Begg et al. : Stock discrimination of Scomberomorus queenstandicus and S. munroi 
663 
Discussion 
Limitations of elemental analysis 
of whole otoliths 
We found evidence of two separate groups of school 
mackerel and one group of spotted mackerel in the 
study region based on the variation in the mean el- 
emental composition of individual fish otoliths of the 
same age. Such variation has been commonly pre- 
sumed to reflect prolonged separation of the popula- 
tions and ultimately stock divergence (Edmonds et 
al., 1991). However, it is not possible to infer from 
such studies alone what environmental, dietary, or 
genetic factors cause spatial patterns. Consequently, 
a number of wholly different but equally plausible 
hypotheses could be considered in determining the 
cause of the patterns observed in the elemental com- 
positions of the otoliths of the two mackerel species, 
depending on what factors are considered important 
in determining otolith chemistry. 
An obvious explanation is that mean otolith com- 
position differs spatially for school mackerel because 
their environment (or diet) varies 
among locations along the east coast, 
whereas there are no such differences 
for spotted mackerel because their en- 
vironment (or diet) is more uniform 
throughout their range. For example, 
Proctor et al. ( 1995) concluded that an 
inability to discriminate among south- 
ern bluefin tuna ( Thunnus maccoyii ) 
samples with trace-element analysis 
was due partly to the uniformity of the 
pelagic environment. Water chemistry 
and food sources along the east coast of 
Queensland might be expected to vary 
among coastal bays, between inshore 
and offshore waters and between deep 
and shallow parts of the water column, 
especially given the strong influence of 
freshwater input in the wet-dry tropics 
(Thorrold and McKinnon, 1995). Indeed, 
the clupeid and engraulid species that 
form major parts of the diet of both 
school and spotted mackerel are gener- 
ally found only in the southern half of 
the study area (Begg and Hopper, 1997). 
At present there is no fishery-inde- 
pendent information on the cross-shelf distribution 
and habitat of school and spotted mackerel and only 
a coarse understanding of the feeding patterns of both 
species. School mackerel are thought to inhabit 
mainly inshore waters, whereas spotted mackerel are 
thought to be more common offshore (Munro, 1943; 
4-, 
O 
Q. 
0- 
- 2 - 
• • 
0 / 
o 
* 
On O ® 
■ 0 U 
I I 
I I I I 
-2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 
PC I score 
Figure 8 
Principal component (PC) analysis of spotted mackerel 
otolith elemental concentrations indicating grouping pat- 
terns. Open circle = Bowen 3-year-old fish; open square = 
Hervey Bay 1-year-old fish; solid square = Hervey Bay 3- 
year-old fish; solid triangle = Innisfail 3-year- old fish; and 
solid circle = Moreton Bay 1-year-old fish. 
Bowen-3 
Hervey Bay-1 
Hervey Bay-3 
lnnisfail-3 
Moreton Bay-1 
Canonical variate I 
Figure 9 
Discrimination between spotted mackerel samples based on the concentra- 
tions of the seven (length corrected) trace elements (+95% confidence el- 
lipses around the sample mean for each area). 
Collette and Russo, 1984), although they do move to 
inshore waters to feed when undertaking seasonal 
migrations along the coastline (Begg and Hopper, 
1997; Begg et al., 1997). School and spotted mack- 
erel are often caught together in the same mixed 
schools during winter in the shallow northern bays 
