Abstract
Tilapia (Oreochromis niloticus) farming on Lake Kariba, Zambia has been practiced since early 2000s. In recent years, the industry has experienced disease outbreaks with clinical signs consistent with streptococcus-like infections. Affected fish were growers weighing approximately 200 g or above from farms designated A and B, reared in cages on the lake. The objective of this study was to identify and characterize the causative bacteria of these outbreaks. A total of eighty-six diseased fish were sampled between 2014 and 2016. Bacteria were cultured from internal organs and were characterized by biochemical tests, PCR and sequencing of the 16S rRNA gene or 16S23S intergenic spacer region. The results showed the prevalence of Lactococcus garvieae at farm A in 2015 and 2016 outbreaks (15/20 and 23/40, respectively) and at farm B (6/20) in 2016. Infection of naïve fish with one isolate of L. garvieae recovered from the outbreaks resulted in clinical signs and mortalities akin of what had been observed during the outbreaks, confirming Koch's postulates. In the 2014 outbreaks, the low prevalence of Streptococcus agalactiae (4/15 and 2/11 respectively at farms A and B) and S. iniae (2/15) at farm A suggests that other pathogens were likely involved in the disease outbreak observed.
Highlights
In many populations, harvesting is the main cause of mortality (Allendorf, England, Luikart, Ritchie, & Ryman, 2008) and may act as a powerful selective force that can influence individual life histories (Darimont et al, 2009; Kvalnes et al, 2016), population dynamics (Biro & Post, 2008) and have evolutionary consequences (Engen, Lande, & Sæther, 2014; Mysterud, 2011)
The goal of the study was to assess the individual consistency in the use of risky habitat types, and how habitat use was related to fitness components and lifehistory strategies
Using data from a closely monitored and harvested population of moose Alces alces, we examined how individual variation in offspring size, reproduction and survival was related to the use of open grasslands; a habitat type that offers highquality forage during summer, but at the cost of being more exposed to hunters in autumn
Summary
In many populations, harvesting is the main cause of mortality (Allendorf, England, Luikart, Ritchie, & Ryman, 2008) and may act as a powerful selective force that can influence individual life histories (Darimont et al, 2009; Kvalnes et al, 2016), population dynamics (Biro & Post, 2008) and have evolutionary consequences (Engen, Lande, & Sæther, 2014; Mysterud, 2011). We predicted that frequent use of open grasslands will increase the exposure and reduce the survival of moose during the hunting season (Ciuti et al, 2012) For such fitness consequences of habitat use to occur, an individual's propensity to use open areas must be consistent over time, that is, that there is high repeatability in the level of use of open areas, and that moose inclined to use open habitats are more often killed by hunters. In a recent study in the same population, Kvalnes et al (2016) found a harvest-induced selection towards smaller calf body masses, presumably because females that were larger as calf lost a higher proportion of calves to hunters as adults This indicates that the costs of using open grassland with respect to survival may be higher than the gains with respect to reproductive performance. We predicted a negative relationship between LRS and the use of open grassland
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