Trophodynamic model of the Black and Azov Sea pelagic ecosystem: consequences of the comb jelly, Mnemiopsis leydei, invasion

The logic, structure and preliminary results from a mathematical model of t he Black and Azov Sea ecosystems are presented in response to the need for a theoretical framework to analyse recent drastic decreases in catches of a nchovy. The modelling approach is dictated by incomplete data for many key processes involved in the disastrous ecosystem changes that have impacted f isheries in these inland seas since the late 1980s. These not only resulted in environmental changes and negative impacts on fisheries, but also in th e introduction of the exotic planktonic predator, the comb jelly Mnemiopsis leydei; however, their relative importance is unclear from the limited dat a available. The objectives of this first project phase are to: reconstruct the introduction of M. leydei, study different hypotheses of its trophic i nteraction with anchovy, and consider alternatives for rehabilitation of th e commercial stocks. The model is modular, with subsystems for the Black and Azov Seas and linke d flows and migrations of energy, organic matter and fish biomass, specifie d, as a system of difference and algebraic equations, both at the system le vel and the level of energy balance of individual organisms. Emphasis is on documenting hypotheses for transformation of incoming energy by pelagic zo oplankton and fish of the two seas to average densities per square metre of surface. The Black Sea trophic chain is more detailed, including 18 compon ents (excluding age groups), but the Azov Sea model is limited to 11 compon ents. The anchovy population is the only one with age structure specified ( larvae, juveniles and three adult stages). The model does not represent det ailed transformations of organics or the biogenic cycle, which, with primar y production, are considered as model driving factors, and impacts of pelag ic production on benthic/demersal ecosystems are only considered incidental ly, via pelagic detrital outflows to sink. The model is aimed at a parsimon ious description of the main processes and mechanisms of inter and intra-an nual population dynamics, the impacts of environmental conditions, of spawn ing and seasonal migrations for anchovy, with or without specified impacts by jelly predators. Simulations considered three scenarios covering 1966-2000, divided into thr ee phases. Model identification was based on 1996-1982 data, verification i n 1983-1993, and the last phase (1994-2000) was used to validate model reli ability. Three scenarios were considered: (i) absence of M. leydei, (ii) on ly food competition between anchovy and the ctenophore and (iii), in additi on to (ii), predation of anchovy larvae by M. leydei. Results are presented as graphic and tabular outputs of annual and seasonal variations, energy b alances, and values for other system variables. The model is available for operation under the Windows 95/NT environment. (C) 1999 Elsevier Science B. V. All rights reserved.