© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Standardization of bluefin tuna, Thunnus thynnus, catch per unit effort in the baitboat fishery of the Bay of Biscay (Eastern Atlantic)
a Instituto Español de Oceanografía (IEO), Centro Oceanográfico de Santander Apartado 240, 39080 Santander, Spain
b Instituto Tecnológico, Pesquero y Alimentario (AZTI), Txatxarramendi ugartea 48395 Sukarrieta, Vizcaya, Spain
c National Marine Fisheries Service (NMFS), Southeast Fisheries Science Center, Miami Laboratory Miami, FL, USA
d Centre for Environment, Fisheries and Aquaculture Science (CEFAS) Pakefield Road, Lowestoft NR33 0HT, UK
*Correspondence to E. Rodríguez-Marín; tel: +34 942 291060; fax: +34 942 275072. e-mail: rodriguez.marin{at}st.ieo.es.
Relative indices of abundance by age class for bluefin tuna from 1975 to 2000 were estimated using generalized linear mixed models. Age class was included as a fixed factor within the model specifications, with the YearxAge interaction as a fixed factor component in order to obtain annual indices by age. Catch and effort data on bluefin tuna were available from two sources, catches by trip, and daily catches from logbooks. Catches were modelled using the delta-lognormal model. The model finally selected included the following explanatory factors: Year, Age, YearxAge, Month, number of Crew, number of Bait Tanks, and with the YearxMonth interaction as a random component. Overall, year trends and coefficients of variation were similar from both datasets by age class. In general, the standardization procedure showed that vessel characteristics and technological advances related to fishing have a relatively minor explanatory effect on the observed catch rates of bluefin tuna in the fishery. Neither geographical distribution nor type of bait had an explanatory effect on the observed catch rates. The 1994 cohort stands out as an exceptionally large year class; it can be followed through the standardized series. This standardized catch rate index at age of bluefin tuna is the most complete yet available and is the only one currently used for juvenile fish in calibrating population models to evaluate the Eastern Atlantic and Mediterranean stock. It is particularly valuable because, apart from improvements in electronic technology, there are no changes in the method of exploitation during the entire series.
Keywords: bluefin tuna, catch per unit effort, generalized linear mixed models, standardization
Received 25 February 2003; accepted 5 July 2003.
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