Mystery of the Disappearing Dogfish: Transboundary Analyses Reveal Steep Population Declines Across the Northeast Pacific With Little Evidence for Regional Redistribution

ABSTRACT Quantifying broad‐scale population trends and distribution change is critical for effective management and conservation of marine species, particularly under climate change. However, fragmented regional survey data often hinder such efforts for transboundary populations. A prime example is Pacific Spiny Dogfish ( Squalus suckleyi , Squalidae), a small shark with a remarkably slow life history and wide‐ranging distribution. Dogfish are now caught incidentally but were heavily fished along the Pacific US–Canada coast ≈80 years ago. Reports on local population trends have conflicted along the coast, suggesting that movement between regions may be responsible. We fit spatiotemporal models integrating data from 10 surveys to synthesise trends in biomass, abundance, distribution and thermal niche for dogfish across their entire eastern Pacific Ocean range. Prior to 2003, Alaskan biomass increased through the 1990s whereas California to British Columbia indices were variable and imprecise. However, during 2003–2023, we found a coastwide 51% (95% CI: 38%–61%) decline in dogfish biomass with mature females and immature dogfish showing the largest proportional declines. Regionally, declines were steepest for the US West Coast (71%–85%) and Canada (58%–82%), while Alaska showed less severe declines (13%–54%). Off the US West Coast, dogfish shifted into deeper waters as temperatures in their habitat increased, but these patterns do not explain the coastwide declines. Our results suggest population declines are primarily driven by reduced abundance rather than between‐region movement, indicating elevated coastwide conservation concern and helping focus investigations of causal mechanisms.