SCRS/2014/108

SCRS/2014/108
Collect. Vol. Sci. Pap. ICCAT, 71(5): 2043-2053 (2015)
UPDATED CATCH RATES OF SWORDFISH (XIPHIAS GLADIUS) CAUGHT BY
MOROCCAN DRIFTNET FISHERY IN THE STRAIT OF GIBRALTAR, 1999-2011
N. Abid1, A. Arakrak2, A. Laglaoui2 & M. Bakkali2
SUMMARY
The catch rates from the Moroccan driftnet fleet targeting swordfish in the Strait of Gibraltar,
from 1999 to 2011, were analyzed using the General Linear Modelling approach (GLM), under
log-normal error assumption in order to compute standardized abundance indices. The relative
abundance index didn’t show any clear trend. In general the index showed a relative stable
trend over the considered time series. The factors year, month and vessel size explained most of
the variability observed in the abundance index.
RÉSUMÉ
Les taux de capture de la flottille marocaine de filets dérivants ciblant l’espadon dans le détroit
de Gibraltar entre 1999 et 2011 ont été analysés au moyen de l’approche de modélisation
linéaire généralisée (GLM), en postulant une erreur log-normale afin de calculer les indices
d’abondance standardisés. L'indice de l'abondance relative n'a fait apparaître aucune tendance
claire. En règle générale, l'indice a dégagé une tendance relativement stable pendant toute la
série temporelle considérée. Les facteurs année, mois et taille du navire ont expliqué la plupart
de la variabilité observée dans l'indice d’abondance.
RESUMEN
Las tasas de captura a partir de la flota de redes de deriva marroquí que se dirige al pez
espada en el estrecho de Gibraltar, desde 1999 a 2011, se analizaron utilizando un enfoque de
modelación lineal generalizado (GLM), con un supuesto de error lognormal con el fin de
calcular índices de abundancia estandarizados. El índice de abundancia relativa no mostraba
ninguna tendencia clara. En general, el índice mostraba una tendencia relativamente estable a
lo largo de la serie temporal considerada. Los factores año, mes y tamaño del buque
explicaban la mayoría de la variabilidad observada en el índice de abundancia.
KEYWORDS
Swordfish catch rates, Strait of Gibraltar, General linear modelling (GLM)
1. Introduction
During the period 1990-2011, the Moroccan driftnet fishery was one of the most important fisheries exploiting
the Mediterranean swordfish. The vessels were operating in the Strait of Gibraltar, in the area comprised
between the longitudes 05° and 06°W (Figure 1).
With an annual average catch of about 2850 TM, Morocco came in the second place among the producing
countries of this species in the Mediterranean Sea after Italy. The fleet targeted swordfish from April to June
when fish is undertaking a genetic migration to the Mediterranean Sea and during its feeding migration from
the Mediterranean to the Atlantic, during July-September (El Hannach, 1987, Abid, 1998, Srour and Abid, 2004;
Abid and Idrissi, 2007).
1
2
National Institute for Fisheries Research (INRH), regional centre of Tangier. P.O. Box 5268, Tangier, Morocco
Faculté des Sciences et Techniques de Tanger (FST). B.P. 416.Tanger, Maroc
2043
During the 90s, the Moroccan swordfish catches taken by driftnet were the most important and represented about
60% on average of the total catches of this species at the national level. Nevertheless, since 2008, swordfish
catches taken by this gear have steadily decreased because of the implementation of the National plan for
banning gradually driftnet due to ICCAT recommendation (Rec. 03-04). In 2012, this gear was totally eliminated
from the Moroccan waters.
The swordfish stocks are assessed at regional level by the International Commission for the Conservation of
Atlantic tunas (ICCAT) mainly by means of Analytical and production models which make use of relative
abundance indices from the major fisheries targeting this species (Anon., 2013).
The scientific monitoring of the Moroccan driftnet fishery operating in the Strait of Gibraltar was of a great
importance at the regional level, especially in terms of updating and analyzing the trend of the relative
abundance index. This relative abundance index has been used by the ICCAT scientific committee for the
Mediterranean swordfish stock assessment in 2007 and 2010 (Anon., 2008; 2011).
The standardization of catches rate of the Mediterranean swordfish by eliminating the effect of the external
factors other than the stock abundance is required for the stock assessment purposes (Anon., 2013). The aim of
this document is to analyze the effect of some factors including the moon phases on the catch rates of swordfish
caught by this fishery in order to compute a standardized index to be used in the 2014 stock assessment.
2. Materials and Methods
2.1 Description of data source
The catch (in weight) and effort data per trip related to the Moroccan driftnet fleet targeting swordfish in the
Strait of Gibraltar during the period 1999-2011, were collected from the commercial fishing statistics recorded in
the fish market at the port of Tangier.
