Growth variation and water mass associations of larval silver hake (Merluccius bilinearis) on the Scotian Shelf

Abstract

Otolith microstructure is shown to be ideal for assessing age and growth in silver hake (Merluccius bilinearis) larvae and is used to examine growth among individuals and cohorts. Larvae collected from Western Bank, Scotian Shelf, in September, October, and November 1997 defined three monthly cohorts identified using inferred hatch dates. Total length-at-age relations did not differ between the September and October cohorts despite substantial differences in growing degree-day (435 versus 318°C·d) and zooplankton (potential prey index) wet biomass (0.15 versus 0.27 g·m-3). Larvae collected off-bank in September exhibited a growth advantage of >0.10 mm·d-1 relative to larvae collected on-bank. Greater variability in growth rate within cohorts and among water masses implies that cohort-averaged growth and survival (based on growth) estimates can be biased by overrepresentation of a single water mass. The focus on growth variability, and its relationship to survival, should be on individuals within cohorts and not on cohort-averaged estimates. We hypothesize that growth, and perhaps survival, in silver hake larvae on the Scotian Shelf is most easily explained by variation in physical oceanographic processes.