© 2019 Elsevier Ltd Ocean resources have been exploited at unprecedented rates, leading to marine biodiversity loss, food web changes, and other alterations of ocean ecosystem functions and structures. The capture of wild fish for human consumption and fishmeal are the primary drivers. Microalgae oil has long been investigated for biofuel production. Its co-product, defatted microalgal biomass, has potential to replace fishmeal from wild fish catch and thus mitigate ocean resource depletion. This study develops a new indicator for assessing consequential impacts on ocean resources in life cycle assessment. The indicator is based on primary production required, a concept previously used in ecological assessments and life cycle assessments to evaluate ecological impacts of fisheries and aquaculture. We estimate the primary production required for fishmeal production from the ocean (166 kg carbon/kg fishmeal), and the potential of defatted microalgae biomass displacing fishmeal. Results show that defatted microalgae biomass can lead to highly variable, but potentially significant, reductions in ocean resource demand. The variability is a function of the potential for replacement, which depends on the cultured fish species considered. As an example of this significance, based on available data for estimating the potential for defatted microalgal biomass to displace fishmeal for cultured tilapia, salmon, shrimp, carp, flounder, yellowtail and cod, by 2020 net primary production demand from the ocean could be reduced by approximately one billion tons of carbon.