Do local environmental factors induce daily and yearly variability in bluefin tuna (Thunnus thynnus) trap catches?

The daily influence of local environmental factors on bluefin tuna fisheries in Portugal was investigated by analysing catches made at two traps, Barril and TUNIPEX. A ‘hurdle’ geometric regression model was fitted, since this class of generalized linear models can deal with zero-inflated data such as daily trap catches. With this model, the influence of environmental factors on the probability of catching tuna and on the expected size of positive catches were analysed separately. Parameter estimation utilized data from the oldest trap (1926–1966) only, whereas data from the more recent trap (1996–2000) was used to validate the model. The obtained results were in agreement with many previously formulated hypotheses regarding the ecology of bluefin tuna, in aspects such as the impacts of turbidity and wind- and tidal-generated currents on the presence of schools close to shore, where they can be caught by traps. Through simulation, it was further found that periodic oscillations observed in yearly trap catches are probably associated with local weather and coastal ocean variability.