664 
Fishery Bulletin 96(4), 1998 
of the Queensland east coast and in summer in the 
southern bays, but the location and habitats of 
juveniles is not well known, particularly for spotted 
mackerel. 
Complementary biological information from other 
stock identification methods is therefore essential to 
help interpret results from whole otolith analyses, 
but some hypotheses regarding stock structure can- 
not be tested in the absence of knowledge about the 
factors governing otolith composition. The resolution 
of the technique is limited further by volumetric con- 
siderations and the nature of otolith growth. Even if 
there were very large consistent differences in com- 
position among individuals at the otolith core dur- 
ing larval and postlarval life, these would be virtu- 
ally undetectable with bulk analysis, whereas rela- 
tively small differences accumulated during later life 
before capture would have a disproportionately large 
effect on mean composition. To overcome these prob- 
lems — which may be accentuated for pelagic species 
with complex ontogenetic movements (Proctor et al., 
1995) — bulk analysis of whole otoliths from juveniles, 
or excision and analysis of the core regions of otoliths 
from adult fish (Dove et al., 1996), may provide other, 
solution-based alternatives to sectioning and elec- 
tron- or laser-probe techniques. 
There may also be temporal variation in stock dis- 
crimination patterns, particularly for pelagic species 
living in coastal areas influenced by boundary cur- 
rents; Edmonds et al. (1995) demonstrated that varia- 
tion in composition of pilchard (Sardinops sagax ) 
otoliths among years was significantly greater than 
the variation among sites. Stock discrimination pat- 
terns of pilchards were not persistent from one sam- 
pling era to the next within a decade. Further bias may 
be introduced by uneven representation of all life his- 
tory stages within collections (Edmonds et al., 1995). 
Age-reSated variation 
Spatial variation in the concentration of trace ele- 
ments of school and spotted mackerel was strongly 
influenced by the age of fish in the samples at time 
of collection. Differential otolith elemental patterns 
were found between 1- and 2-year-old school mack- 
erel, and 1- and 3-year-old spotted mackerel through- 
out the study region. Numerous studies, including 
this one, support Kalish’s (1989) hypothesis that in- 
corporation of trace elements into otoliths is related 
to growth (Grady et al. , 1989; Thresher et al., 1994; 
Edmonds et al., 1995; Fowler et al., 1995). Conse- 
quently, spatial variation in elemental composition 
of otoliths can be difficult to interpret because of bi- 
ases related to the size and age of fish in the samples 
from separate areas. 
School and spotted mackerel of different ages from 
the same area have significantly different patterns 
in the elemental composition of their otoliths. One- 
year-old school and spotted mackerel tended to have 
lower concentrations of P, S, and Sr, while having 
higher levels of Mg, Mn, and Na in their otoliths com- 
pared with older fish. Grady et al. (1989) found simi- 
lar results for king mackerel where heavy metal con- 
centrations were generally higher in otoliths of 
younger fish. Differences in the concentrations of 
trace elements accumulated in the otoliths of fish of 
different ages from the same stock are not unex- 
pected, particularly if irreversible deposition of ele- 
ments in otoliths is assumed. Distinct chemical pat- 
terns in otoliths of fish of different ages may reflect 
exposure to similar environments, but for different 
accumulation periods, or alternatively, may reflect 
life history differences, such as younger fish inhab- 
iting distinct nursery grounds that are separate from 
adult habitats. 
Not all factors affecting deposition of elements in 
otoliths are strictly environmental or ontogenetic, nor 
do they necessarily act in a simplistic manner that 
reflects ambient environmental chemistry (Kalish, 
1989; Campana et al., 1994). Chemical deposition can 
be regulated by many interacting factors, including 
water temperature, salinity, age, physiology, growth 
rates, and activity levels of individual fish (Kalish, 
1989, 1990, 1991; Radtke and Shafer, 1992; Rieman 
et al., 1994; Fowler et al., 1995). 
Stock structure and management 
Although further research is required to determine 
the mechanisms responsible for elemental deposition 
in otoliths of Scomberomorus species, particularly the 
interaction between environmental and genomic con- 
trols, the validity of using stock and site-specific el- 
emental “fingerprints” does not rest upon the mecha- 
nism underlying otolith formation (Campana and 
Gagne, 1995). Optimal groupings of mean elemental 
composition of school mackerel otoliths strongly sup- 
ported the hypothesis of at least two stocks in the 
study region — a hypothesis that has been developed 
with complementary stock identification techniques. 
Localized movements of tagged school mackerel have 
shown that there is little exchange between adult 
fish from different areas throughout Queensland east 
coast waters; most recaptures occur within the same 
area of release (Begg et al., 1997). Studies of the tim- 
ing and location of spawning have also shown that 
school mackerel spawn concurrently at a number of 
localities along the east coast (Begg, in press). The 
species also exhibits differences in growth patterns 
and genetic variation throughout its distribution on 
