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The otoliths (sagittae and lapilli) and the vertebral centra were evaluated as ageing structures in dolphinfish age 0 juveniles (17.2-72 cm FL) caught in the seasonal small-scale fisheries of Majorca and Sicily (western and central Mediterranean). Different structure preparation techniques were also tested. Mounting and polishing lapilli for preparing frontal sections required significantly less processing time than for the sagittae. The increments in lapillus were clearly visible from the primordium to the otolith margin, whereas the intermediate zone in the sagittae was more opaque and prevented the detection of all the increments. The back-calculated hatchdates for otolith ages showed a good correlation with the species reproduction in the area and provided indirect validation for the age determination. The vertebrae centra showed increments with higher periodicity than seasonal. A test of symmetry on the ages derived from the sagitta and vertebra of the same fish indicated that the vertebra significantly underestimated the age. Considering the structure processing time and increment definition, as well as the age-length relationships obtained, it was concluded that the lapilli were the best ageing structure for juvenile dolphinfish. However, validation experiments should be carried out before the accuracy of this method can be determined.
This work is a result of two projects sponsored
by the EU DG XIV (Biological Studies Ref. 94/031
and 95/73). Mr. J. Moranta and Ms. E. Batanero
(IMEDEA) are thanked for their help in data analysis
and manuscript typing respectively. Dr. R. L.
Radtke (University of Hawaii) provided useful
advice on otolith preparation and interpretation during
a sabbatical visit to IMEDEA, granted by the
Spanish Education Ministry, and Dr. C. Rodgers
revised the English version of the manuscript.
Peer reviewed