© 2006 International Council for the Exploration of the Sea
The biology and abundance of kilka (Clupeonella spp.) along the coast of Azerbaijan, Caspian Sea
Institute of Zoology of the National Academy of Sciences of Azerbaijan Passage 1128, Quarter 504, Baku AZE 1073, Azerbaijan
*Tel: +994 12 439 73 71; fax: +994 12 439 73 53. e-mail: elchin_kaspiy{at}yahoo.com.
In the changing environmental conditions of the Caspian Sea, and specifically following the accidental introduction of the comb jelly Mnemiopsis leidyi from the Azov–Black Sea basin in the late 1990s, ecosystem functioning has been disturbed, particularly in the pelagic zone. The main commercial species of kilka in the Caspian, anchovy kilka (Clupeonella engrauliformis), suffered greatly from the introduction. The catch of kilka by Russia, Azerbaijan, and Iran dropped from 182 700 t in 2000 to 74 700 t in 2001, although the total allowable catch remained at 300 000 t. Between 2000 and 2004, the catch by Azerbaijan alone dropped from 18 500 t to 5 100 t. Recruitment of anchovy kilka failed in early 2001, resulting in an increase in the average age of the stock, and a big decline in abundance. During kilka surveys conducted in summer 2003 and summer 2004 along the Azerbaijan coast of the central Caspian, the average cone-net catch varied from 19.8 kg to 30.5 kg at depths of 51–90 m, and along the Azerbaijan coast of the southern Caspian from 6.1 to 33.7 kg at depths of 53–81 m. Information is provided on the biology and apparent environmental preferences of the three kilka species of the Caspian, specifically their age composition, growth rate, and mean weight, off the coast of Azerbaijan.
Keywords: Caspian Sea, ecosystem effects, invasive species, kilka fishery, Mnemiopsis leidyi
Received 11 November 2005; accepted 12 July 2006.
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Introduction |
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 Top Introduction Material and methodsResults and discussionConclusionReferences |
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Three species of kilka (Clupeonella spp.) in the Caspian Sea are important commercially, accounting in the past decade for more than 80% of the total catch, as well as being a crucial part of the food chain. Their sustainable management is vital to the fisheries and to the ecosystem health of the Caspian basin. However, the environment of the Caspian has altered significantly during the past 30 years. Greater anthropogenic pressure, such as pollution, including domestic and industrial run-off, the development of vast oil and gas fields, and uncontrolled fishing, has resulted in poor management, and this, in turn, has caused escalating environmental degradation of the sea and its coastline, and further damage to the trophic base (Salmanov, 1999; Ivanov, 2000).
A further shock to the ecosystem came with the introduction into the Caspian in ballast water from the Azov–Black Sea basin of the comb jelly Mnemiopsis leidyi. M. leidyi has already severely damaged the pelagic ecosystem of particularly the central and southern Caspian, directly or indirectly impacting all links of the trophic chain. This includes the threatened sturgeon (Acipenser spp. and Huso huso) and seal (Phoca caspica) populations, because kilka are important in their diets. According to Prikhodko (1975), the annual consumption of kilka by predators such as sturgeon and seals was about 400 000 t in the early 1970s when sturgeon and seal stocks were large, but with the massive reductions in the populations of these predatory stocks since then, to a large extent the result of uncontrolled exploitation and environment-induced die-off, respectively, that value would now be much lower.
M. leidyi had a similar devastating impact on the ecosystem of the Black and Azov seas when it was accidentally introduced there in the same manner, i.e. in ships' ballast water (Vinogradov et al., 1992; Kideys, 1994, 2002; Niermann et al., 1994; Shiganova, 1998; Shiganova and Bulgakova, 2000; Volovik, 2000). The productivity of the Azov Sea alone dropped exponentially, and within a few years during the 1990s, the annual catches of khamsa (Engraulis encrasicholus maeoticus) and kilka there fell from 165 300 t to around 21 000 t (Volovik, 2000). According to the FAO, the loss of annual fishing revenue to the Black Sea littoral states as a consequence of the introduction of the comb jelly has been about US$350 million.
