Open Journal of Marine Science, 2011, 3, 82-97
doi:10.4236/ojms.2011.13009 Published Online October 2011 (http://www.SciRP.org/journal/ojms)
An Evaluation of the Management Effectiveness of a Stock
Re-Building Strategy for a Multi-Species Demersal Trap
Fishery in Abu Dhabi, United Arab Emirates
Edwin Mark Grandcourt, Thabit Zahran Al Abdessalaam, Stanley Alexander Hartmann, Franklin
Francis, Ahmed Tarish Al Shamsi
Biodiversity Management Sector, Environment Agen cy–Abu Dhabi, Abu Dhabi, United Arab Emirates
Received June 27, 2011; revised September 8, 2011; accepted September 22, 2011
Abstract
The off-shore demersal trap fishery in the southern Arabian Gulf has been managed by gear regulations and
effort constraints which were aimed at rebuilding depleted stocks. In order to evaluate the success of these
regulations, a variety of selectivity and other fishery metrics were compared for the key species (Diagramma
pictum, Epinephelus coioides and Lethrinus nebulosus) before and after their introduction. With the excep-
tion of a minor increase in the mean age at first capture from 1.3 yrs to 1.9 yrs for E. coioides, there were no
significant changes in the values or trends in juvenile retention, mean size or the mean sizes and ages at first
capture. The comparison of selectivity ogives with data derived independently through an experimental fish-
ing program indicated that the failure of juvenile escape panels to modify the selectivity characteristics of the
fishery could not be attributed to a lack of compliance. Furthermore, there were no significant changes in
fishing mortality rates, harvest rates, catch, effort, yield per recruit and relative spawner biomass per recruit
following the introduction of the management regulations. Age structures were highly truncated and the
management measures had failed to increase the relative proportion of older age classes. Stock status indica-
tors suggested that all species were heavily over-exploited with evidence of both growth and recruitment
over-fishing. Given the failure of existing regulations to modify gear selectivity, reduce effort and rebuild
stocks, the results of the study suggest that management authorities should consider alternative measures in-
cluding a moratorium on the use of traps in the off-shore demersal fishery of Abu Dhabi.
Keywords: Demography, Stock Assessment, Per Recruit, Population Dynamics, Tropical Fisheries
Management
1. Introduction
The fisheries of the southern Arabian Gulf provide a
source of income, employment and recreation at the same
time as contributing to the cultural heritage and food secu-
rity of the inhabitants of the coastal states. Fishing in the
waters off the Emirate of Abu Dhabi in the United Arab
Emirates (UAE) is primarily conducted from traditional
wooden dhows and open fibreglass dories. Dome shaped
wire traps are the most commonly used fishing gear al-
though a variety of other methods exist including; fixed
inter-tidal fence nets, gillnets, hand lines and trolling
lines. Landings are typically diverse and characteristic of
a multi-species tropical fishery, with over 100 species
from 35 families being commonly caught. Target species
in the demersal fishery are primarily composed of repre-
sentatives of the families; Carangidae, Lethrinidae, Hae-
mulidae, Serranidae, Siganidae and Sparidae [1].
An extensive fisheries resource survey conducted in
UAE waters during 2002 indicated that the stocks of key
commercially exploited species had declined to 19% of
levels that were present in 1978. The reduction in abun-
dance was even more pronounced (13%) for more vul-
nerable species such as the orange spotted grouper (Epi-
nephelus coioides) [2]. Subsequent fisheries dependent
assessments corroborated these results and suggested that
71% of the resource base was over-exploited [1]. For
many of the key species, such as Lethrinus nebulosus

E. M. GRANDCOURT ET AL.83
and Diagramma pictum, the relative stock sizes were
well below limit biological reference points suggesting
that both growth and recruitment over-fishing had oc-
curred [3].
The principal critical management issue faced by the
off-shore demersal fishery in Abu Dhabi is that the ma-
jority of species are being exploited well above sustain-
able levels. The excessive fishing effort has even im-
pacted the lower trophic groups, such as the Siganidae,
which are typically more resilient [4]. Furthermore, the
selectivity characteristics of the main gear type (fish
traps) resulted in a large proportion of catches being
composed of immature fish that had not achieved their
full growth potential. More than half of the landed ca-
ches of the sparids (Acanthopagrus bifasciatus and Ar-
gyrops spinifer) for example were composed of imma-
ture fish [5].
Prior to the year 2000, the demersal fishery of Abu
Dhabi was largely open access and there were no con-
straints on fishing effort. Information on the depleted
status of the resource base derived from stock assess-
ments prompted authorities to implement a variety of
management regulations. Effort reductions were imple-
mented at the start of 2004 by limiting the number of fish
traps to 125 for each dhow and banning the use of traps
by open dories. Traditional dhows previously used an
average of 220 fish traps. The simultaneous introduction
of gear regulations included the requirement for a
stainless steel juvenile escape panel to be fitted to all
traps. Given the potential for traps to ‘ghost fish’ after
being lost [6], a magnesium/zinc alloy sacrificial anode
pin, which allows the panel to open after 2 weeks was also
mandatory.
In 2001, an on-going catch and effort data recording
system and stock monitoring programme for 3 key spe-
cies in the demersal trap fishery (D. pictum, E. coioides
and L. nebulosus) was implemented [1]. This provided
the opportunity to establish demographic and fishery me-
trics that relate to the resource status and selectivity
characteristics of the demersal trap fishery. In this con-
text, the objective of this study was to use a selection of
time series metrics to provide an objective evaluation of
the effectiveness of effort and gear regulations that were
implemented in 2004 as part of a stock re-building man-
agement strategy for the off-shore demersal fishery of
Abu Dhabi.
2. Materials and Methods
2.1. Study Site and Sampling Protocol
Size frequency data was collected from commercial
catches of D. Pictum, E. coioides and L. nebulosus made
off the coast of the Emirate of Abu Dhabi in the United
Arab Emirates (Figure 1) between January 2001 and
December 2008. Fish were selected at random from
Figure 1. Study site showing the location of the Emirate of Abu Dhabi in the southern Arabian Gulf.

