Local overfishing of the Black Sea sprat (Sprattus sprattus: Clupeidae, Pisces) and intraspecies differentiation
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Abstract
The Black Sea sprat (Sprattus sprattus) is known to be one of the most numerous fish species in the Azov – Black Sea basin. It is an object of fishing in all the Black Sea countries, its catch reaches 100 thousand tons. Therefore, the task to monitor the state of the sprat population and identify the risk factors under existing conditions of increasing fishing intensity and climate change is of great importance. The subject of the study is to assess the impact of fishing on the state of the Black Sea sprat population and estimate the prospects of further exploitation of its commercial stock. The article is based on the results of our research and the annual reports of the European Commission’s Scientific, Technical and Economic Committee for Fisheries (STECF). The catch of sprat in both regions was carried out with mid-water trawls, in the shelf zone in the near-bottom layer of water. In all calculations, the standard length (SL) in cm was measured. Long-term (2003–2013) dynamics of integrated characteristics of the size-age structure of sprat population (average length and average age of sprat) and also sprat catch from two geographically similar in climatic conditions regions, the Crimean region and the Western region (the shelf of Bulgaria and Romania), was investigated. Interregional differences in characteristics of the size-age population structure and fishing conditions were identified. In the Crimean region, the average length of sprat varied from 5.57 to 6.85 cm, the average was 6.43 cm. The interannual amplitude of the SL fluctuations was near 20 %. The average sprat age varied from 0.9 to 1.57 year, the average fish age in the population was 1.4 year. The interannual amplitude of sprat age fluctuations was more than 57 %. Negative trends of the average sprat length and its average age have been determined, indicating deterioration of the population state as a whole and of its commercial quality, in particular. The annual commercial catch of sprat varied from 11.4 to 24.78 thousand tons, the average annual catch was 16.09 thousand tons. The interannual amplitude of the catch fluctuations was more than 2.2 time. A close negative correlation between high catches and low average lengths and, as well as low average sprat age in the population for one year forward, was found (r = -0.81, p < 0.01 and r = -0.82, p < 0.01). In the Western region, unlike the Crimean, the sprat was longer, the average length of fish varied from 7.24 to 7.62 cm, average was 7.46 cm. Interannual amplitude of fluctuations less than 5 % indicated a sustainable state of the population. The annual commercial catch of sprat varied from 2.77 to 4.64 thousand tons, with the fishing effort upon the population did not exceed the maximum allowable level. The average annual catch was 3.75 thousand tons, which was 4.3 time lower than that in the Crimean region. There was no significant correlation between the catch and the average length of sprat (r = -0.27, p > 0.15). Local overfishing of sprat in the Crimean region together with long-term progressive deterioration of the sprat population state suggested division of total commercial fish stock into separate commercial unit stocks. This contradicts the concept of total commercial stock (a single population) of the Black Sea sprat. This conclusion is preliminary and further investigations in this area are to be made.
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References
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