The North Pacific Ocean is undergoing an unprecedented transformation. Multi-decadal changes in the atmosphere and ocean have resulted in a northward expansion of subtropical gyre waters, a northward shift of the Kuroshio Current Extension current, and a decline in phytoplankton in the North Pacific Transition Zone (NPTZ) (Le et al., 2019; Yang et al., 2020; Kawakami et al., 2023). Against the backdrop of these climate-driven changes, discrete periods of anomalously warm sea surface temperatures known as marine heatwaves (MHWs) have produced some of the most intense warming episodes on record (Hobday et al., 2018; Oliver et al., 2018). The accelerated pace of these perturbations has led to profound biological shifts, with cascading effects propagating across the entire marine ecosystem (Smith et al., 2023; Welch et al., 2023; Farchadi et al., 2024; Gomes et al., 2024).

As a consequence of environmental change, a first response for many pelagic species is a geographic shift in search of favorable thermal and/or foraging habitats (Poloczanska et al., 2013; Pecl et al., 2017). The eastern North Pacific is known as a biodiversity hotspot for many highly migratory marine species (Block et al., 2011), such as tuna and sea turtles (Polovina et al., 2001), pinnipeds and sea birds (Block et al., 2011; Clay and Brooke, 2024), flying squid (Ichii et al., 2009; Polovina et al., 2017), sharks, and a variety of other fishes (Brodeur et al., 1999; Whitney, 2015). The rapid rate of change within the region has generated great interest as to how many of these species will be forced to adapt or adjust. Given that many of these higher trophic level species serve as “climate sentinels”, signaling otherwise unobserved shifts to the broader marine ecosystem (Hazen et al., 2019; Early-Capistrán et al., 2024), there is now a growing understanding of how North Pacific environmental variability will trigger species redistributions and thermal displacements (Jacox et al., 2020; Welch et al., 2023), altered predator-prey relationships (Thorne et al., 2016; Gomes et al., 2024) and increase reproductive failures (Hipfner et al., 2020). By the end of the century, projections have signaled significant, irreversible shifts to core habitats for many ecologically and commercially valuable mobile marine species (Hazen et al., 2013; Cheung et al., 2015; Lezama-Ochoa et al., 2024). As a result, there is substantial concern that ocean warming across the North Pacific will present new challenges and risks to species conservation and fisheries management strategies (Cheung et al., 2013; Cavole et al., 2016; Farchadi et al., 2024), as may be the case in the eastern North Pacific.

To date, most of our knowledge in this context comes from the well-studied California Current System (CCS), owing to the exceptional observational and tracking studies that have originated from this productive eastern boundary system (e.g (Block et al., 2011; Hazen et al., 2018; Welch et al., 2023; Lezama-Ochoa et al., 2024). These studies have examined the potential impacts to highly migratory species habitats in relation to both short- and long-term ocean warming within the CCS. Farther offshore, the expansion of the low-chlorophyll-a subtropical biome is expected to have significant ecological and economic consequences to many of these same vulnerable species within the region (Kobayashi et al., 2008; Cheung et al., 2009; Hazen et al., 2013). Due to the North Pacific’s expansiveness and the lack of observational data at scale, our ability to grasp the adaptive capacity of highly mobile species to recent change within the pelagic realm is less well understood (Holser et al., 2022).

Spanning the North Pacific, the NPTZ is the key oceanographic boundary between the subarctic and subtropical gyres. It serves as an important juncture for ocean currents, nutrient dynamics, and marine biodiversity. Roughly located between 30–32°N and 42–45°N latitudes, it contains the eastward flowing Kuroshio Extension Current west of the dateline which becomes the North Pacific Current east of the dateline. The region is also characterized by many temperatures, chlorophyll-a, salinity fronts, and mesoscale eddies which create areas of high biological productivity that attract highly migratory species previously described. As such, within these productive international waters, the region’s fisheries resources are targeted by fleets from many nations.

