Combining green LiDAR bathymetry, aerial images and telemetry data to derive mesoscale habitat characteristics for European grayling and brown trout in a Norwegian river

While many studies provide microscale relationships between fish and habitat characteristics, studies covering longer river reaches are scarce. Modern remote sensing techniques may enable new and effective ways of mapping and assessing mesoscale habitat characteristics. Using green LIDAR-derived bathymetry and hydraulic modelling, we tested how mesoscale depth and velocity were related to fish counts of adult European grayling (Thymallus thymallus L.) and brown trout (Salmo trutta L.) in 500 m river sections in three separate periods during the year. Using riverbank sinuosity from aerial images and a Froude number-based index from the hydraulic model as proxies for mesoscale spatial and hydraulic heterogeneity, we tested for temporal correlations with river section fish counts of adult European grayling and brown trout. Results showed that mesoscale mean depth and velocity were correlated to period fish counts of adult European grayling. Using mixed model analysis we found that riverbank sinuosity and the Froude number-based index were significantly correlated with river section occurrence of adult European grayling during spawning. The results can be used to assess how flow-induced changes and channel adjustments at the mesoscale level can influence access to and use of relevant habitats in rivers occupied by European grayling and brown trout.

Effects of Dietary Protein and Lipid Levels on Growth, Body Composition, Blood Biochemistry, Antioxidant Capacity and Ammonia Excretion of European Grayling (Thymallus Thymallus)

Background: This study evaluated growth, body composition, blood biochemistry, antioxidant capacity, innate immunity and ammonia excretion of European grayling (Thymallus thymallus) fed diets containing different protein and lipid contents. Six diets were produced to contain 30, 40 or 50% protein with 10 or 20% lipid and fed to triplicate groups (100 fish per replicate) of fish (25.2 ± 0.28 g) to visual satiety twice daily for 12 weeks. Results: Fish growth was enhanced (P ˂ 0.05) as protein increased from 30 to 40% and plateaued thereafter. Enhancing protein and lipid content of diet led to reduced feed intake and improved feed efficiency. Moreover, protein efficiency ratio increased at higher lipid level while lower values were recorded at higher protein levels. Increasing dietary lipid content led to the enhancement of viscerosomatic index and intraperitoneal fat ratio. An interaction of protein and lipid was found on whole-body lipid, and muscle lipid was responsive to dietary lipid level. Muscle ARA, EPA and Ʃn-6 LC-PUFA contents enhanced by increasing dietary protein level. Moreover, increasing fat content of diet led to enhanced muscle linoleic acid, linolenic acid, ƩMUFA, Ʃn-6, DHA/EPA and n-6/n-3. However, EPA, DHA, Ʃn-6 LC-PUFA, Ʃn-3, Ʃn-3 LC-PUFA and EPA/ARA decreased at higher dietary lipid level. Serum triglyceride (TG) concentration and lactate dehydrogenase (LDH) activity decreased as dietary protein level increased while an opposite effect was observed for cholesterol (CHO) concentration. Increasing fat content of diet led to enhanced serum TG, CHO and glucose concentrations and reduced alanine aminotransferase, aspartate amino transferase and LDH activities. Serum malondialdehyde concentration was enhanced by increasing both dietary protein and lipid contents. Furthermore, serum myeloperoxidase activity was enhanced at higher dietary lipid level. Water total ammonium nitrogen (TAN) concentration was measured after 5 and 24 h of last feeding, and the results indicated the reduction of ammonia excretion as dietary lipid content increased. Conclusions: These findings suggest that 40% dietary protein can support optimal growth of juvenile European grayling and increasing lipid content from 10 to 20% can improve feed utilization and reduce ammonia excretion to the rearing water.

Genetic population structure of the monogenean parasite Gyrodactylus thymalli and its host European grayling (Thymallus thymallus) in a large Norwegian lake

Understanding how populations are structured in space and time is a central question in evolutionary biology. Parasites and their hosts are assumed to evolve together, however, detailed understanding of mechanisms leading to genetic structuring of parasites and their hosts are lacking. As a parasite depends on its host, studying the genetic structure of both parasite and host can reveal important insights into these mechanisms. Here, genetic structure of the monogenean parasite Gyrodactylus thymalli and its host the European grayling (Thymallus thymallus) was investigated in 10 tributaries draining into the large Lake Mjøsa in Norway. The population genetic structure of spawning grayling was studied using microsatellite genotyping, while G. thymalli was studied by sequencing a mitochondrial DNA gene (dehydrogenase subunit 5). Two main genetic clusters were revealed in grayling, one cluster comprising grayling from the largest spawning population, while the remaining tributaries formed the second cluster. For both taxa, some genetic differentiation was observed among tributaries, but there was no clear isolation-by-distance signature. The structuring was stronger for the host than for the parasite. These results imply that moderate to high levels of gene flow occur among the sub-populations of both taxa. The high parasite exchange among tributaries could result from a lack of strong homing behavior in grayling as well as interactions among individual fish outside of the spawning season, leading to frequent mixing of both host and parasite.