Fishes are ectothermic; therefore, their metabolism and biology are directly affected by thermal exposure (Ficke et al. 2007; Schulte 2015; Little et al. 2020). Fishes exhibit a general trend of increasing metabolism with increases in water temperature, up to a maximum temperature at which metabolism rapidly decreases to a critical thermal maximum (Schulte 2015; Ern et al. 2023). Increasing water temperatures is one general trend linked to climate change (Whitehead et al. 2009; Wu et al. 2012; van Vliet et al. 2013). Thus, understanding thermal tolerance of fishes is crucial for predicting impacts of climate change on their populations and developing effective management strategies. Pacific lamprey (Entosphenus tridentatus Richardson, 1836) is a wide-ranging anadromous species of conservation interest in the United States because of its significance to Indigenous tribes, its important ecological roles, and concerns over significant declines in abundance (Close et al. 2002). Laboratory studies of temperature impacts on embryos and larvae (Meeuwig et al. 2005) and adults (Clemens et al. 2009) and reviews on the thermal ecology of Pacific lamprey conclude that warmwater temperatures (i.e., ≥ 20°C; Clemens 2022) are a threat to the species (Clemens et al. 2016; Clemens 2022). By contrast, three recent papers that fill data gaps on the thermal tolerance of larval Pacific lamprey conclude that warmwater conditions experienced by them and projected under a warming climate are not a significant threat to this life stage (Whitesel and Uh 2023; Whitesel and Sankovich 2025) or the species (Reid and Goodman 2024). Key findings of these studies are listed in Table 1. These disparate conclusions on whether warmwater temperatures are a threat to Pacific lamprey may confuse management efforts, which could result in variable levels of caution when evaluating temperature impacts on the species or implementing instream habitat restoration. For example, different thermal thresholds for adverse impacts may be applied when conducting environmental assessments. Herein we provide a different perspective on how to interpret the findings of these three recent studies. Reid and Goodman (2024) suggest the relatively high thermal tolerance of the larval life stage provides resiliency to the species, playing down the potential population-level consequences of the adverse impacts of elevated water temperatures on the adult and early life stages. The authors conclude that ‘…rising stream temperatures are not likely to restrict the distribution of western lampreys in the foreseeable future, and therefore, conservation strategies should focus on other issues, such as restoring access to historical habitats, avoiding stream desiccation, and restoring natural flow regimes’. Given the remaining uncertainties in how rising temperatures will influence larval lamprey ecology and distribution, along with the likelihood that the distribution and survival of other life stages are adversely impacted by rising temperatures (Meeuwig et al. 2005; Clemens et al. 2009; Clemens 2022), this conclusion overlooks precautionary principles (see below). Reid and Goodman (2024) state further that ‘…temperature may not limit spawning or early rearing under current or projected conditions’, based on the points that (1) ‘…hatching and early development tend to occur before stream temperatures reach seasonal highs’, and (2) observed mean May temperatures in the warmest stream in their study area only reached 22.5°C. Even if mean monthly water temperatures of 22.5°C during spawning in May do not directly reduce the survival of spawning adults (but see Clemens et al. 2016 and Clemens 2022), embryos would be exposed to warmwater temperatures that are detrimental to their survival. Meeuwig et al. (2005) found a sharp decline in the survival of both ‘fertilization-to-hatch’ and ‘hatch-to-larvae’ stages as rearing temperature increased from 18°C to 22°C. Embryos reared at 22°C were also approximately six times more likely to have developmental abnormalities than those reared at lower temperatures. Hence, when water temperatures exceed 20°C during embryonic development, it is likely that larval recruitment will be considerably reduced. Although embryonic development does not typically coincide with the annual thermal maximum, many embryos are still developing during May and June (Brumo et al. 2009), when daily mean water temperatures in many streams exceed 22°C, including the example stream discussed by Reid and Goodman (2024). Thus, under current conditions, and especially under projected future conditions, warmwater temperatures may impact lamprey embryos and could, therefore, limit the abundance and distribution of the species. Whitesel and Uh (2023) state: ‘Since Pacific lamprey may spend as much as 85% of their life as larvae in freshwater, this may be one of the most important life stages when assessing the overall resiliency of the species to climate change’. Similarly, Whitesel and Sankovich (2025) state: ‘Given that the Pacific lamprey likely exists as larva for the majority of its life history, and the relative diversity of environments in which larvae rear, the greatest impact of climate warming may be to this developmental stage’. However, temperature impacts to more thermally-sensitive, shorter-duration life stages, such as embryos and adults (Table 1), may have pronounced population-level consequences. Furthermore, as the authors generally acknowledge, the conclusion that the larval stage is resilient to high water temperatures is based on study of a single causative factor (i.e., water temperature), and does not consider potential additive, synergistic or antagonistic effects of other stressors (such as water quality) on larval survival. We encourage application of precautionary principles when evaluating potential warmwater impacts and developing protective measures for Pacific lamprey. Precautionary principles are used in environmental risk management and conservation. These principles espouse the need to take precautionary measures when dealing with uncertainty, placing the burden of proof on resource users and developers, exploring alternatives to potentially harmful actions, and where possible, increasing public input in decision-making (Kriebel et al. 2001; deFur and Kaszuba 2002). Implementation of precautionary principles stems from the belief that conservation actions are outpaced by threats to ecosystems, and that holistic scientific understanding of threats to the environment is difficult and elusive, often involving complex interactions and feedback loops (Kriebel et al. 2001). In the case of warmwater and Pacific lamprey, we maintain that precautionary principles would be to assume that the stressor to this species (in this case, warmwater and related water chemistry) is harmful and can have negative impacts (either acute or chronic) on individuals and populations, and then to plan studies that can make inferences based on these assumptions. Without complete knowledge of the thermal exposure and influences of other stressors on individual larval lamprey (or of any life stage), we believe that it is impossible to rule out that warmwater temperatures produce chronic, sublethal, delayed mortality, and/or population-level effects on them. Further, observations of differential survival and developmental abnormalities for larvae that were exposed as embryos (Meeuwig et al. 2005), die-offs of adult Pacific lamprey and associations of gonadal morbidity (Clemens et al. 2016; Clemens 2022), and expedited sexual maturation (Clemens et al. 2009) provide evidence that the species is vulnerable to warmwater conditions. Therefore, we encourage fisheries managers to apply precautionary principles when interpreting study results and considering management actions for Pacific lamprey—for example, when evaluating potential temperature impacts of a project or designing measures to avoid temperature impacts during fish salvage. Benjamin J. Clemens and Abel F. Brumo: conceptualization, visualization, writing – original draft and writing – review and editing. This work was funded by the authors' organizations. The authors have nothing to report. The authors declare no conflicts of interest. The authors have nothing to report.
Clemens et al. (Tue,) studied this question.