Beyond lethal temperatures: Factors behind the disappearance of chum salmon from their southern margins under climate change

1. Key Points

• The Tone River in Japan represents one of the southern limit of the distributions of chum salmon (Oncorhynchus keta)^※1 on the western side of the North Pacific, but the number of adult chum salmon observed at Tone River weir^※2 has declined dramatically since 2013 and reached zero in 2024.
• The factors behind the recent decline of the chum salmon population in the Tone River were investigated by using 20-year particle-tracking simulation based on JCOPE2M^※3 reanalysis, in which none of the simulated scenarios reproduced the recent decline in the population regardless of the swimming strategy and addition of lethal temperature thresholds.
• Instead, the decline in population growth rate appears to be correlated with warming water temperature and reduced zooplankton abundance caused by the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio. However, partial correlation analysis indicated that zooplankton abundance was the main influencing factor coherently related to the population growth rate of salmon.

2. Overview

The Tone River in Japan represents one of the southern limit of the distributions of chum salmon (Oncorhynchus keta) on the western side of the North Pacific, but the number of adult chum salmon observed here has declined dramatically since 2013 and reached zero in 2024. The factors behind the recent decline of the chum salmon population in the Tone River were investigated by using ocean reanalysis data and a 20-year particle-tracking simulation. Virtual chum salmon fry were released at the mouth of the Tone River in spring each year with six different swimming strategies to evaluate the effects of ocean currents on the population growth rate of salmon. None of the simulated scenarios reproduced the recent decline in the population regardless of the swimming strategy and addition of lethal temperature thresholds. Instead, the decline in population growth rate appears to be correlated with warming water temperature and reduced zooplankton abundance caused by the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio. Along the coast of northeastern Japan, the warm, nutrient-poor Kuroshio/Kuroshio Extension replaced the cold, nutrient-rich Oyashio on the migration route of chum salmon fry, increasing the water temperature and reducing zooplankton abundance. Partial correlation analysis of the water temperature and zooplankton abundance indicated that the latter was the main influencing factor coherently related to the population growth rate of salmon. The reduced zooplankton abundance would affect the growth and survival of chum salmon fry, which would result in a decline in population growth. The northward shift of the Kuroshio/Kuroshio Extension and Oyashio may continue or return southward depending on the effects of climate change, which will greatly influence the future population growth of chum salmon and whether they come back to the Tone River.

*This study is supported by JSPS KAKENHI (23K03503 and 24K01833).

These findings are scheduled to be published in " PLOS ONE" on September 11 (Japan Time).

3. Background

One of the major effects of climate change on organisms is a shift in their geographic distribution, which is particularly pronounced for marine organisms compared with their terrestrial counterparts. Increasing water temperature have driven a northward shift in the distribution of several marine fish species in the Northern Hemisphere, which has led to their disappearance from the southern limit of their range. Although rising water temperature is frequently correlated with northward shifts in marine species, the underlying mechanisms remain surprisingly unclear. Chum salmon (Oncorhynchus keta) are the most widely distributed species of Pacific salmon. They have an extensive migration range across the North Pacific yet exhibit a remarkable homing ability by returning to spawn in specific natal rivers. Populations at the southern limit of their range face a greater challenge than those at higher latitudes because they not only experience higher temperatures in their natal rivers and adjacent coast but also must migrate longer distances to reach their northern oceanic feeding grounds. The Tone River (35.76° N, 140.86° E), which flows through the Kanto Plain in Japan, is one of the southern limit locations in western Pacific (Fig. 1a). The number of adult chum salmon counted at the Tone River weir increased after 2000 to peak at over 18,000 individuals in 2013, yet the number of returning adults has declined since then and eventually reached zero in 2024 (Fig. 1b). The population growth rate was generally positive before 2010 and became negative in 2011 and remained below the minimum value since then (Fig. 1c). In this study, we investigated the factors behind the recent decline of the chum salmon population in the Tone River by examining the effects of the three factors: ocean currents, water temperature and food availability (zooplankton). A 20-year (2001–2020) simulation of the dispersal of virtual salmon fry released at the mouth of the Tone River was conducted by using a particle-tracking model that incorporated transport by ocean currents and active swimming behavior. We used the results of this numerical simulation along with reanalysis data on water temperature and zooplankton biomass to identify the key factors contributing to the recent population decline of chum salmon at the southern limit of their distribution.

Fig 1. (a) Map of the study area and mean ocean currents over the tracked period (mid-February–mid-July 2001 to 2020 at a depth of 5 m. Possible migration routes of chum salmon (Oncorhynchus keta) from the Tone River (green star) are plotted in magenta, where the solid line is taken from historical record and the dashed line is projected. (b) Number of adult chum salmon counted at the Tone River weir 150 km upstream from the river mouth from 1990 to 2024; (c) population growth rate of chum salmon in the Tone River assuming one generation corresponds to four years. Insets show photos of the (b) Tone River weir and (c) chum salmon in the fish counting ladder window taken in November 2018.

4. Results

The Tone River in Japan represents one of the southern limit of the distributions of chum salmon (Oncorhynchus keta) on the western side of the North Pacific, but the number of adult chum salmon observed here has declined dramatically since 2013 and reached zero in 2024. The causes of the recent decline in chum salmon population of the Tone River were investigated using 20-yr particle-tracking simulations based on ocean reanalysis JCOPE2M conducted at Earth Simulator of JAMSTEC. The simulation could not reproduce the observed decline in population regardless of the selected swimming strategy (Fig. 2a) and addition of lethal temperature thresholds, which indicated that ocean currents and lethal water temperatures are not significant factors. Instead, the decline in the salmon population growth rate was correlated with increase in water temperature and the reduced zooplankton availability, which was caused by the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio. The water temperature, however, was no longer important when partial correlation analysis was applied; only the zooplankton remained significant (Fig. 2b). The results indicated that the initial zooplankton availability is the only factor affecting the population growth rate of chum salmon. The northward shift of the Kuroshio/Kuroshio Extension and Oyashio may continue or return southward depending on the effects of climate change, which will greatly influence the future population growth of chum salmon and whether they come back to the Tone River.

Fig 2. (a) Arrival rates (%) of v-salmon fry from the Tone River mouth at southern Hokkaido (41°N–43°N, 141°E–146°E) from 2001 to 2020. Partial correlation analysis on the effects of the (b) water temperature and (c) initial zooplankton availability on the population growth rate of chum salmon.

5. Outlook

In this study, we focused on the early mortality of chum salmon fry the first few months after they entered the ocean. Under climate change, the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio appear to be causing a critical food shortage for chum salmon fry migrating northward from the Tone River. However, climate change is also likely to impact chum salmon adults by reducing growth and maturity rates and increasing their exposure to high water temperature when they return to their natal rivers, which may prevent them from reaching their spawning sites. In addition, the in-river spawning and survival of newborn fry may be particularly sensitive to warming water temperature at the southern limit of their distribution. A comprehensive understanding of these impacts remains an important challenge for future research.