieved at the density of 4:300. These results indicate that mullets as detritivores can effectively ingest and absorb organic deposits generated by white shrimp culture. Additionally, these findings show that polyculture of mullets at suitable size and density does not lead to decreased yield of white shrimp; instead it increases shrimp survival and food utilization efficiency and improves the water quality of the system. Thus, shrimp-mullet co-culture should be promoted to improve the health and sustainability of intensive shrimp aquaculture.