Harbour porpoises (Phocoena phocoena) are the most common cetacean in the North Sea and are considered useful indicators of ecosystem state due to their high metabolic demands and sensitivity to prey availability. Despite the ecological relevance of the Belgian Part of the North Sea (BPNS) as a recolonised habitat, the extent to which porpoise diet reflects local prey dynamics remains poorly characterised. This study analyses stomach contents from 138 harbour porpoises stranded or bycaught in Belgian territory between 2000 and 2025. The diet was dominated by gobies, gadoids, and sandeels, with secondary contributions from clupeids and squid. Significant seasonal and ontogenetic variation was detected, while sex had no detectable effect on diet composition. Interannual dietary composition was relatively stable across the 25-year study period despite documented fluctuations in North Sea forage fish stocks. Seasonal dietary patterns for gadoids and sandeels were broadly consistent with prey availability dynamics inferred from fisheries survey data. Narrower intra-individual dietary breadth in summer adds nuance to the conventional framing of this species as a generalist predator and has implications for how predator diet–prey availability relationships are interpreted in a changing environment.
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