Tóm tắt
Transverse thin sections (0.5 mm thick) of sagittal otoliths from 290 Murray cod up to 1400 mm
in total length and 47.3 kg in weight were used to establish the age and growth of cod in the lower
Murray-Darling Basin, including comparisons of recent (1986-91) and past (1949-51) growth rates and
growth in different waters. The maximum estimated age was 48 years. Quantitative and qualitative
analysis of the seasonal changes in otolith marginal increments showed that annuli in fish of all ages
were laid down each spring, and 1 October was assigned as the birthday. The thin-sectioning method
was validated by comparing age estimates for 55 Murray cod from Lake Charlegrark (age 0-21 years),
which had been validated by using burnt and polished half-otoliths. The new method had an accuracy
of 96.4% and it offers major advantages in ease of preparation, reading, and batch-handling of large
numbers of otoliths. The precision of the method, estimated as an average error for four readers,
was 5.4% (3.0% after ignoring discrepancies in relation to annuli on otolith edges). There was a linear
relationship between otolith weight and fish age and an exponential relationship between otolith weight
and fish length. Both otolith length and otolith width reached an asymptote at about 15 years, when
fish length also approached its maximum. However, otolith thickness continued to increase throughout
the life of the fish and, after about 15 years, contributed most to the increase in otolith weight.
This confirmed that otoliths continued to grow in thickness and that annuli were laid down throughout
life, and that cod could be aged reliably to the maximum age. The annulus pattern is very clear and
distinct, and the reading techniques are fully described, including recognition of 'larval' and 'false' rings.
Various differences were found in the growth rates, and the length-weight relationships for males and
females, for cod caught in 1986-91 and those caught in 1949-51, and various subpopulations are
discussed. The von Bertalanffy growth parameters (all individuals combined) were estimated at
L∞ = 1202 mm, k=0.108 and t0= -0.832. The availability of a reliable ageing method provides the
first opportunity to determine year of birth and thus to examine the age structure of populations and
to effectively manage cod populations that have declined in abundance.
