SCRS/2016/119 Collect. Vol. Sci. Pap. ICCAT, 73(3): 1115-1132 (2017)

1115

UPDATED CATCH RATES OF SWORDFISH (XIPHIAS GLADIUS) CAUGHT BY

THE MOROCCAN FLEET IN THE STRAIT OF GIBRALTAR, 1999-2015

Noureddine Abid1, Abdelouahed Ben Mhamed2 & Mohammed Malouli Idrissi2

SUMMARY

The catch rates from the Moroccan fleet targeting swordfish in the Strait of Gibraltar, from

1999 to 2015, were analyzed using the General Linear Modelling approach (GLM), under

lognormal error assumption in order to compute standardized abundance indices. The relative

abundance index showed a relative stable trend over the period 1999-2011, but the index

showed an increase in 2012 to remain stable thereafter.

RÉSUMÉ

Les taux de capture de la flottille marocaine ciblant l’espadon dans le détroit de Gibraltar

entre 1999 et 2015 ont été analysés au moyen de l’approche de modélisation linéaire

généralisée (GLM), en postulant une erreur lognormale afin de calculer les indices

d’abondance standardisés. L'indice d'abondance relative présentait une tendance relativement

stable entre 1999 et 2011, mais l’indice affichait une augmentation en 2012 avant de rester

stable par la suite.

RESUMEN

Las tasas de captura de la flota marroquí que se dirigió al pez espada en el estrecho de

Gibraltar, desde 1999 a 2015, 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 presentaba una tendencia

relativa estable durante el periodo 1999-2011, pero el índice mostraba un aumento en el año

2012 y permaneció estable a partir de entonces.

KEYWORDS

Swordfish catch rates, Strait of Gibraltar, General linear modelling (GLM)

1 INRH, regional centre of Tangier, noureddine.abid65@gmail.com 2 INRH Casablanca

1116

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 mainly operating in the Strait of Gibraltar.

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, 2007a).

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, fishery department banned

driftnet 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 (Anonymous, 2013).

The scientific monitoring of the Moroccan swordfish 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. The ICCAT scientific committee has used this index for the assessment of the Mediterranean

swordfish stock in 2007, 2010 and 2014 (Anonymous, 2008; 2011; 2015).

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 (Anonymous, 2013). The

aim of this document is to update the relative annual abundance index to be taken into consideration in the 2016

stock assessment.

2. Material and Methods

2.1 Description of data source

The catch (in weight) and effort data per trip related to the Moroccan driftnet fleet for the period 1999-2011 and

to longliners targeting swordfish in the Strait of Gibraltar during the period 2001-2015, were collected from the

commercial fishing statistics recorded in the fish market at the port of Tangier.

The structure of the dataset is as follow: date of landing, vessel name, GRT, catch in weight, estimated length of

net, estimated number of hooks. A total of 37144 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 1).

2.3 Management regulation

The Moroccan driftnet fishery has known a noticeable development since the early 90s in terms of fishing effort

and the volume of swordfish catches. It recorded a peak of about 5000 TM in 1997. Nevertheless, with the

driftnet ban in 2012, the fishing effort as well as the catches displayed a downtrend during the period 2012-2015

(average catch of 800 TM) where the catches have been made only by longliners.

In order to take into account the effect of the recent management regulation on the catch rates, the CPUE data

related to driftnet for the period 1999- 2011 were included in the analysis, with the gear effect included in the

GLM model.

1117

2.4 Model standardization

As recommended by the SCRS, the fishing effort for drift netters was defined as the total length of net set by a

given 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 was estimated using the following formula:

Unit fishing effort = 1.171xGRT^0.416

Previous analyses showed that 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).

For longliners, the length of each trip is on average 5 days. During one fishing operation, about 1000 hooks were

set.

As to longliners, the nominal CPUE was defined as the total weight of swordfish in kg/1000 hooks, whereas for

drift netters, the nominal CPUE was calculated as the total weight in kg/1000m of net.

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 month, vessel Gross Registered tonnage (GRT), on the catch rate and

compute an annual standardized abundance index under a lognormal error distribution.

As in the previous analyses, we considered 5 levels for the factor month: April, May, June, July and August for

drift netters and 4 levels for longliners: January, February, November and December.

As to GRT, we considered the six (6) levels below:

Vessel size levels GRT

1 GRT< 10

2 10≤GRT<20

3 20≤GRT<30

4 30≤GRT<40

5 40≤GRT<50

6 GRT> 50

The step AIC analysis was performed to select the statistically significant factors in the final model.

