After the 2014–2016 marine heatwave, the CalCOFI larval fish community temperature index followed local temperature in 2017–2023 at 66 stations; an independent analysis reproduces the 0.2 °C/°C slope

Question: after the 2014–2016 northeast Pacific marine heatwave, was the larval fish community off southern California warmer than local temperature predicts? We used CalCOFI ichthyoplankton tows at the 66 core stations (1951 to January 2023; 62 consistently identified taxa). For each tow we calculated a community temperature index (CTI): the log-density-weighted mean of species temperature indices, which come from the long-term spatial and seasonal distribution of each taxon. We fitted the cruise-level relation between CTI and temperature anomalies on the 1978–2013 bongo cruises. We used two temperature sources: in situ 10 m bottle temperature to May 2021, and OISST v2.1 SST from 1982 to 2023. The CTI changed by 0.22 ± 0.02 °C per °C of in situ temperature anomaly (OISST 0.21; R² 0.55). This slope is 54–87% of the spatial slope and far outside a null with STI values shuffled among taxa. The slope was the same in 1951–1977, 1978–2013 and 2014–2021 with in situ temperature. During the heatwave, the CTI increase agreed with the prediction (mean residual +0.03 ± 0.06 °C). After 2016, the residual was +0.035 ± 0.058 °C (2017–2021; OISST +0.016 ± 0.048, 2017–2023), about +0.1 to +0.2 °C as a temperature equivalent. The CTI stayed high in 2017–2020 because the water stayed warm, and it went back to near zero with temperature in 2021–2023. Temperature of the previous year added a small lagged effect. In all ten variants of weighting, taxon set, STI definition and temperature source, the 95% interval of the post-2016 residual included zero. An STI without seasonal temperature gave the largest offset (+0.45 to +0.56 °C equivalent), so a small offset is possible. During the heatwave, warm-affinity taxa were more abundant than temperature predicts, but the warm-to-cold composition followed temperature. Limitations: fewer cruises after 2017; in situ temperature only to May 2021; larvae show spawning and transport, not adult ranges. The slope, lag and 2014–2016 results independently reproduce sci:sha256:16e7fa6aa99994401effde45c652662aa580554e63da0947c4b7aae50a0c6c26 (same data, different code, taxa and second temperature source); the formal 2017–2023 test is new. Code, derived tables and retained raw data are included.

CalCOFI · ichthyoplankton · larval fish · community temperature index · marine heatwave · California Current · hysteresis · thermal affinity · climate tracking · OISST