Background, aim, and scope At tropical latitudes, and especially on the semi-arid coasts of the Brazilian Northeast, the rainfall regime governs the
water quality of estuaries due to the pronounced difference between the rainy and dry seasons. These changes may be responsible
for seasonal changes in bioavailability of mercury (Hg) and other pollutants to the estuarine and coastal biota. Mercury bioaccumulates
along estuarine–marine food chains usually result in higher concentrations in tissues of top predators and posing a risk to
both marine mammals and humans alike. The Goiana River Estuary (7.5° S) is a typical estuary of the semi-arid tropical regions
and supports traditional communities with fisheries (mollusks, fish, and crustacean). It is also responsible for an important
part of the biological production of the adjacent coastal waters.
Materials and methods
Trichiurus lepturus (Actinopterygii: Perciformes) is a pscivorous marine straggler. Fish from this species (
n = 104) were captured in a trapping barrier used by the local traditional population and using an otter trawl net along the
main channel of the low estuary during two dry seasons (D1 = November, December 2005, January 2006; D2 = November, December
2006, January 2007) and the end of a rainy season (R = August, September, October 2006). Fish muscle samples were preserved
cold and then freeze-dried prior to analysis of its total mercury (Hg-T) contents. Total mercury was determined by cold vapor
atomic absorption spectrometry (CV-AAS) with sodium borohydride as a reducing agent.
Results The studied individuals (
n = 104) were sub-adult (30–70 cm, 71 ind.) and adult fish (>70 cm, 33 ind.). Weight (W) (204.1 ± 97.9 g, total biomass = 21,229.7 g)
and total length (TL) (63.1 ± 10.1 cm, range 29.5–89.0 cm) presented a significant (
p < 0.05) correlation. Two-way ANOVA (
n = 81) showed that TL and W had significant differences (
p < 0.05) among seasons, being higher in D1 than in D2 and R, respectively. Moreover, season vs. month interaction were detected
for the variables length and weight. For the variable weight was detected significant difference for the factor month (
p < 0.05). It suggests that the fish enter the estuary at the end of the rainy season and increase in length and weight during
the time they spend in the estuary. Fish from this estuary are shown to be fit for human consumption (125.3 ± 61.9 μgHg-T
kg
–1 w.wt.;
n = 104). Fish mercury contents increased with size and weight. Correlations between TL and Hg-T (
r = 0.37286) and between W and Hg-T (
r = 0.38212) were significant (
p < 0.05). Dryer months showed higher mercury concentrations in fish (D1 773.4 ± 207.5 μgHg-T kg
–1 d.wt.,
n = 27; D2 370.1 ± 78.8 μgHg-T kg
–1 d.wt.,
n = 27; R 331.2 ± 138.5 μgHg-T kg
–1 d.wt.,
n = 27). The variable mercury concentration showed differences in relation to the factor season (
p < 0.05), where fish captured during the first dry season showed the highest concentration of mercury. The correlation between
Hg-T and rainfall (Rf) showed a negative correlation (
r = –0.56;
p < 0.05).
Discussion The main likely source of mercury to this estuary is diffuse continental run off, including urban and industrial effluents.
Since concentration of mercury in fish tissue is negatively correlated to rainfall, but positively correlated with fish length
and weight, it suggests that fish growth in this estuary results in mercury uptake and concentration on the fish tissue. In
the dry season of 2005–2006, when rainfall remained below the historic average, fish bioaccumulated significantly more mercury
than in the dry season 2006–2007, when rainfall was within the predictable historic average. It is suggested that less rainfall,
and consequently less particulate matter and less primary production in the estuary, make mercury more available to the higher
levels of the estuarine food chain. In the case of higher rainfall, when river flow increases and water quality in the estuary
is reduced, mercury probably is quickly exported associated to the particulate matter to the adjacent coastal waters where
it then disperses. This species is a potential routine bioindicator for mercury contamination of the biota, but so far was
used only with a limited number of individuals and contexts.
Conclusions Fish from the Goiana River estuary can still be safely consumed by the local population. However, any further contamination
of this resource might lead to total mercury levels above the recommended limits for pregnant women and small children. The
proposed heavy dependency of total mercury levels in fish on water quality indicates that land use and water quality standards
must be more closely watched in order to guarantee that best possible practices are in place to prevent bioaccumulation of
mercury and its transfer along the food chain. Human interventions and climatic events which affect river water flow are also
playing a role in the mercury cycle at tropical semi-arid estuaries.
Recommendations and perspectives
T. lepturus is largely consumed by coastal populations of tropical and sub-tropical countries all over the world. It is also consumed
by a number of marine mammals over which we have a strong conservation interest. This species is also a link among different
ecosystems along the estuarine ecocline. Therefore, knowledge of its degree of contamination might contribute to public health
issues as well as marine conservation actions. Studies on mercury and other contaminants using this species as bioindicator
(cosmopolitan, readily available) could help elucidating mechanisms through which pollutants are being transferred not only
through the food chain, but also from estuarine–coastal–open waters. In addition, using the same species in marine pollution
studies, especially as part of a mosaic of species, allows for wide range comparisons of marine food chain contamination.
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