Trailing edges projected to move faster than leading edges for large pelagic fish habitats under climate change

There is mounting evidence to suggest that many species are shifting their ranges in concordance with the climate velocity of their preferred environmental conditions/habitat. While accelerated rates in species’ range shifts have been noted in areas of intense warming, due to climate change, few studies have considered the influence that both spatial temperature gradients and rates of warming (i.e., the two components of climate velocity) could have on rates of movement in species habitats. We compared projected shifts in the core habitat of nine large pelagic fish species (five tuna, two billfish and two shark species) off the east coast of Australia at different spatial points (centre, leading and trailing edges of the core habitat), during different seasons (summer and winter), in the near-(2030) and long-term (2070), using independent species distribution models and habitat suitability models. Model projections incorporated depth integrated temperature data from 11 climate models with a focus on the IPCC SRES A2 general emission scenario. Projections showed a number of consistent patterns: southern (poleward) shifts in all species’ core habitats; trailing edges shifted faster than leading edges; shifts were faster by 2070 than 2030; and there was little difference in shifts among species and between seasons. Averaging across all species and climate models, rates of habitat shifts for 2030 were 45–60 km decade⁻¹ at the trailing edge, 40–45 km decade⁻¹ at the centre, and 20–30 km decade⁻¹ at the leading edge. Habitat shifts for 2070 were 60–70 km decade⁻¹ at the trailing edge, 50–55 km decade⁻¹ at the centre, and 30–40 km decade⁻¹ at the leading edge. It is often assumed that the leading edge of a species range will shift faster than the trailing edge, but there are few projections or observations in large pelagic fish to validate this assumption. We found that projected shifts at the trailing edge were greater than at the centre and leading of core habitats in all large pelagic fish included in our study. Faster shifts at species trailing edges were due to weaker spatial gradients in temperature in the north than in the south of the study region, in conjunction with relatively constant rates of warming across latitudes. Rather than assuming that leading edges will always move faster, this study suggests that spatial gradients of temperature could be important in determining differences in shifts at different points in species core habitats.