Species acclimatization pathways: Latitudinal shifts and timing adjustments to track ocean warming

Abstract

Poleward distributional shifts and phenological adjustments are universal ecological responses to ocean warm-ing. Though they are commonly addressed as independent processes, phenology usually depends on the lat-itudinal location where an organism lives. Such interaction can mask the actual impacts of warming and may even constrain the ability of a species to cope with ocean warming. We propose a framework to analyse alter-native acclimatization pathways that accounts for pure distributional shifts, pure phenological adjustments, and mixed response. We tested this analysing drivers of observed changes in the spawning of horse mackerel (Tra-churus trachurus) and Atlantic mackerel (Scomber scombrus) in northeast Atlantic in 1992-2019. Since species had contrasting population trajectories, data from the same survey is suitable to test whether shifts are driven by warming or density-dependence. Species showed contrasting responses to warming; horse mackerel spawned earlier (11.7 +/- 1.5 days/degrees C of warming), whilst Atlantic mackerel shifted northward (367 +/- 22 km/degrees C of warming). The narrower reproductive thermal niche of horse mackerel resulted in a tight coupling between latitude and timing of spawning that is consistent with geographical attachment, and an acclimatization con-strained to an earlier phenology. The northward shift of Atlantic mackerel might be associated to its wider thermal reproductive niche, which enables prolonged migrations and almost synchronous spawning along the Atlantic margin. Results support our framework underpinning that acclimatization can combine or exclude timing and poleward shift, which could depend on species traits. These new insights are useful for fisheries management and to improve habitat projections under climate change.