High sensitivity of a keystone forage fish to elevated CO and temperature.

Sand lances of the genus are keystone forage fish in coastal ecosystems across the northern hemisphere. Because they directly support populations of higher trophic organisms such as whales, seabirds or tuna, the current lack of empirical data and, therefore, understanding about the climate sensitivity of sand lances represent a serious knowledge gap. Sand lances could be particularly susceptible to ocean warming and acidification because, in contrast to other tested fish species, they reproduce during boreal winter months, and their offspring develop slowly under relatively low and stable CO conditions. Over the course of 2 years, we conducted factorial CO × temperature exposure experiments on offspring of the northern sand lance , a key forage species on the northwest Atlantic shelf. Wild, spawning-ripe adults were collected from Stellwagen Bank National Marine Sanctuary (Cape Cod, USA), and fertilized embryos were reared at three CO conditions (400, 1000 and 2100 μatm) crossed with three temperatures (5, 7 and 10 ˚C). Exposure to future CO conditions consistently resulted in severely reduced embryo survival. Sensitivity to elevated CO was highest at 10 ˚C, resulting in up to an 89% reduction in hatching success between control and predicted end-of-century CO conditions. Moreover, elevated CO conditions delayed hatching, reduced remaining endogenous energy reserves at hatch and reduced embryonic growth. Our results suggest that the northern sand lance is exceptionally CO-sensitive compared to other fish species. Whether other sand lance species with similar life history characteristics are equally CO-sensitive is currently unknown. But the possibility is a conservation concern, because many boreal shelf ecosystems rely on sand lances and might therefore be more vulnerable to climate change than currently recognized. Our findings indicate that life history, spawning habitat, phenology and developmental rates mediate the divergent early life CO sensitivities among fish species.