The data set is structured as follow: date of landing, vessel name, GRT, moon phase, catch in weight, estimated
length of net. A total of 35809 daily trips were available for analysis.
2.2 Size/age range of fish
The abundance index is applied to fish whose size ranged from 100 to 240 cm LJ-FL, with an average size of
145cm LJ-FL. These sizes correspond to fish aged 2-9 years (Figure 2).
2.3 Management regulation
The Moroccan driftnet fishery has known a noticeable development since the early 90s in terms of fishing
capacity and the volume of swordfish catches. It recorded a peak of about 5000 TM in 1997. Nevertheless, with
the implementation of the national plan for banning driftnet since 2010, the fishing effort as well as the catches
displayed a downtrend to reach a minimum in 2012 when the driftnet was totally prohibited. Since then the
catches have been made only with the surface longline especially during the fall and the winter time.
In order to take into account the effect of the recent management regulation on the catch rates, only the data
related to driftnet fishing operations from 1999 to 2011 were included in the analysis.
2.4 Model standardization
As regard the unit fishing effort, as recommended by the SCRS, it is defined as the total length of net set by a
vessel. The fishermen spend one fishing day at sea during which one fishing operation is carried out. The fishing
effort per vessel and per trip is estimated using the following formula:
Unit fishing effort = 1.171xGRT^0.416
As it was previously demonstrated there is a strong correlation between the total length of net used by a vessel
and its Gross Registered Tonnage (GRT), (r2=0.80) (Abid and Idrissi, 2007). The bigger the vessel is, the larger
the net used is.
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All the daily trips catch rate data were analysed by means of the General Linear Modelling analysis (Gavaris,
1980; 1988) to analyse the effect of the season, the vessel size(GRT), the moon phases as well as the interactions
between these factors on the catch rate and compute an annual standardized abundance index under a lognormal
error distribution.
As in the previous analyses, we have considered 5 levels for the temporal factor (April-August) which
correspond to the main fishing season of the swordfish in the Strait of Gibraltar and 3 levels for the vessel size
factor, which are displayed below:
Vessel size levels
1
2
3
GRT
GRT< 20
20≤GRT<40
>=40
Regarding the moon phases, we aggregated them into two phases: Dark and light. The dark phase includes the
new moon, the first and the last crescent and the light phase comprises the first and the last quarter, the full
moon, the waxing and waning moon (De la Serna et al., 1992, Di Natale, 1995, Dos Santos, 2005).
The step AIC analysis was performed to select the statistically significant factors in the final model, which is
expressed as follow:
Lg CPUE w= U + Y i + M j + S k + Y i* M j+ Y i* S k+ M j* S k +e ijk
Lg : natural logarithm
CPUE w : catch rate in weight
U : intercept
Y i : year effect
M j : month effect
S k : vessel size effect
Y i* M j : interaction year and month effect
Y i* S k : interaction year and vessel size effect
M j* S k : interaction month and vessel size effect
e ijk : error
3. Results and Discussions
The number of observations (trips) by year, month and vessel size combination levels is presented in the
Table 1. In general, the number of trips analysed by month and by vessel size categories seems to be
satisfactory.
Figures 3, 4, 5 and 6 display the distribution of Log CPUEw by year, month, vessel size and moon phases. The
nominal CPUEs show a relative stable trend during the analysed time series. The highest value of CPUE is
recorded during the month of May of each year, which corresponds to the maximum passage of swordfish
through the Strait of Gibraltar. The biggest vessels seem to have the highest catch rates.
Table 2 shows the results of the deviance analysis. All the factors considered in the analysis: year, month, vessel
size and the moon phases as well as the interactions between these factors are statistically significant at 0.5%
level. The selected model explains about 14% of the total deviance. The factors year, month and vessel- size
contribute with 14%, 33% and 16% of the total deviance, respectively.
The effect of the moon phases on catch rates is statistically significant. This result confirms those found by De la
Serna et al. (1992), Di Natale (1995) and Dos Santos (2005). A slight higher CPUES are recorded during the
dark when the net is not well seen by the fish. But the variability explained by this factor in the abundance index
remained insignificant (0.4%).
The selected model fit well the observed data as the residuals distribution follows a normal pattern (Figure 7).
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The nominal CPUE, the standardized CPUEs, with their corresponding lower and upper confidence interval
(95%), the coefficient of variation (CV) are presented in the Table 3. The trend of the annual relative abundance
index is illustrated by the Figure 8.
The swordfish relative abundance in the Strait of Gibraltar didn’t show any clear trend over the period 19992011, but in general it remained relatively stable over the same period.
