Genetic marking of farmed Atlantic cod (Gadus morhua l.) and detection of escapes from a commercial cod farm

A genetically marked Atlantic cod (Gadus morhua L.) strain was used to identify escapes from commercial cod farms, and to investigate the potential interbreeding between farmed and wild cod. This farmed cod was homozygote for a rare allele (30) in the GPI-1 locus expressed in white muscle tissue. Juveniles were produced from this strain in 2007 and 2008, and 500 000 individuals of each year class were transported to a cod farm in western Norway, where they were raised under commercial conditions. A monitoring fishing program was established from spring 2007 to detect escapees during the farming period. The first farmed cod escapees, identified to the 2007 year class through the genetic mark, age and body size, were detected during the fishing survey in November 2008. The second escape of the same year class was detected during the natural spawning season in early April 2009. A third escape was detected in November 2009, and this time the farmed cod were identified to the 2008 year class. The escapees were spreading through the whole fjord system, including local spawning sites for wild cod. Detailed examination of the escaped cod revealed a substantial degree of sexual maturation, and nearly 1000 cod larvae and early juveniles were therefore collected through spring 2009. The genetic analyses identified eight of these as genetically marked, demonstrating successful reproduction either in the cage or after escape. Interbreeding between escaped and wild cod may also have occurred, but cannot be proven from our material. In all years after the three identified escapes, genetically marked cod were found in the fjord area. In addition, several specimens were observed in adjacent fjord systems, demonstrating long-term survival in the local spawning areas as well as substantial spread over larger distances.