The oldest mention of Lnáře or Schlüsselburg dates back to 1319, when it was referenced to the local Old Stronghold, formerly wooden fort, as the site of governor of Lnáře district (descendants of that governor later became hereditary knights of Lnáře). Lnáře was a village hidden in woods, on the shores of muddy streams in moorland. This moorland was (as in case of town Třeboň) very soon turned into a basin with dozens of ponds and lakes, later numbering almost four hundred. When some decline in fishing industry occurred during the 17th century, many ponds and lakes were dried up and these reclaimed plots have been used for pasture for sheep – numbering about 13,000 of those animals as of 1840. Moreover, a new unusual variant of common carp, with the name "Lnářský modrák" ("Blue carp of Lnáře"), was cultivated at the beginning of the 20th century by specialist Ing. Theodor Mokrý, who substantially contributed to revival of local fishing industry.