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5 Iranian Fisheries Research Organization (IFRO), No. 297, Fatemi St., Tehran, Iran ABSTRACT: 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 preda- tory ctenophore were studied in Caspian Sea water (12.6 ppt salinity) conditions using animals trans- ported 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 consump- tion, respiration and growth rates revealed a mean assimilation efficiency of 0.72 ± 0.1, a gross growth efficiency (K1) of 0.48 ± 0.12 and a net efficiency (K2) of 0.66 ± 0.06. Based on these physio- logical 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 condi- tions (most probably due to poor handling) remains the main obstacle to overcome in the successful establishment of B. ovata in the Caspian Sea.

Physiological characteristics of the ctenophore Beroe ovata in Caspian Sea water