E. M. GRANDCOURT ET AL. 84
landings, lengths were taken using a measuring board
and recorded to the nearest cm fork length (LF) for D.
Pictum and L. nebulosus and total length (LT) for E.
coioides. Biological data was collected from specimens
purchased from commercial catches over the same period.
Standard length (Ls), fork length (LF) and total length (LT)
were obtained using a measuring board and recorded to
the nearest mm. Whole wet weight was measured using
an electronic balance and recorded to the nearest g. Fish
were sexed by macroscopic examination of the gonad
which was dissected out and subsequently weighed to
0.1 g using an electronic balance. Sagittal otoliths were
extracted, cleaned in water, dried, weighed to 0.1 mg and
stored in seed envelopes. One of each pair of sagittae
was weighed to 0.1 mg and embedded in epoxy resin.
Transverse sections through the nucleus of approxi-
mately 200 to 300 m thickness were obtained using a
twin blade saw. Sections were mounted on glass slides
and examined using a low power microscope and re-
flected light.
Catch and effort data were collected through a strati-
fied catch and effort data recording system which was
implemented in 2001. Landings of the key species were
recorded to family level between 2001 and 2004 and
only to species level from 2005 onwards. Consequently,
data used in the analyses of catches were aggregated to
family level (Haemulidae, Lethrinidae and Serranidae)
for the entire time series.
2.2. Age Estimation and Length Weight
Parameters
Age was estimated from the number of opaque bands in
transverse sections of otoliths as these had been previ-
ously validated as annuli in D. Pictum and L. nebulosus
[3] and E. coioides [7]. Three independent age readings
were made and data only included in the analyses if two
or more readings were in agreement. The overall preci-
sion was established using the index of average percent
error (APE) [8].
Parameters of the length weight relationship were ob-
tained by fitting the power function to length and weight
data:
F .
b
WaL
Where W is the wet weight, a is a constant, L
F
is the fork
length (replaced with L
T
for E. Coioides) and b is close
to 3.0 for species with isometric growth.
2.3. Mortality and Selectivity
Size-at-age data were used to construct age length keys
following the method described in [9], and these were
used to convert length frequency data into age frequency
distributions. The annual instantaneous rate of total mor-
tality (Z) for each species by year was subsequently de-
termined using the age based catch curve method [10].
The natural logarithm of the number of fish in each age
class was plotted against the corresponding age and Z was
estimated from the descending slope of the best fit line
using least-squares linear regression. Initial ascending
points representing fish that were not fully recruited to the
fishery were excluded from the analyses.
Backwards extrapolation of age based catch curves
was used to estimate probability of capture data based on
the method described in [11]. Selectivity curves were
generated using the logistic function fitted to plots of the
probability of capture against age and used to derive
values of the mean age at first capture for each species
by year:


1/(1exp() )Prttc
Where: P is the probability of capture, tc is the mean age
at first capture and r is a constant which increases in
value with the steepness of the selection curve.
Length frequency samples for each year were con-
verted into relative age frequency distributions using
parameters of the von Bertalanffy growth function [7,3]
following the method of [12]. The natural logarithm of
the number of fish in each relative age group divided by
the change in relative age was plotted against the relative
age. Backwards extrapolation of the descending limb of
these length converted catch curves was used to estimate
probability of capture data using the method of [11]. Se-
lectivity curves were generated by fitting the logistic
function to probability of capture and size data which
were used to derive values of the mean size at first cap-
ture (Lc50) for each species. The lengths giving the high-
est yields (Lopt) were estimated for the study species us-
ing the empirical relationship of [13] where Lopt =
L(3/(3+M/k)).
Selectivity curves were also generated for pooled size
frequency data collected before (2001-2003) and after
(2004-2008) the introduction of juvenile escape panel
regulations. These were compared to selectivity curves
for the study species that were derived from an experi-
mental fishing program in order to provide an independ-
ent assessment of selectivity and gauge compliance. The
mean size was also calculated for each species by year.
The annual instantaneous rate of fishing mortality (F)
was calculated for each species and year by subtracting
the natural mortality rate (M), estimated as 0.13 and 0.20
for D. Pictum and L. nebulosus respectively [3] and 0.19
for E. coioides [7], from the total mortality rate (Z) de-
rived from age based catch curves. Juvenile retention (J)
was calculated as the proportion of fish in landings that
were below the mean size at first sexual maturity of fe-

E. M. GRANDCOURT ET AL.85
males given as 31.8 cm LF and 27.6 cm LF for D. Pictum
and L. nebulosus respectively [3] and 42.6 cm LT for E.
coioides [14].
The annual harvest rate (H), or percentage removal by
the fishery was estimated as:

