Introduction
The rapid expansion of intensive fish farming has raised concerns about the sustainability of this industry in European coastal waters. Integrated multi-trophic aquaculture (IMTA) combines fed finfish culture with the cultivation of other extractive species (Chopin 2006). This may have environmental and socio-economic benefits, converting farm effluents into commercial products and increasing the efficiency and sustainability of the aquaculture industry (Troell et al., 2009; Wang et al., 2012).
Given the potential benefits, IMTA systems are beginning to be tested in coastal fish farms around the world using seaweeds to take up inorganic waste and invertebrates to utilise excess suspended and deposited organic material (Barrington et al., 2009). Preliminary IMTA trials have begun in Cyprus where oligotrophic conditions limit macroalgal growth and summer surface seawater temperatures are lethal to many commercial bivalve species.
Materials and Methods
Trials were undertaken at fish cages floating 37-43m above the seabed in a bay off south Cyprus. The farm produces ca 1200ty-1 of mostly gilt-head sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax). Monthly environmental monitoring was carried out at six stations for a year to assess water column and sediment characteristics during finfish culture prior to IMTA deployment. CTD profiles were recorded monthly and three discrete water samples were collected at the surface, 15m and near the sea bed for measurements of Total Particulate Matter (TPM) and nutrient concentrations on a monthly basis, and for chl. a concentrations on a seasonal basis. Sediment was collected seasonally with a 0.1m2 Van Veen grab to determine particle size, % organic matter (OM) and to assess the ecological status with a Water Framework Directive (WFD) macrofaunal index.
Preliminary experimentation with potential IMTA candidate species included the deployment of longlines with mussels (Mytilus galloprovincialis) at three depths in the water column and sea urchins (Paracentrotus lividus) at different densities on the seabed at 34m depth ca 100m downstream of the cages. Mussels deployed in the summer 2013 were monitored to observe survivorship and 50 individuals from each depth were analysed ca bimonthly to determine shell dimensions, wet weight and assess mussel meat quality. In autumn 2013, sea urchins were placed in 1m2 benthic cages in three replicates of 25, 50 and 100 individuals. Three to six replicates of sediment for OM and granulometric analyses and five sea urchins for measuring shell dimensions, wet weight and gonadal weight, were collected from each cage ca bimonthly.
Results
Water analysis revealed that the entire water column was oligotrophic in summer (chl. a < 0.1μgl-1), becoming mesotrophic in winter and spring (chl. a 0.1-0.3μg/l). Ammonia was elevated around the fish farms in surface waters and at 15m depth but most other nutrients were close to minimum detection limits and phosphate was below <1μgl-1. At the farm site, TPM ranged from 0.3-3.3mgl-1 and averaged ca 1mgl-1. Temperature was ca 17oC in winter and ranged from 20-28oC in summer with a pronounced thermocline at 15-18m depth.
Many mussels died near the surface and above the thermocline although survivors grew at all three depths. Mussel grew from 33.5mm shell length in August to 46.1mm in March and wet weight increased from 2.1g to 9.3g. Sea urchins hardly grew at all across the treatments and most died when held in high densities (100 individuals per m2). The sediment OM content in cages containing sea urchins was lower than the control; the most effective uptake of organic matter was recorded at 25 individuals per m2.
Discussion and conclusion
The preliminary trial revealed that polyculture at fish farms off Cyprus is challenging. There was high summer mortality of M. galloprovincialis above the thermocline. As well as temperature stress, five day transportation, friction between mussel socks and grazing by sea turtles may have contributed to the losses. Surviving mussels grew rapidly however, and may reach commercial size in a matter of months. The sea urchin trials showed that extremely high densities of P. lividus led to high mortality but that at lower densities these animals reduce organic loading on sediments around fish farms.
The lessons learnt from these trials will be used to inform a full scale IMTA experiment in summer 2014. We aim to deploy an array of native suspension and deposit feeders to select species for commercial production following the trials. Mussels (M. galloprovincialis), oysters (Ostrea edulis), clams (Ruditapes decussatus), abalone (Haliotis tuberculata), and sea urchins (P. lividus) are currently the most likely candidates. In cases where organisms cannot be supplied by hatcheries, they may be tested on a smaller scale with stock collected from wild local populations.
References
Barrington, K., T. Chopin, and S. Robinson. 2009. Integrated multi-trophic aquaculture (IMTA) in marine temperate waters. In D. Soto (ed.). Integrated mariculture: a global review. FAO Fisheries and Aquaculture Technical Paper. No. 529. Rome, FAO. 7-46pp.
Chopin, T. 2006. Integrated Multi-Trophic Aquaculture. What it is and why you should care… and don't confuse it with polyculture. Northern Aquaculture 12 (4): 4.
Troell, M., A. Joyce, T. Chopin, A. Neori, A.H. Buschmann, and J.G. Fang. 2009. Ecological engineering in aquaculture - potential for integrated multi-trophic aquaculture (IMTA) in marine offshore systems. Aquaculture 297: 1-9.
Wang X., L.M. Olsen, K.I. Reitan, and Y. Olsen. 2012. Discharge of nutrient wastes from salmon farms: environmental effects, and potential for integrated multi-trophic aquaculture. Aquaculture Environment Interactions 2: 267-283.