In 2000, when the impact of M. leidyi in the Caspian Sea was beginning to be felt (Kideys et al., 2005), the total biomass of anchovy kilka (Clupeonella engrauliformis), the main commercial species, was 825 000 t, and the exploitable biomass was 600 000 t (Sedov and Paritskiy, 2001). By 2004, the estimated commercially available biomass of anchovy kilka had dropped to some 164 000 t (Sedov et al., 2004), and in 2005, survey-estimated biomass was just 90 500 t. Mnemiopsis leidyi was first recorded in the Caspian Sea in November 1999 by CaspNIRKh scientists conducting an underwater video survey of part of the east coast of the sea, near the border between Kazakhstan and Turkmenistan (Ivanov et al., 2000; Figure 1). By spring 2001, M. leidyi had become the dominant species in the Caspian pelagic ecosystem, and was deemed by many to be the main driver of the reduction in the kilka stocks.
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Research on kilka in the Caspian Sea started in the 1940s (Lovetskaya, 1951). That pioneering work was followed by the research of Prikhodko (1975), Paritskiy (1989), and Sedov et al. (2004). The Caspian Sea ecosystem in which kilka live, however, has now changed, so the aim of the work reported here, which underpins a formal assessment of kilka stocks in the Caspian Sea that is the subject of a sister paper, is to look at the biology and abundance of the kilka resource along the coast of Azerbaijan, traditionally the most important area of commercial capture of Caspian kilka, under current environmental conditions. Historical analyses of size and age composition, reproductive capacity, and catch are undertaken for the purpose.
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Material and methods |
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TopIntroductionMaterial and methodsResults and discussionConclusionReferences |
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The study is based partly on material collected from commercial catches by the FV "Narimanabad", a standard kilka fishing vessel used to survey the resource along the coast of Azerbaijan, from 1995 to 2004. Sampling was carried out at 35 stations along the coast at depths of 20–100 m. Kilka catches were made at night in the traditional commercial manner, using a cone net with a 1500-kW light to attract the fish. Data were collected (Table 1) and analysed according to the standard methodology for former Soviet countries (Pravdin, 1966), but also with due regard to the methodology given by Sparre and Venema (1998). Measurement is therefore of caudal length. Along with standard von Bertalanffy growth equations, the index of Pauly and Munro (1984) was used to investigate the annual growth dynamics of the resource:
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is the maximum asymptotic length attainable, and K is the curvature parameter. Natural mortality was then calculated using the empirical formula of Pauly (1980): |
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The condition factor (Q) was calculated from
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Pravdin's (1966) classification of reproductive stages of ovaries is as follows:
- Stage I. Juvenile.
- Stage II. Ovaries narrow and flat, occupying less than 1/3 of the body cavity, yellow to orange, slightly transparent but dense. Eggs distinct under a magnifying glass.
- Stage III. Ovaries occupy up to 2/3 of the body cavity, dark orange to red, completely transparent, rich in blood vessels. Filled with small, opaque, orange eggs.
- Stage IV. Ovaries occupy more than 2/3 of the body cavity, light orange, firm and fragile (walls easily damaged). Filled with easily separated opaque yellow eggs.
- Stage V. Ovaries fill the body cavity, but they are soft, transparent, and pinkish. They are filled with large, transparent, mature eggs, but there are a few unripe eggs near the walls. Blood vessels clear. Oviduct aperture inflamed, and eggs are extruded under slight pressure to the body.
- Stage VI. Ovaries occupy less than 2/3 of the body cavity, flaccid, with stretched walls. Oviduct aperture inflamed. A few unripe eggs near the ovary walls.
- Stage II. Ovaries narrow and flat, occupying less than 1/3 of the body cavity, yellow to orange, slightly transparent but dense. Eggs distinct under a magnifying glass.
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Results and discussion |
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TopIntroductionMaterial and methodsResults and discussionConclusionReferences |
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The catch of kilka by Russia, Azerbaijan, and Iran dropped from 182 700 t in 2000 to 74 700 t in 2001, although the total allowable catch remained at 300 000 t. Between 2000 and 2004, the catch of Azerbaijan alone dropped from 18 500 t to 5100 t. The arrival of M. leidyi resulted in a reduced biomass of zooplankton, the main food resource of kilka, which between 2000 and 2003 dropped by a factor of 4–10 (Sokolskiy and Kamakin, 2004). The diet of M. leidyi in the southwestern Caspian includes cladocerans (Polyphemus exiguus and Pleopis polyphemoides), copepods (Calanipeda aquae dulcis, Eurytemora grimmi, E. minor, Halicyclops sarsi, and Acartia clausi), and bivalve, crab, and Balanus larvae. Eurytemora, A. clausi, and bivalve larvae dominate the food, the other species accounting for some 11% of the total (Kasymov, 2001).