11
Z
F
H -e00
Z


2.4. Per-recruit Analyses
A yield per recruit (YPR) model [10] was used to esti-
mate YPR and the relative spawner biomass per recruit
(SBR) every year for each species. Published estimates
of natural mortality rates (M) and growth parameters (k
and L) of the von Bertalanffy growth function for the
same stocks [3,7], were used as inputs to the model along
with the selectivity parameters (Lc50) estimated here. The
target biological reference point was defined as the fish-
ing mortality rate at which the slope of the YPR curve
was 1/10th of the value at its origin (F0.1). The limit bio-
logical reference point was defined as the fishing mortal-
ity rate at which YPR would be maximized (Fmax). Addi-
tional target (Fopt) and limit (Flimit) biological reference
points of 0.5M and 2/3M respectively were calculated for
each species for comparison following [15].
2.5. Analyses of Metrics and Age Structures
Selectivity (J, mean size, Lc50, tc) and other fishery met-
rics (F, YPR, SBR, H, catch, effort) were compared be-
fore (2001-2003) and after (2004-2008) the introduction
of juvenile escape panels and effort restrictions using
Mann-Whitney U tests [16]. Partial correlations [16] were
also used to evaluate the impact of the introduction of
juvenile escape panels and effort restrictions on time se-
ries trends of these metrics.
The proportion of fish in each age class was calculated
from pooled data from the same before (2001-2003) and
after (2004-2008) periods. Age structures were examined
using 2 goodness of fit tests. Independent tests were
conducted to determine whether there was a significant
difference from unity for individual age classes follow-
ing the introduction of management measures in 2004.
The probability level was set at .05 ( 0.5) and Yates's
correction factor was used on account of there being only
1 degree of freedom for each comparison.
3. Results
3.1. Age Estimation and Length Weight
Parameters
Transverse sections of sagittal otoliths were character-
ised by well defined alternating translucent and opaque
bands when viewed with reflected light under low power
magnification (Figure 2).
A total of 1,082 otoliths were used to age D. Pictum of
which 83 samples were rejected due to a lack of agree-
ment between readings. Fish ranged in size from 18.9 to
63.0 cm LF and age from 1 to 13 yrs. A high level of re-
peatability was achieved in age readings with an APE
value of 5.7%. A total of 17,273 size measurements were
recorded and the length weight relationship (y = 0.014
x2.99) provided a good fit to length and weight data (r2 =
0.99).
A total of 1,706 otoliths were used to age E. coioides
of which 26 samples were rejected due to a lack of
agreement between readings. Fish ranged in size from
20.6 to 100.2 cm LT and age from 1 to 12 yrs. A high
level of repeatability was achieved in age readings with
an APE value of 3.5%. A total of 36,528 size measure-
ments were recorded and the length weight relationship
(y = 0.006 x3.22) provided a good fit to length and weight
data (r2 = 0.99).
A total of 1,827 otoliths were used to age L. nebulosus
Figure 2. Photomicrographs of transverse sections through
the sagittal otoliths of (a) D. pictum (b) E. coioides and (c) L.
nebulosus viewed with reflected light. Black dots indicate
annually deposited opaque bands and the axes along which
age readings were made (scale bar = 1 mm).

