Salp catch presence showed no clear rise at 16 Palmer survey cells from 1993–2008 to 2009–2024

We tested whether summer salp catches spread while Antarctic krill catches contracted at repeated Palmer LTER survey cells west of the Antarctic Peninsula. We used 753 standard January–February net tows in 30 survey years. We compared 1993–2008 with 2009–2024 at 16 cells with at least five survey years in each era. We gave cells equal weight and years equal weight within cells. Salp catch presence changed from 54.0% to 47.6%, a difference of −6.4 percentage points (95% three-year block interval −22.8 to +11.6). Krill presence changed from 92.8% to 96.7%; its increase was not clear in all sensitivity checks. Salp-only catches changed from 4.7% to 3.3%. Mean salp density was 32.7 and 103.7 individuals per 1000 m³, a ratio of 3.17 (interval 0.65 to 8.73). The largest 10% of later salp tows supplied 95.9% of weighted salp density. Thus the higher mean density estimate did not come with a higher catch-presence estimate. Exact station cells, a common date window, and date, night and volume adjustments gave the same direction for salp presence. The data do not establish a spread of salp catches or local replacement of krill. The result applies to selected cells and sampled summers. A net absence is not true species absence, and the presence interval still permits a rise of about 12 percentage points. We provide code, source hashes, aggregate results, figures and an independent CSV audit. Raw source files are retrieved from fixed archive versions because their reuse terms are mixed.

Antarctic Peninsula · krill · salps · species distributions · catch presence · sampling design · long-term monitoring · reproducible analysis