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Fishery and Sea Resources2.3 Otolith preparation andreadings (2005)The otolith preparation for microincrementand birthdate analyses were performedon data from the 2005 survey andamong the available trawls only 6 weresampled because the collected specimenswere enough to cover the complete sizerange of juvenile size distribution. A totalamount of 310 otoliths, were collectedfor polishing; each otolith was fixed on amicroscope glass slide by means of resinand thin sagittal sections were producedby grinding with decreasing grain paper.Readings and interpretation of dailymicro-increment growth were carried outby transmitted visible light at 400x magnification.2.4 Age, growth and mortalityof juveniles during 2005Thin sagittal sections were obtained asshown in Figure 2. Among the examinedotoliths (n=310), about 21% (n=65)were rejected because they were not readable.Two independent observers accomplisheddaily increment readings, countingtwice per otolith: first from the nucleus tothe edge and then, from the edge to thenucleus; the average of the readings wasadopted as the final result. Counts that differedmore than 5% of the total amount ofthe increments were rejected [24]. In thisstudy less than 3% of the otoliths were rejectedand the remaining 238 were used fordaily reading purposes. The ageing criteriafollowed Campana and Jones [35], whilefor the increment counting the individualmark reading (IMR) method was followed[36].The mortality rate was estimated by meansof the “Catch curve analysis”. A non-linearregression fitting was performed on datawithout considering the descending abundances.The fitted model was the negativeexponential function:N t = N 0 e −Zt ,where Z is the instantaneous mortality rate,N t and N 0 are respectively the number offishes at age t and 0.To yield parameter estimateswith the smallest variance, a weighingfactor equal to the inverse of age wasintroduced in the least-squares analysis, asthe variance in length was not homogeneous[37, 38]. A goodness of fit measurewas provided by the examination ofthe residuals distribution.Daily increment width evolution andgrowth patterns were obtained by measuringa subsample of 121 specimens – 15-25 for each trawl – selected among thosewhich showed the clearest rings. The measurementswere performed on live imagesby means of an image-processing systemcalibrated at 400x (Optimas 4.10) and followingtransects located within 5° of thelongest radius from the core to the posteriormargin of the otolith.The length growth of each juvenilewas backcalculated from micro-incrementwidth data. The backcalculation method,revisited by Francis [39] was applied. Thedimensions of one or more marks in hardbody parts of the fish, together with itscurrent body length, are used to estimateits length at the time of formation of eachof the marks. This technique is based onthe relationship between the fish body size(length or weight) and the size of the hardbody part considered (radius of otoliths,scales, etc). The employed equation inbackcalculation was the following:( ) a + bRiL i =L c ,a + bR c2060