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Otolith microstructure reveals recruitment processes for snapper

Abstract

The daily increments in the otoliths of a young fish remain an extraordinary powerful tool to reconstruct its life history.  This study used otolith microstructure in conjunction with data on spawning of adults and environmental variables to enhance the understanding of the recruitment dynamics of snapper (Chrysophrys auratus) in South Australia.  The population dynamics of this species are driven by interannual variation in recruitment.  The saggital otoliths of 0+ snapper have exceptionally clear microstructure evident in transverse sections prepared by grinding and polishing. 

These can provide estimate of age, spawn date, settlement date, pre-settlement duration and growth rate.  The impact of spawning activity, water temperature and lunar cycle on the timing and strength of recruitment were investigated.  Dramatic interannual variation in the recruitment of 0+ snapper was observed in trawl spawning.  Spawning was continuous from November to early February but peaked in December each year.  From otolith microstructure, birth dates showed the most sucessful recuits were spawned mid-December to mid-January.  Strongest recruitment resulted from spawning during December and January on days when water temperatures were between 21 and 23oC but spawning on days in this range did not necessarily result in recruitment.  Evidence of lunar periodicity was detected in both the spawn and settlement rate frequencies.  The spawn date frequency distributions of sucessful 0+ recruits did not correspond with the measured spawning activity of adults as considerable parts of the spawning season in each year did not produce successful recruits.  The information from the otolith microstructure provided insight into the important relationship between spawning, recruitment and environmental characteristics in this important fishery.