SCRS/2006/161

SCRS/2006/161
Col. Vol. Sci. Pap. ICCAT, 60(4): 1401-1403 (2007)
STANDARDIZED CATCH RATES OF BLUEFIN TUNA (THUNNUS THYNNUS)
FROM THE MOROCCAN ATLANTIC TRAP FISHERY, 1998- 2005
Noureddine Abid1, M’Hamed Idrissi2 and José Maria Ortiz de Urbina3
SUMMARY
Catch rates from the Moroccan trap fishery in the Atlantic from 1998 to 2005 were analyzed
following a General Linear Modeling (GLM) approach under negative binomial (log link) error
assumption in order to compute blue fin tuna standardized abundance indices.
RÉSUMÉ
Les taux de capture de la pêcherie marocaine de madragues de l’Atlantique, relatifs à la
période 1998-2005, ont été analysés en utilisant l’approche du Modèle Linéaire Généralisé
(GLM), avec la supposition de l’erreur négative binomiale (log link), et ce pour l’estimation des
indices d’abondance standardisés de thon rouge.
RESUMEN
Se analizan las tasas de captura de la pesquería Marroquí de Almadraba en el Atlántico
durante el periodo 1998-2005 mediante la aplicación de un Modelo Lineal Generalizado
(GLM) con distribución del error binomial negativo (log link) para la obtención de índices de
abundancia estandarizados de atún rojo.
KEYWORDS
Effort, CPUE, trap fishery, bluefin tuna, east Atlantic
1. Introduction
During the 2006 ICCAT Bluefin Tuna Stock Assessment Session (Madrid, Spain, June 12-18, 2006) the
Working Group, in order to assure coherence between relative abundance indices, asked the Moroccan scientists
to standardize the Moroccan trap index of abundance (Abid and Idrissi 2006, SCRS/2006/078) following the
methodology described in document SCRS/2006/071 (Ortiz de Urbina et al., 2006). This document presents the
results submitted during the above mentioned meeting.
2. Material and methods
Information on catch, both in number of individuals and weight of bluefin tuna, days of fishing, and trap
characteristics was collected in the Moroccan traps located in the area of influence of the Strait of Gibraltar,
along the coastline from Tangier to Moulay Bouselham, for the period 1998- 2005.
As regards to fishing effort, it has been traditionally defined as the number of days elapsed during two
consecutive net lifting operations (Abid and Idrissi 2006). As this measure of effort did not seem to be not
satisfactory (Ortiz de Urbina et al. 2006), it was alternatively defined as the number of days during which the
1
Biologiste des Pêches, INRH, Centre Régional Tanger/M’diq, Maroc; E-mail: [email protected]
Bio-économiste des Pêches, chef, INRH, Centre Régional Tanger/M’diq, Maroc; E-mail: [email protected]
3
Biostatisticien des pêches, IEO, Centre Océanorgaphique de Málaga, Espagne; E-mail: [email protected]
2
1401
trap is set and operative during a given year. This quantification seemed more reasonable since it is known that
traps are passive gears, having been set in the same season and locations for decades and without any significant
technical modifications since the early 20th century.
Data from three traps (codes 1, 2 and 11 in the data base) which covered the whole time series were used for the
analysis. A general linear modeling (GLM) approach to analysis of variance under negative binomial error
assumption (with log link) and factors year and trap was used to analyze bluefin tuna nominal catch in number
and weight.
3. Results
Table 1 and Figure 1 show the standardized relative abundance indices and corresponding coefficients of
variation (CV).
References
ABID, N., M. Idrissi. 2006. Analyse des indices d’abondance standardisés relatifs aux captures du thon rouge
(Thunnus thynnus) par les madragues de l’Atlantique Marocain. Période 1998-2005. ICCAT,
SCRS/2006/078.
ORTIZ DE URBINA, J., J.-M. Fromentin, V.R. Restrepo, H. Arrizabalaga, J.M. de La Serna. 2006.
Standardized CPUE of bluefin tuna (Thunnus thynnus) caught by Spanish traps for the period 1981- 2004.
ICCAT, SCRS/2006/071.
1402
Table 1. Predicted abundance indices in weight and number (scaled to 1998) and corresponding coefficients of
variation.
1998
1999
2000
2001
2002
2003
2004
2005
Index
(weight)
CV
Index
(number)
CV
1
0.52975954
0.75467692
1.95956125
0.50936386
0.17434009
0.12435805
0.42016617
0.22383006
0.22383328
0.22383125
0.22382827
0.22383356
0.22384728
0.22385574
0.22383507
1
0.57685256
0.79556691
1.9805214
0.52253432
0.19962056
0.16484837
0.49643825
0.20570674
0.20633724
0.20592799
0.20527915
0.20649156
0.20911057
0.20999104
0.20657765
2.5
2
1.5
weight
number
1
0.5
0
1997
1998
1999
2000
2001
2002
Figure 1. Predicted abundance indices (number and weight).
1403
2003
2004
2005
2006