Fat fish, thin fish: regime shifts in fish condition and environmental drivers Marine Ecology Progress Series

Fish condition is an indicator of fish health, where fatter fish are generally healthier and produce more viable recruitment. Many factors may contribute to fish condition, including temperature, food availability and quality, competition and even fishing pressure. In this study, relative condition factors (Kn) were analyzed from 1992-2023 for 50 finfish species regularly caught on the Northeast Fisheries Science Center fall bottom trawl survey. Declines in condition occurred around the year 2001 across most fish species on the Northeast US Continental Shelf, with many species recovering around 2009. These changes in condition have direct implications for fish recruitment and mortality, and may in turn impact stock assessments, catch quotas and management. Similar changepoints were found in bottom and sea surface temperatures, zooplankton anomalies and copepod size structure, potentially indicating a bottom-up regime shift. Determining relative condition changepoints along with environmental drivers has enabled stock assessments to base biological reference points on ecosystem-based time stanzas specific to the stock. Empirical dynamic modeling analyses suggest causal relationships of these environmental drivers with Atlantic mackerel and butterfish condition, further strengthening the justification of using these relationships in assessments.