Javier Tomás

Tracking continental habitat shifts of eels using otolith Sr/Ca ratios: validation and application to the coastal, estuarine and riverine eels of the Gironde-Garonne-Dordogne watershed

To enable a relevant interpretation of otolith strontium : calcium (Sr/Ca) variations in terms of habitat shifts of eels, the Sr/Ca-salinity relationship in eel otoliths was validated. Downstream and upstream migrations of young eels were reproduced in the laboratory by transferring groups of fish every 2 months between aquaria filled with water coming from the Dordogne river (salinity = 0), the upper Gironde estuary (salinity = 5), the lower Gironde estuary (salinity = 25) and the coast (salinity = 30), which represented the salinity gradient observed in the Gironde–Garonne–Dordogne watershed. Ontogenetic changes in otolith Sr/Ca were assessed in two groups of control fish that were kept in one of either two constant salinities (fresh water or seawater). X-ray electron microprobe (wavelength dispersive spectrometry, WDS) analyses of Sr/Ca ratios in the otoliths showed that the change of aquarium was recorded as a Sr/Ca increase (downstream migration) or a Sr/Ca decrease (upstream migration). No ontogenetic effect was detected in otoliths of control fish outside glass eel marks in either group of fish. The electron microprobe (WDS) analysis of the Sr/Ca life (transected in several otoliths of eels caught in the Gironde–Garonne–Dordogne watershed) showed that some of them were migrant eels that had experienced one major habitat shift during their continental life.

A new method for robust feature extraction of otolith growth marks using fingerprint recognition methods

Individual and population age structures constitute essential knowledge for proper management of commercial fisheries. Despite the important advances made in age determination using otolith growth structures, there is still a need to improve both precision and accuracy. The problem of increasing precision in age estimations has been addressed via increasing automation in the identification of growth marks. However, approaches based on otolith size, weight, perimeter, and related measurements (including contour analysis) have moderate success in age prediction. Likewise, early attempts of image analysis have reported poor results, both in cases of 1D (grey-intensity profiles) or 2D images. Recent developments in image analysis have broken this trend, and fully automatic techniques could be an alternative for routine ageing in the near future. Here, we propose a new method for 2D feature extraction that provides robust numerical descriptors of the growth structures of otoliths.