Determination of size at first sexual maturation in Flemish Cap cod (Gadus morhua) using the otolith-fish size allometric relation

Poster.-- NAFO/PICES/ICES Symposium on Reproductive and Recruitment Processes of Exploited Marine Fish Stocks, 1-3 Octubre 2007, Lisboa

A progressive decrease in fish age and size at maturity has been observed in many fish stokc during the last decades, as in Flemish Cap cod (Gadus morhua). Fishing is considered as the main cause in this process because of its size selective capacity and changes that produce in the ecosystem structure. The earlier sexual maturation combined with the decrease in the abundance of older mature cod have induced a decline in the mean spawning stock age and size affecting to the stock reproductive potential through a decrease in fecundity, offspring quality and survivorship chances, affecting population status recovery.It is necessary to understand the reasons behind these changes to design better management strategies. To estimate size and age at maturity is, thus, critical in the fisheries management. However, the estimation of this parameters requires the collection of ovaries and it is not possible to estimate in historical data. Otoliths, however, have been collected in regular basis in most of the assessed fish stocks. In this paper a modified method of Bervian et al, 2006, is applied to Femish cap cod. The variable allometric coefficient introduced in the classical allometric equation, allowed us to detect key period in the fish life history, such as the maturation proccess.Female size at maturity in Flemish Cap cod obtained with this methodology for the 1990-1997 period was highly correlated (r= 0.93) with the L50 values determined histologically from ovaries collected during the same period. Apart from low cost and reasonably time consuming, this method, opposite to others, would permit us to know the evolution in size at maturation over the past years, by means of use the available otoliths in long time series. It could be very useful to clarify the responsible factors of the early maturation process, via compensatory response and/or genotypic frequency variation. In the future, this method could collaborate in a more effective management together with other methodologies such the reproductive potential concept in the ecosystem approach view

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