An improved methodology for calculating the nitrogen footprint of seafood

Abstract

Human activities create more reactive nitrogen than is created through natural processes every year. The excess reactive nitrogen in the environment causes various negative effects including eutrophication, climate change, acidification, and human health problems. The sources of the excess nitrogen are mainly fertilizers and animal and human waste generated by food production and consumption. Therefore, our food choices have major effects on the nitrogen load to the environment. To quantify the load, a consumption-based accounting tool, called the nitrogen footprint, has been recently developed. In current nitrogen footprint models, seafood is calculated as a single category using the same simple assumptions as livestock. However, there is a variety of types and methods of production of seafood. In addition, world per capita consumption of seafood is projected to continue expanding. In this paper, we propose a new nitrogen footprint model to evaluate the impact of seafood in detail, present the results of the model applied to Japan as a case study, and explain the important parameters that are needed to accurately evaluate the load of seafood consumption. Our model tracks the feeding steps in detail, considering differences among fed aquacultured seafood, non-fed aquacultured seafood (mainly bivalves and filter-feeding carp), and captured seafood. Our model evaluates the Japanese food nitrogen footprint of fed aquacultured seafood as 0.7kg-N/capita/year, ca. 45% of that of all seafood, whereas the previous model evaluates it as 3.36kg-N/capita/year. Our results demonstrate that the key factors for assessing the nitrogen load of seafood are the proportions of fed aquaculture and of plant protein in feed. In order to enable food choices that will effectively reduce nitrogen release, we provide the virtual N factors (per intake nitrogen release during production) for different seafood as 0.2 (non-fed aquacultured and captured), 4.8 (freshwater and diadromous fish), 3.9 (demersal fish), 3.4 (pelagic fish and other marine fish), and 8.2 (crustaceans). Our results show that eating more non-fed aquacultured and captured seafood and less fed-aquacultured shrimps and prawns could reduce our nitrogen load from food consumption as effectively as choosing poultry instead of beef. Although its evaluation is limited to nitrogen load related to food itself and not including the load from energy use, etc., our detailed nitrogen footprint model for different seafood categories and production sources can quantify the effectiveness of policies and actions that link sustainable consumption and sustainable fisheries and aquaculture, contributing toward the Aichi Biodiversity Targets.