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Fishery Bulletin 106(4) 
that could lead to overexploitation (Gulland, 1977). 
Pauly et al. (2001) demonstrated that ecosystem effects, 
as measured by declines in the mean trophic level of 
landings, were apparent at exploitation ratios (E) of 
0.4 and were substantial at ratios of 0.6. Silvestre and 
Garces (2004) estimated E, E max (exploitation rate as- 
sociated with maximum yield per recruit), and E 01 
(a biological reference point analogous to F 01 ) for 25 
waters of Brunei Darussalam. Fishes were similar in 
size to inshore lizardfish (mean (standard devia- 
tion [SD])=262 mm [143]) but their M estimates were 
much greater (mean M [SD]=1.61/yr [0.48]). Mean (SD) 
estimates of E max and E 0 1 among the species were 0.54 
(0.04) and 0.36 (0.03), respectively. Our estimates of E b 
for inshore lizardfish approach the mean E max level esti- 
mated by Silvestre and Garces (2004) and exceed their 
Figure 6 
Von Bertalanffy growth function (VBGF) fitted to size-at-age 
data for inshore lizardfish ( Synodus foetens ), namely all fish 
(n = 967) sampled from spring 2004 through spring 2005 in 
study habitats in the northern Gulf of Mexico, as well as those 
sampled opportunistically. Resultant VBGF fitted to all data: 
L t = 290. 8( 1-e -0 - 486 W-o.204))^ w here L t = length at age t. 
fish populations exploited in trawl fisheries in coastal mean E 0 : reference point. Therefore, inshore lizardfish 
exploitation rates estimated in our study would 
indicate that the species is fully exploited despite 
there being no directed fishery for the species. If 
other non-exploited fishes have similarly high F b 
rates as inshore lizardfish, then simulations con- 
ducted by Pauly et al. (2001) would indicate that 
substantial trophic effects of trawl bycatch may 
exist in the ecosystem. 
Although it is likely that demersal or benthic 
fishes other than inshore lizardfish have suf- 
fered similarly high F b due to shrimp trawling 
in the north central GOM, the only other spe- 
cies that have been examined quantitatively (At- 
lantic croaker [Micropognias undulatus] and red 
snapper) are also heavily exploited by directed 
fisheries. Bycatch rates for shrimp trawls have 
had significant effects on those two species at the 
population level (Diamond et al., 2000; Porch, 
2007; Wells et al., in press), but distinguishing 
bycatch effects from those caused by the directed 
fisheries can be problematic. Furthermore, shrimp 
trawl fisheries have had significant effects on the 
ecosystem beyond that of bycatch of nontargeted 
species at various life stages (Thrush and Dayton, 
2002; Gray et al., 2006). 
In the northern GOM the inves- 
tigation of bycatch has been cen- 
tered around the overfished status 
of red snapper because of the eco- 
nomic importance of that species 
(Strelcheck and Hood, 2007). Re- 
sults from this study demonstrate 
that nonexploited but ecologically 
important species, such as inshore 
lizardfish, may be significantly im- 
pacted by shrimp trawl bycatch. Re- 
sults presented here demonstrate 
that effects of shrimp trawl bycatch 
may be significant at the population 
level for nonexploited species and 
indicate that a broader, ecosystem- 
scale examination of bycatch effects 
is needed (Diamond, 2004; Wells et 
al., in press). If species other than 
inshore lizardfish show similar di- 
rect effects on their populations, 
or if habitat degradation is caus- 
ing indirect effects on populations 
and communities, a new approach to 
management of the shrimp fishery 
may be warranted. 
0 - 
Inside artificial reef (AR) zone 
Catch curve for inside AR zone 
Outside AR zone 
Catch curve for outside AR zone 
012345678 
Age (years) 
Figure 7 
Catch curves computed for inshore lizardfish ( Synodus foetens) sampled inside 
(open circles, O) and outside (open triangles, A) outside the artificial reef 
zone in the north central Gulf of Mexico from spring 2004 through spring 
2005. Plotted lines are linear regressions of In (number) versus age for the 
fully recruited ages: inside the artificial reef zone In (catch) = (-0.94/yr)age 
+ 8.18; outside the artificial reef zone ln(catch) = (-1.02/yr)crge + 8.31. 