E. M. GRANDCOURT ET AL. 86
of which 48 samples were rejected due to a lack of agree-
ment between readings. Fish ranged in size from 19.0 to
57.8 cm LF and age from 1 to 14 yrs. A high level of re-
peatability was achieved in age readings with an APE
value of 5.2%. A total of 25,432 size measurements were
recorded and the length weight relationship (y = 0.026
x2.89) provided a good fit to length and weight data (r2 =
0.98).
3.2. Selectivity
For D. Pictum, harvest rates ranged from 43%-53% and
the level of juvenile retention reached 64.9% in 2004.
Furthermore, the mean size at first capture (24.7-31.6 cm
LF) was below the mean size at first sexual maturity
(31.8 cm LF) for all years (Table 1). The estimate of the
length giving the highest yield (Lopt) of 53.4 cm LF was
considerably greater than the mean size at first capture
Table 1. Sample sizes (n), total mortality rate (Z), fishing mortality rate (F), target fishing mortality rate (F
0.1
), limit fishing
mortality rate (F
max
), harvest rate (H), juvenile retention (J) and selectivity parameters (Lc
50
, tc) for (a) D. Pictum (b) E.
Coioides and (c) L. nebulosus in the southern Arabian Gulf from 2001 to 2008.
(a)
H Lc
50
tc
Year n Z F (95% CI) F
0.1
F
max
(%) J (cm L
F
) (yrs)
2001 - - - - - - - - -
2002 3567 0.74 0.61 (0.68-0.53) 0.07 0.08 43.0 34.5 28.4 1.1
2003 2919 0.80 0.67 (0.77-0.57) 0.07 0.08 46.0 45.8 24.7 0.7
2004 1653 0.89 0.76 (0.88-0.64) 0.07 0.08 50.4 64.9 24.9 0.7
2005 3108 0.81 0.68 (0.80-0.56) 0.07 0.08 46.7 36.6 28.8 1.1
2006 1117 0.86 0.73 (0.83-0.62) 0.07 0.08 48.7 29.3 31.6 1.5
2007 2602 0.95 0.82 (0.96-0.68) 0.07 0.08 53.0 35.4 31.0 1.4
2008 2309 0.90 0.77 (0.91-0.62) 0.07 0.08 50.6 45.6 29.2 1.2
Mean 2468 0.85 0.72 (0.83-0.60) 0.07 0.08 48.3 41.7 28.4 1.1
(b)
H Lc
50
tc
Year n Z F (95% CI) F
opt
F
limit
(%) J (cm L
T
) (yrs)
2001 19543 1.00 0.81 (1.05-0.56) 0.10 0.12 51.0 31.1 40.4 1.1
2002 5595 0.96 0.77 (1.02-0.51) 0.12 0.13 49.4 30.4 48.4 1.4
2003 3074 1.15 0.96 (1.25-0.66) 0.09 0.11 56.9 44.2 36.6 1.4
2004 1856 1.12 0.93 (1.29-0.58) 0.09 0.11 56.1 53.9 36.3 1.5
2005 3096 0.98 0.79 (1.05-0.54) 0.12 0.13 50.5 23.3 48.3 1.8
2006 866 0.95 0.76 (1.14-0.39) 0.12 0.14 49.2 12.8 50.3 2.5
2007 2658 1.14 0.95 (1.22-0.67) 0.12 0.13 56.5 23.5 48.9 2.0
2008 2361 0.96 0.77 (1.00-0.55) 0.11 0.13 49.7 23.2 47.7 1.8
Mean 4881 1.03 0.84 (1.13-0.56) 0.11 0.12 52.4 30.3 44.6 1.7
(c)
H Lc
50
tc
Year n Z F (95% CI) F
opt
F
limit
(%) J (cm L
F
) (yrs)
2001 5346 0.6639 0.47 (0.57-0.36) 0.160.1834.0 8.1 38.9 1.0
2002 5604 0.6752 0.48 (0.59-0.36) 0.160.1934.6 13.1 39.6 0.7
2003 3194 0.6432 0.44 (0.53-0.36) 0.120.1432.8 16.9 30.1 0.6
2004 1835 0.6819 0.48 (0.60-0.37) 0.110.1435.0 25.1 29.6 1.0
2005 3469 0.6299 0.43 (0.55-0.31) 0.120.1532.0 14.2 32.2 0.7
2006 830 0.6974 0.50 (0.61-0.39) 0.140.1635.9 8.2 35.5 1.5
2007 2753 0.5692 0.37 (0.50-0.24) 0.130.1628.2 6.5 34.6 1.1
2008 2447 0.6583 0.46 (0.58-0.34) 0.140.1733.7 9.6 35.6 1.1
Mean 3185 0.65 0.45 (0.57-0.34) 0.140.1633.3 12.7 34.5 0.9

E. M. GRANDCOURT ET AL. 87
for all years. The results of Mann-Whitney U tests to com-
pare selectivity metrics indicated that there were no sig-
nificant differences in juvenile retention (P = 1.0), mean
size (P = 0.7), mean size at first capture (P =0.12)
and mean age at first capture (P = 0.24) following the
introduction of the mandatory escape panel regulation
(Figure 3). The results of partial correlations to evaluate
the impact of the introduction of the mandatory escape-
panel regulations on trends also indicated that there were
no significant changes in juvenile retention (P = 0.48),
mean size (P = 0.50), mean size at first capture (P = 0.35)
and mean age at first capture (P = 0.32) (Figure 4).
For E. coioides, harvest rates ranged from 49.2%-
56.9% and the level of juvenile retention reached 53.9%
in 2004. Furthermore, the mean size at first capture
(36.3-50.3 cm LT) was below the mean size at first sexual
maturity (42.6 cm LT) for some years (Table 1). The
estimate of the length giving the highest yields (Lopt) of
67.4 cm LT was considerably greater than the mean sizes
at first capture for all years. The results of
Mann-Whitney U tests to compare selectivity metrics
indicated that there were no significant differences in
juvenile retention (P=0.18), mean size (P=0.23) and the
mean size at first capture (P=0.46) following the intro-
0
10
20
30
40
50
60
D. pict umE. coioidesL. nebulosus
Before
After
Juvenile retention (%)
0
10
20
30
40
50
60
D. pict umE. coioidesL. nebulosus
Before
After
Meansize (cm)
(b)
0
10
20
30
40
50
60
D. pict u
m
E. co io id e
s
L. nebulosu
s
Before
After
L
c50
(cm)
(c)
0.0
0.5
1.0
1.5
2.0
D. pict u
m
E. co io id e
s
L. nebulosu
s
Before
After
t
c
(yrs)
(d)
(a )
Figure 3. A comparison of (a) juvenile retention (b) mean size (c) mean size at first capture (L
c50
) and (d) mean age at first
capture (t
c
) for D. pictum, E. coioides and L. nebulosus before (2001-2003) and after (2004-2008) the implementation of juve-
nile escape panel regulations (± SE).

