- Ocean-atmospheric interaction (Regional, speculative). Ocean to atmosphere heat transfer might maintain the atmospheric thermal fields and wind stress maintaining the changed sea surface temperatures (SST). It is suggested that in the Central North Pacific Ocean, the new regime was maintained by increased wind mixing acting upon a significantly cooler pool of water. The strengthened Aleutian low pressure increased westerly winds, which in turn increased turbulent mixing, southward Ekman current advection and sea-surface heat flux. All these enhanced further cooling of SST. In the eastern Pacific Ocean strengthened Aleutian low increased warm, moist southerly winds, which reduced heat loss and warmed SST. This way, the warmer state along the California Coast would have been maintained by reduced heat losses. It is not yet clear how the heat flux anomalies affect atmospheric circulation, but the pattern could be related to the persistence of the post-1976-77 regime. Overall it is not clear yet what is needed to maintain a certain regime.
The main direct drivers that contribute to the shift in the biology include:
- Sea surface temperature (Local, well-established). Oceanic temperature variations not only affect organisms directly (e.g. metabolic rates) but they also influence other important variables such as air-sea heat exchange, sea level (e.g. intertidal organisms), local currents and movement of nutrients and plankton. For example, the multi-decadal stock oscillation for anchovies and sardines in the California and Kuroshio Currents may be caused by the different thermal ranges of these species: different temperature preferences and limits for growth, spawning and distribution contribute to the colder regimes off Japan being dominated by sardines and the same areas during the warmer years being dominated by anchovies. (In other upwelling regions, sardines tend to dominate during warm periods, probably because of the availability of fluctuating food.)
- Stratification / mixing (Local, speculative). The thermal stratification and the depth of the mixed layer affect the variations in light levels and the availability of nutrients, both of which are critical for the primary production. Strong stratification also tends to favour a pelagic dominant system where energy is recycled within the upper layers and a weaker stratification favors a more demersal dominant system. During the 1976-77 regime shift, in the coastal areas the mixed layer depth (MLD) became shallower, and in the central North Pacific the MLD cooled and deepened mainly due to wind mixing. In the California Current system, changes in the abundance of zooplankton have been attributed to variations in the strength of coastal upwelling, variations in the horizontal transport of nutrient-rich water from the north, and/or increased stratification due to SST warming. The deepening of the thermocline may have accompanied the warming and increased the stratification of the water column, leading to a decrease in the supply of plant nutrients to the upper layer. This is suggested to be the most likely mechanism for the observed plankton decline, and subsequent ecosystem changes.
- Changes in currents (Regional, well-established). Changes in atmospheric pressure are expressed through their effects on surface wind stress, which in turn influence wind-driven surface ocean circulation. Changes in the currents change the transport of zooplankton and early life stages of fish and fish distribution, e.g. when the East Korean current transported tropical fish species into the Kuroshio - Oyashio region and caused high yields for commercial fishing.
- Salinity (Local, well-established). Variations in ocean salinity are driven by the currents and upwelling. Salinity affects zooplankton and fish abundance and recruitment.
- Fishing (Regional, well-established). Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, overfishing may affect how and at which scale an ecosystem responds to the climate change. It may alter the response of populations to otherwise natural change through the changes of community and age structures and energy cycling. Overfishing is a severe threat for the North Pacific Ocean ecosystem: many of the commercially important fisheries show critical decreases. Bycatches further reduce fish stocks and even marine mammals, for instance the Stellar sea lion, which primary threat is fishing and absence of fatty fishes in their diet.
The main external indirect drivers that contribute to the shift include:
- Changes in tropical sea surface temperatures (Regional, contested). Large cooling over an elliptical area in the western and central North Pacific Ocean SSTs and warming of the central northeast Pacific Ocean during the 1976-77 regime shift are suggested to be mainly due changes in the tropical SSTs, which were transmitted to the midlatitudes through atmospheric - oceanic teleconnections.
- Atmospheric - oceanic teleconnection patterns (Global, well-established). Shifts of North Pacific atmospheric circulation can drive SST anomalies in the central and eastern North Pacific via changes of surface heat flux, mixing and Ekman advection, and in the Kuroshio - Oyashio system region via oceanic circulation adjustment.
- ENSO (Regional, contested). In 1983 and 1984 West Coast El Nino had extensive biological consequences. In the California Current the thermocline and nutricline deepened and the phytoplankton biomass was largely redistributed from the upper layers to deep chlorophyll maximum. The upper 200m of the system was strongly density-stratified. Many fish populations and invertebrates shifted to the north. Zooplankton and kelp forests declined greatly. The spawning ranges for commercial pelagic fish changed. The breeding season was poor for many seabird species. The pup counts for Californian sea lions and northern fur seals dropped as they or their mothers were starving. 1982 and 1983 were disastrous years for commercial fishing off the west coast North America.
