No abstract

The impact of the invasive ctenophore Mnemiopsis leidyi on the zooplankton community of the Caspian Sea was quantified according to food consumption and other major physiological activities (i.e. respiration and reproduction), coupled with field data on population structure. The adverse effects of M. leidyi on the zooplankton community during the first years of the invasion were tremendous for the Caspian Sea compared to other regions affected by this ctenophore. The impact was highest in summer, due to high water temperatures and a population size structure in which juvenile ctenophores with mean lengths of 2 to 5 mm accounted for most of the population. During winter/spring, these ctenophores could consume the available stock of zooplankton in 3 to 8 d, whereas in summer consumption took only 1 d. The computed critical ctenophore biomass that does not affect (decrease) the abundance of mesozooplankton in the Caspian Sea is about 4 g m-3 (or 120 g m-2 , assuming most of the ctenophores occur in the upper 30 m layer). As is clear from the monitoring data, the M. leidyi biomass in summer in different regions of the Caspian Sea is far in excess of this value. Such a high abundance of ctenophores, if maintained, would constantly keep the nongelatinous zooplankton biomass at very low levels, and, as a consequence, no recovery could be expected in the pelagic fishery.

The invasive ctenophore Mnemiopsis leidyi (Agassiz), which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001–2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (four Copepoda and one Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of Mnemiopsis leidyi into the Caspian Sea. Cladocera species seemed to be highly affected by the invasion of Mnemiopsis leidyi; only one species, Podon polyphemoides, remained in the study area, whereas 24 Cladocera species were found in the study carried out in 1996. Whereas among the Copepoda Eurytemora minor, Eurytemora grimmi, Calanipeda aquae dulcis and Acartia tonsa that were abundant before the Mnemiopsis leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maximum in zooplankton abundance (22,088 ± 24,840 ind·m−3) and biomass (64.1 ± 56.8 mg·m−3) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001–2006 was in the range of 3361–8940 ind·m−3; this was two‐ to five‐fold less than the zooplankton abundance in 1996. During 2001–2006, the highest abundance and biomass of Mnemiopsis leidyi were observed during summer‐autumn months coincident with warm temperatures and generally when the abundance of other zooplankton organisms was low.

Riparian countries of the Caspian Sea have been evaluating the pros and cons of the predatory ctenophore Beroe ovata as a control agent against the invasive ctenophore Mnemiopsis leidyi, which has enormous adverse impacts on the fishery resources as well as on the biodiversity in this once fertile sea. To assess the viability of B. ovata establishment in the Caspian Sea, the survival and some physiological characteristics (feeding, respiration, reproduction and growth) of the predatory ctenophore were studied in Caspian Sea water (12.6 ppt salinity) conditions using animals transported from the Black and Marmara Seas to a laboratory on the Iranian Caspian coast. The findings of the study showed that when salinity was gradually decreased from 22 to 12.6 ppt, B. ovata were able to adapt well to Caspian Sea water. Most of the predatory ctenophores began to swim actively and to feed on M. leidyi within 15 to 30 min following each step of acclimation. The feeding rate of B. ovata ranged from 14 to 765% of body wet weight and was highest for smaller individuals (i.e. 13 to 16 mm). Over the measured weight range of 0.23 to 3.87 g wet wt, the weight-specific respiration rate was independent of weight. The daily specific growth rate of adult ctenophores was 7 to 11% of body wet wt. B. ovata specimens were spawned and their eggs were hatched in Caspian Sea water, but the larvae survived for only a few hours. The energy budget of B. ovata calculated from food consumption, respiration and growth rates revealed a mean assimilation efficiency of 0.72 ± 0.1, a gross growth efficiency (K 1 ) of 0.48 ± 0.12 and a net efficiency (K 2 ) of 0.66 ± 0.06. Based on these physiological data, we suggest that in the Caspian Sea, B. ovata will be able to ingest M. leidyi intensively. However, concerning the reduction of the M. leidyi population and consequently the reversal of its adverse impact on this ecosystem, the failure of larval growth observed under experimental conditions (most probably due to poor handling) remains the main obstacle to overcome in the successful establishment of B. ovata in the Caspian Sea.

Changes in length, weight and fecundity of mature females of the Kutum (Rutilus frisii kutum) were compared for the periods 1948-1950, 1974 and 2007 in Iranian waters of the Caspian Sea. The average fork length decreased from 54.1 over 44.7 to 43.9 cm and the average weight from 2181 over 1295 to 1210 g. However, the average condition factor has remained fairly constant and varied between 1.35 and 1.38. The potential fecundity of the Kutum for the three periods also decreased from 106,800 over 74,600 to 64,400 eggs. Size and fecundity of the Caspian Kutum has therefore sharply declined during the last six decades. The reduction of fecundity is apparently the consequence of the shift towards smaller fish size.