Abstract:
This thesis evaluates the current break-and-burn ageing method for the southern
stock (U.S. west coast) of sablefish (Anoplopoma fimbria). Differences in growth
rates between the northern (north of Vancouver Island, BC) and southern stocks (south
of Vancouver Island, BC) and results from a radiometric study conducted on fish from
the southern stock suggest that the ageing method, developed from fish from the
northern stock, may not be appropriate for fish in the southern stock. An edge
analysis, using fish landed in Oregon, confirmed the formation of one hyaline zone
annually. The timing of the deposition of the hyaline zone, May through September, is
unique for fish from temperate waters. Sablefish tagged and injected with
oxytetracycline (OTC) did not indicate ageing error significant enough to warrant the
development of a new ageing method but some amendments to the current ageing
method are recommended: do not excessively split marks in the transition zone (the
area on the otolith that corresponds to the change from early fast growth to older slow
growth); if multiple marks originate from the same spot on the ventral edge of the
otolith, concentrate on the narrowing opaque zones when determining which marks to
count as annuli; and assign age ranges or discard otoliths that display odd growth
patterns. These amendments may increase the accuracy and precision of sablefish age
estimates. Neither final age nor any of the auxiliary catch data (sex, size, depth, and
latitude at recapture) contributed significantly to the ageing error. It is also
recommended that the thin section method of preparing otoliths be used in future
sablefish OTC studies.
