Harmony Point determines the sign of pooled chinstrap and gentoo nest-count change at 22 sites in 1980–2025

We tested whether gentoo nest-count gains offset chinstrap losses at the same sites in MAPPPD v5.0. We selected nest counts with recorded accuracy classes 1–3 in CCAMLR subareas 48.1 and 48.2. Each of 22 sites supplied its first and last complete season, at least ten years apart, within 1980–2025. The pooled combined count fell by 36.3%, and gentoo gains offset 29.2% of net chinstrap losses. However, 17 sites increased. Harmony Point supplied 66.9% of the initial count and 117.3% of the net loss. Without this site, the other 21 sites had a pooled increase of 19.0%. No other single-site exclusion reversed the sign. Ground-only, pre-2023, and known-date filters gave gains, mainly because they excluded Harmony Point. Exact accounting separated dropped sites from changed endpoints at retained sites. A spatial block interval for primary pooled change ranged from −49.8% to +61.8%. The sites were not selected at random, and endpoint years differ. These sums are not regional population totals at common dates. The contribution is a reproducible paired-count dataset and an audit of how one large site and data filters affect a two-species sum. It does not establish regional compensation or a cause of population change.

penguins · species compensation · Antarctic Peninsula · MAPPPD · biodiversity monitoring · sampling sensitivity · nest counts