Biomagnification of organochlorine pollutants in farmed and wild gilthead sea bream (Sparus aurata) and stable isotope characterization of the trophic chains
Introduction
Currently, in the Western Mediterranean fish culture operates in parallel to traditional fisheries, as a consequence of the increase in fish consumption and decrease of wild stocks, remaining unchanged fisheries capacity. In this area, aquaculture activities are mainly focused on two species: gilthead sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax). Both cultured and wild fish are important components of the Mediterranean diet.
Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) are non-polar, highly lipophyllic and ubiquitous environmental pollutants. Both are classified as Persistent Organic Pollutants (POPs) and are present in the contamination pattern of marine environments world-wide (Hernández et al., 2000, Pandit et al., 2001, Hoekstra et al., 2003, Bocquené and Franco, 2005, Yang et al., 2007). Organochlorine compounds are dangerous pollutants due to their potential toxicity and chemical stability, long biological half life and high liposolubility. This leads to high biomagnification in the food chain, and involves a wide range of trophic levels (Borga et al., 2001, Kidd et al., 2001, Serrano et al., 2003a, Hoekstra et al., 2003, Bordajandi et al., 2006).
Fish consumption is a possible source of accumulation of organochlorine compounds (OCs) in humans. In fact, dietary intake, especially of marine organisms, is considered the most important source of OCs in humans (Bjerregard et al., 2001, Johansen et al., 2004, Tsukino et al., 2006) and, as a consequence, OCs are frequently detected in human lipid tissues and fluids (Hernández et al., 2002a, Hernández et al., 2002b, Hernández et al., 2002c, Pitarch et al., 2003, De Felip et al., 2004, Tsukino et al., 2006, Muñoz de Toro et al., 2006).
Previous works have detected these pollutants in fish feed used in aquaculture and in cultured fish (Santerre et al., 2000, Easton et al., 2002, Serrano et al., 2003a, Hites et al., 2004, Antunes and Gil, 2004, Maule et al., 2007). Specifically, fish feed used in fish production on the Spanish Mediterranean Coast has been analysed by Serrano et al. (2003a) who found DDTs and PCBs at the ng/g concentration level. Moreover, differences between the levels of pollutants in wild and farmed fish have been pointed out by several authors (Santerre et al., 2000, Antunes and Gil, 2004, Serrano et al., submitted for publication). These differences have been attributed to the level of contaminants present in the diet and different feeding characteristics due to the intensive culture in the farms. Fish are cultured in sea-cages sited close to the coastline and a calculated amount of feed is supplied to ensure the maximum growth rate, which has as a consequence of increased tissues lipid content.
Stable isotope analysis represents a powerful tool to study trophic interactions (Michener and Schell, 1994, Smit, 2001, Guiguer et al., 2002, Sweeting et al., 2007) and can be used to elucidate the different link interactions of the marine aquaculture trophic chain (Serrano et al., 2007). δ13C and δ15N provide information on the source of carbon in the diet and trophic position, assuming that a predator contains heavier isotopes than what it feeds on.
In the present study, we have determined the levels of PCBs and OCPs in liver and white muscle from wild and cultured sea bream from the Western Mediterranean region over one year. In addition, PCBs and OCPs in fish feed used in farms and natural diets (composed mainly by bivalves and crustaceans from Western Mediterranean shores) of gilthead sea bream have been determined as well as the stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in white muscle samples from wild and farmed specimens of gilthead sea bream over the same period. Likewise, isotopic ratios for C and N were measured in fish feed and natural prey.
Biomagnification of organochlorine pollutants in aquaculture facilities has been studied and compared with the pollutant load present in wild specimens from the same area. Moreover, carbon and nitrogen stable isotope ratios have been studied to obtain information about the trophic position of farmed and wild fish, which could provide information about the possible effect of intensive culture on feeding features and on differential organochlorine accumulation.
Section snippets
Sample collection
Real world samples of wild and cultured gilthead sea bream (S. aurata) came from populations located off the Castellón Coast (Spanish Mediterranean Coast) (Fig. 1) either by trawl catch or from sea-cage farms located in the same area. They were purchased from local commercial markets in January, March, May, October and December 2005 (mean weight ± standard deviation of wild specimens: 619 ± 91 g, length: 34 ± 1 cm; farmed specimens: 608 ± 40 g and 33 ± 1 cm). Collection of wild fish samples was not
Organochlorine pesticides and PCBs
The analysis of fish feed used in fish farms during the study period gave us a mean total load of organochlorine pollutants of 12.0 ± 0.6 ng/g fresh weight (62.9 ± 0.9 ng/g lipid weight). The total load of organochlorine in natural sea bream prey (mean value of bivalve and crustacean contents) was found to be 10.3±1.1 ng/g fresh weight (689 ± 7 ng/g lipid weight).
Fig. 2 shows the total load of organochlorines in white muscle and liver of farmed and wild specimens of sea bream. Differences in
Discussion
The presence of organochlorine pollutants in fish feed and natural preys leads to the accumulation of these compounds in the next link of the trophic chain, in both aquaculture and the natural food chain.
Farmed fish presented homogeneous organochlorine concentration during the year in white muscle and in liver (Fig. 2). This could be due to the continuous supply of food during the year on the farms, without any relation to the season. On the contrary, wild fish presented a contamination profile
Acknowledgements
The authors are very grateful to the Serveis Centrals d'Instrumentació Científica (SCIC) of University Jaume I for the use of the Isotope ratio mass spectrometer, Micromass Isoprine, and the elemental analyzer EuroEA 3000.
M.A. Blanes is very grateful to the Generalitat Valenciana for his junior research contract.
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