Commercial Fishery Impact on the Modern Black Sea Ecosystem: a ...

78 V. N. Eremeev and G. V. Zuyev / Turk. J. Fish. Aquat. Sci. 7: 75-82 (2007)Table 1. Species composition of the Black Sea landings in the end of 1960s - beginning of 1970-s and in 1988s (%) (Ivanovand Beverton, 1985; FAO yearbook, 2004)Species1960 - 1970ss 1988sThousand tonnes % Thousand tonnes %Anchovy 53.6* 67.7*Sprat 1.6* 69.88.6*93.4Horse mackerel 12.5* 12.6*Whiting 2.1*4.5*Atlantic bonito 16.1 1.7Mackerel 1.2 1.5Bluefish 3.2 30.21.26.6Shad 0.8 0.2Turbot 2.0 0.2Picked dogfish 1.5 0.7Thornback ray 0.9 0.2Mugils 2.6 0.3Red mullet 1.7 0.5Sturgeons 0.2~ 0.01Total 182.8 100 783.8 100* Small short cycle fish speciesAnother proof of the Black Sea fish overexploitationevidences the decrease of anchovy of midsize and mid age in the end of 1980s testifyingthereby “rejuvenation” of the population. On thecontrary, weakening of fishing pressure on itspopulation in the following years challenged a reverseeffect, namely, the size- age structure rehabilitationand its catch increase (Figure 5). Thus drastic fall oftotal catches in 1989s might have been predictedbeforehand and only the emergence of two highlyproductive generations of horse mackerel in 1983sand 1984s arrested it for some years.In addition, the imbalance in a “stock and catch”relationship in 1980s is exhibited in peculiarities ofthe long-term “stock and catch” dynamics indices. Asit is seen in the figure, the catch curve is equal in formto that of the stock retarding for 4-5 years (Figure 6).Thus the peak for the stock in 1979s wasaccompanied by peak for the catch in 1984s,minimum of the stock in 1983s was attended with thatof the catch in 1987s. And again the peak for thestock in 1984s was followed by the catch peak in1988s. It could hardly be regarded as incidentalcoincidence. Present temporal intervals correspond tothe life span of one generation of the maincommercial species - anchovy and horse mackerelconstituted in the end of 1970s more than 75% of totallandings. Correlation coefficient value between stockwithin 1979-1987ss and catch within 1983-1991sswas equal to 0.748 (p < 0.02). In this connection, theposition of a number of scientists holding to the ideaof “the fatal” role of Mnemiopsis leidyi in sharpdecline of Black Sea catches doesn’t seem to be ratherconvincing.Phenomenon of Mnemiopsis “outburst” attainingits high point in 1989s suggests the directconsequence of biomass decrease in small fishplanktophagous (anchovy, horse mackerel and others)resulted from overfishing (Figure 7). Only due tosmall numbers of these fish, Mnemiopsis leidyi turnedout to be competitive to these planktophagous in agiven ecological niche. In 1988s as compared to1979s, the commercial fish stocks reduced more thanone and a half-fold: from 2200 thousand tonnes to1400 thousand tonnes. Finally, the ecological nicheoccupied by fish planktophagous appeared to bepartly empty and correspondingly, according to thegeneral ecological rule “nature abhoret vacuum”,was occupied by Mnemiopsis.Along with Mnemiopsis, the crucial role instructural and functional alterations of the ecosystemin the north-west of the Black Sea played theextensive commercial bottom sprat trawlingscommenced in 1970s of the 20th century. Since 1979still 1986s, more than 120 thousand operations wereperformed in the NW shelf of the Black Sea. Ascompared to the previous period, the volume of spratcatch appeared to be increased as much as 15-20times during some years. Bottom trawling affecteddirectly the sea ecosystem by mechanic destruction ofbottom biotops by trawlboard and low headline,respectively damaging or completely destroyingbenthic communities.Much more deleterious influence on theecosystem was exerted by bottom sediments (peliticfractions) raised into the pelagic layer. It inducedsecondary eutrophication and water pollution, declineof its transparency, depression of photosynthesisprocesses. However, the main negative factor ofbottom trawlings was recognized silting of vastbottom areas. Thus pelitic particles from TarkhancutCape of the Crimea peninsula spread with the currentfor the distance of 150-200 km (Figure 8). Totally,more than 5 thousand square km of the shelf weresilted as an outcome: at larger parts, the silt layerthickness varied from 2 till 5 cm, attaining up to 40-50 cm in spots. In a period of 1976-1984ss, some siltdepositions raised into the water layer constituting