When was Stellbrookmoor established?
CREATE TABLE "table1_26013618_1" (
  "nsg_nr" real,
  "name_of_the_nature_reserve" text,
  "district_town" text,
  "area_ha" text,
  "date_established" text
);
SELECT "date_established" FROM "table1_26013618_1" WHERE "name_of_the_nature_reserve"='Stellbrookmoor';