Mapping Immune and Stress Gene Profiles in Atlantic Cod, Gadus morhua: Effects of Overcrowding

Serum-mediated reduction of Listonella anguillarum, plasma cortisol levels and transcriptional profiles of selected stress and immune-related genes were studied in Atlantic cod, Gadus morhua, a cold-water fish species, following exposure to short-term overcrowding. Fish were subjected to overcrowding for 1 h by increasing the stocking density from 8-10 kg m-3 to 100 kg m-3. The stressor was repeated thrice over a 12 h interval period. Blood samples were collected before overcrowding and at 2, 24 and 72 h post-crowding. There was significant reduction in L. anguillarum when incubated with sera obtained at 2 h after crowding compared with the initial group. Plasma cortisol levels significantly increased at 2 h post-crowding but returned to baseline levels beginning 24 h after exposure. There was differential expression of the genes analyzed, with most of them significantly increasing at 2 h post-crowding. These results suggest that short-term overcrowding in Atlantic cods leads to an increased but short-lived resistance against a bacterial pathogen, which could be mediated in part by the transcriptional upregulation of the different genes studied.