References
Abid, N.1998. Contribution à l’étude de la pêcherie marocaine de l’espadon dans le détroit de Gibraltar. 217
Mémoire de troisième cycle pour l’obtention du diplôme d’ingénieur d’État, spécialité : Halieutique. I.A.V,
218 Hassan II. Rabat 92 p.
Abid, N and. Idrissi, M. 2007. Situation récente de la pêcherie marocaine de l’espadon (Xiphias gladius).
Période: 1996-2005. Col. Vol. Sci. Pap. ICCAT, 60(6): 2018-2028.
Abid, N and Idrissi, M. 2008. Standardized catch rates of swordfish (Xiphias gladius) from the Moroccan
driftnet fishery operating in the Mediterranean Sea during the period 1998-2006. Col. Vol. Sci. Pap.
ICCAT, 62(4): 1107-1111.
Abid, N and Idrissi, M. 2009. Analysis of the size data of swordfish (Xiphias gladius) caught by the Moroccan
driftnet fishery operating in the Mediterranean Sea. Period 1999-2006. Col. Vol. Sci. Pap. ICCAT, 64(6):
2093-2104.
Abid, N. and Idrissi, M. 2011. Standardized catch rates of swordfish (Xiphias gladius) caught by the Moroccan
driftnet fleet in the Mediterranean Sea. Period 1999-2009. Col. Vol. Sci. Pap. ICCAT, 66(4): 1480-1488
Anon. 2008. Report of the 2007 Mediterranean swordfish stock assessment session. Col. Vol. Sci. Pap.
ICCAT, 62(4): 951-1038.
Anon. 2011. Report of the 2010 ICCAT Mediterranean swordfish stock assessment meeting. Col. Vol. Sci.
Pap. ICCAT, 66(4): 1405-1470.
Anon. 2013. Report of the 2012 meeting of the ICCAT working group on stock assessment methods. Col.
Vol. Sci. Pap. ICCAT, 69(3): 1354-1426.
de la Serna, J.M, Alot, E and Rivera, E. 1992. Análisis de las CPUES por grupos de tallas del pez espada
(Xiphias gladius) capturado con artes de superficie y enmalle a la deriva en el área del estrecho de Gibraltar
durante los años 1989 y 1990. Relacion con la fase lunar y otros factores ambientales. Col.Vol.Sci.Pap.
ICCAT, 39(2): 626-634.
Di Natale, A. and Mangano, A. 1995. Moon phases influence on CPUE: a first analysis of swordfish driftnet
catch data from the Italian fleet between 1990 and 1991. Col. Vol. Sci. Pap. ICCAT, 44(1): 264-267.
Dos Santos, M. N. and Garcia, A. 2005. The influence of the moon phase on the CPUEs for the Portuguese
swordfish (Xiphias gladius L. 1758) fishery. Col. Vol. Sci. Pap. ICCAT, 58(4): 1466-1469.
El Hannach, A. 1987. Données biologiques et écologiques sur l’espadon (Xiphias gladius) L.1758 à partir de la
pêcherie marocaine dans le détroit de Gibraltar. Thèse de Doctorat, spécialité halieutique. ENSA, Rennes,
France. 162 p.
Gavaris, S. 1980, Use of multiplicative model to estimate catch rate and effort from commercial data. Can.J.Fish.
Aquat.Sci.37: 2272-2275.
Gavaris, S. 1988, Abundance indices from commercial fishing. Collected papers on stock assessment methods.
CAFSAC Res. Doc.88/61. 167 pp.
Srour, A. and Abid, N. 2004, Situation de la pêcherie de l’espadon (Xiphias gladius) des côtes marocaines.
Col. Vol. Sci. Pap. ICCAT, 56(3): 898-903.
2046
Table 1. Number of observations by factors levels combination.