Previous investigations of species-environment shifts within the NPTZ have relied on short-term animal tracking data to examine seasonal or interannual relationships (e.g Polovina et al., 2001; Holser et al., 2022; Clay and Brooke, 2024). Simulations using climate-scenario models have added to our long-term (i.e., decadal or longer) expectations of how species may acclimate (Hazen et al., 2013). But forecasting distributional shifts can be challenging, as models may depart from anticipated patterns of range shift or movement, especially if climate-driven change occurs faster than expected (Chen et al., 2011; García Molinos et al., 2016). Given the multiple lines of evidence that signal biophysical change within the region (Polovina et al., 2008; Le et al., 2019; Yang et al., 2020), our inability to accurately capture long-term ecological response on the order of decades or longer leaves a critical gap in understanding how these species will adapt.

Addressing this gap, juvenile North Pacific loggerhead sea turtles (Caretta caretta) offer an excellent study species to monitor ecosystem change within the region. Loggerhead sea turtle populations in the Pacific Ocean are genetically distinct and Japan represents the sole nesting ground for individuals from the North Pacific population (Bowen et al., 1995). Their long-term dataset of satellite tracking within the North Pacific, spanning several decades (1997–2024), represents the most comprehensive population-level datasets of any marine species. Additionally, their affinity to the well-defined oceanographic properties of the North Pacific has been described in detail (Polovina et al., 2000; Polovina et al., 2001; Polovina et al., 2004; Polovina et al., 2006; Howell et al., 2008; Kobayashi et al., 2008; Abecassis et al., 2013). Decades of research on the species including electronic tagging studies shows that North Pacific loggerheads exhibit a well-established 1,000-km north-south seasonal movement that closely tracks the movement of the Transition Zone Chlorophyll Front (TZCF), a dynamic feature that marks the sharp surface chlorophyll-a gradient between the oligotrophic subtropical waters with surface chlorophyll-a less than 0.2mg/m³ to the south and the temperate waters with surface chlorophyll-a exceeding 0.2mg/m³ to the north (Polovina et al., 2001; Polovina et al., 2017). Such tracking data has shown that these turtles travel east and west across the NPTZ as they undertake seasonal north-south movements, occupying developmental habitats that span the open ocean and the mid latitudes of the North American coast (Polovina et al., 2001; Kobayashi et al., 2008; Briscoe et al., 2016).

As ectotherms, loggerheads are particularly sensitive to thermal cues, as temperature is one driver of habitat and distribution (Epperly et al., 1995) while prey abundance is the other (Plotkin et al., 1993; Parker et al., 2005; Mariani et al., 2023). North Pacific loggerhead sea turtles are known to be opportunistic foragers within their oceanic habitats, with a diet consisting of neustonic animals including Janthina spp., Carinara cithara, Vella vella, Lepas spp., Planes spp., and pyrosomas (see Parker et al., 2005). Given their sensitivity to ecosystem perturbations, conspicuity as an upper trophic level species, and vulnerable status as an endangered species (NOAA, 2021), they emerge as potent climate sentinels for gauging the on-going change to the overall structure and function of the NPTZ ecosystem (Sydeman et al., 2015; Hazen et al., 2019; Early-Capistrán et al., 2024). Through the application of our long-term data set, they may also serve as proxies for other highly mobile and protected species that utilize the region but are difficult to track.

This study combines almost 3 decades of loggerhead sea turtle data with concurrent ocean observations to investigate higher trophic level response to environmental change within the NPTZ. We first identify temporal trends in SST and chlorophyll-a within the eastern NPTZ. We then characterize the animals’ response to these changing ocean conditions. Our results identify a significant change to the NPTZ and thus latitudinal location of turtle habitat in the last quarter century and provide the first empirical evidence of important multi-decadal trends within the NPTZ.

Over the past 27 years, various subsets of this loggerhead sea turtle electronic tracking data set have played pivotal roles in defining the ecological significance of the NPTZ and how these animals use this region (Polovina et al., 2000; Polovina et al., 2001; Polovina et al., 2004; Polovina et al., 2006; Kobayashi et al., 2008). Since then, our understanding of this dynamic but persistent basin-scale ocean feature has grown into a well-studied marine productivity hotspot for migratory marine animals and commercial fisheries (Polovina et al., 2001; Kappes et al., 2010; Block et al., 2011; Hazen et al., 2013; Xu et al., 2017). Across this same period, the eastern North Pacific has experienced an unprecedented rate of change. The combined impacts of MHWs and long-term climate change threaten key ecological structure and functions that have defined the NPTZ. While evidence of physical forcing and lower trophic level alterations has been mounting (Polovina et al., 2008; Polovina et al., 2011), our ability to map these changes onto higher trophic levels has been limited. To date, our knowledge has been informed by short-term studies and end-of-century simulations, but there has been scarce long-term information (decadal-scale or longer) that integrates across these climatic extremes.

