Herring (Clupea harengus) recruitment failure in the western Baltic Sea as a consequence of a drastic zooplankton decline

Climate-induced shifts in zooplankton communities can influence the survival and recruitment of commercially important fishes worldwide by altering the prey field, potentially causing severe food limitation. Due to the recruitment failure of the western Baltic herring stock over the last decade, we investigated the role of zooplankton and potential environmental drivers of early life stage survival in a major nearshore nursery area in the western Baltic Sea. The seasonal synchrony between predator and prey structures annual secondary production. However, studying the cause-and-effect relations is challenging, as many monitoring programs are static in time and limited to a short period every year. We analysed weekly data from 2008 to 2020, including zooplankton density and community composition together with herring larvae survival. Larval herring productivity has been notably low in recent years, particularly since 2014, reaching a record low in 2020. Annual mean total zooplankton density has also decreased drastically, with the largest decline (~84%) occurring between 2012 and 2013. This decrease was predominantly seen in small organisms (copepod nauplii, bivalve veligers, and rotifers). The total density of these major prey organisms was positively correlated with the larval herring survival index for early herring life stages. The decrease in zooplankton taxa density was most likely induced by changes in phytoplankton phenology expressed by earlier seasonal chlorophyll a peaks. Using these high-resolution time series, we provide a potential mechanism of cascading climate change effects on the survival of young herring in the western Baltic Sea.