Slow internal system changes that contribute to the regime shift include:
- The transition-zone chlorophyll front (TZCF) (Local, contested) in the North Pacific subtropical gyre marks the boundary between the subtropical gyre (low productivity) and the high-latitude ecosystems, where increased productivity is driven by deep winter mixing. Variations in the position of the TZCF can have important ecological consequences because many fish and marine mammals forage along the front. The position of the TZCH changes during El Nino and may also vary on multidecadal time scales. The northern margin between the vertically stratified oligotrophic subtropical gyre waters and the higher surface chlorophyll Transition zone waters vary latitudinally on interannual and decadal time scales. These shifts have significant consequences for example on the Hawaiian monk seals.
- Change of the Pacific Decadal Oscillation phase – The threshold at which the PDO change causes a major (i.e. 1976-77) or minor (1989) regime shift.
- Intensification of the Aleutian Low – The threshold at which the deepening and eastward movement of the Aleutian Low was significant enough to cause large changes in the surface-heat flux, ocean current advection, turbulent mixing and horizontal transport, which attributed to the regime shift.
- Maintaining species diversity (Regional, speculated). It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management. 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.
- International management of fisheries and scientific management approach (Regional, speculative). The North Pacific Ocean is a region surrounded by several countries. Therefore, the North Pacific fisheries are not managed as one unit. Many states have responded to the changes in fish stocks in a variety of marine policies such as TACs, fisheries co-management, regional cooperative fishery management etc. International responses have appeared in the form of convections and laws, e.g. North Pacific Marine Science Organization and Pacific Anadromous Fish Commission in 1992. Agreements between certain nations (e.g. Korea, China and Japan) show that the regional, cooperative resource management is important in international marine ecosystems with migratory fish species. When it comes to international operation, some studies propose that a new management institution should be created for the North Pacific Ocean. It has been suggested that this international organ should do research on ecological interactions among species sharing the same resources to create a framework for decision-making and it should ensure equal benefits. It has been indicated that one difficulty in managing the North Pacific fisheries is the absence of scientific knowledge on what are the drivers of the ecosystem changes. It should be accepted that the regime shifts are natural and reoccurring part of North Pacific ecosystems. An observational program to monitor changes in climate, ocean systems and ecosystems should be developed and maintained.
- Decreasing resource competition (Regional, speculative). Another difficulty in North Pacific fishery management is the resource competition among commercial fisheries, endangered species protection, private industry, public sector etc. The interest of environmental organizations in the protection of Pacific ecosystems against trawl fishery began in 1980s in the North Pacific and later on expanded to require protection of endangered species in the ecosystem as a whole. Marine reserves (for example to protect the seals) and fisheries closures may increase species diversity and consequently fish production. The governmental interest towards fisheries in the early days was to obtain the optimum harvest of fish, but over time it has changed to responsible and sustainable harvesting of fish. Local communities are interested in the fishery management for the welfare of their residents. Complex legal and regulatory legislature has been created for example in the USA for the management of marine mammals and their interaction with fisheries.
- Protecting locally adapted species (Regional, speculative). Eliminating locally adapted fish populations and by-catch species (e.g. seals) by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment.
- Sustainable fish harvest during the regime shift (Regional, speculated). It is suggested that regime shifts should be incorporated into fishery management. 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. Various studies have been made to find out what constitutes an optimum management strategy for fisheries that undergo regime shifts. Two different strategies have been suggested: the constant harvest rate strategy and the regime-specific harvest rate strategy, which has been generally preferred to the constant harvest rate. One example species for the regime-specific harvest is Pacific sardine, which together with anchovy has decadal cycles in abundance. Sardine has much higher reproductive rates during the warm years (ca 25 years regime) than cold years, so the fishery can achieve large population size and harvest during the warm regime. Additionally, fish stock can be managed even better if the harvest rate switch is delayed some years after the regime shift so that the fishing stock is allowed to rebuild the population levels. The variable harvest rate will allow higher yields during the periods of high productivity but the fisheries may need to be closed down during the unproductive periods. The constant harvest rate may work well for long-lived species but if it is used, it must be relatively low (ca 10% of the exploitable biomass) so that overfishing does not result during the periods of low productivity. The disadvantage of regime-specific harvest rate strategy is that scaling down form the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Additionally, if the low biomass is overestimated, the stock might be overharvested.
Ecosystem service impacts
The 1976-77 and 1989 regime shifts affected humans mainly through changes in North Pacific fisheries. Large human populations live in the countries bordering the North Pacific Ocean and the ocean even provides a great proportion of the world fish catch for international trade. The fish catches there are larger than those in the other oceans; by the late 1980s the North Pacific Ocean accounted for one third of world marine fish landings. The groundfish of the North Pacific are valued at more than 1 billion USD annually. Salmon fisheries are important in the United States, Japan, Russia and Canada.
Fish production responded differently to the regime shifts in question: The 1976-77 regime shift fish response was nearly balanced (some species increased and others decreased in biomass) but in 1989 there were widespread declines. Changes in physical environment affected fish recruitment timing and success differently according to species and habitats. In the central North Pacific Ocean, the fish populations increased during the 1976-1989 regime, after which they started declining. In the subarctic North Pacific, fish production increased: there were large increases in the landings of some commercial fishes after the 1976-77 regime shift. Salmon catches were high. The catches in post 1976-77 regime increased in the Kuroshio - Oyashio system a.o. for sardines, filefish and salmon.On the contrary, fish populations in the California Current system declined. The decline continued in post-1989 regime. For example Canada experienced some of the lowest salmon catches in the history.
