Oregon Sea Grant

This study investigated long-term (1961-2008) changes in albacore (Thunnus alalunga) abundance and distribution in relation to local environmental and large-scale climate indices in the Northeastern Pacific using time series and spatial analyses. Prior to the time series analysis, a wavelet analysis was conducted to examine nonrandom patterns of cyclical variability which revealed that monthly and annual time scales had the highest nonrandom variability. Thus, the time series analysis was done at these two scales using nonlinear generalized additive models (GAMs) and threshold GAMs. At the monthly scale, sea surface temperature (SST) was found to be the variable with the strongest (positive) association to albacore catch per unit effort (CPUE). This association was likely driven by the seasonal migrations of juvenile albacore into and out of the U.S. coastal waters. At the yearly time scale over large geographical areas, the SST association broke down, and the scalar wind speed cubed (an indicator of mixed layer depth) at a five year lag became the dominant variable. The scalar wind speed cubed index explained 65% of the variability and was highly significant, even after adjusting for multiple tests (Bonferroni corrected P-value<0.001). These results suggest that a deeper mixed layer in the Northeastern Pacific may provide favorable foraging habitat for juvenile (mostly age 3) albacore, resulting in successful growth, spawning, and recruitment into the fishery in later years. This mixed layer depth association could help managers and stock assessment groups in their efforts to integrate environmental factors into the estimate of albacore population size. The spatial/spatio-temporal analyses involved modeling the CPUE with four competing GAM formulations, each representative of a different hypotheses for albacore distribution: 1) spatial, 2) spatial and environmental (SST, PDO, and MEI), 3) spatially variant, and 4) nonstationary, as indicated by the North Pacific regime shift of 1977. Results indicate that SST had a predominantly positive but spatially-variable effect on albacore CPUE, while the PDO had a negative overall effect. Specifically, CPUE was found to increase with increased SST, particularly off of Oregon and Washington. These results imply that if ocean temperatures continue to increase, west coast communities reliant on commercial albacore fisheries are likely to be negatively impacted in the southern areas but positively benefited in the northern areas, where current albacore landings are highest.