Migratory behaviour and otolith chemistry suggest fine-scale sub-population structure within a genetically homogenous Atlantic Cod population

The question whether temperate marine fishes typically consist of self-sustaining populations or "open" populations still remains unresolved. At the heart of this population connectivity problem lays the nature of the stock separation mechanisms. Fish populations could be segregated either by environmental forcing, accompanied with opportunistic recruitment of juveniles to spawning areas, or by philopatric behaviours (i. e., inclination of an individual to return to or remain in its natal area). Here we report three, partly independent, studies on Atlantic Cod (Gadus morhua) stock separation in the Kattegat and Öresund (eastern North Sea): characterisation of spawning aggregations with genetic markers, tagging experiments, and analysis of chemical constituents in otolith cores of recaptured fish that could be linked to a specific spawning site. While the genetic investigation showed no population segregation, the observed migratory patterns indicated three separate spawning sites at close distances. The natal dependence on the choice of spawning site was tested by measuring the contents of various trace elements in the otolith core of recaptured tagged fish. Quantification of the trace elements: Ba, Br, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sr, Ti, and Zn expressed as ratios to Ca were obtained using scanning micro PIXE. These results indicated that natal origin could be differentiated between spawning sites, supporting the hypothesis that natal homing is an important stock separating mechanism even over short distances (<100 km).