Food production significantly impacts Earth's systems and accounts for approximately a quarter of all greenhouse gas emissions. To create a more sustainable food system, scientific evidence emphasizes replacing consumption of certain types of protein with high environmental impact such as beef and pork, with protein with less-impact alternatives such as fish or vegetable protein. This study evaluates a nudge intervention at a medium sized grocery store designed to increase purchases of fish. Alongside it also examines other relevant factors influencing protein choice, such as values, attitudes, habits, demographics and price. To assess the nudge's impact a natural field experiment was designed, and the effect was measured by observing changes in sales of fish (over 59,000 items sold over 143 days). Additionally, data was collected from a selected sample of customers (N = 147) to further explore protein choice determinants. The results fail to demonstrate a significant effect of the nudge intervention. Instead, values, habits, attitudes and price significantly influence protein selection. These findings underscore the complexity of shopping decisions and how nudge interventions are not always easy to implement, adding important null findings to the available literature. Policy implications and possible improvements are discussed, emphasizing the need to account for habits and habit-breaking when designing interventions that aim to steer similar shopping decisions.