Ahmet E. Kideys

Recent dramatic changes in the Black Sea ecosystem: The reason for the sharp decline in Turkish anchovy fisheries

Abstract As a result of eutrophication caused by increased nutrient input via major rivers during the last few decades, the Black Sea ecosystem has been subject to extreme changes in recent years. These changes first became evident in the 1980s, with abnormal phytoplankton blooms and a large increase in medusae ( Aurelia aurita ) biomass. Then, the introduction of a new species (a lobate ctenophore, Mnemiopsis sp.) into the Black Sea radically affected the whole ecosystem. This species competes with anchovy for the edible zooplankton as well as possibly consuming anchovy eggs and larvae in the Black Sea. The mass occurence of Mnemiopsis appears to be one of the most important reasons for the sharp decrease of anchovy and other pelagic fish stocks in the Black Sea. Although the future of the Black Sea ecosystem seems rather bleak, it is suggested that in addition to reducing anthropogenic impact, systematic studies are essential if the Black Sea fisheries are to recover.

A review of zooplankton investigations of the Black Sea over the last decade

Abstract Investigations performed in the last decade indicate that there have been important changes in the zooplankton composition and structure in the Black Sea. However, contrasting events taking place in different regions of the Black Sea indicate a non-uniform structure of its ecosystem. Several fodder zooplankton species have either disappeared from or substantially decreased in number at different sampling sites of the Black Sea over the last one or two decades. Some other species adapted to thrive in eutrophic conditions have either appeared or increased in quantity. Meanwhile the biomass of the fodder zooplankton has also fluctuated considerably through the years. However, there seems to be a reverse trend in the long-term variation of fodder zooplankton between the shallow western and deep eastern areas. Over the last few decades the abundance of fish larvae has decreased significantly when compared either to past records or with larval abundances of other seas. This was shown to be due mainly to malnutrition of larvae. One of the most striking changes in the ichthyoplankton has been the shift in the spawning areas of the main fish species, the anchovy Engraulis encrasicolus from the northwestern to the southeastern Black Sea. Even the invading ctenophore Mnemiopsis were found to be starving. The condition of other species ( Calanus euxinus and Pleurobrachia pileus ) disclosed the fact that cyclonic regions where chlorophyll and nutrient concentrations are high, provide better nutrition than anticyclonic regions.

Chemical composition, respiration and feeding rates of the new alien ctenophore, Beroe ovata , in the Black Sea

Maximum daily rations of the ctenophore Beroe ovata Brugiere and predatory impacts on the Mnemiopsis leidyi A. Agassiz population were estimated via digestion time, prey biomass and predator and prey density in Sevastopol Bay and adjacent water regions. Digestion times ranged from 0.5 to 5.2 h and depended on the prey/predator weight ratio. Overall, the mean daily ration was 45% of B. ovata wet weight. Preliminary conclusions are given on the B. ovata population as an effective control of the M. leidyi population and on the dynamics and structure of the planktonic community as a whole.

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, ∼50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×103 cells l−1 in spring to 110×103 cells l−1 in autumn and biomass from 250 μg l−1 in summer to 1370 μg l−1 in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.

Increased Chlorophyll Levels in the Southern Caspian Sea Following an Invasion of Jellyfish

A significant correlation was observed between satellite derived chlorophyll a (Chl a) concentrations and the biomass of the invasive comb jellyfish Mnemiopsis leidyi in the southern Caspian Sea. By consuming the herbivorous zooplankton, the predatory ctenophore M. leidyi may have caused levels of Chl a to rise to very high values (∼9 mg m−3) in the southern Caspian Sea. There might also be several other factors concurrent with predation effects of M. leidyi influencing Chl a levels in this region, such as eutrophication and climatic changes which play major roles in nutrient, phytoplankton, and zooplankton variations. The decrease in pelagic fishes due to overfishing, natural, and anthropogenic impacts might have provided a suitable environment for M. leidyi to spread throughout this enclosed basin.

Effect of starvation on the biochemical compositions and respiration rates of ctenophores Mnemiopsis leidyi and Beroe ovata in the Black Sea

The proximate biochemical composition and metabolic rates of ctenophores Mnemiopsis leidyi and Beroe ovata from the Black Sea were examined with respect to starvation conditions. Although organic matter content in B. ovata was two times higher than that of M. leidyi (2.51 ±0.53 and 1.14 ±0.17 mg g - ¹ of wet weight, respectively), these species did not significantly differ in their biochemical composition. In both species protein formed about 80% of the total organic matter, lipids amounted to about 10%. Carbohydrate and amino acids measured separately made up less than 6.5% of the total organic matter. Under experimental starvation (18 days at 16–18°C for B. ovata and 8 days at 12.4°C for M. leidyi), wet weights of both ctenophore species were reduced by 9.4% and 9.3% d-¹, respectively. The rate of organic matter decrease was nearly two times lower than that of wet weight being on average 5.9% d - ¹ in M. leidyi and 5.5% d - ¹ in B. ovata. There was no trend in percentage of the four major biochemical categories with starvation time. The glycogen content in polysaccharides reached maximum values in freshly collected ctenophores (76.0 ±7.9% in B. ovata, and 86.6% in M. leidyi), but it was reduced substantially (34.4 ±2.7% in B. ovata and 18.3–28.8% in M. leidyi) with starvation. Monosaccharide content, expressed as a percentage of total carbohydrate, decreased from 39.9% to 13.5% in B. ovata, and from 45.8% to 14.3–23.2% in M. leidyi. The relationship between respiration rate (R) and wet weight (W) of individuals during the starvation can be expressed by power function R = R 1 Wk (r²=0.85–0.94; P<0.001) for both ctenophore species. On average, k values were 0.95 and 0.83 in B. ovata and in M. leidyi, respectively. By the end of the starvation, metabolic rate per unit wet weight decreased by 33% in B. ovata and 46% in M. leidyi. Organic matter utilization was almost totally explained by respiration of ctenophores in the experiments and exceeded metabolic requirements of studied species by 11% and 15%, correspondingly. As compared with Mnemiopsis, Beroe has better tolerance to starvation which explains to some extent the success of the species survival during prolonged periods of food shortage in the Black Sea conditions.