Anchovy kilka too are planktophagous. Their main food (>90% annually) is copepods, Eurytemora alone contributing 70% on average annually (Prikhodko, 1975; Sedov and Paritskiy, 2001). Following the M. leidyi expansion into the Caspian in 2000, E. grimmi and E. minor, the favoured prey of kilka, virtually disappeared. Currently the dominant zooplankton species in the southern and central Caspian Sea is Acartia clausi (>95%; Tinenkova and Petrenko, 2004), and that species now contributes some 55% of the diet of kilka off Azerbaijan (unpublished data).
Kilka stocks are declining year on year. The stock of the main commercial species, anchovy kilka, is seemingly unable to recover from its total recruitment failure in April and May of 2001, when 100 000–270 000 t of kilka (10–40% of the stock) died (Tarasov, 2001), possibly as a result of a natural event. Earthquake data reveal that, in the first quarter of 2001, the local Absheron seismic plate was active, and water and gas systems in the soil were unstable and indicative of hydro-volcanic events or significant gas blow-outs (Katunin et al., 2002). Hydrogen sulphide and methane probably rose through and from the sea, and examination of dead kilka revealed that their intestines and tissue had been impregnated by these gases, and this probably brought about the mortality (Katunin et al., 2002).
Anchovy kilka are traditionally the most abundant fish species in the Caspian, contributing some 70–85% of the total catch of kilka (Figure 2). In the past 20 years, the level of the Caspian Sea has risen, and catches of common kilka (Clupeonella cultriventris caspia) now account for 14–29% of the total catch, whereas in the 1970s common kilka catches were just 1% of the total (Prikhodko, 1975). Stocks of a third species, big-eye kilka (Clupeonella grimmi), are declining under the present environmental conditions: recently they have constituted just 0.2–2% of the total kilka catch, well below their contribution in the 1970s, when the species was the second most important in the kilka catch (20%; Prikhodko, 1975).
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Kilka inhabit the entire shelf of the Azerbaijan coast of the Caspian Sea. According to survey results, common kilka are found mainly (22–24% of total kilka biomass) in the central Caspian at depths of 10–92 m, and in the southern Caspian (5–17%) at depths of 23–100 m. Anchovy kilka concentrations in the central Caspian are about 75% of the total kilka biomass in the sea, and their depths of greatest concentration are between 20 and 30 m, mainly in the southern Caspian. Big-eye kilka are caught sporadically in the central Caspian, just 0.5–1.5% of the catch, but more often in the western part of the southern Caspian around the Borisov shoal (see Figure 1), at depths of 30–56 m (1.5–3%), and over the Kornilov–Pavlov shoal (1.5–3.5%) at depths of 46–97 m. The traditional distribution of the three kilka species in the central and southern Caspian has clearly changed from what it was some 20–30 years ago (Prikhodko, 1975). The probable reasons are the rise in sea level and the consequent change in the hydrological regime over the past decade. The consequence is that the area preferred by common kilka would have expanded, but through the increased inflow of freshwater from the Volga River in particular, conditions would have become less optimal for anchovy kilka (Sedov et al., 1998).
Length, weight, and age
During surveys along the coast of Azerbaijan between 1995 and 2004, the length of anchovy kilka ranged between 6.3 and 13.0 cm, with an average of 10 cm (CV 5.5%), and weight between 3 and 16 g, with an average of 7.4 g (CV 7.4%). More than 80% of the catch was of fish 9.5–11.0 cm long. Female anchovy kilka were mainly 9.5–11.0 cm long, with an average of 10.2 cm, and their weight 5–16 g, with an average of 8.6 g. Generally, the males caught were slightly smaller than females, on average 9.9 cm long (7.5–11.0 cm), weighing on average 7.7 g (5.0–9.4 g). Figure 3 shows the length–frequency distribution of anchovy kilka before (1995–2000) and after (2001–2004) the introduction of M. leidyi in the Caspian. Slight, but clear, changes are evident. The sex ratio of anchovy kilka did not vary much over the survey period reported here (Table 2). Average length and weight, and condition of anchovy kilka in the survey catch along the coast of Azerbaijan for the period 1995–2004 are shown in Figure 4.
