Decreased mortality of Lake Michigan Chinook salmon after bacterial kidney disease challenge: evidence for pathogen-driven selection?

In the late 1960s, Chinook salmon Oncorhynchus tshawytscha from the Green River, Washington, were successfully introduced into Lake Michigan. During spring from 1988 to 1992, large fish die-offs affecting Chinook salmon occurred in the lake. Multiple ecological factors probably contributed to the severity of the fish kills, but the only disease agent found regularly was Renibacterium salmoninarum, the causative agent of bacterial kidney disease. In this study, survival after challenge by R. salmoninarum was compared between two Chinook salmon stocks: a Lake Michigan stock from Wisconsin (WI) and the progenitor stock from the Green River. We found that the WI stock had significantly greater survival than the Green River stock. Next, the WI and Green River stocks were exposed to the marine pathogen Listonella anguillarum (formerly Vibrio anguillarum), one of the causative agents of vibriosis; survival after this challenge was significantly poorer for the WI stock than for the Green River stock. A close genetic relationship between the Green River and WI stocks was confirmed by analyzing 13 microsatellite loci. These results collectively suggest that disease susceptibility of Lake Michigan Chinook salmon has diverged from that of the source population, possibly in response to pathogen-driven selection.

Effects of rearing temperature on immune functions in sockeye salmon (Oncorhynchus nerka).

To determine if the defences of sockeye salmon (Oncorhynchus nerka) raised in captivity are affected by the rearing temperature or their life-cycle stage, various indices of the humoral and cellular immune functions were measured in fish reared at either 8 or 12 degrees C for their entire life-cycle. Measures of humoral immunity included the commonly used haematological parameters, as well as measurements of complement, and lysozyme activity. Cellular assays quantified the ability of macrophages from the anterior kidney to phagocytise Staphylococcus aureus cells, or the activities of certain bactericidal systems of those cells. The T-dependent antibody response to a recombinant 57 kDa protein of Renibacterium salmoninarum was used to quantify the specific immune response. Fish were sampled during the spring and fall of their second, third and fourth years, corresponding to a period that began just before smolting and ended at sexual maturation. Fish reared at 8 degrees C tended to have a greater percentage of phagocytic kidney macrophages during the first 2 years of sampling than the fish reared at 12 degrees C. During the last half of the study the complement activity of the fish reared at 8 degrees C was greater than that of the 12 degrees C fish. Conversely, a greater proportion of the blood leucocytes were lymphocytes in fish reared at 12 degrees C compared to the fish reared at 8 degrees C. Fish reared at 12 degrees C also produced a greater antibody response than those reared at 8 degrees C. Results suggested that the immune apparatus of sockeye salmon reared at 8 degrees C relied more heavily on the non-specific immune response, while the specific immune response was used to a greater extent when the fish were reared at 12 degrees C. Although a seasonal effect was not detected in any of the indices measured, varying effects were observed in some measurements during sexual maturation of fish in both temperature groups. At that time there were dramatic decreases in complement activity and lymphocyte numbers. This study was unique in its scope because it was the first quantitative assessment of salmon immune functions for an entire life-cycle.