This study aims to provide a quantitative analysis of the amino acids (AAs) and fatty acids (FAs) found in by-products of the Patagonian marine polychaete Abarenicola pusilla. Samples of polychaetes were taken in different seasons and processed to generate three by-products: eviscerated whole body, press cake, and press liquid. The results showed a clear seasonal difference, with essential nutrients decreasing significantly in winter. For example, from eviscerated whole body specimens, essential fatty acids, eicosapentaenoic acid (EPA), and arachidonic acid (ARA) maintained a relationship of 1 in summer, 1.24 in winter, and 1.5 in spring and autumn. In comparison, docosahexaenoic acid (DHA) was undetected in some extracts in winter. However, the fatty acid content in press liquid resulted in high variation, with the lowest content during the autumn. In contrast, the press liquid and press cake responded similarly. DHA decreased from spring to winter (from 0.1 to 0.08), whereas EPA resulted in high variation along the seasons (from 0.57 to 0.03) from summer to autumn. The highest protein values were observed in the spring and summer. In contrast, lipid values remained under 10% constant throughout all seasons, unless in the eviscerated carcass, where 5 to 3% higher lipids were found, indicating that A. pusilla uses protein more efficiently than lipids as an energy source. The most abundant EAAs were lysine (LYS) and leucine (LEU), while glycine (GLY), glutamic acid (GLU), and alanine (ALA) were the most abundant NEAAs. Finally, A. pusilla harvest conditions and the type of extract recommended to produce a valuable ingredient for marine aquafeeds are discussed.