The noble crayfish is obviously the only local species in Latvian waters. It has been found in many lakes and rivers throughout the territory of Latvia (Bērziņš 1936, Mazītis 1955, 1967, 1971).

In surveys undertaken from 1962 to 1968 (Jurane 1967, Jurane & Dzene 1969), 31 lakes where noble crayfish could be found were mentioned. These lakes were distributed fairly evenly throughout the territory of Latvia.

In parallel to this, a survey of local fishermen was undertaken via questionnaires. From the collated data, crayfish were ascribed to 163 lakes and 109 rivers in Latvia. From the sites indicated on the map included in the publication (Jurane 1967), noble crayfish could be found in 95% of them, with narrow-clawed crayfish in the rest.

Judging from the Baltic Fish Conservation and Reproduction Administration’s archival materials from the 1970s and 1980s, distinguishing between the species of crayfish has not always been correct. The local residents and crayfish fishermen’s surveyed called the smaller sized noble crayfish “swamp crayfish”, and the specimens with regenerated claws, as narrow-clawed crayfish. Therefore, the data from various surveys and questionnaires, as can be observed nowadays too, is not certain enough in relation to the systemic affiliation of the crayfish found in this or that water body.

In the surveys that we conducted from 1992 to 2018, noble crayfish was found in a total of 158 lakes, 52 rivers, 6 reservoirs on rivers and 5 quarry reservoirs throughout the territory of Latvia, in all of the largest river basins, (Fig. 2.).

In general, a trend can be observed, that the occurrence and local distribution of noble crayfish is decreasing, which is caused by their periodical mass mortality, and the increased distribution and occurrence of other species of crayfish.

In neighboring Estonia, noble crayfish are the only local species (Paaver & Hurt 2010, Kouba et al. 2014). Similarly, in Lithuania too, it is considered to be the only local crayfish species (Burba 1994, Taugbøl 1998, Arbačiauskas et al.

2012). In Belarus, the noble crayfish populations have co-existed with narrow-clawed crayfish (Šteinfeld 1957, Aklehnovich & Razlutskij 2013).