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During the study period, the length of common kilka in the central Caspian off Azerbaijan varied between 6.0 and 11.5 cm, averaging 8.8 cm (CV 7.4%); the weight was between 2.5 and 9.0 g, averaging 5.0 g (CV 13%). In the southern Caspian, the length of common kilka varied from 6.0 to 10.5 cm, averaging 8.6 cm, and the weight from 3.5 to 9.0 g, averaging 6.1 g. Big-eye kilka were scarce in the catch, but their length varied from 5 to 12.5 cm (average 9.1 cm, CV 8.5%), and their weight from 3 to 20 g (average 8.5 g, CV 9.1%).
Length–weight relationships for the three species of kilka are shown in Figure 5. The equations of the curves are: anchovy kilka W = 0.0257L2.467, common kilka W = 0.0256L2.583, and big-eye kilka W = 0.0025L3.567.
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Analysis of the age composition of commercial and survey catches reveals that anchovy kilka were mainly 1–6 years old (Figure 6), but recently fish 2–5 years old have dominated (92–100% of the catch; Table 3), and there have been few 1 year olds. The average age of the population was 3.2 years. Common kilka in commercial catches tend to be between 1 and 5 years old, though more than half the recent catches were 1–3 years old. Big-eye kilka 1–7 years old are caught, but >84% are between 1+ and 3+ years old. Weight increases in kilka most during the first two years of age (Sedov and Paritskiy, 2001), but later, at the onset of maturity, it slows.
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Linear growth of the three kilka species off Azerbaijan during the survey years 1995–2004 is best represented by the von Bertalanffy equations:
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) is fastest in big-eye kilka (1.802), slower in anchovy kilka (1.495), and slower still in common kilka (1.251; Table 3).
Reproduction and mortality
Caspian kilka mature young (Lovetskaya, 1951). The gonads of most common kilka are mature by the end of their first year, anchovy kilka mature at 1–2 years, and although most big-eye kilka mature at 2 years of age, a few (10%) attain maturity during their first year (Sedov and Paritskiy, 2001). Observations during the current study have shown that most kilka (of all three species) currently mature at 2 years of age.
Of the three species, anchovy kilka spawn over the longest period: May–December in water temperatures of 15–25°C and salinity 8–13. Along the coast of Azerbaijan, however, the results of the current study show that some spawning takes place in May and June. This is a different spawning season from that of the same species in the southeast Caspian where, according to Sedov and Paritskiy (2001), most (80%) anchovy kilka spawn between October and December.
Analysis of the reproductive stages showed that, in summer, most female anchovy kilka (61.7%) were at stage II, 21% at stage II/III, 10.1% at stage IV, and just 2.7% at stage V. In autumn, 77.2% were at prespawning stages (III–IV), and the percentage mature was 3.1, higher than in summer (2.6%). Anchovy kilka spawn in all areas of the central and southern Caspian (Sedov et al., 1998). According to those authors, the basic area of spring and summer spawning is the central Caspian, especially in its northwestern and northeastern parts. In May and June in those areas, about 20% of anchovy kilka spawn. Along the coast of Azerbaijan in the central Caspian in the same months, some 11.2% of anchovy kilka spawn, but in the southern Caspian in May and June, the spawning proportion is just 3.5% (Sedov et al., 1998).
Natural mortality of kilka in the Caspian can be calculated from the formula of Pauly (1980), which for anchovy kilka gives a value of 0.54, for common kilka 0.48, and for big-eye kilka 1.04 (Table 3).
Recruitment and the ecosystem
Kilka catches over the past few years have contained virtually no juvenile anchovy kilka. Such fish now account for only 0.2–0.6% of the total catch: clear evidence of poor recruitment of the species. For the entire Caspian kilka harvest, just 20 anchovy kilka were caught per cone-net haul in 2004, a massive decrease from the average historical rate of capture of 1900 fish per haul (Sedov et al., 2004).