Our results show that over the past 27 years the NPTZ has become more oligotrophic with the rate of greatest increase during the winter and especially since 2011 ( Figure 3 ). Specifically, the eastern NPTZ has gradually warmed by an average of 1.59°C but experienced a much more substantial biological change with mean chlorophyll-a density declining by almost 19.58% ( Figures 2A, B ), with the greatest signals of change occurring in the winter months. Across the study period, the 5-year average winter SSTs in the NPTZ have increased from approximately 12.4°C from the beginning of the study period to 13.8°C at the end of the study period while average chlorophyll-a concentrations have decreased from approximately 0.22 mg/m³ to 0.185 mg/m³ during this same time. This represents a significant increase in oligotrophic waters, from ~3 million km² to 4.5 million km², with a notable increase since ~2011 ( Figure 3 ).

Results from our study align with previous research signaling significant biophysical change within the North Pacific Ocean. Polovina et al. (2008); Polovina et al. (2011) and Yang et al. (2020) identified climate-induced poleward expansion of the subtropical gyre and reduced productivity due to ocean warming. Le et al. (2019) and Kawakami et al. (2023) reported a northward shift of the Kuroshio Current Extension current and a decline in phytoplankton abundance in the NPTZ. The impacts of the increasing trend in recent MHW events (Smith et al., 2023) are evident in the further reduction of chlorophyll-a since 2012 ( Figure 2 ), however our results highlight a longer-term change within the region over the past few decades.

In response to these changes, loggerhead sea turtles have shifted their foraging grounds northwards by 450 km (from the GAMM) to 600 km (from the year group), or about 116–200 km per decade ( Figures 4A, B ). Our results indicate that this population-level shift is not just a northern limit range extension of habitat, but rather indicative of a pervasive northward shift across both the lower and upper bounds of their habitat range ( Figure 4B ). The 200 km/decade northward shift is considerably more than the global mean rate for marine organisms of less than 100 km per decade (Poloczanska et al., 2013; Pinsky et al., 2020), but is closer to the upper-end rate of leading edge expansions for other higher trophic level species (i.e., bony fish, 277.5 km per decade) (Poloczanska et al., 2013). These rates suggest that the ecology of the eastern NPTZ may be changing more rapidly than other regions and our ability to measure the habitat of a higher trophic level species moving northwards in response to these changes is unique. Even though the NPTZ has warmed and experienced an almost 20% decline in chlorophyll-a, the loggerhead sea turtles despite their northward movement, continued to occupy waters within preferred habitat ( Figures 5A, B ). Broadly, these results suggest that turtles are adapting to a changing ocean by undertaking significant geographic shifts to maintain suitable habitat.

