Sutherland and Richards: Aging Lophius americanus based on length-mode progression of a strong cohort 21 
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A S O N 
Month 
Figure 7 
Average relative marginal increment on illicia by month for goosefish 
(Lophius americanus) captured along the Atlantic coast from Virginia to 
Georges Bank during May 2016—May 2017. Numerals in parentheses along 
the x-axis indicate monthly number of samples. Error bars indicate 95% con- 
fidence intervals. 
Hatch date 
The average hatch date, 1 June 2015, supports our assump- 
tion that fish in the strong length mode were hatched in 
2015. This spawning date is consistent with other survey 
observations and studies of goosefish biology. The strong 
2015 recruitment was first observed in the scallop dredge 
survey conducted in early June 2015 (Fig. 4) but was not 
observed in the NEFSC bottom trawl survey that took 
place about 2 months earlier in the same region. This find- 
ing indicates that settlement occurred sometime between 
early April and early June. The similarity between the 
lengths of the modes representing the 2015 and 2018 year 
classes in June and September of those years implies that 
the 2015 year class was growing in a typical way. Able 
et al. (2007) observed that goosefish settle to the benthos 
at lengths of about 7 cm TL (range: 5-8 cm TL), a size 
that approximates the lower bound of the length mode we 
observed in June 2015 (Fig. 4). 
Results of additional studies of goosefish biology also 
support our interpretation that the strong length mode 
was composed of fish hatched in 2015. Goosefish along 
the Atlantic coast of the United States from Virginia to 
Georges Bank spawn from late winter through summer 
(Johnson et al., 2008; McBride et al., 2017). Hatching 
occurs from June through October with a peak in July 
(Able et al., 2007), larval abundance peaks in May—June 
(Able et al., 2007; McBride et al., 2017), 
and juveniles settle to the benthos 5-10 
weeks after hatching (Able et al., 2007). 
Therefore, although our sample size 
for estimating hatch date was small, 
our results are consistent with expec- 
tations from many additional lines of 
evidence. 
Age estimation 
Illicium diameter increased with fish 
length (Fig. 8), indicating that this 
structure continues to accrete, one of 
the characteristics of a structure that 
is effective for age determination (Van 
Oosten, 1928). The diameters of both 
the settlement check and the first ring 
point to a weak positive relationship 
with fish length, indicating that the 
internal portions of the illiclum may 
have changed as the fish grew or that 
the marks may not have been identified 
consistently. However, in our study, this 
change was minimal, as there was little 
variation in these diameters across all 
the samples, and the values were sim- 
ilar to those obtained from larger fish 
(>50 cm TL) (Bank et al., 2020). Ofstad 
et al. (2013) also noted changes in the 
size of the first ring in samples of fish 
under 50 cm TL. 
Bank et al. (2020) suggested that the illictum might 
prove to be a useful structure for aging of goosefish because 
they had greater success with illicia than with vertebrae 
in finding the winter increments after the chemical mark. 
They also described how recent advances in preparation 
and interpretation of illicia may have made age interpre- 
tation easier for this structure. However, we did not find 
the expected number of rings on illicia in 50% of samples 
(Fig. 5A), and precision was only 9% (Fig. 6A). These levels 
of accuracy and precision are not adequate for age esti- 
mation. Ring counts from illicia tend to overestimate age. 
The bias is not consistent, precluding use of a standard 
correction. 
Although studies on other Lophius species have been 
able to validate yearly marks on illicia (Jénsson’; Landa 
et al., 2013), the structure remains a challenge to read 
(Duarte et al.'; Landa et al., 2013). Many rings are visible, 
and it is difficult to choose which ones to count. Unlike 
with typical aging structures, the spacing between annuli 
does not decrease with age and is consistent across the 
section; increment width may even increase near the edge 
(Duarte et al.'). Disagreements between otolith- and 
illicium-derived ages of white anglerfish were not resolved 
until Wright et al. (2002) showed that an additional ring 
(i.e., false annulus) is formed on illicia. However, it is pos- 
sible that goosefish illicia may have multiple false annuli. 
This complexity in reading illicia may also account for the 