Fluctuations of Pelagic Species of the Open Black Sea During 1980–1995 and Possible Teleconnections

The drastic changes in the Black Sea ecosystem, i.e. the harsh decline of the Black Sea fishery in 1989 and the dramatic changes in the zooplankton were often related to the outburst of the accidentally introduced ctenophore Mnemiopsis leidyi and to other man made events, such as pollution, eutrophication, regulation of river outflows (irrigation, damming) and overfishing. Beginning with the question as to why such changes in the Black Sea ecosystem occurred specifically at the end of the 1980s, the fluctuation of zooplankton stocks in other regions of the world are reviewed and compared with the changes in the Black Sea ecosystem. It transpires that changes in the zooplankton community and in small pelagic fish stocks in the second half of the 1980’s until the beginning of the 1990s were evident in all seas under consideration. These changes were discussed in connection with changes in the climatic regime. Striking changes were observed in the NAO (North Atlantic Oscillation), SO (Southern Oscillation), ENSO (Southern Oscillation (El Nino Index), and ALPI (Aleutian Low Pressure Index) in the second half of the 1980s resulting in changes of the hydrological and meteorological regime (river run off, salinity, sea- and air temperature, atmospheric pressure, precipitation and strength of westerly winds) in the northern hemisphere. It is concluded (hypothetically), that possibly, changes in the weather regime during the 1980s could have triggered the changes in the phyto- and mesozooplankton communities of the Black Sea, which caused the conditions for the outburst of M. leidyi and the decline of the anchovy stock.

Summer ichthyoplankton, food supply of fish larvae and impact of invasive ctenophores on the nutrition of fish larvae in the Black Sea during 2000 and 2001

Qualitative composition and abundance of both ichthyoplankton and small forms of zooplankton were evaluated by field studies in the northern (the Crimea near Sevastopol) and southern (Sinop region and TEEZ) Black Sea during the summers 2000 and 2001. A tendency of increasing the species richness, abundance offish eggs and larvae as well as zooplankton (which is the food for fish larvae) was observed over a period of Mnemiopsis leidyi and Beroe ovata co-existence. The eggs and larvae of the Mediterranean migrants-bonito and bluefish appeared again in the coastal waters near Sevastopol, which testified to favourable conditions for the spawning and nutrition of these fish species and their larvae. Aborigen copepod Oithona nana was found in tlie Crimean coastal waters although earlier in the 1990s it had completely vanished. Although rare in the 1990s copepods Centropages ponticus and Paracalanus parvus appeared in inshore waters as well as Pontellids species. Observed increases in species number and abundance of both ichthyoplankton and small zooplankton (≤500 μm), which promoted survival and development of fish larvae, were attributed to reduced predatory impact of Mnemiopsis on prey zooplankton after tlie arrival of Beroe in the late 1990s. However, the influence of Mnemiopsis continued to be significant during the short period of its peak occurrence in late summer. When this period coincided with the appearance of fish larvae, a negative impact on their survival could be predicted due to a low concentration of food items for larvae feeding.

In situ grazing pressure and diel vertical migration of female Calanus euxinus in the Black Sea

Gut pigment and abundance of the female Calanus euxinus (Hulsemann) weremeasured from several water layers (defined by density values), with3–5 h intervals during 30 h and 21 h at a station in the southwesternBlack Sea in April and in September 1995, respectively. The female C.euxinus was observed to begin migration to the upper phytoplankton-richlayer approximately 3 or 4 hours before the sunset. Only a fraction of thefemale Calanus population (0.2% in April and 3.6% inSeptember) did not migrate but remained at the depth of the oxygen minimumzone during the nighttime. The migrating population was determined to havespent 7.5 h in the euphotic zone in April and 10.5 h in September. Thegrazing rate of female Calanus euxinus was measured from the gut contentdata collected from the layers which contain the euphotic zone. Thepercentage of primary production grazed by the female C. euxinus wascalculated as 14.5% in April and 9.5% in September.

Seasonal distribution of fish eggs and larvae off sinop (the southern Black Sea) in 1999-2000

The seasonal distribution of fish eggs and larvae off Sinop, the central southern Black Sea, was studied by using vertical and horizontal tows at two stations at biweekly intervals between January 1999 and November 2000. Besides basic oceanographic parameters, macro gelatinous was also sampled to evaluate their relation to fish eggs and larvae. Eggs or larvae of a total 23 fish species were present in the samples. Most of the species were found to occur during spring and summer. It is observed that especially anchovy (Engraulis encrasicolus) eggs and larvae were highly abundant during this period. Additionally, beginning from May, the eggs and larvae of species, such as Mullus barbatus, Solea lascaris, Trachurus mediterraneus, Ctenolabrus sp., Blennidae and Gobiidae were represented in samples. In the colder period of the year, early life stages of sprat (Sprattus sprattus) were dominant in ichthyoplankton. According to overall quantitative evaluation, anchovy (87%) had superiority in abundance among egg samples and Gobius sp. (35%) among larvae samples.