Introduction
Observations on sturgeon by-catch during sprat trawl fishing operations in the Black Sea were conducted in framework of Discard monitoring program as part of the GFCM project “Select Activities Implemented of the Mid-term Strategy towards the Sustainability of Mediterranean and Black Sea Fisheries” (BlackSea4Fish). Studies were carried out by observers aboard of Ukrainian SChS and PTR-type vessels (about 25 m length). The study period lasted from August to November in 2018, from April to November in 2019 and in August-September 2021.
Materials and methods
Observations were performed according to the methods described in the FAO manual (FAO, 2019). The sprat fishery was conducted in the north-western Black Sea in the area bounded by longitude 31.3200 E from the east, latitude 46.0000 N from the south, and the coastline from the north and west. The trawling operations were carried out using midwater trawl in the near-bottom layer at the depths of 9 to 33 m at a distance of 1.5 to 50 km from the coast. The vessel speed during tows was 3.5–4.0 knots. All fishing operations were conducted during daylight hours. The duration of one trawling operation varied from 0.5 to 3–4 hours. The sprat trawl fishery in the NWBS is characterized by high selectivity. The proportion of all other fish species in catches (whiting, anchovy, sturgeons, turbot, sand smelt, shad, horse mackerel, etc.) did not usually exceed 1.0–1.5%.
The catch composition was studied during 342 fishing operations (see Table 1). The average sprat catch per hour of trawling (catch per unit effort, CPUE) was 0.57 ton in 2018; 0.44 ton in 2019 and 0.85 ton in 2021.
Results and discussion
The most undesirable phenomenon registered during the period of observations was the by-catch of young sturgeons - the most "vulnerable" group of fish in the Black Sea. The Starry sturgeon Acipenser stellatus (Pallas, 1771) and the Russian sturgeon Acipenser gueldenstaedtii (Brandt & Ratzeburg, 1833) were quite regularly presented in mid-water trawls in 2018-2019. Beluga Huso huso (L., 1758) was also caught in 2021 and even in greater numbers than Russian sturgeon. In 2018-2019, sturgeons were recorded in 50 trawls out of 283 (approximately in one haul out of six). And in 2021, they were registered in 52 hauls out of 59, that is, in almost every trawl.
Individuals of Starry sturgeon absolutely dominated among all sturgeons in trawl by-catches. Their share was 89.7% in 2018, 98% in 2019 and 93.8% in 2021. The share of Russian sturgeon in this by-catches was 5.1% in 2018-2019 and 2.8% in 2021. The share of beluga was 3.4% in 2021.
All sturgeon species in the by-catch were represented mainly by young individuals, which were characterized by relatively small size and weight (Table 2).
Only one Russian sturgeon and two Starry sturgeons had a body length of more than 1 m and could potentially reach sexual maturity among all the sturgeons caught in 2018-2019. The number of large individuals in the by-catch slightly increased in 2021: Russian sturgeon – 2 ind., starry sturgeon - 13 ind. Among 10 belugas caught in 2021 two had a body length exceeding 2 m, which allows them to be classified as sexually mature. Thus, we can state a slight upward trend in the size and weight characteristics of sturgeons over the past two years. The extremely low proportion of adult fish in this by-catch can be explained by their elimination by illegal fishing in the Danube and Dnieper rivers and at sea.
Proportion of sturgeons among all by-catch fish weight amounted to 61.2% in 2021. The number of sturgeons caught per trawl operation has increased significantly over the past two years. It amounted to 0.24 ind. in 2018, 0.28 ind. in 2019, and already 4.91 ind. in 2021. If in 2018 the by-catch of one individual of sturgeons accounted for 5.8 tons of sprat, in 2019 - for 2.3 tons, then in 2021 - only for 1.54 tons of sprat. So the fact of a noticeable increase in the number of at least one species - starry sturgeon in recent years is difficult to deny.
Obviously, the observed recovery of the Starry sturgeon population in the NW Black Sea was made possible by relatively successful natural spawning in the Danube River. Beside it can be assumed that the growth of sturgeon populations has been facilitated by restrictions on fishing in this marine area provided by the Russian Navy.
Some special study was conducted to search for CWT tags using T-wand detector to assess the role of artificial sturgeon stocking carried out massively in Romania with tagging of all individuals released in 2008-2015. 69 Starry sturgeons, 4 Russian sturgeons and 3 belugas were examined in trawl by-catches. The presence of tags was detected for two individuals of the Russian sturgeon only. Nothing tags were found in individuals of the other two species. This fact may indicate that the rate of the natural spawning of Russian sturgeon in the Black Sea basin is extremely low, and artificial stocking of this species can significantly help to preserve its population.