Overfishing drives a trophic cascade in the Black Sea

Abstract

During recent decades, environmental conditions have deteriorated in the Black Sea. Population explosions of phytoplankton and jellyfish have become frequent and several fish stocks have collapsed. In this study, literature sources and long-term data are explored in order to find empirical evidence for ecosystem effects of fishing. Inverse trends of decreasing predators, increas- ing planktivorous fish, decreasing zooplankton and increasing phytoplankton biomass are revealed. Increased phytoplankton biomass provoked decreasing transparency and nutrient content in surface water. A massive development of jellyfish during the 1970s and 1980s had a great impact on con- sumption and consequent decrease in zooplankton. The turning point for these changes occurred in the early 1970s, when industrial fishing started and stocks of pelagic predators (bonito, mackerel, bluefish, dolphins) became severely depleted. A 'trophic cascade' is invoked as a mechanism to explain observed changes. According to this hypothesis, reduction in apex predators decreases con- sumer control and leads to higher abundance of planktivorous fish. The increased consumption by planktivorous fish causes a consequent decline in zooplankton biomass, which reduces grazing pres- sure on phytoplankton and allows its standing crop to increase. The effects of fishing and eutrophi- cation are explored using a dynamic mass-balance model. A balanced model is built using 15 eco- logical groups including bacteria, phytoplankton, zooplankton, protozoa, ctenophores, medusae, chaetognaths, fishes and dolphins. Ecosystem dynamics are simulated over 30 yr, assuming alterna- tive scenarios of increasing fishing pressure and eutrophication. The changes in simulated biomass are similar in direction and magnitude to observed data from long-term monitoring. The cascade pat- tern is explained by the removal of predators and its effect on trophic interactions, while the inclusion of eutrophication effects leads to biomass increase in all groups. The present study demonstrates that the combination of uncontrolled fisheries and eutrophication can cause important alterations in the structure and dynamics of a large marine ecosystem. These findings may provide insights for eco- system management, suggesting that conserving and restoring natural stocks of fish and marine mammals can contribute greatly to sustaining viable marine ecosystems.