Status of the burbot ( L.) in the Lower Danube (Bulgaria) - a species threatened by climate change.

The study provides data on the catch composition, length-weight relationship, age structure, gender structure, growth, maturation, fecundity, distribution and conservation status of the burbot (Linnaeus, 1758) in Bulgaria. During six consecutive winters (2008-2014) a total of 395 burbot specimens were caught. The total length and the weight of the specimens ranged from 16 to 51 cm and 29.8 to 1057 g, respectively. Seven age classes were represented (3- to 9-years-old), with 3-, 4-, and 5-years-old most abundant. The maximal life expectancy was estimated as 12 years. Male-female ratio was 1:1. Maturity happens at the age of four at the earliest, valid for both sexes. One-quarter of the fish, older than 5 years were determined as non-reproducing in the particular year. The absolute fecundity varied between 47 462 and 810 236 eggs for females ranging from 5 to 7 years old and from 25.7 to 41.5 cm in length. A dramatic decrease of burbot population was observed in the last two decades. Warming water temperatures of the Danube, together with fragmentation in its tributaries are considered as the major threats affecting the species. Therefore, the burbot in Bulgaria is classified as Endangered.