Reassessing regime shifts in the North Pacific: incremental climate change and commercial fishing are necessary for explaining decadal‐scale biological variability

Abstract

In areas of the North Pacific that are largely free of overfishing, climate regime shifts - abrupt changes in modes of low-frequency climate variability - are seen as the dominant drivers of decadal-scale ecological variability. We assessed the ability of leading modes of climate variability [Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation (NPGO), Arctic Oscillation (AO), Pacific-North American Pattern (PNA), North Pacific Index (NPI), El Niño-Southern Oscillation (ENSO)] to explain decadal-scale (1965-2008) patterns of climatic and biological variability across two North Pacific ecosystems (Gulf of Alaska and Bering Sea). Our response variables were the first principle component (PC1) of four regional climate parameters [sea surface temperature (SST), sea level pressure (SLP), freshwater input, ice cover], and PCs 1-2 of 36 biological time series [production or abundance for populations of salmon (Oncorhynchus spp.), groundfish, herring (Clupea pallasii), shrimp, and jellyfish]. We found that the climate modes alone could not explain ecological variability in the study region. Both linear models (for climate PC1) and generalized additive models (for biology PC1-2) invoking only the climate modes produced residuals with significant temporal trends, indicating that the models failed to capture coherent patterns of ecological variability. However, when the residual climate trend and a time series of commercial fishery catches were used as additional candidate variables, resulting models of biology PC1-2 satisfied assumptions of independent residuals and out-performed models constructed from the climate modes alone in terms of predictive power. As measured by effect size and Akaike weights, the residual climate trend was the most important variable for explaining biology PC1 variability, and commercial catch the most important variable for biology PC2. Patterns of climate sensitivity and exploitation history for taxa strongly associated with biology PC1-2 suggest plausible mechanistic explanations for these modeling results. Our findings suggest that, even in the absence of overfishing and in areas strongly influenced by internal climate variability, climate regime shift effects can only be understood in the context of other ecosystem perturbations.

Marine growth patterns of southern British Columbia chum salmon explained by interactions between density-dependent competition and changing climate

“…Following standard convention (Agler et al 2013;Litzow et al 2014;Jeffrey et al 2017), values of winter season variability were calculated by averaging monthly values from November to March for PDO (available from research. jisao.washington.edu/pdo/)andNPGOindices(availablefromwww.…”

“…December to May monthly values were averaged for the NOI index (available from http://www.pfeg.noaa.gov/ products/PFEL/modeled/indices/NOIx/noix.html); this allowed for easier comparison of results with previous studies. The propagation of El Niño Southern Oscillation takes approximately a year to affect physical processes in the North Pacific Ocean (Litzow et al 2014), so changes in annual increment scale growth were aligned with annual (January to December) mean values of MEI from the year previous to the growth year (available from www.esrl.noaa. gov/psd/enso/mei/).…”

Forty years of change in forage fish and jellyfish abundance across greater Puget Sound, Washington (USA): anthropogenic and climate associations

“…Extensive commercial harvest of forage fish has been implicated as a cause of declines in forage fish abundance across the world (Pikitch et al 2012) and in the North Pacific in particular (Litzow et al 2014), as well as increases in jellyfish biomass resulting from re lease from predation (Purcell & Arai 2001) or com petition (Daskalov 2002). Mixed effects models suggested that commercial landings were less consequential than human population density, although both were important predictors of forage fish and jellyfish CPUE.…”

Globalization, marine regime shifts and the Soviet Union

“…The regime shift in the North Pacific is a classic example of a climate-driven shift, but a recent re-analysis [34] illustrates the importance also of fishing (electronic supplementary material, table S2). The Soviet Union was the largest catch nation in the North Pacific prior to the 1977 regime shift, but only a minor actor at the time of the 1989 regime shift in this region, when this role had been taken over by the USA (see the electronic supplementary material, table S2, also for details of the Soviet Union in the California Current, the Gulf of Alaska and the Eastern Bering Sea, all part of the described North Pacific regime shift).…”