Determining the state of the population of softmouth trout

Determining the state of the population of softmouth trout
Determining the state of the population of softmouth trout (Salmo obtusirostris) using eDNA and visual census method.

Research to date that has significantly contributed to determining the structure and diversity of the indigenous ichthyofauna of Krka National Park, and the abundance and presence of alien and translocated species in the Krka River basin, has not detected the presence of the softmouth trout (Salmo obtusirostris), recorded in the literature to inhabit these waters. The aim of this research was to apply new methods to detect the presence of softmouth trout using the eDNA method.

eDNA samples contain a mixture of the DNA of all species found in a given locality. In order to detect the presence of the softmouth trout, it is necessary to find specific molecular markers for this species. Sampling was performed at multiple locations, primarily those listed in the literature as the habitat for this species. In order to increase the degree of efficacy and precision of this method, it was necessary to take samples of softmouth trout, and other salmonid species, both in the Krka River and at other localities where this species lives, i.e., the Jadro, Žrnovnica and Vrljika Rivers. Pectoral fin samples were taken from several fish species: rainbow trout (Oncorhynchus mykiss), Danube trout (Salmo trutta or S. labrax), Adriatic brown trout (Salmo farioides), Krka minnow (Phoxinus krkae), and softmouth trout (Salmo obtusirostris).

The next step was to develop a special method to detect softmouth trout, and to conduct a data and GeneBank search, and amplification of specific molecular markets for this species and related species from the study area, and to perform a bioinformational analysis to determine appropriate regions specific to the species in question, in relation to other species, and in comparison with the softmouth trout from the Vrljika and Jadro Rivers. The DNA was isolated from a fin clip from species of the family Salmonidae collected in the Krka, Vrljika and Jadro Rivers where the softmouth trout resides, and served as a positive control Finally, sections of the restrictive enzyme were tested for the selected molecular markets in the target species and other related species in the family, in order to determine whether there are clear differences. During the research, a number of potential markers were identified, and the clearest were cytochrome b, somatolactin, and lactate dehydrogenase. Of these markers, restriction digestion of somatolactin was able to detect the presence of softmouth trout and differentiate it from other salmonid species in the Krka River area in the eDNA samples.

The eDNA marker method is most effective when combined with other standard ichthyological methods as a complementary research method, or with monitoring of an area or fish community, with any invasive impacts or over-selection. In this study, the visual census method was selected, and scuba diving and the use of an underwater drone (ROV) were applied. The visual census method proved to be good for the detection of small benthic species with a small range of movement, such as gobies, and for pelagic fish that form schools and that move in the water column. However, this method proved to be less successful for the detection of less abundant species that do not school, and often appear only individually in samples using other ichthyology methods.