Interactions of cage aquaculture in Nile Delta lakes: Insights from field data and models
Introduction
Aquaculture has been rapidly increasing in Egypt in the last 30 years and, at present, contribute to about 70% of the total fish production, which reached the tonnes in 2009 (GAFRD, 2011). Semi-intensive fish farming in earthen ponds still represents, by far, the most important contribution to the total aquaculture production, but intensive cage culture is definitely on the rise. In fact, in the last twenty years, the production from fish cages has grown (El-Sayed, 2012), from 2103 tons in 1997 to 62 276 tons in 2007 (GAFRD, 2007), and presently includes also marine species, such as European seabass, Dicentrarchus labrax (herein seabass) and Gilthead seabream Sparus aurata (herein seabream), besides the fresh/brackish water species traditionally farmed, i.e. tilapia, (Oreochromis niloticus), two mullet species, (Mugil cephalus,Liza ramada), and carps (Cyprinus carpio). In 2010, seabass and seabream production from fish cages reached 31,000 tons (GAFRD, 2011), which represented about 3% of the total Egyptian fish production.
A relevant fraction of seabass/seabream production is harvested in the four main coastal lakes located in the Nile Delta, i.e. Maryut, Edku, Burullus and Manzala. These ecosystems are still very productive ones, even though their environmental status is being threatened by several anthropogenic pressures. Fishery and aquaculture are regarded as main drivers and, according to (e.g. Shaltout, 2010), their impacts need to be assessed in order to implement a management strategy based on the Ecosystem Approach to Aquaculture (EAA), endorsed by FAO (Soto et al., 2008).
The impact of fish cage culture has been extensively studied both in coastal waters (Hargrave, 2010) and in tropical lake systems (Troell and Berg, 1997, Beveridge, 2008). However, thus far, they have not been assessed in Egyptian coastal lakes, where only the impact of farming of herbivorous/detritivorous species, mainly Nile tilapia and mullets, were investigated (El-Sayed, 2006).
Fish cages release in the surrounding both organic matter and dissolved nutrients: in the case of Nile Delta Lakes, their local impact on both the water column and benthic ecosystems may concern a restricted area but be very relevant, due to high residence time and poor hydrodynamic circulation. For example, according to El-Adawy et al. (2013), the residence time is higher than 60 days in Lake Burullus western sector.
In general, the assimilative capacity of these ecosystems could be much lower than that of coastal marine ecosystems, in which cage culture is usually practiced. In turn, this means that stocking densities in Nile Delta Lakes should be kept at lower levels, compared with coastal waters, in order to avoid excessive impacts on their environmental status.
In this study, we investigated how the impact of seabass and seabream cages depends on local hydrodynamic and on husbandry practices in lake Maryut, in order to:
- (1)
identify a subset of indicators, to be used for Environmental Impact Assessment and monitoring also in the other coastal lakes;
- (2)
provide a preliminary assessment of the impact in relation to the stocking density, by means of a mathematical models which simulates the transport and fate of the organic matter released by a fish cage.
From the methodological point of view, the simulation model plays a key role, as it was used for: (1) designing the sampling of the indicators; (2) relating the impact to the stock density and husbandry practices.
Section snippets
Study site and husbandry practices
Lake Maryut is located about 20 km West from the city of Alexandria and covers a surface of (see Fig. 1). It receives runoff waters from the Nile delta, as well as wastewaters discharged by Alexandria and its industrial area. Water depth ranges between 0.4 and 2 m (EA Engineering, 1997), and the salinity is very low, ranging between 2 to 5 psu (EA Engineering, 1997). At present, artisanal fishery and aquaculture still play a relevant role in the local economy, even though Maryut total
Materials & methods
The methodology here proposed was tested and previously applied in coastal areas in the framework of the ECASA EU project (e.g. Rampazzo et al., 2013, Brigolin et al., 2008). It involves three steps, namely:
- (1)
the collection of local hydrodynamic data;
- (2)
the use of a simulation model for estimating “a priori” the likely area of impact and, on this basis, designing the sampling program;
- (3)
the collection of biogeochemical and ecological data along a transect, in order to test the model findings and
Design of the sampling survey
As one can see from Fig. 2, the main direction of water currents is oriented S-SE. The average velocity module, computed over the whole time series, is (; min–max range ). These values are in the lower end of the range characterizing Mediterranean fish farms located in sheltered environments (e.g. Holmer et al. (2008) reported average velocities comprised between 4.7 and ).
Results from the deposition module are shown in Fig. 3(a), (b): due to the
Discussion
Starting from a set of indicators designed to assess the impacts of aquaculture in marine coastal areas, this work allowed us to identify the subset of indicators which performs better in a shallow water transitional environment characterized by a limited exchange with the sea. These are water column DIN, TOC in surface sediments, and macrobenthos abundance, all presenting higher values nearby the fish cage. As far as dissolved nutrients is concerned (NH4, NO2, NO3, PO4, SiO2), finfish
Conclusions
Results showed that the impact of the fish cage in lake Maryut is limited to a radius of from the edge of the cage. Higher values of water column DIN were detected within this area, with respect to the control station, located at 25 m distance from the edge of the cage along the major current axis. An higher TOC and lower macrofaunal abundances were also recorded in the proximity of the farm.
This study allowed us to derive useful indications to optimize the sampling effort for a
Acknowledgments
This work has been supported by EU Projects No. 295004 “Fisheries & aquaculture Oriented Research Capacity in Egypt” (FORCE), and No. 282977 “Marine Ecosystem Dynamics and Indicators for North Africa” (MEDINA).
References (39)
- et al.
A marine biotic index to establish the ecological quality of soft-bottom benthos within European estuarine and coastal environments
Mar. Pollut. Bull.
(2000) - et al.
The application of a Marine Biotic Index to different impact sources affecting soft-bottom benthic communities along European coasts
Mar. Pollut. Bull.
(2003) - et al.
Assessing the suitability of a range of benthic indices in the evaluation of environmental impact of fin and shellfish aquaculture located in sites across Europe
Aquaculture
(2009) - et al.
Optimization of shellfish production carrying capacity at a farm scale
Appl. Math. Comput.
(2008) - et al.
Modelling the deposition and biological effects of organic carbon from marine sewage discharges
Estuar. Coast. Shelf Sci.
(1998) - et al.
Effects of fish farmwaste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools
Mar. Pollut. Bull.
(2008) - et al.
Impact of aquacultures on the marine ecosystem: modelling benthic carbon loading over variable depth
Ecol. Modell.
(2007) - et al.
Manual for the geochemical analyses of marine sediments and suspended particulate matter
Earth-Sci. Rev.
(1992) - et al.
Increasing nutrient concentrations and the rise and fall of a coastal fishery; a review of data from the Nile Delta, Egypt
Estuar. Coast. Shelf Sci.
(2008) - et al.
Impact of mussel farming on sedimentary geochemical properties of a Northern Adriatic area influenced by freshwater inflows
Estuar. Coast. Shelf Sci.
(2013)