For drift netters, the final model is as follow:

Log CPUE w= U + Y i + M j + S k + Y i: M j + Y i : S k + M j : S k + e ijk

For longliners, the final model is as follow:

Log CPUE w= U + Y i + M j + S k + e ijk

Log : natural logarithm

CPUE w : catch rate in weight

U : intercept

Y i : year effect

M j : month effect

S k : vessel GRT effect

Y i: M j : interaction between year and month

Y i : S k : interaction between year and vessel GRT

M j : S k: interaction between month and vessel GRT

e ijk : error

1118

3. Results and Discussions

The numbers of observations (trips) by year, month and vessel GRT class combination levels for drift netters and

longliners are summarised in the Table 1A and 1B. In general, the number of trips analysed by level

combination seems to be satisfactory.

Figures 2, 3 and 4 display the distribution of Log CPUEw by year, month, vessel GRT class and gear.

Differences in CPUE can be observed among years, months, vessel GRT classes and Gear types.

Tables 2A and 2B shows the results of the deviance analysis. All the factors considered in the analysis: year,

month, vessel GRT class and gears are statistically significant at 0.5% level. The selected model explains about

9% of the total deviance. The factors month, vessel- GRT class and year contribute with 45%, 32% and 22% of

the total deviance, respectively. The factor gear explained only 0.25% of the variability in CPUE.

The selected models fit well the observed data as the residuals distribution follows a normal pattern (Figures 5A

and 5B).

The nominal CPUE, the standardized CPUEs, with their corresponding lower and upper confidence interval

(95%), the coefficient of variation (CV) by fishing gears are presented in the Tables 3A and 3B. The trend of

the annual standardized index is illustrated by the Figures 6A and 6B.

1119

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. 2007a. Situation récente de la pêcherie marocaine de l’espadon (Xiphias gladius).

Période: 1996-2005. Collect. 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. Collect. Vol. Sci. Pap.

ICCAT, 61(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. Collect. 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. Collect. Vol. Sci. Pap. ICCAT, 66(4): 1480-1488.

Anon. 2008. Report of the 2007 ICCAT Mediterranean swordfish stock assessment session. Collect. Vol. Sci.

Pap. ICCAT, 62(4): 951-1038.

Anon. 2011. Report of the 2010 ICCAT Mediterranean swordfish stock assessment meeting. Collect. Vol. Sci.

Pap. ICCAT, 66(4): 1405-1470.

Anon. 2013. Report of the 2012 meeting of the ICCAT working group on stock assessment methods. Collect.

Vol. Sci. Pap. ICCAT, 69(3): 1354-1426.

Anon. 2015. Report of the 2014 ICCAT Mediterranean Swordfish Stock Assessment meeting (Heraklion,

Greece, 21-25 July 2014). Collect. Vol. Sci. Pap. ICCAT, 71(5): 1870-1879.

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.

Collect. Vol. Sci. Pap. ICCAT, 56(3): 898-903.

1120

Table 1A. Number of observations by factors levels combination for driftnet (1999-2011).