E. M. GRANDCOURT ET AL. 88
0
20
40
60
80
2000 2002 2004 2006 2008
D. pict umE. coio id e sL. nebulosus
Year
Juvenile retention (%)
30
40
50
60
2000 2002 2004 2006 2008
D. pict umE. coio id e sL. nebulosus
Year
Mean size (cm)
(b )
20
30
40
50
60
2000 2002 2004 2006 2008
D. pict umE. coio id e sL. nebulosus
Year
L
c50
(cm)
(c)
0
1
2
3
2000 2002 2004 2006 2008
D. pict umE. c o io id e sL. nebulosus
Year
t
c
(yrs)
(d)
(a)
Figure 4. Trends in selectivity metrics between 2001 and 2008 (a) juvenile retention (b) mean size (c) mean size at first cap-
ture (L
c50
) and (d) mean age at first capture (t
c
) for D. pictum, E. coioides and L. nebulosus in the southern Arabian Gulf.
duction of the mandatory escape panel regulation. There
was, however, a significant increase (P=0.03) in the mean
age at first capture for E. Coioides from 1.3 yrs to 1.9 yrs
following the introduction of the mandatory escape panel
regulation (Figure 3). The results of partial correlations
to evaluate the impact of the introduction of the manda-
tory escape panel regulations on trends indicated that
there were no significant changes in juvenile retention

E. M. GRANDCOURT ET AL.89
(P= 0.35), mean size (P = 0.47), mean size at first cap-
ture (P = 0.38) and mean age at first capture (P = 0.29)
(Figure 4).
For L. nebulosus, harvest rates ranged from 28.2%-
35.9% and the level of juvenile retention reached 25.1%
in 2004. The mean size at first capture (29.6-39.6 cm LF)
was above the mean size at first sexual maturity (27.6 cm
LT) for all years (Table 1). The estimate of the length
giving the highest yield (Lopt) of 41.2 cm LF was consid-
erably greater than the mean sizes at first capture for all
years. The results of Mann-Whitney U tests to compare
selectivity metrics indicated that there were no signifi-
cant differences in juvenile retention (P = 0.88), mean
size (P = 0.30), mean size at first capture (P = 0.30) and
mean age at first capture (P = 0.07) following the intro-
duction of the mandatory escape panel regulation (Fig-
ure 3). The results of partial correlations to evaluate the
impact of the introduction of the mandatory escape panel
regulations on trends indicated that there were no sig-
nificant changes in juvenile retention (P = 0.22), mean
size (P = 0.30), mean size at first capture (P = 0.70) and
mean age at first capture (P = 0.74) (Figure 4).
Comparisons of selectivity curves generated using
pooled size frequency data collected before and after the
introduction of juvenile escape panels with selectivity
curves generated with data collected from an experimen-
tal fishing program, indicated that there was a negligible
change in the selectivity characteristics of traps fitted
with escape panels by comparison with traps having
regular mesh for D. Pictum and E. Coioides (Figure 5a
&b). For L. nebulosus, there was a detectable reduction
in the selectivity characteristics of traps fitted with es-
cape panels (Figure 5c) although this could not be veri-
fied independently as there was insufficient data from the
experimental fishing program for this species.
3.3. Catch and Effort
Whilst reductions in catches occurred for the families
Haemulidae (P = 0.46), Serranidae (P = 0.10) and Leth-
rinidae (P = 0.53) following the introduction of effort
reductions (Figure 6a), none were significant. The mean
annual number of dhow trips reduced from 7,372 to
5,944 following the introduction of effort reductions
(Figure 6b), although the drop was not significant (P =
0.10). The results of partial correlations to evaluate the
impact of effort reductions on trends in catch and effort
indicated that there were no significant changes in catch
trends for the Haemulidae (P = 0.65), Serranidae (P =
0.80) and Lethrinidae (P = 0.80) (Figure 7a). There were
also no significant changes in effort trends in terms of
of the number of dhow trips per year (P=0.71) (Figure
7b).
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1020304050607
Probability of capture
Fork length (cm)
0
Experimental
Before
After
(a)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 102030405060708090
Probability of capture
Total length (cm)
Experimental
Before
After
(b)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1020304050607
Pr obabilit y of c apt ure
Fork length (cm)
0
Before
After
(c)
Figure 5. Selectivity curves for (a) D. pictum (b) E. coioides
and (c) L. nebulosus before (2001-2003) and after (2004-
2008) the implementation of juvenile escape panel regula-
tions. The dashed line ‘Experimental’ indicates independent
selectivity curves established using data collected from an
experimental fishing program. Note there was insufficient
data from the experimental fishing program to be able to
generate a selectivity curve for L. nebulosus.

E. M. GRANDCOURT ET AL. 90
0
500
1000
1500
2000
2500
HaemulidaeSerranidaeLethrinidae
Before
After
Mean annual catch (mt)
0
2000
4000
6000
8000
Before After
Mean annual effort (no.dhow trips)
(b)
(a)
Figure 6. Comparisons of (a) the mean annual catch of the families Haemulidae, Serranidae and Lethrinidae (b) mean annual
effort in terms of the number of dhow trips and (b) mean annual effort in terms of the number of trap sets before (2001-2003)
and after (2004-2008) the implementation of fishing effort reductions.
0
1000
2000
3000
4000
2000 2002 2004 2006 2008
Haemulidae Serranidae Lethrinidae
Year
Totalcatch (mt )
0
2000
4000
6000
8000
10000
2000 2002 2004 2006 2008
Year
Total effor t ( no. dhow trips)
(b)
(a)
Figure 7. Trends in (a) the total catch of the families Haemulidae, Serranidae and Lethrinidae (b) total effort in terms of the
number of dhow trips and (b) total effort in terms of the number of trap sets in the southern Arabian Gulf between 2001 and
2008.

