80 
Fishery Bulletin 1 14(1) 
Figure 1 
Transverse section of a sagittal otolith, taken from a wreckfish (Polyprion americanus ) 
caught in the North Atlantic in 2005, that shows the typical pattern of increment for- 
mation. Each black dot (opaque growth increment) represents 1 year of growth. The 
fish from which this otolith was removed was estimated to be 48 years old. 
Natural mortality 
We investigated 4 distinct M estimators, 2 age-constant 
estimators from Then et al. (2015, eqs. 4 and 5), 
M — 4.899U max ~° 916 ) and 
(4) 
M = (4.118& 073 ) (L^' 033 ), 
(5) 
and 2 age-varying M estimators from Gislason et al. 
(2010, eq. 6) and Charnov et al. (2013, eq. 7), 
jyj- g(0.55-1.61xLn(L t )+1.44xLn(L (Xl )+Ln(^)) (g) 
M — 
x k. 
(7) 
Each of these estimators used either estimates for 
VBGM parameters ( k and L„) or maximum age W max ) 
to predict M. 
Results 
Examination of otoliths and growth increments 
External examination of 3 of the whole otoliths from 
wreckfish that were selected for use in the age and 
growth analysis revealed a misshaped translucent oto- 
lith with white crystalline lumps along the external 
surface. We sectioned one otolith with these character- 
istics to examine growth increment formation, only to 
discover that the otolith contained no distinguishable 
growth increments. We excluded these 3 specimens 
from further analysis. 
Otolith sections exhibited a distinct opaque core, 
trapezoid in shape, between 5 and 6 mm in length and 
from 2 to 3 mm in height. Growth increments in trans- 
verse otolith sections were most visible along the medi- 
al surface ventral to the sulcus, although the first few 
increments were highly variable in width and spacing 
in any plane (Fig. 1). The first 4-6 opaque bands were 
broad and tended to widen or begin to bifurcate as 
they extended to the ventral surface of the section. Of- 
ten, the first 4-6 opaque increments were followed by 
a distinct, crack-like structure. Thereafter, the growth 
bands continued to be broad and diffuse until an in- 
crement count of 10-15, after which the growth incre- 
ments became noticeably more compact and regularly 
spaced but harder to discern. 
Earlier, broader increments (<15) were read more 
easily at a lower magnification (40x) by bringing the 
increments in and out of focus and allowing the reader 
to focus on the more distinct increment patterns. The 
later and more compact opaque increments (> 15 ) were 
more easily read at higher magnification (100x), which 
