Publication Abstract

Early stage eggs of cod (Gadus morhua L) are impossible to visually differentiate from those of haddock (Melanogrammus aeglefinus L.) or whiting (Merlangius merlangus L). Here we use species-specific genetic probes to distinguish eggs of the three fish species, as part of a project evaluating the use of egg production surveys to monitor the status of the Irish Sea cod stock during the EU stock recovery program. Cod-like eggs were collected during five ichthyoplankton surveys during the spawning season in 2006. At each plankton station, a subset of the eggs caught was fixed in individual vials filled with ethanol at sea and brought back to the laboratory where DNA was extracted and purified. Using universal primers and Taqman species-specific minor grove binding (MGB) probes in a multiplex, high through-put assay, the samples were processed on an Applied Biosystems 7900 real time sequence detection system. Analysis of hatchery egg standards interlaced with the field sampled eggs indicated a 95% accuracy in determining species composition of cod, haddock and whiting. Out of 4700 eggs collected on the surveys, 22% were identified, as cod, 15% as haddock and 42% as whiting, and approximately 20% were other species. Unidentified eggs are being sequenced on an Applied Biosystems 3100 genetic analyser to confirm their species. Application of DNA results will allow estimation of the stock size of gadoids in the Irish Sea using the Annual Egg Production Method.