Anchovy kilka fecundity studied on the basis of survey hauls varied from 8510 to 58 340 mature oocytes per fish, averaging 35 876 oocytes, close to Sedov and Paritskiy's (2001) recorded fecundity of 9800–61 000, with a mean of 38 400. Clearly, therefore, it would be difficult to ascribe the recruitment failure to a decrease in egg production. Further, during the kilka survey of summer 2003, large numbers of anchovy kilka larvae were caught by cone net off the coast of Azerbaijan, raising hopes of a recovery. Unfortunately, however, the 2004 survey caught very few juvenile anchovy kilka. One of the reasons for this may be the predation pressure exerted on juvenile anchovy kilka by Mnemiopsis leidyi, a species known to eat fish eggs and larvae. As already stated, there is some anchovy kilka spawning along the coast of Azerbaijan in May and June, the same time of year as M. leidyi populations tend to appear along the same coast. Much work has been carried out and documented in search of how this predatory or competitive interaction with M. leidyi may be negatively impacting kilka populations in the Caspian Sea (e.g. Kasymov, 2001; Sokolskiy and Kamakin, 2004; Negarestan et al., 2001; Vostokov et al., 2002; Kuliyev, 2004). One finding has been that the population of M. leidyi in the deep central areas of the southern Caspian is about three times bigger than that in the southwestern coastal area off Azerbaijan (Vostokov et al., 2002; Kideys and Moghim, 2003). Further, an unusually large concentration of M. leidyi was found in the pelagic zone above a water depth of 700–800 m associated with an algal bloom; it contained mainly juveniles <5 mm long (1600 specimens per m2). Moreover, in July and August, concentrations of M. leidyi have been recorded in which 89.1% are <15 mm long. The length of the largest M. leidyi found off Azerbaijan is 61–65 mm (near Siazan in 5 m of water; Kuliyev, 2004). Work conducted in 2003 and reported by Sokolskiy and Kamakin (2004) showed that the limiting temperature for survival of M. leidyi in the southern Caspian is 8.3°C, and in the central Caspian 5.3°C. Lower temperatures in winter, especially in shallow water over the shelf of the southern and central Caspian, deter the population of M. leidyi from blooming.
According to Vostokov et al. (2002), onboard measurements of chlorophyll concentration, turbidity, and water colour, as well as remote sensing of optical properties of the sea surface (SeaWiFS data), have shown that the waters of the western shelf south of Azerbaijan's Absheron Peninsula are the most productive in the whole Caspian Sea. Consequently, the zooplankton suitable as food for pelagic fish such as kilka is generally very abundant in that part of the sea (Vostokov et al., 2002), perhaps revealing the reason why the kilka resource has traditionally been so abundant off Azerbaijan.
Stock size, the fishery, and Mnemiopsis leidyi
Nowadays kilka are fished commercially mainly in the southern Caspian Sea, and at least until the recent sudden decline in catches, the annual harvest was 80 000–150 000 t. However, the kilka catch peaked in the 1970s at more than 400 000 t, of which traditionally about 60% was taken off the coast of Azerbaijan, although it was often countries other than Azerbaijan that caught it. Figure 7 shows the time-series of kilka catches in the Caspian.
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Recently, unfavourable hydrological conditions could have had a negative impact on the sustainability of exploitable concentrations of kilka: the thermal structure of the upper layers of the sea has been detrimental to the species, and there has also been poor vertical mixing of surface and deeper water (pers. obs.). During some recent surveys the temperature of the upper layers reached 26–27°C, and as a consequence, kilka migrated to deeper water where the temperature was lower. There, though, they could not be fished with cone nets.
