BENEFIT Horse Mackerel age - determination workshops, May 2000 and November 2001

MAY 2000 WORKSHOP

All specimens used for age determination in this workshop were sampled
from fish caught between 25 May and 14 June 1998 during the annual
horse mackerel routine survey conducted on board the R/V Dr. Fridtjof
Nansen. The survey area was between 25.00 S and 17.25 S. The area
under investigation was divided into a northern and a southern part
according to catch and size of the fish.

A total of 275 otoliths from Trachurus trachurus capensis ranging
between 6 - 40 cm were examined. However, otoliths from specimens
greater than 31 cm were only discussed and viewed under the Image
Analysing System (IAS).

Otoliths from specimens less than 31 cm were read twice without prior
knowledge of information regarding size, sex and catch locality /
station number. The reason for omitting specimens above 31 cm was
because of difficulties reading the larger otoliths regarding the
current method of otolith preparation. Slides were prepared for
otoliths from specimens larger than 31 cm, but due to difficulties in
slicing the otoliths with the present single bladed saw we decided not
to include these otoliths in the individual reading exercise. Otoliths
from specimens greater than 21 cm were burned in order to enhance the
contrast between the opaque and hyaline zones (annuli) (Christensen,
1964). Each pair of otoliths (the burned one and the unburned one)
were then viewed; immersed in glycerin or water, with a stereo
dissection microscope under reflected light (figure 4.1).

In addition, two readers (reader 1 and reader 4) read 28 otoliths from
Cunene horse mackerel (Trachurus trecae). The specimens were caught by
commercial vessels in Angolan waters in 1991 Whole otoliths were
immersed in glycerine and viewed through a dissection microscope under
reflected light. At first, the otoliths were not treated before the
age reading and they were read unburned. For the second reading one of
the otoliths from each pair was burned over an alcohol flame until
dark brown in appearance.

NOVEMBER 2001 WORKSHOP
*report currently in process