E. M. GRANDCOURT ET AL. 91
3.4. Mortality, Harvest Rates, Yield and
Spawner Biomass Recruit
For D. pictum, the fishing mortality rate estimates
(0.61-0.82) were considerably greater than both the tar-
get (F0.1) and limit (Fmax) biological reference points for
all years (Table 1), indicating that the population is heav-
ily overexploited. Consequently, estimates of the relative
spawner biomass per recruit, which ranged from 3.2%-
6.6%, were particularly low (Table 2). Values of the
alternative target and limit biological reference points of
0.5M (0.07) and 2/3M (0.09) respectfully, closely resem-
bled mean values of F0.1 (0.07) and Fmax (0.09). The re-
sults of Mann-Whitney U tests to compare fishery met-
rics indicated that there were no significant differences in
fishing mortality rates (P = 0.053), YPR (P = 1.00), SBR
(P = 0.70) and harvest rates (P = 0.53) following the
introduction of effort reductions (Figure 8). The results
of partial correlations to evaluate the impact of effort
reductions on trends also indicated that there were no
significant changes in the fishing mortality rate (P =
0.49), YPR (P = 0.65), SBR (P = 0.81) and harvest rate
(P = 0.32) (Figure 9).
For E. coioides, the fishing mortality rate estimates
(0.76-0.98) were considerably greater than the target
(F0.1) (0.09-0.12) and limit (Fmax) (0.11-0.14) biological
reference points for all years (Table 1), indicating that
the population is heavily overexploited. Consequently,
estimates of the relative spawner biomass per recruit,
which ranged from 1.4%-5.9%, were particularly low
(Table 2). Values of the alternative target and limit bio-
logical reference points of 0.5M (0.10) and 2/3M (0.13)
respectfully, closely resembled mean values of F0.1 (0.11)
and Fmax (0.12). The results of Mann-Whitney U tests to
compare fishery metrics indicated that there were no
significant differences in fishing mortality rates (P=0.55),
YPR (P = 0.053), SBR (P = 0.18) and harvest rates
(P=0.66) following the introduction of effort reductions
(Figure 8). The results of partial correlations to evaluate
the impact of effort reductions on trends also indicated
that there were no significant changes in the fishing
mortality rate (P = 0.75), YPR (P = 0.33), SBR (P = 0.56)
and harvest rate (P = 0.95) (Fi gur e 9).
For L. nebulosus, the fishing mortality rate estimates
(0.37-0.50) were considerably greater than the target
(F0.1) (0.11 - 0.16) and limit (Fmax) (0.14 - 0.19) biologi-
Table 2. Relative spawner biomass per recruit (SBR) and yield per recruit (YPR) for D. Pictum, E. Coioides and L. nebulosus
in the southern Arabian Gulf from 2001 to 2008.
Year Species SBR (%) (95% CI) YPR (g)
D. pictum - -
2001 E. coioides 1.8 (4.9-0.7) 890.8
L. nebulosus 6.9 (10.8-4.7) 119.9
D. pictum 6.6 (8.3-5.3) 387.2
2002 E. coioides 2.5 (6.9-1.0) 1059.0
L. nebulosus 5.9 (9.8-3.8) 99.3
D. pictum 4.2 (5.9-3.1) 266.8
2003 E. coioides 1.4 (4.0-0.6) 985.8
L. nebulosus 6.3 (9.6-4.3) 97.1
D. pictum 3.2 (4.6-2.3) 231.4
2004 E. coioides 1.5 (5.5-0.5) 997.3
L. nebulosus 6.4 (10.5-4.2) 115.0
D. pictum 5.4 (7.5-4.0) 350.6
2005 E. coioides 3.2 (7.7-1.5) 1295.6
L. nebulosus 7.1 (12.8-4.3) 111.8
D. pictum 6.5 (8.2-5.3) 438.4
2006 E. coioides 5.9 (8.0-2.5) 1787.0
L. nebulosus 7.8 (11.8-5.5) 152.4
D. pictum 5.0 (6.8-3.9) 379.4
2007 E. coioides 2.4 (5.4-1.1) 1300.2
L. nebulosus 11.0 (20.4-6.7) 166.5
D. pictum 4.6 (6.7-3.4) 336.9
2008 E. coioides 3.7 (7.8-1.9) 1365.6
L. nebulosus 7.6 (12.9-4.9) 134.7

