Spatial and temporal heterogeneity of the cod spawning environment in the Bornholm Basin, Baltic Sea

The present study quantifies the spatial heterogeneity of the environmental conditions associated with successful spawning by cod Gadus morhua in the Bornholm Basin. Quarterly means from 1989 to 2003 of the thickness of reproductive volume that enabled egg survival indicate that most favourable spawning conditions were located in the central area, inside the 80 m isobath. On average, spatial patterns were similar in the second and third quarters, but with overall lower thicknesses of reproductive volume and less horizontal extension of conditions suitable for egg survival in the third quarter. The observed basin-wide variation in thickness of reproductive volume and oxygen content inside this volume can result in marked horizontal differences in oxygen-related egg mortality, especially during stagnation years. The spawning habitat selected by adult Baltic cod in the Bornholm Basin was characterised by comparing data on egg abundance with environmental variables measured concurrently with egg collection. A clear preference for spawning at locations in the deep basin is evident after both inflow events, as well as for stagnation periods. In consequence, the closed area for fishing implemented in the Bornholm Basin during the main spawning periods from 1995 to 2003, although located in the northern centre of the basin, did not necessarily ensure undisturbed spawning in stagnation years.