Dennis et al.: Using otolith chronologies to identify extrinsic drivers of growth 
145 
productivity in this region (Ridgway and Hill*). The large 
range and timescale in which eddies exist may facilitate 
the high growth synchrony observed in fish from NSW. 
In comparison, KI is influenced by the Flinders Current, 
a northern boundary current, and the Leeuwin Current, a 
seasonal shelf-break current (Middleton and Cirano, 2002; 
Middleton and Bye, 2007). This region is characterized by 
short events (3-10 d) of summer upwelling and winter 
downwelling along the southern coast of Eyre Peninsula, 
Bonney Coast, and western KI, resulting in increased pro- 
ductivity in these regions (Kampf et al., 2004; Ward et al., 
2006; Middleton and Bye, 2007). In comparison with the 
magnitude and temporal extent of the eddies in NSW, the 
smaller size and shorter period of the upwelling off KI 
may not have the strength to drive the growth synchrony 
in these fishes, likely explaining the difference in growth 
synchrony between these regions. 
Both species grew larger off KI than off southern NSW. 
Such regional differences have previously been documented 
in several temperate fish species, including the Pacific sar- 
dine (Izzo et al., 2017), Australian salmon (Arripis trutta) 
(Hughes et al., 2017), and sand whiting (Stocks et al., 2011). 
Variation in available resources (e.g., food and habitat) can 
be the driving force that alters the demographics of a pop- 
ulation (Ruttenberg et al., 2005; Hughes et al., 2017) and 
results in differences across their distributions. Although the 
diets of both common jack mackerel and redbait are similar 
across their distributions, with krill being their main prey 
item (Ward and Grammer’), differences in growth across 
their distributions were observed in our study—between 
populations of common jack mackerel and within the popu- 
lation of redbait (Ward et al.%?°). 
The more variable growth in both species off NSW, in 
comparison to that off KI, can likely be explained by the 
EAC and associated eddies. Upwelling and biological pro- 
ductivity are driven by increases in vertical mixing of the 
epipelagic zone due to water circulation of eddies and their 
interaction with the continental shelf and coastline. As a 
result, environmental conditions vary among years depend- 
ing on the extent of the EAC southward extension and ensu- 
ing location of eddies (Tilburg et al., 2001; Ridgway and 
Hill*). Findings of studies on the larval growth of Pacific 
sardine (Uehara et al., 2005), white trevally (Psewdocaranx 
dentex), and jack mackerel (Trachurus novaezelandiae) 
indicate the effects of the EAC and its upwelling regions on 
° Ward, T. M., G. L. Grammer, A. R. Ivey, J. J. Smart, and P. Keane. 
2018. Spawning biomass of jack mackerel (Trachurus declivis) 
and sardine (Sardinops sagax) between western Kangaroo 
Island, South Australia and south-western Tasmania. Report 
to the Australian Fisheries Management Authority. South 
Aust. Res. Dev. Inst., SARDI Publ. F2018/000174-1, SARDI 
Res. Rep. Ser. 983, 51 p. SARDI Aquat. Sci., Adelaide, Australia. 
[Available from website.] 
10 Ward, T, G. Grammer, A. Ivey, and J. Keane. 2019. Spawning 
biomass of redbait (Emmelichthys nitidus) between western 
Kangaroo Island, South Australia and south-western Tasma- 
nia in October 2017. Report to the Australian Fisheries Man- 
agement Authority. South Aust. Res. Dev. Inst. SARDI Publ. 
F2019/000053-1, SARDI Res. Rep. Ser. 1011, 38 p. SARDI 
Aquat. Sci., Adelaide, Australia. [Available from website.] 
growth and the amount of interannual variability (inter- 
annual variability in increment growth relative to the 
long-term mean) among species (Syahailatua et al., 2011). 
In comparison, the reduced temporal and spatial scale of 
upwelling events off KI (Middleton and Bye, 2007), in com- 
bination with the lack of basin-wide changes, has likely 
resulted in the lower interannual variability in growth of 
fishes off KI than off NSW. 
Similar to differences between regions, the higher inter- 
annual variability in growth of common jack mackerel 
compared with that of redbait in both regions may reflect 
the conditions they are exposed to in their respective habi- 
tats. Although the species occur in similar habitats, redbait 
are also found around seamounts, mid-ocean ridges, and 
islands and at deeper depths than common jack mackerel 
(Pullen and TDPIF, 1994; Welsford and Lyle, 2003). This 
difference in habitat means that common jack mackerel 
are more likely to be influenced by the environmental con- 
ditions of surface waters, which are in turn more likely 
to vary between years than the deeper waters in which 
redbait reside (Ridgway and Dunn, 2003). Because of this 
increased distribution depth, SST may not be the most 
appropriate temperature variable for redbait; however, 
bottom temperature records covering the chronologies in 
our study areas are not available. As such, SST is currently 
the most suitable proxy we have for temperature changes 
in these regions for the chronologies in this study. 
The principle factor influencing somatic growth in ecto- 
therms is commonly thought to be environmental tem- 
perature (Hughes et al., 2017). Marine organisms require 
specific temperature ranges to maintain control of physi- 
ological processes and avoid thermal stress (Calosi et al., 
2008). In South Australia, snapper growth declined at tem- 
peratures higher than 18—20°C, which is the likely pejus 
temperature for snapper in South Australia (Martino et al., 
2019). Results from the linear models in our study indi- 
cate that the optimal temperature for growth of common 
jack mackerel at KI is approximately 16—18°C (KI actual 
SST range: 16.42-18.01°C), a range at which the growth 
rate was still increasing. However, the growth rate of com- 
mon jack mackerel at 18-20°C (NSW actual SST range: 
18.40—20.05°C) was lower. This result indicates that a 
mean temperature higher than 18°C might start to cause 
some thermal stress, decreasing growth rates. Within the 
NSW region, the growth and metabolic costs of the red 
moki (Chirodactylus spectabilis) has already been affected 
by the intensification of the EAC (Neuheimer et al., 2011). 
As such, a continued increase in water temperature may 
result in the decline of the growth rates of common jack 
mackerel off NSW; however, a specific study investigating 
the thermal tolerance of common jack mackerel is required 
to define the optimal temperature for this species. 
Indirect effects, such as changes in productivity (e.g., 
food), structure (e.g., habitat and abundance of predators), 
and composition (e.g., abundance of competitors), can also 
affect fish growth (Brander, 2010). For example, produc- 
tivity or food availability can trigger massive changes in 
a population (Sanchez-Garrido et al., 2019), as has been 
reported for populations of anchovy and sardine species 
