Herve Troadec

Otoliths imprinting of sole (Solea solea) from the Bay of Biscay: a tool to discriminate individuals from nursery origins?

Sole nurseries are located in the main bays and estuaries of the Bay of Biscay, where juveniles from the same stock concentrate and constitute temporarily isolated groups. This context being favourable for elemental analyses of juvenile otoliths, this study has been initiated with the aim of obtaining environmental imprints of the main nurseries of origin of juveniles recruiting to the adult stock, and of evaluating the relative contribution of these nurseries to the stock. The objectives were to compare (i) the otolith elemental imprints obtained by sampling juveniles in the Loire and the Gironde nurseries, and (ii) two multi-elemental analysis techniques: laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and solution-based ICPMS (SB-ICPMS) adapted to small volumes. Depending on the fish origin, differences in Sr and Rb concentrations were shown using LA-ICPMS. Comparisons between the antero-dorsal and postero-ventral sites from where otolith material was ablated also suggested a spatial heterogeneity in otolith composition at least for some metals. From linear discriminant analyses, 73 % and 79 % of individuals (bootstrap estimations) were correctly classified with respect to their origin from the composition of the antero-dorsal and postero-ventral areas, respectively. The SB-ICPMS analysis was more powerful, which resulted in an 89 % rate of correct classification from a 2-variable model (Mg and Cd), whereas a 5-variable model (Li, Mg, Rb, Cd, Th) resulted in a 91 % rate of correct classification (bootstrap estimations). These results confirm that sole juveniles from the main estuaries of the French Atlantic coast could be discriminated by the elemental fingerprints of their otoliths.

Growth ring detection on fish otoliths by a graph construction

We present an algorithm for the detection of fish otolith growth rings based on a graph construction method. The identification of growth rings, for age estimation, is routinely achieved in fishery laboratories by human readers. One of the major problem encountered during an automatic image processing is the lack of ring continuity perception. We present an approach to this continuity perception based on the 2D reconstruction of rings from the restoration of the connectivity of nodes detected in polar coordinates. The node connection is based on an a priori knowledge of ring geometry.

Continuity perception using a multi-agent system an application to growth ring detection on fish otoliths

One of the major problems encountered during an automatic contour detection is the lack of structure continuity perception. Active contours are currently used to represent this continuity, but they point out some problems for contour detection in textured, noisy and low contrast images. We present an approach to perceive continuity based on a 2D reconstruction of closed contours using a multi-agent system. Each agent is provided with sensors on the image which allow it to follow local extrema. The purpose is to detect alternative light and dark concentric structures in an image. In the first version of the method, the closed contour was validated when the agent had found again its initial position after having described a circular path. This condition was not easily satisfied in badly contrasted images. We have added high level information about the shape of the contour to improve the detection. Application to fish otolith growth ring detection is presented in the paper.