Lytton et al.: Age validation of Polyprion americanus based on bomb radiocarbon 0 4 C) 
87 
due to a disparity in growth between the 2 stocks. The 
difference in estimates between the Vaughan et al. 
(2001) and our study may have resulted from the lack 
of larger fish (>1100 mm TL) in our sample; however, 
the length frequency of the subsample used in age and 
growth analysis in our study mirrored the length fre- 
quency of our total sample population. It is more likely 
that the difference in values between the afore- 
mentioned study and ours was caused by incorrect age 
assignments, which in turn would impact the VBGM 
parameters. Either way, this difference in L«, values 
has implications for estimating related life history pa- 
rameters and highlights the importance of validation 
efforts, as well as the importance of the inclusion of 
sufficient samples from the oldest age groups when de- 
veloping a VBGM. 
Our samples contained only 3 specimens that were 
less than 500 mm FL, the proposed size at settlement 
for wreckfish from both the North and South Atlantic 
(Sedberry et al., 1998; Peres and Haimovici, 2004). The 
smallest specimen was 452 mm FL and was estimated 
to be 3 years old. The lack of smaller and younger fish 
(only 14 specimens <2 years old) within the sample is 
consistent with the notion that juvenile North Atlan- 
tic wreckfish settle at a size around 500 mm TL and 
at an age of 1-2 years. If juveniles are present at the 
same locations as adults, gear selectivity is unlikely to 
have excluded them from our sampling because juve- 
nile wreckfish have relatively large mouths and would 
be susceptible to hook-and-line capture. 
Natural mortality is a fundamental life history pa- 
rameter used in stock assessments. Without a realistic 
estimate of M, fishing-induced mortality cannot be es- 
timated from the age or size composition of commercial 
catches, and the effects of fishing mortality on future 
yields cannot be predicted. Methods used to estimate 
M treat it either as a constant value (Pauly, 1980; Hoe- 
nig, 1983; Alagajara, 1984; Polovina and Ralston, 1987; 
Hewitt and Hoenig, 2005; Then et al., 2015) or as an 
age- or size-varying parameter (Lorenzen, 1996; Gisla- 
son et al., 2010; Charnov et al., 2013), but the latter is 
the generally preferred method for estimating M. 
Although treating M as a constant value has been a 
historically common practice, today researchers gener- 
ally accept that M is highest during larval stages and 
decreases as a fish ages, finally arriving at some steady 
state (Gislason et al., 2010). Assuming M varies with 
age, managers can consider the effects of size compo- 
sition when examining alternate management strate- 
gies. Data from our study indicated that age-based es- 
timates of M reached an asymptote by approximately 
age 15. The value of the asymptote varied according to 
the aging method used, ranging from 0.07 for Gislason 
et al. (2010) to 0.12 for Charnov et al. (2013), respec- 
tively. Most fish captured along the Charleston Bump 
are greater than 900 mm FL (Sedberry et al., 1999), 
corresponding to an age of around 10-15 years from 
the VBGM. If managers were to select a point estimate 
of M, because of the absence of younger age classes, it 
is likely that the point estimate of M used to determine 
the allowable biological catch would fall between the 
asymptotic values of M from the age-varying methods. 
This value would represent the M experienced by the 
fished portion of the stock. 
When selecting the appropriate method for estima- 
tion of M, researchers are faced with a daunting se- 
lection of estimation techniques, most of which involve 
the use of estimates of one or multiple parameters from 
the VBGM, t max , or weight-at-age data. Proper selec- 
tion of a technique will first depend on the information 
available and then on the confidence in the accuracy 
of the parameter estimate being used (e.g., whether 
a researcher is more confident in the accuracy of the 
VBGM parameters or of t max ). However, Then et al. 
(2015) asserts that f max is the best proxy for estimating 
M when a value is available. Because this study vali- 
dated age estimates of wreckfish captured in the North 
Atlantic and because the M estimate based on VBGM 
parameters (M=0.091) was close to the M estimate that 
was based on t max (M=0.088), 0.09 is an appropriate 
value to use for M in stock assessments of wreckfish. 
The new information presented here on the life 
history of wreckfish in the North Atlantic represents 
the North Atlantic stock more accurately because it is 
based on recent samples collected during 2000-2011, 
because new aging criteria and resulting estimates 
have been validated with bomb radiocarbon analysis, 
and because the new age estimates are similar to es- 
timates for other Polyprion species. Several aspects of 
the life history of wreckfish in the North Atlantic are 
still in need of study. The validation of the maximum 
age of 80 years reported here cannot be validated by 
radiocarbon analysis until the year 2038. However, we 
have validated the aging criteria used to determine 
ages of wreckfish through bomb radiocarbon analysis. 
In addition, the structure and formation of the first 
annual increment needs to be elucidated and investi- 
gated because it may potentially affect age estimates 
by up to 3 years. Samples from the eastern Atlantic 
are needed to compare potential differences in life his- 
tory parameters within the North Atlantic population 
and to investigate connectivity between populations. 
Finally, the determination of size and age at maturity 
and sex-specific differences in age and growth is essen- 
tial for future stock assessments and requires samples 
from whole, rather than gutted, fish. 
Acknowledgments 
We thank J. Potts (NOAA Southeastern Fisheries Sci- 
ence Center in Beaufort, North Carolina) for the do- 
nation of otoliths used in this study. We also thank 
the commercial fishermen who collected the samples 
used in this study. G. Sedberry is thanked for shar- 
ing his knowledge on wreckfish and his assistance with 
the preparation of this manuscript. This research was 
supported by NOAA grants NA11NMF4540174 (MAR- 
MAP) and NA11NMF4350043 (SEAMAP-SA) and by 
the South Carolina Department of Natural Resources. 
