Reproductive interactions between fugitive farmed and wild Atlantic cod (Gadus morhua) in the field Justin J. Meager, Jon Egil Skjæraasen, Anders Ferno ¨ , and Svein Løkkeborg Abstract: Atlantic cod (Gadus morhua) are being increasingly farmed in net pens adjacent to coastal populations that are currently at historic lows. One concern is that farmed escapees enter local spawning shoals and mate with wild cod. We tested for the potential of escaped farmed cod to interact and hybridize with wild fish by examining the spatial dynamics of, and associations between, fish tagged with ultrasonic transmitters. Based on these data, we also investigated the basic mating system of cod in the field. The spawning ground was best described as a lekking arena. Wild males aggregated near the seafloor and associations between individuals were frequent. Wild females had a pelagic and dispersed distribu- tion and rarely associated with each other. Associations between individual wild males and females were also infrequent. Farmed males rarely associated with wild fish and had core usage areas above the wild males, suggesting that they were not admitted into the spawning arena. Farmed females were over the spawning arena more frequently than wild females and often associated with wild males at the depth of the spawning arena, indicating potential mating with wild males and the possibility of courtship interference. Hence, hybridization between escaped farmed and wild cod is likely. Re ´sume ´: On fait de plus en plus d’e ´levages de morues (Gadus morhua) dans des enclos en filet pre `s des populations co ˆ- tie `res qui se retrouvent actuellement a ` des densite ´s historiquement basses. On se pre ´occupe de ce que des poissons e ´chap- pe ´s d’e ´levage se joignent aux bancs locaux de reproducteurs et frayent avec des morues sauvages. Nous avons teste ´ la possibilite ´ que des morues e ´chappe ´es d’e ´levage interagissent et s’hybrident avec les poissons sauvages en examinant la dy- namique spatiale et les associations de poissons marque ´s d’une e ´tiquette e ´mettrice d’ultrasons. Avec ces donne ´es, nous avons aussi e ´tudie ´ le syste `me d’appareillade de base des morues en nature. Les zones de fraie peuvent le mieux e ˆtre de ´cri- tes comme des are `nes de lek. Les ma ˆles sauvages se rassemblent pre `s du fond marin avec de fre ´quentes associations entre individus. Les femelles sauvages ont une re ´partition pe ´lagique et disperse ´e et s’associent rarement l’une a ` l’autre. Les as- sociations entre des individus sauvages ma ˆles et femelles sont aussi peu fre ´quentes. Les ma ˆles d’e ´levage s’associent rare- ment aux poissons sauvages et leurs zones principales d’utilisation se situent au-dessus de celles des ma ˆles sauvages, ce qui laisse croire qu’ils ne sont pas admis dans l’are `ne de fraie. Les femelles d’e ´levage se retrouvent au-dessus de l’are `ne de fraie plus fre ´quemment que les femelles sauvages et s’associent fre ´quemment a ` des ma ˆles sauvages dans le fond de l’are `ne de fraie, ce qui indique la possibilite ´ d’une appareillade avec les ma ˆles sauvages et de comportements de cour d’in- terfe ´rence. L’hybridation entre les morues e ´chappe ´es de culture et les morues sauvages est donc probable. [Traduit par la Re ´daction] Introduction The number of accidental and intentional releases of hatchery-produced fish into marine environments is increas- ing, driven by declines in exploited wild stocks and subse- quent development of mariculture, stock enhancement, and restocking (Brown and Laland 2001; Naylor et al. 2005). However, there is surprisingly little known of the natural re- productive behaviour of many of these species, particularly of oceanic spawners. Many of the current candidate species for mariculture such as flatfishes, sparids, sciaenids, scom- brids, and gadoids (Food and Agriculture Organisation of the United Nations (FAO) 2007) spawn at night or at depths that prevent direct observation. They also attain sizes at ma- turity that can make observations in the laboratory difficult. Without this knowledge, it is difficult or impossible to pre- dict the reproductive success of farmed fish in the ocean. Resolving the behaviour of individual fish that move rapidly Received 30 October 2009. Accepted 26 May 2010. Published on the NRC Research Press Web site at cjfas.nrc.ca on 20 July 2010. J21494 Paper handled by Associate Editor C. Tara Marshall. J.J. Meager 1,2 and J.E. Skjæraasen. Department of Biology, University of Bergen, Bergen N-5020, Norway. A. Ferno ¨. Department of Biology, University of Bergen, Bergen N-5020, Norway; Institute of Marine Research, Bergen N-5817, Norway. S. Løkkeborg. Institute of Marine Research, Bergen N-5817, Norway. 1 Corresponding author (e-mail: justin.meager@gmail.com). 2 Present address: Faculty of Science, Health and Education, University of the Sunshine Coast, Maroochydore DC, Queensland 4558, Australia. 1221 Can. J. Fish. Aquat. Sci. 67: 1221–1231 (2010) doi:10.1139/F10-066 Published by NRC Research Press