The 1976-77 regime shift was an onset for another sardine regime. Decade-scale fluctuations of Pacific salmon populations have been shown to be consistent with the changes in the North Pacific climate. Landings of northern North Pacific salmon stocks began to increase in the mid-1970s after a low of three decades (positive PDO phase), and at the same time decline at the North American west coast; the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The positive variations in the salmon stocks of the late 1970s are explained either due to the improved feeding conditions (copepods) or due to improved marine survival of migrating salmon. It has also been proposed that the increased food supply in the Gulf of Alaska has increased salmon abundance, resulting in greater competition and smaller individual body sizes. The groundfish fisheries experienced an increase in biomass and a change in species composition (the dominance shifted from shrimp and capelin to cod, pollock, halibuts etc.) as studied in the Gulf of Alaska between the late 1970s and late 1990s. Groundfish increased greatly in three different study sites in the Bering Sea. The groundfish stocks that had strong year classes were successful before 1977 but not after, in opposite to a.o. herring in the Gulf of Alaska. There appears to have been a north-south shift in fish production. Another example is sablefish, which clearly related to climate-ocean conditions. Sablefish had weaker than average year classes before the 1976-77 regime shift and stronger than average year classes during the 1977-1989, and below average again since 1990. Saury stock collapsed in post 1976-77 regime. Saury's migration matched poorly with the spring bloom changes in the Kuroshio - Oyashio system in 1976, which together with the increase in fishing effort lead to a recruitment failure and stock collapse. Filefish, a subtropic species, increased and walleye pock decreased when the warm currents became stronger in 1976. Similar kind of species exchanges took place in Russian and Japanese waters. 1989 regime shift started widespread declines in productivity. There were declines in the catches of Western Alaska chinook, chum, and pink salmon, British Columbia coho, ink and sockeye salmon, West Coast salmon catches and groundfish recruitment. Regulative services It is suggested that the marine ecosystem response to the PDO-related environmental changes starts with phytoplankton and zooplankton and works its way up to the top level predators like salmon. This bottom-up control of overall productivity appears to be closely related to the upper ocean changes characteristic to the positive PDO.
Zooplankton biomass increased in North Pacific in general since the 1950s even though there were spatial differences. Around the time of the 1976-77 regime shift, phytoplankton production nearly doubled in the central North Pacific (near the Hawaiian Islands) due to deepening of the mixed layer, which lead to mixing of nutrient-rich water into the euphotic zone. At the same time, the mixed layer in the Gulf of Alaska was shallower than normal, but the primary and secondary production still increased due to the increased exposure of phytoplankton to light. In the California Current, the biomass of zooplankton decreased heavily. In the California Current system the variability in the structure of the zooplankton community may have been larger than the changes in the total zooplankton biomass. In the subarctic Pacific the zooplankton biomass and phytoplankton concentration increased in the mid-1960s and remained high until the end of the 1980s, when the values started to decline. In the Oyashio system and Japanese waters, phytoplankton and zooplankton biomass were small from the late 1970s to the late 1980s and increased in the 1990s. Changes in the mixed layer depths reported throughout the North Pacific likely affected the production and biomass of zooplankton.
The direct impacts of the regime shifts in 1976-77 and 1989 were changes in North Pacific fish populations on people whose livelihoods depend on them. Fisheries are not essential only for the commercial fish enterprises but also for many coastal communities and indigenous people. Fish stock changes may cause large costs for human communities for example in terms of long-term unemployment for fishermen and through other fishery-based businesses. The effects of widespread overfishing add to the natural fish declines. In Alaska alone, some 30 coastal communities have economic stakes in the ocean fisheries and in their sustainable use. Many community residents are Aleuts, who have now become dependent on the groundfishery as salmon fisheries have declined in magnitude and value. An example of the major importance of fisheries is a coastal city called Unalaska, where 90 percent of the employment depends on fishing industry. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially Salmon species are iconic for many people. Salmon has sustained the indigenous people of Russian Far East and northwestern North America for thousands of year. Therefore, declines in the salmon stocks affect negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs. The returns of salmon to rivers and migrations to deep inland each year have been important events for the coastal societies. The importance of animals for the cultural and religious traditions has been acknowledged in federal legislation and conventions. It has been estimated that appr. 65 000 people are affected by ecosystem changes. Changes in fish stocks don't affect human societies only directly through seafood availability and economy; the declines in species richness, genetic diversity and productivity also decrease the ecosystem stability. An unstable ecosystem may in turn increase the risk for stock collapses - only a few are estimated to be able to recover through protection - and extinctions of commercially important marine species. Additionally, it has been suggested that an ecosystem with rich diversity might be capable of providing a larger number of ecosystem services and economic opportunities. The Pacific Ocean also supports some of the world's most important trade routes.