Trace elements in oceanic pelagic communities in the western Indian Ocean
Introduction
Located in the western-central Indian Ocean, the Republic of Seychelles is one of the 52 member states of the Small Island Development States' group of the United Nations. With a land surface of only 459 km2 divided into 115 tropical islands scattered within an Exclusive Economic Zone (EEZ) of 1.3 million square kilometres, the Republic of Seychelles is staking his future growth on the development of a national Blue Economy strategy. The main inter-related challenges for this country include the conservation of ocean ecosystems and biodiversity, the implementation of an integrated cross-sectorial national spatial planning, the increase utilisation of bioresources with respect for those potential new ocean industries as well as for existing ones (eg, shipping, offshore petroleum) of sustainable practises to ensure minimal environmental impact related to their operations, and the maximisation and sustainability of revenues derived from fisheries and fisheries-related sectors. The latter has yet been seriously facing with intense governmental support in the last years to increase national semi-industrial fishing fleets targeting large pelagic species, to facilitate fish export that already represents more than 50% (mainly tuna and swordfish) of Seychelles total exports, and to encourage research and development in the field of seafood value addition, processing and quality. While such actions should directly benefit to coastal ecosystems by reducing fishing pressure on demersal species that are showing signs of overfishing in Seychelles, they have to be balanced with another important source of revenues that comes from the foreign fishing agreements. Indeed, the Republic of Seychelles is receiving economic benefits through access and licence fees, stemming from the large catches of tuna made by European and Asian purse-seine and longline fleets within its EEZ.
Purse-seine and longline fisheries also take non-target, associated and dependent species, referred hereafter as bycatch. In the case of purse-seiners, it has been estimated to be around 5% of the total catches which gives a figure of around 12,500 tons of fish species per year in the western Indian Ocean, with some components discarded at sea (Amandé et al., 2012). Although the magnitude and type of discarded bycatch is fishery-specific and highly variable in space and time, fish species with the highest capture probability are the dolphinfish (Coryphaena hippurus), the wahoo (Acanthocybium solandri), the rough triggerfish (Canthidermis maculata), the silky shark (Carcharhinus falciformis), the rainbow runner (Elagatis bipinnulata) and marlins (Makaira nigricans, M. indica, Tetrapturus audax). Nowadays, it becomes evident that discard practices constitute a purposeless waste of valuable living resources. It is in this context, and with the aim of promoting the responsible and sustainable management of fishing, that the International and European instances are taking a number of actions oriented to the implementation of “no-discard” and “zero-waste” policies to be followed by the fishing fleets in the near future (IOTC, 2010). With those fisheries policies, a significant increase in bycatch landings is expected in Seychelles. In view of the increasing local population coinciding with a significant drop in the total fish landings from the artisanal fishery, the Republic of Seychelles anticipates that the new influx of bycatch will be a new source of high quality protein for the Nation, hence contributing greatly towards the self-sufficiency and food security of the country where 47% of current daily animal protein stems from fish (Monnereau and Failler, 2014). One other potential advantage of bycatch landings for the country is to guarantee a reliable availability of raw material to support the initiative of Seychelles to sustainably develop its fisheries post-harvest sector. In such an economic context, it appeared essential for Seychelles to first conduct a nutritional composition study to better estimate the real potential of fish bycatch.
Among the multitude of essential nutrients provided by fish, mineral elements are surprisingly poorly documented despite their important roles in our life functioning as part of numerous enzymes, cofactors and vitamin constituents (Goldhaber, 2003). For instance, zinc (Zn) is a cofactor to more than 300 enzymes involved in important functions such as RNA and DNA metabolism and plays a major role in the stabilization of the structure of a large number of proteins (Chasapis et al., 2012). Manganese (Mn) is involved in protein, lipid and carbohydrate metabolism (NAS, 2002), and its deficiency results in poor reproductive performance, growth retardation, congenital malformations in offspring, and abnormal function of bone and cartilage (Goldhaber, 2003). Selenium (Se) is a critical component of numerous selenoproteins which play major roles in antioxidant systems that actively protect against damage from free radicals and reactive oxygen species, and ultimately could protect against cancer or cardiovascular diseases (Flores-Mateo et al., 2006) (Greenwald et al., 2007). Despite their importance in human nutrition and increasing interest in the last decade in the food industry, most of scientific studies focus on the occurrence in fish flesh of the toxic mineral elements that are mercury (Hg), cadmium (Cd) and lead (Pb), and ultimately conclude with the risks for human associated with fish consumption. The nutritional–toxicological conflict related to seafood consumption is being largely discussed at the Commission of the Codex on Contaminants in Food (Codex Alimentarius Commission, 2016), and is of primary concern for both the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO). Efforts should now concentrate on the co-evaluation of essential and toxic nutrients in the different commercial fish species with reference to their fishing origin to better advise the general public.
In this context, the present study aims (i) to determine the levels of 11 essential elements (arsenic As, cobalt Co, chromium Cr, copper Cu, iron Fe, Mn, nickel Ni, Se, silver Ag, vanadium V, and Zn) and the three non-essential ones (Hg, Cd, Pb) in the muscle of target and non-target oceanic fish species caught in the western-central Indian Ocean and being landed in Seychelles, (ii) to study the influence of fish size and feeding habits on the bioaccumulation of those elements, and (iii) to discuss human exposure benefits and risks with regards to International food safety regulations. A particular attention is given to the balance between Hg and Se levels in fish flesh as Se is believed to protect against the toxic effects of Hg, particularly its organic methyl-Hg form (Park and Mozaffarian, 2010).
Section snippets
Sample collection
Thirteen oceanic species were caught by industrial fishing vessels in the western-central Indian Ocean during the northeast monsoon season (November 2014–February 2015) (Fig. 1). Samples and data were carefully collected by fishermen and observers onboard longliners and purse-seiners, respectively. Fishing location and length measurement (lower-jaw-fork length - LJFL - for billfishes; fork length – FL - for other pelagic species) to the nearest cm were recorded for each fish. A sample of around
Biological data
The characteristics of the 13 oceanic pelagic fish are presented in Table 2. Species differed with their muscle fat content (Kruskal-Wallis, H = 72.56, p-value < 0.001) and were categorized into three groups: (ii) lean fish with a mean TLC below 5% dw includes the four tunas (skipjack, yellowfin, bigeye and albacore), the dolphin fish, the blue marlin and the wahoo, (ii) low fat fish with a mean TLC between 5 and 10% dw (silky shark, rainbow runner, rough triggerfish and giant grouper), and
Discussion
The Food and Agriculture Organization and the World Health Organization both recommend that governments monitor the levels of chemical contaminants in food. This recommendation comes in response to increasing concern over the health effects of various contaminants, including metals, found in the food supply. The present study determined the levels and patterns of essential and non-essential trace elements in the flesh of oceanic pelagic communities caught in the western-central Indian Ocean and
Acknowledgments
The authors wish to thank the fishermen and observers onboard the EU purse-seiner Belouve and the Seychelles longliner Albacore for their help with fish sampling, in particular Elvis Hoareau and Emmanuel Chassot. We are also very grateful to Emmanuel Chassot for drawing the map and editing the manuscript. This study is part of the SEYFISH project co-funded by the Institute for Research and Development (IRD) and the Seychelles Fishing Authority (SFA).
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