In summer 2004, the kilka catch 100 m deep off Divichi (87 kg per cone-net haul) was the biggest in the whole central Caspian. In the southern Caspian, the largest research catch was over the Makarov Bank (Figure 1): 9.8 kg at a depth of 49 m. During the kilka surveys of summer 2003 and summer 2004 along the Azerbaijan coast of the central Caspian, the average catch varied from 19.8 to 30.5 kg at depths of 51–90 m, and along the Azerbaijan coast of the southern Caspian from 6.1 to 33.7 kg at depths of 53–81 m. Virtual population analysis (Sparre and Venema, 1998) reveals that the average annual catch of anchovy kilka off Azerbaijan during the period 1994–2004 was 20.8 billion fish, or 119 200 t. The mean annual size of the anchovy kilka stock then can be calculated by Pope's (1972) method to have been 92.7 billion fish, or 537 500 t. Fishing mortality calculated by the same method ranged from 0.22 to 1.21, with a mean for the period of 0.59. Taking the values of M from Pauly's (1980) method, total mortality Z for anchovy kilka ranged between 0.76 and 1.75, with a mean of 1.13. The steady decline in exploitable biomass is clearly related to the poor environmental conditions in the Caspian Sea during this period, especially with the expansion of the ctenophore M. leidyi population, and overexploitation during the years 2000–2002 (when F is calculated to have been 0.80–1.21). A similar picture emerged for anchovy (Engraulis encrasicolus ponticus) in the Black Sea, where decreasing exploitable biomass and catch were associated with overfishing and the emergence of large blooms of M. leidyi during the years 1987–1989 (F was 0.81–1.44; Prodanov et al., 1997).
That kilka stocks have decreased off Azerbaijan is confirmed by the statement above that catches there between 2001 and 2004 dropped from 18 500 t to 5500 t (Figure 8). The stock in the Caspian Sea is therefore clearly in a critical condition and is suffering more damage every year. The situation off Iran is similar to that off Azerbaijan. According to Bagheri et al. (2004), the decline in kilka catches off the coast of Iran during the period 1998–2001 caused economic losses of at least US$15 million.
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In order to prevent further reduction of the kilka stock and to combat the spread of Mnemiopsis leidyi, it was proposed at an international meeting in Baku in 2001 that another ctenophore, Beroe ovata, a natural predator of M. leidyi, be introduced to the Caspian Sea. However, gobies, natural predators of M. leidyi, already exist in the shallower, coastal areas of the Caspian Sea. Analyses of the stomach contents of the three main species of goby (Neogobius fluviatilis pallasi, Neogobius melanostomus, and Neogobius kessleri gorlap) have shown that they do indeed consume M. leidyi. They consumed the ctenophore along the east coast of the southern Caspian and in the northwestern Caspian, 8.8% and 6.7% of their diet at 7–8 m and 25–50 m, respectively, being ctenophores. Preliminary calculations have shown that gobies in the northwestern Caspian and along the coast of Turkmenistan (southeastern Caspian) can consume 15 600 t of ctenophores in summer, some 10–15% of the current total biomass of M. leidyi in those areas (Stepanova and Sokolskiy, 2003). Given these values, although gobies are mainly shallow-water species, it does beg the question as to whether there would be benefit in introducing another exotic species to the Caspian to "control" M. leidyi.
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Conclusion |
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TopIntroductionMaterial and methodsResults and discussionConclusionReferences |
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Analysis of historical data confirms that kilka stocks in the Caspian have been reduced recently by both natural (climatic and perhaps seismic in origin) and anthropogenic influences. The biggest change has been to the anchovy kilka population. According to the age and length compositions of the recent catch, there has been no recent recruitment of note, resulting in an increase in the age of the population and a massive reduction in total biomass. However, since 2003 there have been some positive signs, perhaps heralding some attempt by the population to recover. All the biological parameters, including age composition, confirm that the anchovy kilka stock is in a sufficiently good condition to recover. Growth rates and the weights of length and age groups are at higher levels than expected for a stock under such severe stress. However, given the competitive or predatory interaction that anchovy kilka experience with Mnemiopsis leidyi, it may be necessary to take urgent measures to combat the spread of the ctenophore, which seemingly seriously jeopardizes the survival of juvenile anchovy kilka.
Such a problem of conservation and rational exploitation of Caspian kilka stocks is of global interest given the uniqueness of this water body. Sustainable fisheries management and conservation of biodiversity in the Caspian Sea are real challenges now.
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Acknowledgements |
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I am grateful to two anonymous reviewers for their valuable comments on the submitted manuscript, to Andrew Payne (Cefas) for extensive grammatical revision of all drafts, to Georgi Daskalov (also Cefas) for his mentorship of the work recently, and to Zulfugar Kuliyev, my supervisor at the Azerbaijan Academy of Sciences, for his support and encouragement throughout.
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References |
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TopIntroductionMaterial and methodsResults and discussionConclusionReferences |
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