| Abstract |
Metals and trace element concentrations in sediment, in tissues of farmed fish (seabass,
Dicentrarchus labrax and gilthead seabream, Sparus aurata), in tissues of wild fish
aggregating in the vicinity of fish farm cages and in benthic macroinvertebrates were
investigated at fish farms in the Eastern Mediterranean Sea. The main objective of the present
study was the detection of the distribution of metals in the the marine environment and of the
relationship between their bioaccumulation and bioavailability with various environmental
variables. Samples were collected from two fish farms in two different sites in the Aegean
Sea (AEG1 and AEG2) and two in the Ionian Sea (ION1 and ION2), Greece. Sediment
samples were collected by scuba divers under the cages (0 m) as well as at 5, 10, 25 and 50 m
from the edge of the cages downstream in the residual current direction and from reference
stations located at 400-1000 m from each farm. Fish tissues (muscle, liver, gills, bone,
intestine, gonads, stomach, stomach content) and macrofaunal taxa were also sampled.
Sediment geochemistry plays an important role in metal bioavailability and in metal
distribution in sediments. Based on elemental distribution, sediments from the farms were
separated into coarse oxidized and silty anoxic ones. In silty and anoxic sediments, metal
concentrations were higher than in coarse and oxic ones. In anoxic sediments, all metals had
higher concentrations at the impacted stations than at reference stations while in oxic
sediments, many elemental concentrations were lower at impacted stations than at reference
stations. Increased silt content, high organic matter content and low redox in sediments reduce
metals bioavailability due to their coupling with sediments. Fish reared in semi-exposed sites
with anoxic substratum accumulate a greater number of these elements than fish reared in
exposed sites with oxic substratum. Wild fish aggregating in sites with anoxic substrata
accumulate metals from the ambient habitat in their gills whereas those aggregating in sites
with oxic substrata concentrate these elements through their diet in their intestine. However,
elemental concentrations of benthic organisms were increasing with silt content, refractory
organic matter and chlorophyll-a of sediment.
Fish farm effects on sediments were negligible beyond 25 – 50 m from the edge of the
cages. Furthermore, our results seemed to show that tissues of wild fish aggregating around
farm cages accumulated a greater number of these elements and with higher concentrations
than farmed fish. Macroinvertebrates accumulated lower concentrations of most metals and
elements than their respective sediment. Tolerant macrobenthic species were found to
accumulate higher concentrations of most metals and elements than equilibrium species. The
ecological (habitat, mobility, feeding type, bioturbation) and morphological (body form and body size) characteristics of the benthic invertebrates can affect the bioaccumulation of
metals and elements in macrobenthos.
As in oxic and anoxic sediments and Cu, Zn and Fe in anoxic, fine-grained sediments can
cause unwanted effects on aquatic life in fish farm sites. In farmed fish it was detected
bioaccumulation through feeding of Hg and Cs in seabass muscle, Cu in seabass liver and P,
Ca and Sr in the bones of both species. Furthermore, in the present study, it was found that Hg
was biomagnified in tissues of wild fish aggregating around fish cages feeding on zoobenthos.
Thus, Hg in farmed and wild fish may increase significantly through their diet and may affect
the fish health as well as the health of consumer – human. Most concentrations were lower in
muscle and higher in liver and bone than in the other body tissues of both farmed and wild
fish species. Nevertheless, the edible part (usually muscle) of farmed fish did not exceed
permissible limits and therefore their consumption should be considered as safe for human
health. However, elevated concentrations of Fe, As (inorganic) and Hg in muscle of wild fish
aggregating around fish farms may impose a risk for human health. Farmed and wild fish are
good sources of P, K, Cr and Se while flathead grey mullet (Mugil cephalus), picarel (Spicara
smaris) and comber (Serranus cabrilla) are excellent sources of Ca and Se.
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