Publication Abstract

Otolith micro-increment formation in herring (Clupea harengus L.) larvae growing under high and low growth rates

C. J. Fox, A. Folkvord and A. J. Geffen

The estimation of the age of individual fish larvae by analysis of micro-increments in the otoliths has become a widespread tool in ecological studies on the early life history of fish. Reliable age determination assumes that there are identifiable micro-structures in the otolith which have been formed with regular (usually daily) periodicity. Before applying the technique to field samples, it has been recommended that validation trials are undertaken to test these assumptions. Whilst many validation trials have demonstrated daily periodicity in micro-increment formation, most of these have not examined larvae growing at sub-optimal rates. Where such studies on slow growing larvae have been undertaken, the assumption that recognisable micro-structures are formed daily has often not been supported. This is particularly true for the larvae of herring (Clupea harengus). Most validation of the studies carried out to date on slow growing herring larvae have only examined the otoliths using optical microscopy. It has been suggested that increments are in fact being laid down daily but that they are too narrow to be resolved by optical microscopy. In this study we raised groups of herring larvae under high (1200 prey l^-1) and low (100 prey l^-1) prey conditions. The larval otoliths were marked using immersion in alizarin complexone at 16 and 37 days old. The trial was terminated at 51 days post-hatch. The otoliths were examined both by optical and scanning electron microscopy. Feeding conditions influenced growth rates from day 16 onwards. Micro-increment counts undertaken using optical microscopy were lower than expected (assuming daily deposition) in larvae growing at less than 0.50 mm day^-1 in the period from hatch to 37 days old and in those growing at less than 0.25 mm day^-1 from day 37 to day 51. These results are in accord with previously reported results in the literature. Examination of otoliths by SEM produced slightly higher increment counts between the alizarin markings (average +2) but this was not sufficient to support the assumption of daily deposition. Rather than leading to the deposition of narrow increments, slow growth in herring larvae appears to alter otolith structure. The growth rates attained in this study are not atypical of those which may be experienced by cohorts of larvae in the wild, particularly from autumn spawning stocks. Absolute ageing of herring larvae in field studies by otolith micro-increments should therefore be used cautiously and a separate estimate of overall growth rates may be required to support the assumption of daily increment formation.

Reference:

C. J. Fox, A. Folkvord and A. J. Geffen (2003) Otolith micro-increment formation in herring (Clupea harengus L.) larvae growing under high and low growth rates. Marine Ecology Progress Series, 264: 83-94