Population structure of the European anchovy, Engraulis encrasicolus, in the SW Mediterranean Sea, and the Atlantic Ocean: evidence from otolith shape analysis

Abstract

Understanding the influence of oceanographic features on the structure of fish population is of basic importance to population dynamics studies and fisheries management. The European anchovy (Engraulis encrasicolus) exhibits a complex population structure which has produced conflicting results in previous genetic studies. This study examines the variability in the shape of the anchovy's otolith as a tool for identifying different stocks, and investigates the effects of oceanographic features on population structure. Anchovies were analysed from seven locations in the SW Mediterranean Sea and Atlantic Ocean along the northwestern African (Morocco) and Portuguese (Bay of Cadiz) coasts. A combination of otolith shape indices and elliptic Fourier descriptors were investigated by multivariate statistical procedures. Within the studied area, three distinct anchovy stocks were identified: the Algero-Provençal Basin, the southern Alboran Sea, and the Atlantic Ocean (Morocco and Gulf of Cadiz). The separation of the stocks was based on non-parametric discriminant analysis returning a classification percentage. Over 81% of the separation of the stocks could be explained by oceanographic features. Shape variability of anchovy otoliths was associated with the presence of the Almeria-Oran front, and the strait of Gibraltar. The Alboran stock was distinct from the Algero-Provençal Basin and from the closest Atlantic stocks (Gulf of Cadiz or Atlantic coast of Morocco). Results are discussed and compared with those previously obtained by genetic studies. This study supports the efficiency of otolith shape analysis for the stock identification of anchovy, and highlights the role of oceanographic features in stock separation.

Insights on stock structure of round sardinella Sardinella aurita off north‐west Africa based on otolith shape analysis

“…Few fish species form single homogeneous populations and most are composed of discrete stocks. There is growing recognition that many marine fish stocks, including small pelagic fishes, consist of amalgamations of several isolated geographic components even if they are genetically identical (Baibai et al , ; Bacha et al , ; Jemaa et al , 2015 a ). Relatively low levels of exchange between stocks, negligible from a management perspective, may be sufficient to ensure genetic homogeneity (Begg & Waldman, ).…”

“…Among the methods used to identify stocks, the study of the morphological characteristics of otoliths has been considered an efficient tool for fish stock identification (Campana & Neilson, ). Otolith shape analysis has recently been widely used with success in stock identification of various marine fish species, including small pelagic fishes such as anchovy Engraulis encrasicolus (L. 1758) (Gonzalez‐Salas & Lenfant, ; Bacha et al , ), European sardine Sardina pilchardus (Walbaum 1792) (Jemaa et al , 2015 b ), Atlantic herring Clupea harengus L. 1758 (Libungan et al , ) and horse mackerel Trachurus trachurus (L. 1758) (Stransky et al , ). Although otolith shape provides a phenotypic basis for stock separation, factors affecting otolith shape are not fully understood.…”

“…Indeed, various environmental factors such as water temperature, depth and feeding conditions (Gauldie & Crampton, ; Cardinale et al , ; Gagliano & McCormick, ) influence fish growth which in turn can affect otolith growth, hence producing variations in otolith shape in the absence of genetic differences (Simoneau et al , ). The presence of hydrographic features may act as barriers to fish dispersal and migration (Bacha et al , ). As such, geographic variation in otolith shape reflects the conditions under which the stock developed and could reveal areas within which mixing is restricted.…”

Otolith shape: a population marker for Atlantic herring Clupea harengus

“…Shape analysis has been applied in stock and population identification of several marine fish species, such as cod Gadus morhua L. 1758 (Campana & Casselman, 1993;Cardinale et al, 2004;Jonsdóttir et al, 2006), Atlantic salmon Salmo salar L. 1758 (Friedland & Reddin, 1994), anglerfish Lophius piscatorius L. 1758 (Cañás et al, 2012), comber Serranus spp. (Tuset et al, 2003), mackerel Scomber scombrus L. 1758 (Turan, 2006;Stransky et al, 2008), anchovy Engraulis encrasicolus (L. 1758) (Bacha et al, 2014) and C. harengus (Bird et al, 1986;Turan, 2000;Burke et al, 2008a;Eggers et al, 2014). Otolith shape is influenced by genetic factors (Cardinale et al, 2004) and ontogenetic processes where otolith size changes in relation to body growth, temperature and food quantity (Einarsson, 1951;Geffen, 1982;Folkvord et al, 2000;Feet et al, 2002;Fox et al, 2003;Vignon, 2012).…”

Validation of a simple and well‐suited chemical cleaning method for fish otoliths

“…To minimise error, all the otoliths were placed on a microscope slide with the sulcus facing the slide and the rostrum pointing upward (Bacha et al, 2014;Gonzalez-Salas and Lenfant, 2007;Jemaa et al, 2015;Stransky et al, 2008). We used a Zeiss Axio Scope A1 microscope with 10× magnifier lens, coupled with a Canon EOS 550D camera.…”