E. M. GRANDCOURT ET AL. 92
0.0
0.2
0.4
0.6
0.8
1.0
D. pictumE. coioidesL. nebulosus
Before
After
Fishingmortality rate (F)
0.0
0.4
0.8
1.2
1.6
D. pic t umE. coioidesL. nebulosus
Before
After
Yield per recruit (kg)
(b)
0
2
4
6
8
10
D. pic t umE. coioidesL. nebulosus
Before
After
Spawner biomass
per recruit (%)
(c)
(a)
0
10
20
30
40
50
60
D. pictu
m
E. c o ioide
s
L. nebulosu
s
Before
After
Harvest rate (%)
(d)
Figure 8. A comparison of (a) fishing mortality rates (b) YPR (c) SBR and (d) harvest rates for D. pictum, E. coioides and L.
nebulosus before (2001-2003) and after (2004-2008) the implementation of juvenile escape panel regulations and effort reduc-
tions (± SE).
cal reference points for all years (Table 1), indicating
that the population is heavily overexploited. Conse-
quently, estimates of the relative spawner biomass per
recruit, which ranged from 5.9%-11.0%, were particu-
larly low (Table 2). Values of the alternative target and
limit biological reference points of 0.5M (0.10) and 2/3M
(0.13) respectfully, were less than the mean values of F0.1
(0.14) and Fmax (0.16). The results of Mann-Whitney U
tests to compare fishery metrics also indicated that there
were no significant differences in fishing mortality rates
(P = 0.76), YPR (P = 0.10), SBR (P = 0.053) and harvest
rates (P = 0.88) following the introduction of effort re-
ductions (Figure 8). The results of partial correlations to
evaluate the impact of effort reductions on trends indi-
cated that there were no significant changes in the fishing
mortality rate (P = 0.65), YPR (P = 0.57), SBR (P = 0.31)
and harvest rate (P = 0.44) (Fig ure 9).
3.5. Age Structure
Analysis of the age structure of D. pictum indicated that
truncation was exacerbated following the introduction of
management measures in 2004. While there were no sig-
nificant differences in the relative proportions of age

E. M. GRANDCOURT ET AL. 93
0.0
0.2
0.4
0.6
0.8
1.0
1.2
2000 2002 2004 2006 2008
D. pict umE. c o io id e sL. nebulosus
Year
Fishing mortality rate (F)
0.0
0.5
1.0
1.5
2.0
2000 2002 2004 2006 2008
D. pict umE. co io id e sL. nebulosus
Year
Yield per recruit (kg)
(b)
Year
(b)
0
2
4
6
8
10
12
2000 2001 2002 2003 2004 2005 2006 2007 2008
D. pict umE. c o ioid e sL. nebulosus
Year
Spawner biomass per recruit (%)
(c)
0
10
20
30
40
50
60
70
80
2000 2002 2004 2006 2008
D. pict umE. c o ioidesL. nebulosus
Year
Harvest rate (%)
(d)
(a)
Figure 9. Trends in (a) fishing mortality rates (b) YPR (c) SBR and (d) harvest rates for D. pic tum , E. coioides and L. nebulo-
sus, between 2001 and 2008 in the southern Arabian Gulf.
classes up to 9 yrs, there was a significant reduction in
the proportion of the oldest (10 to 13 yr) age classes
(Figure 10a). The same pattern was identified for E.
coioides which also had no significant differences in the
relative proportions of age classes up to 9 yrs and a sig-
nificant reduction in the proportion of the oldest (10 and
12 yr) age classes (Fi gure 10b). For L. nebulosus, there
were no significant differences between the 1 yr to 13 yr

E. M. GRANDCOURT ET AL. 94
0.52
0.63
0.85
0.84
0.84
0.83
0.74 0.58 0.13 0.04* 0.01*
0
10
20
30
40
123456789101112
Before
After
Age Class (yrs)
Proportion (%)
(b)
0.87 0.67
0.87
0.65 0.610.560.29 0.200.07 0.01*0.01*0.01*0.01*
0
10
20
30
40
12345678910111213
Before
After
Age Class (yrs)
Proportion (%)
(a)
0.70
0.62 0.52
0.77
0.84
0.66
0.67
0.54
0.490.490.38 0.380.060.01*
0
5
10
15
20
1234567891011121314
Before
After
Age Class (yrs)
Proportion (%)
(c)
Figure 10. Comparison of age structures before (2001-2003) and after (2004-2008) the introduction of management regula-
tions for (a) D. pictum (b) E. coioides and (c) L. nebulosus. Data labels above bars show the results of 
2
goodness of fit tests (P
values) for individual age classes, * indicates a significant difference.
age classes and a significant reduction in the oldest (14
yr) age class following the introduction of management
regulations (Figure 10c).
4. Discussion
The principal conclusion of this study is that gear regula-
tions and effort reductions implemented between 2004
and 2008 have failed to achieve the desired management
targets for the key species in the demersal trap fishery of
the southern Arabian Gulf. With the exception of a minor
increase in the mean age at first capture from 1.3 yrs to
1.9 yrs for E. coioides, there were no significant changes
in the values or trends of any of the selectivity and other
fishery metrics in the 5 yr period following the introduc-
tion of juvenile escape panels and limits on the number

