Dennis et al.: Using otolith chronologies to identify extrinsic drivers of growth 
137 
species and investigated potential environmental drivers 
of their growth. Specifically, in this study, we 1) compared 
growth rates of each species between KI, South Australia, 
and New South Wales (NSW) and 2) investigated the 
influence of local environmental conditions on the growth 
rates of both species in each region. 
Materials and methods 
Study sites and sample collection 
Common jack mackerel and redbait were collected from 47 
trawl tows at depths between 100 and 303 m off KI and 
from 49 trawl tows at depths between 60 and 283 m off 
southeast NSW throughout each year between 2014 and 
2016; the samples were taken from catch of a single mid- 
water trawling vessel that is part of the Commonwealth 
small pelagic fishery (Fig. 1). From the haul of each trawl 
tow, 50 fish of each species were randomly selected and fro- 
zen. Fish were then subsampled (10 fish per trawl sample) 
for age and growth analysis, in the laboratory; fork length 
(FL, in millimeters), total weight (in grams), and sex were 
recorded, and otoliths were extracted. 
South Australia 
T. declivis—KI: 
n=98 (4-13 years) 
E. nitidus—kI: 
n=52 (3-7 years) 
140° 
Otolith preparation and increment analysis 
Sagittal otoliths were removed from each fish, rinsed with 
water, and air-dried. Otoliths were batch embedded in clear 
epoxy resin and thin-sectioned (sections were ~300 pm) by 
using a single-blade sectioning saw (Gemmasta’, Shell-Lap 
Supplies, Adelaide, Australia) through the primordium (i.e., 
core) (Fig. 2). Up to 3 transverse sections were taken from 
each otolith to ensure that a section was taken through the 
core. Sections were mounted on glass slides, polished, and 
viewed by using a stereo microscope (3.2x magnification; 
Olympus SZX7, Olympus Corp., Tokyo, Japan). 
To ensure aging accuracy and count reproducibility 
prior to aging otoliths, the readers were trained with a 
reference library containing 100 otoliths of known ages 
for each species. The ages at capture of the fish caught 
for this study were within the age range for otoliths in 
the reference library. All otoliths were aged by 2 individ- 
uals with extensive experience in aging both species 
(coefficient of variation of <5%), providing a third reader 
° Mention of trade names or commercial companies is for identi- 
fication purposes only and does not imply endorsement by the 
National Marine Fisheries Service, NOAA. 
Pacific Ocean 
Nan Sanity Wales T. declivis—NSW: 
n=97 (3-9 years) 
E. nitidus—NSW: 
n=70 (3-14 years) 
Figure 1 
Map of sampling areas (indicated with gray rectangles) where common jack mackerel (Trachurus declivis) 
and redbait (Emmelichthys nitidus) were collected by a mid-water trawling vessel off Kangaroo Island (KI) 
and New South Wales (NSW) in Australia during 2014-2016. Sample sizes (n) and age ranges are provided 
for the fish sampled for each species in each region. 