Month/GRT class 1 2 3 4 5 6 Total

1999 280 161 70 24 8 27 570

4 47 20 10 2 1

80

5 74 49 18 8 3 7 159

6 48 30 14 4 2 6 104

7 36 25 12 6

6 85

8 75 37 16 4 2 8 142

2000 88 112 68 36 10 15 329

4 3 32 22 14 5 6 82

5 15 39 25 10 2 1 92

6 27 18 10 6 2 2 65

7 29 16 8 5 1 5 64

8 14 7 3 1

1 26

2001 1355 1147 584 275 84 185 3630

4 281 208 101 38 11 40 679

5 618 408 200 79 23 49 1377

6 111 195 85 54 21 36 502

7 55 96 57 33 9 23 273

8 290 240 141 71 20 37 799

2002 1604 1355 775 414 124 241 4513

4 174 168 99 51 14 39 545

5 509 497 246 121 32 81 1486

6 384 282 166 84 26 59 1001

7 96 62 46 26 7 10 247

8 441 346 218 132 45 52 1234

2003 1002 958 645 360 108 172 3245

4 218 219 147 109 28 53 774

5 374 350 204 121 30 63 1142

6 146 161 111 73 20 34 545

7 98 102 75 28 16 11 330

8 166 126 108 29 14 11 454

2004 964 993 638 381 209 149 3334

4 173 161 139 72 49 27 621

5 405 434 239 145 81 60 1364

6 139 158 96 56 28 28 505

7 30 32 15 7 1 1 86

8 217 208 149 101 50 33 758

2005 475 775 578 345 172 146 2491

4 41 119 125 81 33 29 428

5 248 384 262 138 71 65 1168

6 52 87 65 50 30 27 311

7 33 55 19 9 7 3 126

8 101 130 107 67 31 22 458

2006 608 1074 688 408 221 231 3230

1121

4 163 305 168 112 44 61 853

5 205 378 215 139 63 84 1084

6 42 120 102 70 43 45 422

7 43 62 47 16 17 6 191

8 155 209 156 71 54 35 680

2007 514 969 737 410 200 259 3089

4 53 188 153 95 47 69 605

5 274 456 268 124 69 84 1275

6 47 107 97 56 29 35 371

7 5 11 14 1

31

8 135 207 205 134 55 71 807

2008 467 925 821 564 272 327 3376

4 65 156 135 89 33 63 541

5 192 471 285 192 105 143 1388

6 54 88 85 60 36 48 371

7 47 59 92 54 21 12 285

8 109 151 224 169 77 61 791

2009 347 807 819 505 261 286 3025

4 62 188 214 168 98 92 822

5 141 308 254 163 63 100 1029

6 63 138 123 77 47 49 497

7 39 59 69 20 23 2 212

8 42 114 159 77 30 43 465

2010 252 744 836 439 188 260 2719

4 26 114 158 99 43 47 487

5 115 347 291 160 71 106 1090

6 48 150 186 101 47 68 600

7 24 43 48 20 4 10 149

8 39 90 153 59 23 29 393

2011 35 620 700 438 225 240 2258

4 7 125 126 74 35 47 414

5 14 195 196 108 72 64 649

6

88 86 69 37 41 321

7 10 76 115 66 25 30 322

8 4 136 177 121 56 58 552

Total 7991 10640 7959 4599 2082 2538 35809

1122

Table 1B. Number of observations by factors levels combination for longliners (2001-2015).

Month/GRT class 1 2 3 4 5 6 Total

2001 1 29 12 4 3 24 73

1

5

8 13

2 1 14 9 1 1 9 35

11

7 3 3 2 5 20

12

3

2 5

2002 1 26 35 16 4 18 100

1

3

1

3 7

2 1 16 16 5 2 8 48

11

7 16 2 1 4 30

12

3 8 1 3 15

2003

11 29 22 1 15 78

1

1 3 4

4 12

2

3 10 3

2 18

11

4 9 4

4 21

12

3 7 11 1 5 27

2004

22 19 20 13 19 93

1

6 4 4

5 19

2

1 2 2

1 6

11

7 8 6 7 6 34

12

8 5 8 6 7 34

2005

8 21 29 6 23 87

1

6 11 9 2 9 37

2

2 3 3 1 2 11

11

1 6 1 5 13

12

6 11 2 7 26

2006

8 51 25 11 19 114

1

7 4 2 1 14

2

2 7

2 11

11

4 27 6 5 10 52

12

4 15 8 4 6 37

2007 1 4 26 21 20 40 112

1

2 10 6 3 7 28

2

2 1

3 6

11 1 2 7 4 8 9 31

12

7 10 9 21 47

2008

32 45 30 36 143

1

6 8 10 19 43

2

7 3

1 11

11

16 22 14 13 65

12

3 12 6 3 24

2009 1 13 52 59 24 29 178

1

1 3 2 2 8

2

1 3 1 5

1123

11

1 3 3 2 1 10

12 1 12 48 52 17 25 155

2010

14 53 36 21 23 147

1

3 13 11 7 8 42

2

2 3 2 1 2 10

12

9 37 23 13 13 95

2011 1 7 30 26 18 15 97

1 1

9 10 9 3 32

2

3 17 13 6 6 45

12

4 4 3 3 6 20

2012 1 1 16

10 13 41

1

2

2

2

1

1

3

2

2

9

2

3 2 7

12 1 1 10

6 11 29

2013

5 12

6 16 39

1

1 3

4 3 11

2

2

2 2 6

4

2

2

5

1

2 3

9

1 3

1 5

12

2 2

8 12

2014

7 2 5 3 9 26

1

1

2 3

2

1

1

4

1 1

9

1 1

12

7 1 5 2 5 20

2015

1

3 2 1 7

1

1

2 1 4

12

3

3

Total général 6 156 390 311 172 300 1335

1124

Table 2A. Analysis of deviance results for driftnetters.

Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Table 2B. Analysis of deviance results for longliners

Table 3A. Least square means, standard errors, standardized abundance indices with corresponding 95% upper

and lower confidence limits and coefficient of variation. Driftnetters, 1999-2011.

