Environmental and anthropogenic factors regulating spatiotemporal variation in length-at-age and condition of herring ( Clupea harengus ) in the Central Baltic Sea

Abstract

Marine fish populations display large spatial and temporal variation in key biological traits such as growth. These variations influence individual fitness and can determine population dynamics by affecting survival, maturation, and reproduction. Ignoring spatial heterogeneity in growth when conducting stock assessment can lead to substantial bias in stock productivity estimates. Here we explored spatiotemporal changes in length-at-age (LAA) and body condition of Central Baltic herring (Clupea harengus; CBH) using fisheries-dependent data collected between 1984 and 2021. Our analysis showed that a long commercial data time series of individual length, age, and weight (n = 439 748) can be successfully used to depict significant spatiotemporal variation in key traits such as LAA and condition. We also quantified the influence of environmental and anthropogenic factors on these traits and discussed their implications for the management of the stock. We find a declining trend in LAA, but a relatively stable or recovering trend in condition, suggesting different drivers of these body size metrics in CBH. Environmental drivers, particularly temperature, play a key role in regulating both length-at-age and condition but do not seem to explain the observed decline in LAA. Moreover, adult body condition is shaped by sprat abundance and fishing mortality, highlighting the complex response of LAA and condition in CBH. Because patterns of change in the herring growth and condition vary across different areas of the Baltic, we suggest that integrating spatial structure into stock assessment and management is essential to better characterize the stock dynamics and track possible spatial depletion. This is increasingly relevant when considering that CBH is characterized by genetically differentiated subpopulations, which are also generally associated with certain areas of the basin.