Spatial variation in growth, condition and maturation reaction norms of the Baltic herring, Clupea harengus membras

Understanding of spatial patterns in life-history traits can help fisheries management focus on biologically and functionally relevant stock units. In the present study, we examined lifehistory variation in growth, condition and maturation of the Baltic herring Clupea harengus membras among different areas of the Baltic Sea. As expected based on environmental gradients, herring grew faster in southern than in northern areas. The condition factor for young individuals was higher in the north, but higher for older individuals in the south. Probabilistic maturation reaction norms (PMRNs) based on age, length and condition indicated counter-gradient variation: young herring in the northern areas reached the size at which they had a 50% probability of maturing when they were comparatively smaller than the southern specimens. However, the north–south differences in PMRNs were reversed in older age groups. This indicated that maturation of herring in the north was more size dependent (zero PMRN slope) than it was for herring in the south, where maturation was predominantly determined by age (negative PMRN slope). The geographical differentiation in maturation schedules would potentially translated into divergent changes in recruitment in response to changes in density-dependent growth and, hence, also fishing patterns