Structure and growth of Pacific halibut otoliths: Identifying spatial and temporal variation

Abstract

Otoliths are polycrystals of calcium carbonate and protein that grow through the process of biomineralization within the otic capsule of teleost fish. Otoliths of Pacific halibut (Hippoglossus stenolepis) are routinely collected to provide age information, but other information has not been examined in detail. The purpose of this study was to investigate whether otolith structural patterns reveal information about otolith growth, and by inference, about fish growth and habitat during its early life. Variation in the increment widths of the first five annuli of adult halibut otoliths over a 26 year period were partitioned in two ways: the year the growth took place and the year-class to which the fish belonged. The year of growth explained temporal variation in the youngest ages and was attributed to changes in temperature which may influence recruitment success, while the year-class of growth explained temporal variation in older juveniles, but could reflect sampling bias. An analysis of microstructure increments indicated that relative otolith growth rate was an indicator of larval somatic growth. Young halibut from the Gulf of Alaska exhibited similar larval growth histories, though individual and nursery area differences were apparent. Specimens from the Bering Sea had slower larval growth rates than halibut from the Gulf of Alaska. Trace levels of strontium within otoliths were associated with ontogenetic changes of larvae and winter annuli formation of adults. Levels of potassium and sodium varied by nursery area of capture suggesting some utility for stock separation, though there was indication of significant interannual variation. The shape of the larval crystal within the otolith microstructure of young halibut was found not to be associated with nursery area of capture, and thus is not a good candidate as a stock separation tool. The high variation within individuals suggests that the shape of the crystal is not determined by external events. Overall, several patterns preserved in otoliths can provide insight into processes that influence the growth of halibut and distribution of individuals and these patterns can be recovered from adult fish. However careful interpretation is still required to separate meaningful information from spurious data.