INFLUENCE OF ENVIRONMENTAL, SPATIAL AND TEMPORAL FACTORS ON BLUE SHARK, Prionace glauca, CATCH RATE IN THE SOUTHWESTERN ATLANTIC OCEAN

Resumen

A Generalized Additive Model (GAM) was fitted to blue shark, Prionace glauca (Linnaeus, 1758), CPUE data (shark numbers per 100 hooks) from tuna longliners based in São Paulo State, Brazil, between 1998 and 2006, with the aim to observe their relationship with environmental, spatial and temporal information. GAM model accounted for 42% of the variance in nominal CPUE. Stepwise GAM revealed the relative importance of eight variables by decreasing magnitude, namely latitude, year, month, longitude, chlorophyll-a concentration, sea surface temperature, wind speed and ocean depth. Spatial and temporal factors accounted for 91.5% of the cumulative deviation explained by the model, while environmental factors responded for only 8.5%. The highest blue shark relative abundance was observed between January and June, with a peak in April. It increasedsoutheastward, below 20°C, between 1,500 and 4,000 meters depth. Chlorophyll-a showed highest CPUE values between 0.35 and 0.90 mg.m -3 . Wind speed presented a positive effect on catch rate up to 2 m.s -1 , followed by a steep decline. Standardized yearly CPUE trend was stable, with two peaks, one in 2001 and another in 2006. Both were followed by a high variance.