Begg et a!.: Stock discrimination of Scomberomorus queenslandicus and S. munroi 
655 
Materials and methods 
Otolith chemistry 
School and spotted mackerel were collected monthly 
from commercial and recreational fisheries along the 
Queensland east coast, south of 16°S, between June 
1992 and February 1995 (Fig. 1). School mackerel 
were sampled from three geographic areas and spot- 
ted mackerel from four (Table 1). Samples were ob- 
tained from independent collections at each area, 
usually representing fish caught from several differ- 
ent schools. Specimens were kept on ice, then frozen 
until length to caudal fork (LCF, mm) was measured, 
sex was determined, and both sagittae were removed, 
washed, dried, and weighed. Fish were aged from 
whole and sectioned sagittae according to Begg and 
Sellin (1998). 
Precautions were taken to prevent confounding of 
results based on sexual maturity, size, and age. All 
samples were sexually mature, between the lengths 
of 492 mm and 790 mm LCF (Table 1). Fish of the 
same age (2-year-old school mackerel; 3-year-old spot- 
ted mackerel) were selected to minimize age-related 
variation amongst samples. Additional samples of 
1 -year-old school mackerel from Rockhampton and 
1-year-old spotted mackerel from Moreton Bay and 
Hervey Bay were examined in relation to the older 
samples from the same areas to investigate the ef- 
fects of age on the concentrations of elements in 
sagittae. 
Whole otoliths used in the analysis were cleaned 
of oil and organic residuals in an ultrasonic bath, 
rinsed in distilled water, and oven-dried at 60°C for 
15 hours. Otoliths were dissolved in 0.5 mL of a Lefort 
aqua regia solution (75% nitric and 25% hydrochlo- 
ric acid) and placed in an AIM50Q block digester at 
90°C for 30 minutes. After cooling, the solutions were 
made up to 10 mL with distilled water and analyzed 
with a Varian Liberty 220 inductively coupled plasma 
atomic emission spectrometer (ICP-AES). Eleven el- 
ements (Ba, Ca, Fe, K, Li, Mg, Mn, Na, P, S, and Sr) 
were analyzed from each otolith, and the concentra- 
tions were standardized for individual otolith 
weights. 
Minimum detection limits for the elements were 
determined by averaging a series of acid blank solu- 
tions for both species that were regularly inter- 
spersed throughout the samples. The minimum 
detectable solution concentration for each element 
(mg/L) was converted to the minimum detectable 
otolith concentration (mg/Kg) by using the solution 
volume and the smallest whole otolith weight actu- 
ally analyzed. This procedure ensured that the most 
conservative (largest) minimum detection limits were 
used in the analyses. Three standard solutions were 
included regularly among the samples to calibrate 
measurements. The standards covered the entire 
weight range of otolith material used in the analy- 
ses. Only those elements measured in concentrations 
above the minimum detection limits of the ICP-AES 
were used in the statistical analyses. 
Data analysis 
The chemical composition of school and spotted mack- 
erel whole otoliths sampled from the different areas 
was analyzed to identify the optimum groupings of 
fish in order to make inferences about stock struc- 
ture. All data were examined for normality and ho- 
mogeneity of variances, and concentrations of Mg and 
Na in school mackerel otoliths, and K, Mg, Na, and 
P in otoliths of spotted mackerel, were logMrans- 
formed to enable statistical analysis. Analysis of co- 
variance (ANCOVA) was used to determine the ef- 
fect of area of collection (the main effect) on the con- 
centration of each element, while controlling for ef- 
fects due to fish length (the covariate). Elemental 
concentrations for which area-by-length interactions 
were significant were not included in further analy- 
Tafole 1 
Data from school and spotted mackerel samples used in otolith trace element analyses. 
School mackerel 
Spotted mackerel 
Length range 
Age 
Length range 
Age 
Date 
Area 
n 
(mm) 
(yr) 
Date 
Area 
n 
(mm) 
(yr) 
Nov 1993 
Moreton Bay 
19 
512-614 
2 
Feb 1994 
Moreton Bay 
30 
555-685 
1 
Jun 1993 
Rockhampton 
25 
492-574 
1 
Feb 1994 
Hervey Bay 
28 
544-715 
1 
Jun 1993 
Rockhampton 
19 
552-640 
2 
Dec 1993 
Hervey Bay 
29 
552-790 
3 
Aug 1993 
Bowen 
22 
545-632 
2 
Aug 1993 
Bowen 
32 
605-670 
3 
Jul 1993 
Innisfail 
28 
585-665 
3 
