Modelling biogeochemical fluxes across a Mediterranean fish cage farm

D Brigolin,Vl Meccia,P Tomassetti,R Pastres,S Porrello,C Venier

doi:10.3354/aei00093

D Brigolin, Vl Meccia + Show 4 more

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https://doi.org/10.3354/aei00093

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Abstract

An integrated approach is described for modelling interactions between off-shore fish cages and biogeochemical fluxes of carbon (C), nitrogen (N) and phosphorus (P). Two individ- ual-based population dynamic models for European seabass Dicentrarchus labrax and gilthead seabream Sparus aurata were coupled with a Lagrangian deposition and a benthic degradation model. The individual models explicitly take into account the effects of water temperature and feed availability on fish growth. The integrated model was tested at a Mediterranean fish farm where a comprehensive set of in situ environmental and husbandry data was available. Tests were performed to compare the predicted and observed total organic carbon (TOC) concentrations in surface sediment under and near fish cages. At a local scale, the model output simulated the spa- tial distribution of 4 biogeochemical indicators, namely: TOC concentrations, C fluxes towards the seabed and C:N and C:P ratios. These allowed the most impacted areas and more extended areas of intermediate organic enrichment to be identified. The model was also used for estimating the mass balance of C, N and P, in order to determine the potential cumulative effects of multiple fish farms in the same area. The C, N and P fluxes among feed, fish and environment were calculated for each fish species over 24 mo of farm activity. The results showed that the amount of dissolved N directly released into the water column in inorganic form (ammonia/urea) was comparable to that deposited on the seafloor in particulate form as uneaten feed and faeces. A larger fraction of P (about 65%) was released as faeces. Results from the integrated model yielded useful informa- tion for assessing the sustainability of an area for aquaculture activities that could be used to pro- vide a scientific rationale for fish farm development in new areas.

Highlights

European seabass Dicentrarchus labrax L., 1758 and gilthead seabream Sparus aurata L., 1758 account for 95% of the Mediterranean finfish mariculture production
Two individual-based population dynamic models for European seabass Dicentrarchus labrax and gilthead seabream Sparus aurata were coupled with a Lagrangian deposition and a benthic degradation model
The integrated model includes: (1) Two individual-based population dynamic models (IB-PDM), one each for seabass Dicentrarchus labrax and seabream Sparus aurata, based on a previously described model for seabream by Brigolin et al (2010) (2) A Lagrangian model, which was applied for simulating the deposition of faeces and uneaten feed on the seabed (Jusup et al 2007) (3) A steady-state benthic degradation model, which was used for estimating the organic enrichment of surface sediment (Brigolin et al 2009b) The present methodological section focuses on the aspects related to the integration of the above models and the testing of the integrated model

Summary

Introduction

European seabass Dicentrarchus labrax L., 1758 and gilthead seabream Sparus aurata L., 1758 account for 95% of the Mediterranean finfish mariculture production (around 105 tons yearly during the last decade; FAO FishstatJ 2012). Several previous publications provide evidence that the carrying capacity of a site must be correctly assessed in EIAs in order to avoid negative effects on the pelagic and benthic ecosystem due to the sedimentation and mineralization of feed particles and faecal pellets (Karakassis et al 2000, Papageorgiou et al 2010). Such consequences should be minimized in order to avoid ecosystem degradation and negative effects that reduce the social acceptability of aquaculture and, may hinder further investment in that sector. Mathematical models and the Geographical Information System represent key resources for supporting decision making and augmenting site-selection transparency (Ferreira et al 2010, Aguilar-Manjarrez et al 2008), and these tools are already being used to define the shape and size of the so-called Allowable Zone of Effect of a fish farm (AZE) (GFCM 2012)

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