Gape morphology of cod Gadus morhua L., haddock Melanogrammus aeglefinus (L.) and whiting Merlangius merlangus (L.) through metamorphosis from larvae to juveniles in the western Irish Sea

Variations in standard length (LS), gape size (SG) and jaw length (LJ) were studied in larval and juvenile gadoids (cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus) from 4 to 70 mm. The increase in SG and LJ was not linear with respect to LS. The relationship was best described by segmented regression lines in all three species, with an inflection point at c. 10·5 mm. The SG and LJ increased more rapidly in relation to larval LS for individuals smaller than this inflection point size. The rates of increase slowed significantly post-inflection, an effect more noticeable in SG data compared to LJ data. In each case, the inflection point fell in the intermediate period of development between the larval and juvenile stages, which could be considered as metamorphosis. Published equations that have been used to predict SG from LJ lead to the overestimation of gape. New relationships are presented, which may be used to predict SG from measurements of either LS or upper jaw length in cod, haddock and whiting.