Fish stocks have decreased substantially over the last decades due to human exploitation of the ocean. This declining trend has been exacerbated by climate change, with acidifying waters harming marine life. This paper exploits exogenous variation in water acidity across time and space to study how the ocean impacts early-childhood mortality. We collate and analyze more than 1.5 million births between 1972 and 2018 in communities near the shore of 36 developing countries. By comparing children born in the same location but on different dates and controlling for a set of high-dimensional fixed effects, we identify the causal impact of in utero exposure to the ocean’s acidity on mortality. In coastal areas, a 0.01 unit increase in acidity causes 2 additional neonatal deaths per 1,000 live births. This result is robust to within-siblings comparisons. It is selectively affecting the weakest children as the effect gradually vanishes after the first month of life. Mothers do not compensate with any additional health investment during the gestation period. Reduced access to nutrients derived from fish that are essential to fetal growth is the key mechanism behind our findings. While fish is critical to global food security, humanity’s relationship with the ocean remains poorly understood. This paper provides the first quantitative evidence linking the exploitation of natural resources with malnutrition and neonatal selection.

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