E. M. GRANDCOURT ET AL.95
of traps used per vessel.
The use of selectivity ogives established through an
experimental fishing program, being independent of
those derived from sampled fisheries data were of tre-
mendous utility to the analyses, particularly from a
management perspective. The ogives for E. coioides and
D. pictum showed that there was a negligible change in
the selectivity characteristics of traps fitted with escape
panels by comparison with traps having a regular mesh
and no panel. This independently corroborates the results
of comparisons of selectivity parameters before and after
the introduction of the gear regulations and clearly indi-
cates that the failure of escape panels to modify the se-
lectivity characteristics of the fishery cannot be attributed
to a lack of compliance by fishers.
The large differences between estimates of the sizes at
first capture with those which would give the highest
yields, emphasizes the need to revise existing gear regu-
lations for the trap fishery. This is of critical importance
to a stock rebuilding strategy, particularly given the det-
rimental impact that the depletion of the older age classes
can have on reproductive capacity and output, as has
been demonstrated for E. coioides [14]. However, be-
cause of the differences among species, it is not possible
to have a gear regulation that would select all species at
the optimum sizes and ages at first capture. For this rea-
son, alternative management regulations also need to be
considered.
The utility of the catch and effort data was limited as
there were no reliable estimates of the total number of
traps used before and after the introduction of effort re-
strictions. Furthermore, species specific catch data across
the entire time series and catches made by vessels from
other emirates were not available. Nevertheless, as there
were no significant changes in the values or trends in
catch and effort or any of the other fishery metrics (F,
YPR, SBR, H) following the introduction of effort con-
straints, this management regulation has also clearly
failed to achieve any progress in terms of stock rebuild-
ing.
A more detailed understanding of recruitment would
have helped to elucidate the variability in relative spaw-
ner biomass per recruit over the study period. An associ-
ated limitation of the YPR analyses is the assumption
that there is no relationship between the size of the
spawning stock biomass and subsequent recruitment over
the range of fishing mortality rates used [17]. This is
particularly restrictive for small, fast growing tropical
species with high rates of natural mortality, for which
predictions suggest that high yields per recruit may be
achieved at levels of exploitation where the remaining
spawning stock biomass may not be capable of sustain-
ing recruitment [11]. As values of the target and limit
biological reference points (F0.1 and Fmax) approximated
the precautionary alternatives (0.5M and 2/3M) for D.
pictum and E. coioides, the failure of the model was not
apparent for these species. Conversely, the target and
limit biological reference points (F0.1 and Fmax) for L.
nebulosus were greater than the precautionary alterna-
tives (0.5M and 2/3M) suggesting that the YPR analyses
may have over-estimated sustainable fishing mortality
rates for this species.
Other potential sources of error include the use of a
single age-length key and the associated assumption that
there is no significant inter-annual variability in growth.
This would be particularly important in the developmen-
tal phases of a new fishery where density dependant ef-
fects may occur. Given that the existing fishery is well
established, this assumption may have been less of a
limitation to the analyses than the absence of fish in
samples that were at the extremes of the size and age
ranges.
Whilst increments in the sagittal otolith sections of the
study species have been shown to be formed annually
[3,7], it is important to distinguish between the validation
of increment periodicity and absolute age [18]. As the
absolute age of the study species was not validated, im-
provements in the age data and associated analyses could
have been achieved using an independent validation
means such as a mark recapture study using chemical
marking of juvenile fish with a known age.
The maximum age estimate of D. pictum (13 yrs) was
considerably less than the maximum age of 31 yrs esti-
mated by [19] for this species in New Caledonia. Like-
wise, our longevity estimate of 14 yrs for L. nebulosus
was less than those of [20] (20 yrs) and (21 yrs) [21] for
this species in the northern Arabian Gulf and Gulf of
Aden respectively. Additionally, the maximum age of E.
coioides here (12 yrs) was considerably less than the
maximum age of 22 yrs [20]. The large discrepancy be-
tween the maximum observed ages in our study by com-
parison with those recorded elsewhere can be attributed
to the truncation of the age structures through intensive
fishing. Whilst size specific selectivity cannot be dis-
carded as a possible explanation for the small proportion
of larger and older fish in size frequency and biological
samples, the impact of fishing on the size and age struc-
ture of the respective populations is considered a more
likely explanation, particularly given the historic absence
of regulation in the fishery. The analyses of age struc-
tures of the key species indicated that truncation was
exacerbated following the introduction of management
measures in 2004, further highlighting the failure of ex-
isting regulations to rebuild stocks. Whilst sample sizes
of the oldest age classes were small, significant reduc-
tions were observed for all key species following the

E. M. GRANDCOURT ET AL. 96
introduction of regulations, in contrast to the intended
impact.
YPR and SBR analyses indicated that the study spe-
cies were all heavily over-exploited. As the existing fi-
shing mortality rates were all well in excess of the limit
biological reference points, growth over-fishing had
clearly occurred. Furthermore, with current values of the
relative spawner biomass per recruit being less than 10%
in all cases, recruitment over-fishing would also have
occurred, based on meta-analyses [22]. The critical ma-
nagement issues that relate to the stock status established
in earlier assessments [3,7] are clearly still pertinent.
There is widespread recognition that target species are
over-exploited in places where wire-mesh fish traps have
been used extensively [23,24]. Given the failure of ex-
isting regulations to modify gear selectivity, reduce ef-
fort and rebuild stocks, management authorities should
consider alternative measures including a moratorium on
the use of traps in the off-shore demersal fishery of Abu
Dhabi.
5. Acknowledgements
This study forms part of the activities of the ‘Fish Land-
ings and Population Dynamics Project’ (project # 02-
23-0008-09) which is implemented by the Biodiversity
Management Sector―Marine, of the Environment Agen-
cy―Abu Dhabi. The management of the Environment
Agency―Abu Dhabi is thanked for their support for this
work. The Abu Dhabi Fishermen’s Cooperative Society
facilitated size frequency sampling and catch data collec-
tion. The UAE Coast Guard and later CNIA (Critical
National Infrastructure Authority) provided effort re-
cords. All the EAD enumerators are thanked for their
contribution to data collection; Khalid Al Ali, Sultan Al
Ali, Hamad Al Shamsi, Mohamed Al Zaabi, Sabah Has-
san, Jihad Hassan, Mohamed Ahmed Hassan, Abdulla Al
Blooki, Abdulla Muhyiddeen, Eid Al Romaithi, Yaqoob
Yousif Al Hammadi.
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