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1 Updated Standardized CPUE of Atlantic Bluefin Tuna Caught SCRS/08/100 Updated standardized CPUE of Atlantic bluefin tuna caught by the Spanish baitboat fishery in the Bay of Biscay (Eastern Atlantic). Time series from 1975 to 2007. Enrique Rodriguez-Marin1, Mauricio Ortiz2, Cristina Rodríguez-Cabello1 and Santiago Barreiro1 SUMMARY Updated standardized relative abundance indices by age are presented for bluefin tuna baitboat fishery in the Bay of Biscay from 1975 to 2007. Standardization was carried out using generalized linear mixed models. Catch and effort data on bluefin tuna were available from catches by trip; this catch by commercial category was converted to catch at age by applying seasonal age length keys to the length distribution by commercial category. In this update the age length keys of summer of the entire study period were reviewed. Age class was included as a fixed factor within model specifications because the fishery operates on all available stock fractions, and age determination of the tuna landings is done after removal of the catch. Splitting of the catches into age classes introduces a large number of zero values, so data were modeled using the delta- lognormal model. The model finally selected included the following explanatory factors: Year, Age, Month and Year × Age fixed factors, plus a selection of other factors contributing with a significant percentage of the total explained deviance in the aggregated model. All Year interactions besides the Year × Age factor were considered as random. CVs of present standardized index are less variable than the previous standardization time series from 1975 to 2004, but still some variability was found for the last years when the vessels built more recently and larger are included in the analysis. The revised age length keys seem to be reducing CVs variability in the study period. The standardized index of ages 2, 3 and 4 show a downward tendency in the last years. RESUMEN Se presenta la actualización del índice de abundancia relativa por edad de atún rojo de la pesquería de cebo vivo del Golfo de Vizcaya desde 1975 hasta 2007. La estandarización se realizó utilizando modelos lineales mixtos generalizados. Se utilizaron datos de captura y esfuerzo de atún rojo procedentes de capturas por marea; esta captura por categoría comercial fue convertida en captura por edad, aplicando claves talla edad estacionales a las distribuciones de talla de cada categoría comercial. En esta actualización se han revisado las claves talla edad de verano de todo el periodo de estudio. La edad fue incluida como un factor fijo en el modelo puesto que la pesquería captura toda la población disponible y la determinación de la edad se realiza a posteriori. Al dividir las capturas en edades se introduce un elevado número de ceros, por lo que los datos se modelaron utilizando el modelo delta-lognormal. El modelo final seleccionado incluye los siguientes factores explicativos: año, edad, mes y la interacción fija año × edad, más una selección de factores que contribuyen a explicar la variabilidad del modelo. Todas las interacciones consideradas después de la interacción fija año × edad fueron consideradas como aleatorias. Los CVs del índice actualizado son menos variables que los obtenidos en la pasada estandarización (1975-2004), pero todavía hay un incremento en los mismos a partir de la inclusión en el índice de nuevos barcos más grandes y construidos recientemente. La revisión realizada el las claves talla-edad reducen la variabilidad en los CVs. Los índices estandarizados de las edades 2,3 y 4 muestran una tendencia descendente en los últimos años. KEYWORDS ABFT, catch per unit effort, generalized linear models, standardization. 1 Instituto Español de Oceanografía. Apdo. 240. 39080 Santander. Spain. [email protected] 2 National Marine Fisheries Service (NMFS), Southeast Fisheries Science Center. Miami Laboratory. United States. 1 Introduction At the end of spring, Atlantic bluefin tuna (Thunnus thynnus) (ABFT) reaches the southeast corner of the Bay of Biscay and remains till autumn. The geographical distribution of catches coincides with the accumulation of temperate waters occurring annually in this area of the Bay (Figure 1). ABFT migrates to the Bay of Biscay for feeding, since the influx of continental water from French rivers together with coastal upwelling processes and the formation of anticyclonic gyres in the Cantabrian Sea gives rise to high productivity values. Small pelagic species such as anchovy, sardine, horse mackerel and mackerel take advantage of the phyto- and zooplankton bloom, which takes place in spring and the start of summer, for spawning. Pelagic crustaceans and the recruits of these pelagic species, born in the same year or in the previous spring, constitute the reason for the ABFT trophic migration. The ABFT that migrate to the Bay of Biscay are juveniles, mainly of 1 to 5 years. The most abundant ages in catches are specimens of 2 