However, for loggerheads the ability to maintain an adaptive response to a changing environment may become more difficult over time. Globally, the rate of climate-driven change continues to accelerate the poleward expansion of many tropical and subtropical species, a phenomenon known as tropicalization (Osland et al., 2021; Zarzyczny et al., 2023). For such an important biogeographic transition zone such as the NPTZ, the expansion of warmer, more oligotrophic waters may create latitudinal alterations in prey availability and composition. These changes can appear subtle but can push many upper trophic level predators into new habitats and forage communities, with potentially significant top-down impacts to the structure and function of the rest of the marine ecosystem (Zarzyczny et al., 2023; Gomes et al., 2024). The composition and energetic density of prey at new latitudes can impact metabolic rates and reproductive outputs, given well-established energetic costs for long-distance migrators (Alerstam et al., 2003; Hays, 2008). As a sentinel species, the northern shift of juvenile loggerheads to access forage habitat may serve as an early warning sign of tropicalization within the NPTZ. This is especially true as species such as sea turtles have a narrower thermal tolerance compared to other highly mobile species within the region (Hazen et al., 2013). If prey assemblages continue to shift polewards at a rate faster than warming temperatures, sea turtles may be one of the first mobile species to lose the ability to synchronize the availability of forage habitat with thermal displacements. For example, Brodeur et al. (2018) and Gomes et al. (2024) reported an observed northward expansion of several gelatinous loggerhead prey species, such as pyrosomes, within the CCS. The increased abundance of these prey species coincided with the occurrence of juvenile loggerhead sea turtles in the Pacific Northwest further north than usual (Arden, 2023; Early-Capistrán et al., 2024; Ramirez, 2024). Within the past 5 years, a total of 20 stranded loggerheads have been recorded off of the US and Canada West Coast (NOAA Sea Turtle Stranding Network, 2024). Scat analysis of one particular individual found as far north as Vancouver, Canada, revealed the presence of pyrosomes (Parker et al., 2024) suggesting loggerheads may be following these typically tropical prey sources further north. If forage habitat continues to change at a faster rate than thermal tolerances, winners and losers will emerge, with many species struggling to keep pace. For North Pacific loggerhead sea turtles, a northward expansion becomes highly relevant in terms of energetic costs to forage at new latitudes while maintaining connectivity to known coastal habitats, such as the well-established developmental grounds of Baja California, Mexico (Nichols et al., 2000; Peckham et al., 2011; Wingfield et al., 2011; Seminoff et al., 2014; Tomaszewicz et al., 2015). Individuals will be faced with the need to travel farther south or establish new coastal habitats along the California coast (Allen et al., 2013; Eguchi et al., 2018; Welch et al., 2019).

Our study provides critical information as to how animals are responding to significant long-term change in one of the oceans most productive regions. To date, most studies have emphasized the role of temperature in reshaping habitat. Indeed, temperature plays a significant role in marine biodiversity patterns (Tittensor et al., 2010) and geographic redistributions (Poloczanska et al., 2013; McHenry et al., 2019). But recent insights have broadened our understanding of other environmental covariates (i.e., salinity, currents, productivity, pH) in species’ responses to climate change (see Poloczanska et al., 2016; McHenry et al., 2019). For sea turtles, a conventional approach might be to overemphasize temperature-based models when projecting species’ responses to change, especially given extrinsic thermal vulnerabilities (Goudarzi et al., 2024). Here we also underscore the importance of including other environmental variables in anticipating species’ shifts in a changing ocean.

Within the last few decades, the North Pacific has exhibited unprecedented change. The changes that we are seeing in the NPTZ oceanography and the northward shift in latitude of loggerhead sea turtles appear to be part of a broader northward expansion of the subtropical gyre and northward shift of the NPTZ as has been projected in climate models and observed with SST and altimetry data trends (Polovina et al., 2011; Yang et al., 2020). The documented northward shift in loggerhead presence is likely indicative of broader ecosystem impacts within the NPTZ including impacts to the foraging habitat of commercially valuable pelagic fishes, and other protected species. Understanding how sentinel species will respond and adapt to such an accelerated transformation is imperative to effectively maintaining and managing healthy ecological connections across their entire North Pacific habitat. To date, our ability to track changes to the environment and animal response has been limited. Future research is needed to fully understand these important changes within the context of a warming ocean. The outcomes of this study provide first-order documentation of a significant change to environmental conditions that have defined this ecologically important region and a population-level response by a high trophic level predator species that is highly sensitive to ecosystem change. In a warmer, less productive NPTZ, range shifts and expansions will result in habitat gains and losses and will pose new management challenges within international jurisdictions (Santos et al., 2024). For example, as loggerheads move further north, those that head eastward may enter the California Current at higher latitudes and face new mortality risks from different fisheries and cold stunning. Continued observation of decadal-scale animal response can be assimilated with short-term (i.e., seasonal, interannual) and longer-term information (i.e. climate modeled predictions). Such information will be critical for conservation assessments and management strategies for this endangered species. Sentinel species like loggerhead sea turtles, highly migratory and exceptionally sensitive to environmental changes, offer invaluable insights into the health and stability of the North Pacific ecosystem, and may provide early warning signals to improve forecasted species’ responses in a warming ocean. The exceptional dataset used in this study offers the first longitudinal perspective of its kind, giving scientists the ability to track subtle yet significant shifts in oceanic conditions and animal distributions, facilitating more accurate predictions, and enhancing dynamic management strategies (Howell et al., 2015; Maxwell et al., 2015).