1. Putmins, R. - Die hydrographischen Ergebnisse der lettischen Terminfahrt im Frühjahr 1928. - Folia zoologica et hydrobiologica, I. pp. 53?62. 1929. Riga
2. Putnins, R. - Les croisséres thalassologiques latviennes au printemps de 1929. - Folia zoologica et hydrobiologica, I. pp. 149?150. 1929.
3. Originaldate.
4. Here fishermen take their richest catches. The way of catching is as follow: Across the river five to twenty nets are being cast, ca. 50?70 m from each other. Each net is ca. 200?250 m long and 70?80 cm high. All the nets being cast, they beat with clubs the surface of the water above the nets. That strange noise frightens the perch and they move to the bottom where they crawl into the nets. When the beating is ceased, the nets are immediately drawn up, cleared of fish, and moved to other fishing-places. Only in times when from the sea along the bottom brackish-water flows in, resp. in the river there is the so called ebbing stream, the perch do not move on the bottom but raise themselves a little higher and in such a way they escape the nets. Late in autumn, in December and in the end of November, in the cold water they do not respond to the act of beating, and that ends that kind of fishing.
5. Berzins, B. Note on the Hydrography of the Rivers of Latvia. - Contributions of Baltic University, Nr. 66. 1949. Pinneberg.