and 1 years (in 2007 the regulations changed regarding minimum length and age 1 can no longer be caught). Albacore (Thunnus alalunga) also migrate towards the Bay of Biscay in summer, but they reach the Bay a little later than ABFT and although their distributions overlap, most of the Albacore catches take place outside the warm water area, to the west of 4ºW. Both species leave the Bay of Biscay in mid-autumn. Catches targeting ABFT are made exclusively by baitboat vessels operating from the easternmost ports of the Cantabrian Sea, Hondarribia and Getaria (Figure 1). The latter began to target ABFT in 1996. Catches landed at both ports now make up 90 % of ABFT catches in the Bay of Biscay (although some vessels from nearby ports, such as Ondarroa and Bermeo, also sporadically target ABFT). The remainder is from incidental troll and baitboat fishing for albacore in the Bay of Biscay. This paper updates the previous standardized ABFT cpue index by age-class for the baitboat fishery of the Bay of Biscay (Rodriguez-Marin et al., 2003; Rodriguez-Marin et al., 2007). Material and methods The same methodology detailed in Rodriguez-Marin et al. (2003) was followed. Catch and effort data were collected from the ports of Hondarribia and Getaria between 1975 and 2007. Data were obtained from landing returns, interviews and logbooks. The information collected for each trip included: name of the vessel, date of landing, number of ABFT by commercial category and number of fishing days. Catch by commercial category was converted to catch at age by applying seasonal age length keys to the length distribution by commercial category. This methodology was employed since changes took place in commercial categories during the study period and new ones were established in 2008 ( Table 1). Direct ageing was estimated from spine sections interpretation. Ages 5 and older were combined as 5+, since ages above are scarcely represented in the fishery for the period of study. In this update the age length keys of summer of the entire study period were reviewed. Vessel characteristics were assembled using information from official fleet directories and personal interviews with skippers. The information by boat and year includes: the name of the vessel, its home port, the year it was built, horsepower (HP), gross registered tonnage (GRT), length, construction material, crew number, number of bait tanks and name of the skipper, the presence or absence of radio direction finder, and number of the following devices: navigation radar, GPS, plotter, colour echo- sounder, monochrome sonar and colour sonar. In order to reduce the number of variables related to type of vessel to be included in the model, analyses were done for the reduction of these variables (described in Rodriguez-Marin et al., 2002). The variables GRT, HP and length were used to define three types of vessels. A criterion of baitboats with five or more years in the catch history of the time- series was used for data selection. Nominal cpue was calculated as the number of fish of a particular age caught by day of fishing trip. The number of days that are counted as effort in a fishing trip of one vessel exclusively catching ABFT is 100%. Nevertheless, if the vessel catches several species, albacore or less often big-eye tuna (Thunnus obesus), the assignation of effort is made taking into account the proportion in number of ABFT with respect to the remaining species and their size: 1. If ABFT catches are less than 5% and 2 smaller than 7 kg, the fishing trip is discarded. 2. If ABFT represents values of less than 5% but they are tunas greater than 7 kg and if they represent values between 5 and 15% independently of size, one third of the effort is assigned. 3. If ABFT represents between 15 and 50% half of the effort is assigned. 4. If it is greater than 50%, then all the effort is assigned. This criterion was applied because tuna fishing requires time for searching, and because they are taken one by one and the large ABFT are harder to catch, they need a specific fishing technique. There is a difference in fishing areas between the vessels targeting albacore and those targeting ABFT and there are also differences in fishing gear and in the live bait (Rodríguez-Marín et al., 2005). Relative abundance indices for ABFT by age-class were estimated using generalised linear mixed models (GLMM). Age class was included as a fixed factor within model specifications, with the Year * Age interaction as a fixed factor component, in order to obtain yearly indices by age. This model specification was selected against standardization models for each age class independently, because the fishery operates on all available stock fractions, and age determination of the tuna landings is done after removal of the catch. The proportion of positive cpue to total number of trips, grouped by year, age and month, was high for all ages 1 to 5+, and ages 2 and 3 made up the highest values (Figure 2).
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