Introduction

Blue shark (Prionace glauca) and mako short fin shark (Isurus oxyrinchus) are pelagic, oceanic and highly migratory species [1]. These fishes have a wide geographic range and play an important role in marine food webs as apex predators, feeding on squid, tuna and other fishes [23456–7]. The importance of these sharks in structuring marine food webs has been demonstrated, thus their biomass decrease throughout the world's oceans is a concern [8,9]. Acuña [10] reported that sharks comprise over 70% of the total catch in the swordfish long-line fisheries. Sharks caught as by-catch in long-line fisheries are sold for shark fins in the Asian market and as trunk sales for local meat consumption and fish meal [11].

Sharks accumulate trace elements in their tissues such as mercury (Hg), arsenic (As) and lead (Pb)[12,13], largely through the diet [12]. Although blue and mako short fin sharks are the most abundant and are highly exploited throughout the oceans, knowledge of their heavy metal concentrations is very poor. The mercury and lead concentration of P. glauca and I. oxyrinchus tissues since 1974 in different oceans is summarized in Appendix 1. Mercury is the most studied element, principally in the Northern Hemisphere; in contrast lead is the least studied. Mercury and lead are volatile and highly toxic environmental contaminants present in marine ecosystems [13] where sharks are more susceptible to uptake and biomagnification of these heavy metals, because they incorporate the metals very efficiently and eliminate them slowly.

In the southeastern Pacific Ocean concentrations of mercury and Lead in fresh fish tissues for human consumption are not regulated, but international agencies (European Union and Food and Agriculture Organization/World Health Organization) have established limits for these heavy metals, which are 1.0 µg·g⁻¹ and 0.3 µg·g⁻¹ wet weight for mercury and lead, respectively [151617–18]. In addition, since well-documented incidents of heavy metal exposure of human communities in Japan and Iraq have resulted in severe toxic effects [19], there has been widespread public concern over the bioaccumulation of heavy metals through consumption of shark meat by humans as well as by animals. The main goal of this study was to determine the mercury and lead concentrations in different tissues of two highly migratory sharks that are consumed by humans.

Methods

During March 2011 and December of 2011 one hundred and eight sharks (n=69 I. oxyrinchus and n=39 P. glauca) were collected as by-catch from industrial long-line swordfish (Xiphias gladius) fisheries off Chile, in a geographic range between 21°– 35°S and 78°– 118°W. The total length (TL) was measured and sex determined on board. Muscles from the dorsal part and liver samples (1.0 g) were taken from both species of sharks; however, stomach tissue was obtained only from mako shark specimens. All tissue samples were stored at −80 °C until processing in the laboratory.

Results

Tissue/organ concentrations

Considering all tissues analyzed (muscle, liver and stomach), P. glauca showed 0.048 ± 0.03 (mean ± standard deviation) µg·g⁻¹ of Hg and 1.996 ± 0.67 µg·g⁻¹ of lead, while I. oxyrinchus presented 0.034 ± 0.023 and 0.922 ± 0.44 µg·g⁻¹ of mercury and lead, respectively. The statistical test did not find differences in the mercury concentration of specimens of blue and mako sharks (F=2.08; p=0.1516) (Fig 1a). However, blue sharks exhibited a significantly higher accumulation of lead than mako sharks (F=24.7; p<0.001) (Fig. 1b).

Fig. 1.

Mercury and lead concentration comparison of blue sharks and mako shortfin sharks off Chile in 2011. a) mercury and b) lead concentration. p-value corresponding to one way ANOVA.

Analyzing metal concentrations separately, blue shark mercury concentrations in the liver (0.104 ± 0.03 µg·g⁻¹) were greater than in muscle tissues (0.014± 0.09 µg·g⁻¹), and significantly different (F=40.6; p < 0.001). In contrast, lead concentration was greater in muscle tissue (2.244 ± 0.81 µg·g⁻¹) than in liver (1.602 ± 0.298 µg·g⁻¹) but statistically similar (F=2.17; p = 0.1491).

In mako shark individuals, mercury concentrations were highest in the liver, followed by stomach tissue and muscle tissue with 0.108 ± 0.02, 0.06 ± 0.01 and 0.006 ± 0.001 µg·g⁻¹, respectively. Lead concentration was greater in liver (1.67 ± 0.28 µg·g⁻¹) than in muscle (0.848 ± 0.47 µg·g⁻¹) and stomach tissues (0.448 ± 0.16 µg·g⁻¹); the statistical analysis also revealed that differences in levels of mercury and lead were highly significant (p <0.001).