Year Nominal

CPUE Std.CPUE Upper Lower CV (%)

1999 57.32291

56.2554

79.4133

39.8506 4.6

1.2%

1.3%

1.4%

1.4%

1.9%

1.6%

1.7%

1.8%

1.7%

2.2%

4.7%

2.8%

2000 71.61301

64.8483

55.3103

55.473

53.0692

60.7784

55.4009

64.8643

61.0197

69.5657

63.1766

65.038

51.6316

123.3478

120.9122

113.9048

117.4016

71.0641

59.1762 1.2

2001 54.47799

56.1695

62.2694

50.6671 1.3

2002 54.33921

42.6038

46.9444

38.6645 1.4

2003 52.55259

57.2728

63.5672

51.6016 1.4

2004 65.04154

57.2728

61.4932

41.3861 33.4251

67.46 51.8576

58.7345 45.1325

71.6191 54.6035

65.0892 46.9382

79.4891 39.5927

96.4963 60.737

46.1178 1.9

2005 61.00112

37.1932

41.3861

33.4251 1.6

2006 70.06632

59.1466

67.46

51.8576 1.7

2007 68.67038

51.4863

58.7345

45.1325

1.8

2008 77.81795

62.5352

71.6191

54.6035

1.7

2009 68.73809

55.2736

65.0892

46.9382 2.2

2010 71.99499

56.0998

79.4891

39.5927 4.7

2011 54.49585

76.5565

96.4963

60.737 2.8

Factors df Residual

deviance

Change in

deviance F Pr(>F)

% in the

total

deviance

Null

27112

Year

12 26645 467

59.1725 < 2.2e-16 ***

12.79

Month

4 25557 1088

413.4694 < 2.2e-16 ***

29.80

GRT_class

5 24836 721

219.1715 < 2.2e-16 ***

19.75

Year:Month 48 24289 547

17.3216 < 2.2e-16 ***

14.98

Year:GRT_class 60 24018 271

6.8581 < 2.2e-16 ***

7.42

Month:GRT_class 20 23461 557

42.2960 < 2.2e-16 ***

15.26

Factors df Residual

deviance

Change in

deviance F Pr(>F)

% in the

total

deviance

Null

1151.13

Year

14 1064.78 86.347 8.5491 < 2.2e-16 ***

41.61622

Month

7 1034.64 30.139 5.9680 7.407e-07 ***

14.52594

GRT_class

5 943.64 90.998 25.2267 < 2.2e-16 ***

43.85784

1125

Table 3B. Least square means, standard errors, standardized abundance indices with corresponding 95% upper

and lower confidence limits and coefficient of variation. Longliners, 2001-2015.

Year Nominal

CPUE Std. CPUE Upper Lower CV

2001 40.1178

26.4668

60.9507

11.4927

14.43

2002 52.4140

31.8979

74.196

13.7133

13.74

2003 64.1436

36.0351

83.3281

15.5833

13.13

2004 78.9591

38.6269

89.164

16.7336

12.83

2005 62.1609

30.4991

70.347

13.2229

13.79

2006 84.5596

43.9984

100.5334

19.2559

12.19

2007 79.1482

41.2178

94.4756

17.9825

12.47

2008 62.4420

33.106

75.6302

14.4917

13.28

2009 75.6978

34.469

78.883

15.0617

13.14

2010 58.9551

34.5673

79.2475

15.078

13.16

2011 64.6639

40.1586

95.0174

16.9728

13.07

2012 157.9268

72.7279

175.2437

30.1828

11.35

2013 119.1333

70.8365

174.7483

28.7145

11.75

2014 105.8385

63.5715

190.4096

21.2244

14.81

2015 108.3714

67.2613

155.6885

29.0586

11.03

1126

Figure 1. Monthly size distribution of swordfish landed at the port of Tangier (Strait of Gibraltar), 2006-2011.

1127

A

B

Figure 2. Box plots indicating the distribution of Log CPUEw by year for driftnetters (A) and longliners (B).

1128

A

B

Figure 3. Box plots showing the distribution of Log CPUEw by month for driftnetters (A) and longliners (B).

1129

Figure 4. Box plots showing the distribution of Log CPUEw by vessel GRT class for driftnetters (A) and

longliners (B).

1130

A

B

Figure 5A. Diagnosis plots: normal qq plots (A) and residuals vs fitted positive catch distribution (B) for

Driftnetters.

1131

A

B

Figure 5B. Diagnosis plots: normal qq plots (A) and residuals vs fitted positive catch distribution (B) for

longliners

1132

Figure 6A. Standardized abundance index, with its corresponding upper and lower confidence limits for

driftnetters, 1999-2001.

Figure 6B. Standardized abundance index, with its corresponding upper and lower confidence limits for

longliners, 2001-2015.