Is shifting climate driving changes in salmon population dynamics?
In a nutshell
- Prevailing ocean conditions in the North Pacific are highly influenced by complex interactions between various meteorological phenomena expressed across vast spatial scales
- Increasing global temperatures have led to shifts in the relationships between many of these phenomena, including a reduction in the influence of the Pacific Decadal Oscillation (PDO) relative to that of the North Pacific Gyre Oscillation (NPGO) on the oceanic conditions of the North Pacific
- Evidence suggests that the population dynamics of Chinook and coho salmon, which were traditionally associated with teleconnections linked to the PDO, are now more closely linked with the NPGO, and that the population dynamics of the two species have begun to converge as a result of the physical and subsequent ecological changes in the North Pacific
- These transformations in climatic and oceanic conditions, and the subsequent response of salmon population dynamics, have important implications for salmon fisheries management and conservation
The population dynamics of two Pacific salmon species have been markedly transformed as a result of changing ocean conditions at the equator since the 1980s, with potentially important implications for salmon fisheries and conservation efforts. Shifts in long-term tropical El Niño-related events, which, through various complex interactions, greatly influence variability in the physical conditions of the northern Pacific Ocean, have led to a reduction in the role of eastern Pacific warming (EPW) and a concomitant rise in the importance of central Pacific warming (CPW) over recent decades; CPW and EPW, in turn, stimulate different teleconnections – interactions between various meteorological events that occur at great distances from one another – to the northern Pacific Ocean. As a consequence of these changes in the comparative influence of the CPW and EPW, the relative intensities of the Pacific Decadal Oscillation (PDO), defined as a long-term fluctuation in sea-surface temperatures (SSTs) in the northeast and tropical Pacific Ocean characterized by alterations between warm and cool SST phases over 20–30 year periods, and the North Pacific Gyre Oscillation (NPGO), a decadal climate pattern characterized by fluctuations in salinity and nutrient concentrations that affect the variability of phytoplankton abundance and thus higher trophic levels, have also changed. “For years it was taught that El Niño occurred when the trade winds stopped blowing toward the west at the equator and the consequent seawater that had built up to a higher level in the eastern Pacific because of those winds then began sloshing back to the western side of the Pacific, where its effects – warm, unproductive water – were transmitted northward along the coast of North America and southward along the coast of South America,” explains Louis Botsford, a marine biologist with the Coastal and Marine Sciences Center at UC Davis, and a coauthor of the study. “In the last decade or so, however, the warm water of El Niño has frequently been occurring in the center of the Pacific rather than at the eastern side of the Pacific, which seems to have a teleconnection to the NPGO instead of to the PDO.”
Botsford and his colleagues examined the ocean survival rates of tagged juvenile coho and Chinook salmon released from 72 and 104 hatcheries, respectively, along the west coast of North America, from central California to Alaska, between 1980 and 2006. Analysis of the data collected from the tagged salmon indicated that variability in salmon survival in the Pacific, which was largely associated with the PDO prior to the 1980s, is now more strongly associated with the NPGO. Increasing variance in the North Pacific Oscillation (NPO) – the atmospheric phenomenon that drives NPGO occurrence – has been linked to global warming; for instance, NPO activity was at its highest level yet in the winter of 2013–2014, resulting in the warmest SST anomalies ever recorded in the northeastern Pacific. Because of this link to rising atmospheric temperatures, such changes in the relative influence of the PDO and the NPGO may become more frequent and pronounced as global temperatures continue to increase.
Moreover, the researchers found that ocean survival rates of Chinook and coho salmon, which previously varied independently of one another due to considerable differences in their life histories, began to converge in the 1990s, and have become increasingly more synchronous. What specific mechanisms are driving the growing synchrony of the two populations remains unclear, but the researchers believe that it may be due to modifications in coastal food-web linkages or to undetermined changes in the salmon species themselves.
Regardless of the underlying causes, the convergence in population dynamics between the two salmon species bodes ill for commercial fisheries; given that the two populations now appear to rise and fall simultaneously, catch rates of one species may no longer function to offset low catch rates of the other. Equally problematic, the mechanisms underlying the ocean climate effect are likely to be beyond the scope of management, according to Botsford. “Something has changed in the lives of coho or Chinook salmon, and their interactions with the environment, and identification of this increase in the synchrony of ocean survivals points to something happening in the ocean. As such, it identifies an element of the loss in salmon diversity that managers are probably not going to be able to reverse by management actions in freshwater.