Universidad de Oviedo, Departamento Biologia Funcional, Oviedo, Spain
D. Danancher1 and E. Garcia-Vazquez1
Biology, ecology and genetics Distribution The turbot, Scophthalmus maximus, also called Psetta maxima (Scophthalmidae, Pleuronectiformes), is naturally distributed in European waters, from Northeast Atlantic to the Arctic Circle (30º to 70ºN; 23ºW-42ºE). It occurs in the Baltic and in the Mediterranean, as well as in the Black Sea, where a subspecies Psetta maxima maeotica has been described (1). It also exists in the Southeast Paciﬁc Ocean (Chile) and in China, where it has been introduced for farming. Wild populations inhabit along all European coasts to North West Africa (Morocco), where it is also farmed. Clear geographicaldiscontinuities have not been reported between populations. Though this species is not considered endangered, declines in wild catches (2) and some genetic evidence (3) suggest the existence of historical population reductions for European turbot. Capture With some marked oscillations during the 1980s, turbot catches yield more than 7 000 metric tonnes annually (Fig.1). In the last years a considerableproportion of total production derives from aquaculture (more than 35%), principally in Spain and France.
Fig. 1. Turbot capture ﬁsheries in Europe (4)
Biology The turbot is a predator species that lives on sandy, rocky or mixed bottoms; it is common in brackish waters. When juvenile, its diet is based on crustaceans: Malacostraca and Decapoda. Adults feed mainly on other bottom-living ﬁshes(sandeels, gobies, etc.), and, to a lesser extent, on crustaceans and bivalves. Its trophic level,
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estimated from a number of food items using a randomized resampling routine, is 2.8. The turbot exhibits one of the most important growth rates observed in ﬂatﬁsh (around 30 cm every 3 years). It exhibits a medium resilience, with aminimum population doubling time of 1.4 - 4.4 years (K=0.15-0.28; Fecundity=5 million). Juveniles migrate short distances, less than 10 km in the ﬁrst two years of life as shown by tagging-recapture in Danish waters (5). Their migration seems to be related to tidal cycles in nursery grounds and is also associated to foraging activity. This migration pattern is followed by an increase in the oﬀshoremigration distance, likely associated to spawning behaviour. The spawning season occurs between April and August in Mediterranean populations and between May and August in Atlantic areas. Females reach maturity at 3 years old (around 46 cm length) and males at 2 years old (around 30 cm long). Fecundity is generally over 5 million eggs. Their eggs are pelagic, smooth and spherical, of 1.1 mm diameterand an oil globule of 0.18 mm. Population genetics With 2n=44 chromosomes, its karyotype has been described and many chromosomes characterized by diﬀerential banding (6). There are various polymorphic genetic markers available for turbots. Variability at 17 allozymes has been described for wild European populations (7, 8). Some microsatellite loci are available in publications (3, 9, 10, 11, 12)and in the GenBank databases. Population variation at mitochondrial DNA sequences (control region) has also been described (13). Very little is known about population structure in wild turbot. In the wild, most analyzed populations are in Hardy-Weinberg equilibrium at all loci (coding and non-coding), indicating panmixia (at random mating). In Europe, no genetic diﬀerentiation was detected betweenpopulations. It was not detected with allozymes (7), neither employing more polymorphic markers such as microsatellites; for example between Atlantic and Cantabric populations (3), and between much more distant populations such as Ireland and Norway (9). Most of the genetic variance is distributed within samples. Genetic diﬀerentiation between neighbouring populations has been reported only in...