Species misidentification in mixed hake fisheries may lead to overexploitation and population bottlenecks
Highlights
► Species misidentification may lead to inaccurate estimates of stock size. ► Atlantic hakes caught together in mixed fisheries are confounded. ► SNP methodology serves for unambiguous Merluccius species identification. ► Genetic tools for identifying species at landing are recommended.
Introduction
Industrialized fisheries typically reduce community biomass by 80% within 15 years of exploitation and as a consequence, large predatory fish biomass today is only about 10% of pre-industrial levels (Myers and Worm, 2003). Depletion of marine stocks is due to many different factors, some of them anthropogenic. For example, exploited natural populations are in decline in many marine areas due to factors which include climate change, pollution and overfishing (e.g. O’Brien et al., 2000), as well as illegal fisheries (e.g. Baker and Palumbi, 1994). Inaccurate stock size estimates may also lead to incorrect management decisions and endanger exploited populations in the long term. When eggs and larvae are correctly identified, methods based on egg production are useful for estimating stock sizes in marine species and can provide accurate estimates of sustainable catches, that is, a level of catch that does not endanger the self-sustaining capacity of a population. The problem is that eggs and larvae are not always easy to identify. Eggs and larvae of different species with overlapping spawning areas are often morphologically similar, and methods of species identification in addition to visual identification are needed for accurate stock assessment (Fox et al., 2005, Perez et al., 2005, Von der Heyden and Lipinsky, 2007).
Mislabelling can also mislead estimates of exploitation rates when they are based on reported catch data. In some cases, such as in some sharks, mislabelling is not a problem and high concordance between trade and specific names allows the use of market records for monitoring exploitation rates (Abercrombie et al., 2005). But this method cannot be generalized. Sometimes the adults of two species caught simultaneously, for example in trawl fisheries, are so similar that it is difficult to identify them and mislabelling may occur. Once mislabelled at landing, the error persists along the entire seafood chain to the consumer, who buys a marketed product which does not correspond to the species marked on the label. Mislabelling not only defrauds consumers but could also adversely affect estimates of stock size if it influences the reporting of catch data that are used in fisheries management (Marko et al., 2004).
Genetic methods for species identification can help from the beginning (egg production) to the end (commercial label in stores) of the fisheries lifespan. DNA offers many possibilities from the technical point of view (e.g. Rasmussen and Morrissey, 2008, Teletchea, 2009). The potential utility of species-specific markers in assessment of real fisheries has been explored in some cases. For example, illegal whale hunting was detected employing mitochondrial DNA markers (Baker and Palumbi, 1994). Egg identification in plankton surveys is carried out by means of molecular markers for stock assessment of cod, whiting and haddock in the southern North Sea (Taylor et al., 2002). Another example is sharks, for which trade data in combination with species-specific genetic identification are a good fishery-independent estimate of worldwide catches (Clarke et al., 2006).
The aim of this study was to analyze in detail two cases of mixed hake fisheries where application of species-specific markers to fisheries science seems necessary because the species are morphologically similar. Species misidentification has been reported for the North American M. albidus and M. bilinearis (Garcia-Vazquez et al., 2009), and also for juveniles of the South African M. capensis and M. paradoxus (Von der Heyden and Lipinsky, 2007). In this study we have identified genetically products of these two mixed hake fisheries, and assessed the possible consequences of inadvertent errors for long-term sustainability of hake stocks.
Section snippets
Samples analyzed
Reference samples for each species (Table 1) were obtained from research cruises for the European Project MARINEGGS (M. capensis, M. paradoxus), and from the NMFS NEFSC bottom trawl survey (Azarovitz, 1981) (M. bilinearis and M. albidus; Garcia-Vazquez et al., 2009). The specimens were obtained from at least four different locations covering roughly the Atlantic distributional range of each species and identified by local experts in fish taxonomy: scientists from the Northeast Fisheries Science
Species-specificity of the markers assayed
Species-specificity of the markers, already validated by Machado-Schiaffino et al. (2008), was confirmed here with more individuals of the two species pairs. The seven SNPs analyzed in the control region exhibited species-specific nucleotide differences (Table 2). Three sites were informative for differentiating the species within a pair. The sites 145 and 370 allowed us to distinguish M. capensis and M. paradoxus, and also M. bilinearis and M. albidus. The site 280 served to differentiate M.
Discussion
The level of mislabelling was different in the two groups of species considered in this study, higher in African and lower in North American hakes. However the potential implications for fisheries science are similar, because in the two cases there was a noticeable difference between estimated catches and actual marketed products.
Mislabelling in these hakes is likely accidental (e.g. Garcia-Vazquez et al., 2009), as the species in each pair are morphologically similar and often difficult to
Acknowledgements
We thank the scientists at the Northeast Fisheries Science Center (NMFS, USA) for collecting our North American hake samples, and Dr. Robin Tilney in the Department of Environmental Affairs (Cape Town, South Africa) for South African samples. This study was supported by the General Directorate of Fisheries (Asturias, Spain) and the Regional PCI project IB09-0023. Ivan Gonzalez Pola provided help with laboratory analyses. Gonzalo Machado-Schiaffino had an AECI Grant (Spanish Agency for
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