Concentration by sexes and size

Male blue sharks had the highest concentration of mercury with 0.07 ± 0.03 µg·g⁻¹, followed by female mako sharks with 0.04 ± 0.02 µg·g⁻¹ (Appendix 2). Male blue sharks had statistically significant (p=0.012) greater mercury values than females. No differences were found in mercury concentration in males and females of mako shark (p=0.2969) (Appendix 3). In contrast to blue sharks, female mako sharks had higher mercury concentrations than males (Appendix 2). In comparison with individuals of mako shark, females and males of blue sharks had higher concentrations of lead with 2.07 ± 0.84 and 1.9 ± 0.4 µg·g⁻¹, respectively (Appendix 3). Nevertheless, it should be noted that female blue and mako sharks had higher concentrations of lead than males.

In large-sized sharks (Fig. 2 and 3), the highest concentrations of mercury were found in blue sharks with 0.05 ± 0.03 µg·g⁻¹, and 0.04 ± 0.02 µg·g⁻¹ for mako sharks. In contrast, the highest lead concentrations were encountered in small sized blue sharks and mako sharks (Appendix 2). Statistical differences were not found for mercury (p=0.986) or lead concentrations (p=0.835) between small and large blue shark specimens. A similar situation was found in mako sharks (Appendix 4).

Fig. 2.

Mercury and lead concentration found in different length individuals of blue shark off Chile, in 2011.

Fig. 3.

Mercury and lead concentration found in different length individuals of Mako shortfin shark off Chile in 2011.

Discussion

The significant differences of mercury and lead found in tissues/organs between blue and mako sharks are due to an organotropism phenomenon in which mercury or lead are distributed differentially in shark organs[13]. In fact, organotropism in some marine species is related to the uptake (gill, stomach, intestine) or excretion rate (liver, kidney)[24], but in sharks it is well known that heavy metals are incorporated by dietary habits in a dose-dependent manner and consequently they accumulate in the internal organs (through the blood)[13], producing the organotropism phenomenon in these predators.

All tissues of blue and mako sharks showed lower concentration of mercury and were below the limit value proposed by international organizations (1 µg·g⁻¹) and also with other reports [1516–17]. In contrast, the mean lead concentration in both sharks was greater than the limit proposed by WHO/FAO and European commission (0.3 µg·g⁻¹). A similar situation occurred in females and males of the studied sharks, with a lower mercury concentration than lead. The differences in mercury concentration found between female and male blue shark may be due to the type of food they consume, as suggested by Barrera-Garcia[12], who attributed these parameters to the differences between sexes and maturity stages, where females consume more invertebrates and males feed on fish such as mackerel.

Large size individuals of P. glauca had greater mercury and lead concentrations than small individuals. This is a well-documented process [12,13,20,252627282930313233–34] known as bioaccumulation [25,28,35]. The bioaccumulation process can take different pathways, one of which is through the diet (biomagnification). Thus, the prey of small or large P. glauca and I. oxyrinchus individuals play an important role in size concentrations. In fact, the diet of these predators consists mainly of bony fishes in small individuals and squid in large specimens [3,4]; moreover, Maz-Courrau [28] found that different prey produced variable concentrations of heavy metals in blue sharks.

Implications for conservation

Sharks are apex predators in all oceans; they play an important role structuring the ecosystems which they inhabit [23–4]. The ecological impacts of eliminating these top predators are already indicated in the literature [49], such as predator control and induction of subsequent cascades of indirect trophic interactions [50]. Fishing pressure has disproportionately reduced abundances of these predators mainly near the coast [51], which in turn could produce the above-mentioned effects. Despite a rich ecological and fishery literature on trophic cascades, consequences of removing oceanic apex predators remain uncertain. Moreover, exploitation of large sharks (principally P. glauca and I. oxyrinchus) has been intensified worldwide in recent decades, driven by an upsurge in demand for shark fins and meat [11] and in by-catch in many fisheries. Also, data to assess direct impacts of exploitation on these large sharks are limited, but consistently indicate that they have been driven to low levels of abundance [49,50]. In fact, when fisheries affect indirectly the mean trophic levels of the large sharks, an increase begins because they feed on high trophic level prey, which produce a major accumulation of the trace metals in their tissues, which is finally consumed by human as meat and fins or indirectly as fish meal.