Baltic herring experiences differences in environmental conditions across its habitats e.g. through temperature, food availability and salinity. The interesting paper by Vainikka et al. (2009) investigated phenotypic differentiation among fisheries and survey samples collected in different areas, with the aim to understand if Baltic herring might express counter-gradient variations in growth and maturation. They found that fish in north grew slower but also tended to mature smaller than those in south, which might indicate counter-gradient variation. The practical implication of this would then be that northern and southern stock components might respond differently to management actions.
Vainikka A, Mollet F, Casini M, Gårdmark A (2009) Spatial variation in growth, condition and maturation reaction norms of the Baltic herring Clupea harengus membras. Marine Ecology Progress Series 383: 285-294.