Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits.