title: Impacts of water temperature and other environmental parameters on proliferative kidney disease in wild brown trout (Salmo trutta) populations
Creators: name: Rubin, Aurélie Amandine
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 630 Agriculture

description: At the global scale, biodiversity is massively threatened. Pollution, deforestation, habitat loss are only some of the dangers hovering on terrestrial and aquatic biodiversity. Among it, freshwater ecosystems are at severe risk. Habitat quality, food supply, overfishing, water quality and temperature are some of the multiple drivers that impact the biodiversity of lakes and rivers. In addition, infectious diseases are also a major threat for aquatic organisms. One key challenge for salmonids is the emergence of Proliferative Kidney Disease (PKD), caused by the myxozoan parasite Tetracapsuloides bryosalmonae. This infection provokes a significant fish mortality, either in the wild or in fish farms. In Switzerland, this infection plays a major role in the decline of wild brown trout (Salmo trutta) populations.
Water temperature is considered as the main driver for PKD manifestation. Prevalence, disease symptoms and related mortality are enhanced at high temperature, especially when values reach or surpass 15°C. Up to now, this statement mainly derives from laboratory experiments, using constant water temperature and achieved with rainbow trout (Oncorhynchus mykiss) as the vertebrate host. However, conclusions from these studies may not be applied to the field, where brown trout populations must face fluctuating temperature and a multitude of other potential stressors. In this thesis, we thus focused on the effects of PKD on wild brown trout populations under varying water temperature and possible combining impacts of aggravating factors in the field.
In Chapter 2, we investigated the optimal sampling period for a reliable assessment of PKD in the field. Indeed, if samplings are performed too early in the season (when the infection may not be sufficiently developed or not have taken place yet at this moment) or too late (when fish have already died or the parasites have been excreted), the fish status may be wrongly assessed. We sampled young-of-the-year (YOY) brown trout over a three-years period and assessed the fish status by histology. We showed that the optimal period for the detection of T. bryosalmonae-infected fish was when a mean of 1500 degree days (dd) or 30 days with a daily mean temperature ≥15°C (ndays15) was reached. Based on long-term temperature values, this time-point could thus be employed in particular streams independent of altitude, location or weather characteristics. This threshold is therefore a useful tool for field researchers and should thus be taken into consideration when planning PKD sampling campaigns.
In Chapter 3, we explored the implication of varying water temperature and potential other environmental parameters on PKD infection. We sampled YOY in 45 stations through the canton of Vaud and recorded long-term water temperature values, water quality data and ecomorphology indicator at each site. PKD prevalence and infection intensity were significantly correlated with the mean water temperature of June, ndays15 and presence of a wastewater treatment plant (WWTP). This chapter hence demonstrates that water temperature influences PKD manifestation in the field and suggests that additional environmental stressors, such as water quality, may be considered in the T. bryosalmonae-brown trout system.
In Chapter 4, we focused on the influence of water quality on PKD manifestation. We sampled and tagged fish in two thermal similar stations, upstream and downstream of a WWTP. PKD prevalence and parasite intensity were significantly higher in the downstream site receiving the effluent compared to the upstream station. The apparent survival rate was also reduced in the downstream site. Our results thus show that even a minor alteration of water quality is sufficient to induce consequences on PKD. This factor should thus be considered alongside water temperature as a driver for PKD manifestation.
In conclusion, investigations in wild brown trout populations under varying water temperature regime and possible cumulative influences of environmental parameters need thus to be undertaken in the context of aquatic fieldwork. These outcomes are of primary importance for a better understanding of PKD impacts on fish population dynamics, which could lead to applied protective actions to maintain brown trout populations and to limit their extinctions, particularly in the context of climate change.