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

Abstract

The logic, structure and preliminary results from a mathematical model of the Black and Azov Sea ecosystems are presented in response to the need for a theoretical framework to analyse recent drastic decreases in catches of anchovy. The modelling approach is dictated by incomplete data for many key processes involved in the disastrous ecosystem changes that have impacted fisheries in these inland seas since the late 1980s. These not only resulted in environmental changes and negative impacts on fisheries, but also in the introduction of the exotic planktonic predator, the comb jelly Mnemiopsis leydei; however, their relative importance is unclear from the limited data available. The objectives of this first project phase are to: reconstruct the introduction of M. leydei, study different hypotheses of its trophic interaction with anchovy, and consider alternatives for rehabilitation of the commercial stocks. The model is modular, with subsystems for the Black and Azov Seas and linked flows and migrations of energy, organic matter and fish biomass, specified, as a system of difference and algebraic equations, both at the system level and the level of energy balance of individual organisms. Emphasis is on documenting hypotheses for transformation of incoming energy by pelagic zooplankton and fish of the two seas to average densities per square metre of surface. The Black Sea trophic chain is more detailed, including 18 components (excluding age groups), but the Azov Sea model is limited to 11 components. The anchovy population is the only one with age structure specified (larvae, juveniles and three adult stages). The model does not represent detailed transformations of organics or the biogenic cycle, which, with primary production, are considered as model driving factors, and impacts of pelagic production on benthic/demersal ecosystems are only considered incidentally, via pelagic detrital outflows to sink. The model is aimed at a parsimonious description of the main processes and mechanisms of inter and intra-annual population dynamics, the impacts of environmental conditions, of spawning and seasonal migrations for anchovy, with or without specified impacts by jelly predators. Simulations considered three scenarios covering 1966–2000, divided into three phases. Model identification was based on 1996–1982 data, verification in 1983–1993, and the last phase (1994–2000) was used to validate model reliability. Three scenarios were considered: (i) absence of M. leydei, (ii) only food competition between anchovy and the ctenophore and (iii), in addition to (ii), predation of anchovy larvae by M. leydei. Results are presented as graphic and tabular outputs of annual and seasonal variations, energy balances, and values for other system variables. The model is available for operation under the Windows 95/NT environment.