Mapping the spawning grounds of North Sea cod ( Gadus morhua ) by direct and indirect means

Abstract

Despite recent evidence for sub-stock structuring, North Sea cod are assessed as a single unit. As a consequence, knowledge of sub-stock trends is poor. In particular, there are no recent evaluations of which spawning grounds are active. Here we report results from the first ichthyoplankton survey to cover the whole North Sea. Also, this survey, conducted in 2004, was the first to make extensive use of DNA-based molecular methods to unambiguously identify early developmental stage cod eggs. We compare the findings from the plankton survey with estimated egg production inferred from the distribution of mature cod in contemporaneous trawl surveys. Results from both approaches were in general agreement and showed hot spots of egg production around the southern and eastern edges of the Dogger Bank, in the German Bight, the Moray Firth and to the east of the Shetlands. These areas broadly coincide with known spawning locations from the period 1940 to 1970. We were, however, unable to directly detect significant numbers of cod eggs at the historic spawning ground off Flamborough (northeast coast of England). The results demonstrate that most of the major spawning grounds of cod in the North Sea are still active but that some localized populations may have been reduced to the point where it is now difficult to detect the presence of eggs in the plankton.

The predictive potential of early life stage individual-based models (IBMs): an example for Atlantic cod Gadus morhua in the North Sea

“…Cod spawning in the North Sea occurs in the Southern Bight, German Bight, Dogger Bank and the English Channel (Fig. 1), which constitutes the southernmost edge of Atlantic cod spawning distribution (Fox et al 2008, Brander et al 2010 in the Northeast Atlantic. Spawning appears to be not equally distributed in these areas but restricted to specific regions (Daewel et al 2011).…”

Parallel adaptive evolution of Atlantic cod on both sides of the Atlantic Ocean in response to temperature

“…Fox et al 2009) and adaptive structuring (Hutchings et al 2007) remain contentious. Tagging and oceanographic studies indicate a high proportion of resident non-migratory stocks (Robichaud & Rose 2004) as well as discrete spawning locations within single management units (Fox et al 2009); both observations support a hypothesis of unrecognized diversity.…”

“…Fox et al 2009) and adaptive structuring (Hutchings et al 2007) remain contentious. Tagging and oceanographic studies indicate a high proportion of resident non-migratory stocks (Robichaud & Rose 2004) as well as discrete spawning locations within single management units (Fox et al 2009); both observations support a hypothesis of unrecognized diversity. In contrast, studies of neutral microsatellite loci and mtDNA generally indicate weak spatial structuring over large spatial scales (Bentzen et al 1996;O'Leary et al 2007;Carr & Marshall 2008) consistent with a significant mismatch between the spatial scale of genetic differentiation and management (Reiss et al 2009).…”

Inter-annual and inter-specific differences in the drift of fish eggs and yolksac larvae in the North Sea: A biophysical modeling approach

“…Tagging studies of plaice have revealed a close proximity of annual spawning locations by specific females on Dogger Bank (see Hunter et al, 2003). Similarly, a recent survey of Atlantic cod eggs suggested that spawning was restricted to a number of relatively small "pockets" of activity in the North Sea including regions of Dogger Bank and different areas within the German Bight (Fox et al, 2008). Importantly, the results of that survey suggested that the traditional spawning sites of cod are still being utilized despite poor recruitment in recent years (ICES, 2005).…”