ABSTRACT: Identifying marine resource stocks is crucial for developing effective fisheries management strategies. Biological tracers, such as stable carbon and nitrogen isotopes, capture the phenotypic expression of an individual’s trophic history throughout its lifespan or part thereof. Consequently, these tracers are valuable tools for examining fish stocks from a trophic perspective (trophic units). Our study used stable isotope analyses (SIA) of soft tissues (white muscle) to assess different trophic units of the Fuegian sprat Sprattus fuegensis, a crucial forage species essential for regulating food webs in the Southwest Atlantic Ocean. We focused on 4 marine areas: the estuarine Beagle Channel (inner and outer sectors), the Atlantic coast and shelf areas off Tierra del Fuego, the Malvinas Islands, and the Namuncurá-Burdwood Bank Marine Protected Area over 2 consecutive years during the austral spring of 2018 and 2019. SIA showed that the sampled sprat groups in these areas represent distinct trophic units that remained consistent across both years. The observed differences between units are unlikely to be due to intrinsic factors (e.g. biological variables), rather suggesting a strong spatial association influenced by physical and geographical variables. Furthermore, changes in niche breadth were likely related to variations in resource consumption in the same area, influenced by shifts in ecosystem productivity. Our findings suggest a notable isotopic footprint denoted by contrasting baseline isotopic values that affect marine consumers. This approach can aid in identifying fish stocks and conducting geographical assignments, enhancing its effectiveness by employing a combination of techniques.