1999
Size vessel/month
4
5
6
7
8
Total
1
2
3
67
12
1
123
26
10
78
18
8
61
18
6
112
20
10
Total
80
159
104
85
142
441
94
35
570
2000
Size vessel/month
4
5
6
7
8
Total
1
2
3
35
36
11
54
35
3
45
16
4
45
13
6
21
4
1
Total
82
92
65
64
26
200
104
25
329
2001
Size vessel/month
4
5
6
7
8
Total
1
2
3
489
139
51
1026
279
72
306
139
57
151
90
32
530
212
57
Total
679
1377
502
273
799
2502
859
269
3630
4
5
6
7
8
Total
1
2
3
342
150
53
1006
367
113
666
250
85
158
72
17
787
350
97
Total
545
1486
1001
247
1234
2959
1189
365
4513
2002
Size vessel/month
2003
Size vessel/month
4
5
6
7
8
Total
1
2
3
437
256
81
724
325
93
307
184
54
200
103
27
292
137
25
Total
774
1142
545
330
454
1960
1005
280
3245
Size vessel/month
4
5
6
7
8
Total
1
2
3
334
211
76
839
384
141
297
152
56
62
22
2
425
250
83
Total
621
1364
505
86
758
1957
1019
358
3334
2004
2047
2005
Size vessel/month
4
5
6
7
8
Total
1
2
3
160
206
62
632
400
136
139
115
57
88
28
10
231
174
53
Total
428
1168
311
126
458
1250
923
318
2491
2006
Size vessel/month
4
5
6
7
8
Total
1
2
3
468
280
105
583
354
147
162
172
88
105
63
23
364
227
89
Total
853
1084
422
191
680
1682
1096
452
3230
2007
Size vessel/month
4
5
6
7
8
Total
1
2
3
241
248
116
730
392
153
154
153
64
16
15
342
339
126
Total
605
1275
371
31
807
1483
1147
459
3089
2008
Size vessel/month
4
5
6
7
8
Total
1
2
3
221
224
96
663
477
248
142
145
84
106
146
33
260
393
138
Total
541
1388
371
285
791
1392
1385
599
3376
4
5
6
7
8
Total
1
2
3
250
382
190
449
417
163
201
200
96
98
89
25
156
236
73
Total
822
1029
497
212
465
1154
1324
547
3025
4
5
6
7
8
Total
1
2
3
140
257
90
462
451
177
198
287
115
67
68
14
129
212
52
Total
487
1090
600
149
393
996
1275
448
2719
2009
Size vessel/month
2010
Size vessel/month
2011
Size vessel/month
4
5
6
7
8
Total
1
2
3
132
200
82
209
304
136
88
155
78
86
181
55
140
298
114
Total
414
649
321
322
552
655
1138
465
2258
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Table 2. Analysis of deviance results.
Factors
df
Null
Residual
deviance
Change in
deviance
F
Pr(>F)
% in the
total
deviance
27147.7
Year
12
26680.3
467.4
58.6395
2.2e-16***
13.585
Month
4
25590.5
1089.8
410.1616
2.2e-16***
31.676
Vess.size
2
25018.7
571.9
430.4733
2.2e-16***
16.623
Moon phases
1
25006.2
12.5
18.8136
1.445e-05 ***
0.363
Year*Month
48
24445.1
561.0
17.5965
2.2e-16***
16.306
Year*Vess.size
24
24244.4
200.7
12.5911
2.2e-16***
5.833
Year.Moon phases
12
24186.6
57.8
7.2467
1.990e-13***
1.680
Month*Vess.size
8
23726.0
460.7
86.6913
2.2e-16 ***
13.390
Month*Moon phases
4
23712.4
13.5
5.0953
0.0004209***
0.392
Vess.size*Moon
phases
2
23707.2
5.2
3.9073
0.0201029 *
0.151
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Table 3. Least square means, standard errors, standardized abundance indices with corresponding 95% upper
and lower confidence limits and coefficient of variation. Scaled standardized CPUE to the average of the time
series.
Year
Nominal
CPUE
Std.CPUE
Upper
Lower
CV
1999
41.395
58.256
102.013
33.268
7.5%
2000
52.870
66.671
77.241
57.547
1.8%
2001
39.692
43.149
48.919
38.059
1.8%
2002
39.742
56.034
65.954
47.606
2.1%
2003
37.852
48.181
55.014
42.197
1.8%
2004
45.280
58.411
67.898
50.250
2.0%
2005
42.720
70.678
80.264
62.237
1.6%
2006
47.051
66.164
74.536
58.731
1.5%
2007
46.095
54.056
54.056
47.806
47.806
50.542
50.542
37.734
37.734
63.163
70.715
56.418
1.4%
69.178
79.043
60.545
1.7%
55.582
64.302
48.045
1.9%
51.887
58.854
45.746
1.7%
46.505
66.888
32.333
5.1%
2008
2009
2010
2011
2049
Figure 1. Fishing grounds (shaded area) of the Moroccan driftnet fleet targeting swordfish in the Strait of
Gibraltar.
Figure 2. Monthly size distribution of swordfish landed at the port of Tangier (Strait of Gibraltar),
2006-2011.
2050
Figure 3. Box plots showing the distribution of LogCPUEw by year.
Figure 4. Box plots showing the distribution of LogCPUEw by month.
2051
Figure 5. Box plots showing the distribution of LogCPUEw by vessel size.
Figure 6. Box plots showing the distribution of LogCPUEw by moon phases.
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Figure 7. Diagnosis plots: residuals vs fitted positive catch distribution and normal qq plots.
120
100
Std.CPUE
80
60
40
20
Std.CPUEw
UpperCPUE
LowerCPUE
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
0
Observed
Figure 8. Standardized abundance index, with its corresponding upper and lower confidence limits,
1999-2011.
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