The global aquaculture is dominated by high trophic-level species (finfishes and shrimps), representing more than 50% of the production (FAO, 2012). In those intensive practices, where the monoculture is spatially and managerially often the norm, only 25% of the nutrients input are used by the fed species.
The wastes of fed animals (non-consumed feed and excreta) are available in the water. Those residuals nutrients (mainly, nitrogen e phosphorus), which should be considered economic losses and contribute for the environmental eutrophication, are used and suitable for seaweeds growth (Buschmann et al., 2008; Troell et al., 2003). T
he bioremediation technology Integrated Multitrophic Aquaculture (IMTA), with an ecosystem approach, combines in the appropriate proportion the cultivation of fed aquaculture species and extractive aquaculture species (e.g. seaweeds), offering mutual benefits for the cultured organisms. The efficiency, productivity and profitability of the farm are improved. This solution has been proposed for the mitigation of environmental effects, economic diversification and social acceptability (Chopin et al., 2008). The present study, developed in the state of Rio Grande do Norte (Brazil), was carried out to evaluate the potential benefits of the shrimp/ seaweeds co-culture for the
Materials and methods
The performances of juvenile shrimps Litopenaeus vannamei and native seaweeds Gracilaria birdiae and Ulva fasciata were evaluated, in an IMTA experimental system. The organisms were cultured in nine outdoor tanks (100L) filled with filtered seawater and with constant air supply, during four weeks. A tenth tank, only with water, served as control. Three treatments – in triplicate – were tested: shrimp monoculture (mono), co-culture with Gracilaria birdiae (coGB) and coculture with Ulva fasciata (coUF).
Five shrimps (2.78g ± 0.29 initial weight) were introduced in each experimental unit, and 100g of seaweeds in each co-culture. All experimental groups were fed with a commercial pellet, at a daily ration of 5% shrimp biomass. Dissolved oxygen concentration, temperature, pH, salinity, nutrients dissolved (NH4 +, NO2 -, NO3 -, dissolved inorganic nitrogen – DIN – and PO4 3-) and shrimp and seaweed biomass were measured weekly.
Results and discussion
The average temperatures, dissolved oxygen concentration, salinity and pH were 25.34ºC ± 0.92, 4.25mg.L.-1 ± 0.58, 38.90psu ± 1.81 and 8.12 ± 0.17, respectively. The pH value showed significant differences (P=0.001), being higher in cocultures by the presence of seaweeds.
The shrimp specific growth rates observed were 1.37%.d.-1 ± 0.49 (mono), 1.29%.d.-1 ± 0.37 (coGB) and 1.88%.d.-1 ± 0.29 (coUF). The shrimp zootechnical parameters (final weight, weight gain, specific growth rate and artificial feed conversion ratio) showed significantly higher in the treatment coUF (P<0.01). Additionally, cuts on Ulva structures, green colour of the faeces and the digestive tract from coUF shrimps and negative correlation between the Ulva biomass and the coUF shrimps biomass (rs=-0,608, P=0.036) suggested the ingestion of Ulva fasciata by the shrimps. This co-culture improved the specific growth rate in 40% and allowed the saving of 1 gram of feed per gram of weight gain, comparing with mono.
Other experimental shrimp/seaweeds co-cultures demonstrated the palatability of fresh green seaweeds (Cruz-Suarez et al., 2010; Tsutsui et al., 2010). The survival was 100% in all treatments.
The Gracilaria birdiae grew constantly over the four weeks, showing a 1.13%.d.-1 ± 0.12 specific growth rate. In the other hand, the Ulva fasciata didn’t show a significant growth with wide variation of its biomass (0.19%.d.-1 ± 0.88), justified by its consumption.
The two seaweeds significantly removed the nitrite (NO2 -) and nitrate (NO3 -) from the water (P<0.001). During the study, the concentrations of those two nutrients were always lower in co-cultures, comparing with mono. Those concentrations corresponded to a removal efficiency of 93% NO2 - and 97% NO3 - for G. birdiae and 97% and 94%, respectively, for U. fasciata.
High biofiltration capacity is reported for different Gracilaria and Ulva species, which are frequently evaluated as biofilters for aquaculture effluents (Neori et al, 2003; Marinho-Soriano et al, 2009). No significantly differences between the treatments were obtained in ammonia and dissolved inorganic phosphorus concentrations.
The present study observed mutual benefits between the organisms incorporated in the IMTA system. The two seaweeds significantly removed the residual nitrogen from the water and improved its quality. Furthermore, the seaweed Ulva fasciata complemented the shrimp nutrition.