Back-calculated length at age in Norwegian spring-spawning herring (Clupea harengus L.) : Estimating the common intercept or origin by tuning back-calculated length frequencies to observed ones

In Norwegian spring-spawning herring the common intercept of the back-calculating formula, as estimated by linear regression of the fish-scale relationship, grosssly overestimated the back-calculated length frequencies distributions as compared to the corresponding observed ones. By tuning the back-calculated length frequency distributions to fit optimally with the corresponding observed ones, an "optimum fit" intercept was estimated. This intercept was observed to increase with the age back-calculated. Thus, the growth trajectory of single fish becomes non-linear, and the origin of the growth trajectory is probably situated close to the "biological intercept" as proposed by Campana (1990). The quality of the "optimum fit" intercept depend upon factors as size-selective mortality and representative sampling and should be evaluated further