Introduction
The special importance of Danube River is given by the fact that it represents the habitat for 6 of 26 sturgeon species: Beluga Sturgeon (Huso huso), Russian Sturgeon (A gueldenstaedtii), Starry Sturgeon (A. stellatus) and European Sturgeon (A. sturio), Sterlet sturgeon (Acipenser ruthenus) and Ship Sturgeon (A. nudiventris). All these species are critically endangered or some of them even probably extinct from Danube and Black Sea. Worldwide, the captures of sturgeon had the same drastic decreasing trend in the last century, as in Romania. In 1978 the fished quantity worldwide reached at 32 thousand tonnes and in 2016 decreased to only 2.2 thousand tonnes (FAO, 2018; Ionescu, 2019).
Materials and methods
The data related to the sturgeons commercial catches of are very few, we found data in old books, difficult to obtain and in database archives that are not available online. We studied and analyzed in 3 years all the specific literary sources but also the fishery databases and we found valuable data about Romanian sturgeon fishery, which have scientifically processed and interpreted, resulting very pertinent conclusions.
Results
In the seventeenth century, near Silistra (Danube, km 375), every day were caught 70 big beluga, (Decei, 1976) and, in comparation, in the year 2004, in all Romania, were commercial caught only 100 specimens. Analysing the commercial catches data, we can conclude that after a century, the Russian sturgeon commercial catches decreased with 99.7%, from 10 570 specimens (145 000 kg) in 1903–1904 to only 28 specimens (555 kg) in 2003–2004. In the case of the Beluga sturgeon, the number decreased with 97%, from 4 254 specimens (230 000 kg) in 1903–1904 to just 111 specimens (11060 kg) in 2003–2004 (Antipa, 1909; Manea, 1980; MPADR, 2006; Ionescu, 2019). The situation is also confirmed by caviar production, quantity decreased from 17 tonnes/year to 3.4 tonnes exported/average/year (Daia, 1926; FAO, 2018; Ionescu, 2019).
The Danube sturgeon population was affected by many factors like: overfishing, habitat loss, poaching, spawning site alteration, pollution, dikes, chemical changes of Danube stream water, alien species etc. We analysed the first three causes because we consider them to be the most important, with clear evidence.
Sturgeons commercial fishing, that was practiced in the last centuries without a sustainable vision, directly affected the sturgeon spawners and subadults. This was possible because of the legislation gap, due to which this activity turned into overfishing. As an example, from 1896 to 1974, the Romanian legislation stipulated too small minimum catch size of sturgeons.
For Beluga sturgeon the minimum catch size, regulated by law from 1896, was 100 cm (approx. 5–6-year-old) standard length and in 1974 increased to 170 cm. Analysing commercial catch data, it appears that before 1974, for 80 years, more than 80% of beluga caught commercially, were sub-adults (Beluga sturgeon reaches sexual maturity at an average age of 15 years). For Russian sturgeon, the minimum catch size, regulated by law from 1896, was 80 cm (approx. 6-year-old), after 80 years increased to 140 cm. Analysing the same data, in the case of this species, over 50% specimens, caught commercially, were sub-adults (Russian sturgeon reaches sexual maturity at an average age of 13 years).
Reaching sexual maturity is a primary condition of the species to ensure a sustainable population and this type of overfishing has produced consequences that is difficult to recover.
Habitat loss by blocking migration corridors and destroying the spawning sites is the second major cause that has directly affected the Danube Sturgeons. On Danube River the biggest obstacle for sturgeons is the Iron Gates I (1972) and Iron Gates II (1984) dams. The construction of the IG I (Km 942) and IG II (Km 863), blocked the migration corridor, reducing the migration with 64 %.
Iron Gates it wasn’t just a crossroads for sturgeons, there were the best spawning sites, which now are permanently lost (IG I dam accumulation lake). In 1726, Marsigli describes that in the Iron Gates area, 50–100 Beluga were caught every day, near Adakaleh Island (Marsigli , 1726). Antipa also described that sturgeon are mainly reproduced in the Iron Gate cataract. At the beginning of the twentieth century, in spring, fishermen caught in this area very largest specimens of sturgeons, with a lot of amounts of caviar (Antipa, 1909).
The first and only complete study of benthic fauna from Danube cataract (km 955-1042) was realised between 1958-1960 (Popescu & Prunescu, 1961). The structure of the substrate, in this sector, was composed of boulders, gravel, coarse sand, fine sand to muddy-sandy. This structure is the ideal substrate for sturgeons spawning grounds but also for hatching, pre-development and sturgeon growth in the early stages. For example, beluga fingerlings switch to active feeding at the size of 2-4 cm and in the first stage the main food consists of tubular worms. After 3-5 cm, gammarids begin to appear in the food structure after 8 cm the larvae of chironomids and fish (Leonte, 1959). The structure of the benthos was composed by tubular worms, Gammaridae and Chironomide larvae (Popescu & Prunescu, 1961). There were many spawning sites in this area but there was also a nursery for sturgeon fingerlings, before migrating downstream to the Black Sea. Loss of these sites directly affected natural productivity and is a major cause of sturgeon populations decline, which is unfortunately irreversible.
The third major cause is poaching, which continues to cause losses of Danube Sturgeons wild populations. The poaching phenomenon was encountered in Romania, especially after the fall of communism, when the chaotical transition allowed illegal fishing and trade. The poaching persisted also after 2005, when the Romanian law banned the sturgeon fishery, and it has become more complex and harder to stop. Most of those who catch sturgeons illegally are commercial fishermen, targeting other species (eg carp), but, intentionally or not, bycatch sturgeons, because the fishing gear and method are the same. It is hard to identify this activity because they have all the legal documents for commercial fishing, transformation into poachers is made by keeping the sturgeon specimen and trafficking it on the black market. There are also fishermen who fish for scientific purposes and whom, through this cover and the negligence of the authorities, manage to traffic sturgeon specimens, especially females, which normally must be monitored, tagged and released back into the wild. In recent years, 477 adult sturgeons have been monitored by scientific fishing and tagged and of them, 91% are male (M.A.D.R., 2021), in the context in which the sex ratio is aproximativ1: 1.
Discussions
Many efforts are made for sturgeon conservation, from a general ban, stoking programs, improved legislation, enforcement to awareness, but nevertheless, there are still causes which need to be resolved.
In this context, after hundreds of years of overfishing, irrational fishing, habitat destruction, poaching and other causes that have affected Danube sturgeon populations, recovery will be slow, difficult and it will require a lot of involvement.