Jeffers et al. : Habitat and bycatch effects on population parameters of Synodus foetens in the Gulf of Mexico 
423 
the variability in size-at-age reported for inshore 
lizardfish is representative of the variability in 
the population given the high APE computed be- 
tween reader age estimates. 
Results from catch curve analysis and calcula- 
tions of F b appear to indicate that shrimp trawl 
bycatch mortality was substantial for inshore 
lizardfish. However, that interpretation depends 
on the assumption that the sampling gear has 
a logistic selectivity-at-age function for inshore 
lizardfish, and that there was no other source 
of fishing mortality beyond that of shrimp trawl 
bycatch. If the selectivity-at-age function was 
domed shaped, for example, then some of the 
decline in numbers of larger, older fish in our 
sample would have been due to older fish not 
being fully selected by the gear; however, our 
interpretation was that the observed decline, be- 
yond that due to M, was caused by considerable 
bycatch mortality. Experiments could be designed 
to estimate the selectivity function of our sam- 
pling gear for inshore lizardfish, but no data 
currently exist to evaluate selectivity. However, 
given the small size of even the largest inshore 
lizardfish sampled in relation to the size of the 
sampling trawl, it seems reasonably safe to as- 
sume that a logistic selectivity function existed. 
Furthermore, it seems unlikely that other sources 
of fishing mortality, beyond shrimp trawl bycatch, 
are substantial for inshore lizardfish in the north 
central GOM. Inshore lizardfish are not targeted 
by recreational fishermen in the region and we 
have no knowledge of them being captured in 
commercial or recreational hook-and-line fisher- 
ies as bycatch. 
The difference in Z, hence F b , was not statisti- 
cally significant between fish sampled inside and 
those sampled outside the AR zone. Nonetheless, 
a difference of Z = 0.17/yr clearly is biologically 
meaningful. The lack of a larger difference may 
indicate that either the assumption that no trawl- 
ing occurs inside the reef zone was false, or that 
inshore lizardfish moved between habitats inside 
and outside the AR zone. Results of electronic 
tracking of shrimp trawler GPS coordinates in- 
dicate some shrimping effort may occur inside 
the AR zone (NRC, 2002, Fig. B.9 in that report). 
However, shrimping effort was shown to be orders 
of magnitude greater outside than inside the AR 
zone. Therefore, the lack of difference in Z, hence 
F b , between habitats inside and outside the AR 
zone most likely was caused by movement of in- 
shore lizardfish into and out of the AR zone. 
Exploitation ratios computed for inshore lizard- 
fish appear to indicate that the species is heavily 
fished in the northern GOM despite the lack of a 
directed fishery for it. A general rule of thumb is 
that an E approaching 0.5 indicates that a fished 
population is fully exploited, whereas a ratio 
greater than 0.5 indicates heavy fishing pressure 
400 
350 
300 
250 
200 
150 
100 
o 
Sand 
A 
Low-relief shell 
□ 
High-relief shell 
V 
Reef 
> ft 
Spring 04 Summer 04 Fall 04 Winter 04 Spring 05 
<S 400 
£ 
350 
300 
250 
200 
150 
100 
B 
5 5 
s. 
Spring 04 Summer 04 Fall 04 Winter 04 Spring 05 
Quarter 
Figure 4 
Mean (±standard error) total length (mm) of inshore lizardfish 
(Synodus foetens) sampled from spring 2004 through spring 
2005 from study habitats in the northern Gulf of Mexico for 
(A) areas inside the artificial reef zone that were not subjected 
to trawling and (B) areas outside the artificial reef zone that 
were subjected to trawling. 
c 
Figure 5 
Results from marginal condition analysis of otolith transverse 
thin sections of inshore lizardfish ( Synodus foetens). Data 
plotted indicate the mean number of otolith samples for a 
given month that had opaque rather than translucent margins. 
Monthly sample sizes are indicated on the figure. 
