Species misidentification in mixed hake fisheries may lead to overexploitation and population bottlenecks

Abstract

Accurate species identification is required for stock assessment to monitor exploited species. Genetic tools are useful for species identification; employing SNP (single nucleotide polymorphisms) as markers, here we describe two cases of species misidentification which may lead to inaccurate estimates of stock size and adversely affect sustainable hake fisheries management in the Atlantic Ocean. North American hakes and African hakes show evidence of erroneous labelling that may obscure exploitation estimates of the species caught together in mixed fisheries (Merluccius albidus and Merluccius bilinearis, and Merluccius capensis and Merluccius paradoxus in North America and Africa respectively). Use of genetic methodology for species identification is recommended for improving accuracy of stock estimates in these two pairs of hakes.

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.