Yearly variations in abundance and fish sizes of late pelagic cod juveniles (Gadus morhua L.) in the Faroe area described by the otolith microstructure

Abstract

This study focused on identifying possible causes for the observed interannual variations in size and abundance of late pelagic cod juveniles in the Faroe area. A “good” year of growth, 2001, and a bad year of growth, 1997, was analyzed. The age of the juveniles was estimated by use of the otolith microstructure. There were found significant differences in the hatching distribution between the sampling years, a higher proportion of the surviving cod juveniles were hatched in the early season in 2001 compared to 1997. The onset of the spring bloom was also earlier and the production much higher in 2001 than in 1997. If it is assumed that the spawning duration did not change between the sampling years, then the result might indicate that the larvae have meet more matched feeding condition in 2001, while early season larvae in 1997 might have been exposed to mass starvation. The lack data on the prey that first feeding larvae consume, especially the egg production of the copepod C.finmarchicus makes the Match/Mismatch suggestion tentative. However, the otolith- and somatic growth showed differences between the years. The otolith growth is influenced by temperature and feeding conditions, the temperature did not vary between the years; hence the otolith growth in 2001 is in agreement with the hypothesis that bad feeding condition should have caused the year variations. The average growth estimated by use of the Laird-Gompertz equation demonstrated higher growth rates in 2001 compared to 1997. The maximum growth rate was 0.60 mm/day in 2001 and much lower 0.35 mm/day in 2001. The higher growth rate influenced the time of metamorphosis; the study showed that the larvae metamorphosed at a significant younger age in 2001. Thus the year variation in fish sizes and abundance might be a result of combination of differences in survival in the early season and higher growth rates in 2001 compared to 1997. The results from the back calculation model on the Faroe Plateau were uncertain and should more be regarded as a new approach to estimate size selective mortality. The abundance and size of pelagic juveniles did not seem to correlate to recruitment of the 0-groups. Faroe Bank showed similar trends as found on the Faroe Plateau, but Faroe Bank has a low priority in research, thus it was not possible to conclude anything from this study.

Date of Award	2005
Original language	English
Awarding Institution	University of Copenhagen
Supervisor	Eyðfinn Magnussen (Supervisor), Henrik Gislason (Supervisor) & Henrik Mosegaard (Supervisor)