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, increasing 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 consumption 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 consumer 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 pressure on phytoplankton and allows its standing crop to increase. The effects of fishing and eutrophication are explored using a dynamic mass-balance model. A balanced model is built using 15 ecological groups including bacteria, phytoplankton, zooplankton, protozoa, ctenophores, medusae, chaetognaths, fishes and dolphins. Ecosystem dynamics are simulated over 30 yr, assuming alternative 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 pattern 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 ecosystem management, suggesting that conserving and restoring natural stocks of fish and marine mammals can contribute greatly to sustaining viable marine ecosystems.

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

“…As the above examples have demonstrated, recent examinations of the roles of spatial heterogeneity and the movement of marine populations have implications for the initiation and detection of regime shifts, and some common features emerge. Based on spatio-temporal analyses within the Benguela, Baltic and Black Seas, seasonal and/or interannual shifts in the spatial distribution of predatory populations have contributed to initiating regime shifts and may explain shifts in associations between key population dynamics and environmental drivers [37,49,52,53]. In the North Sea [38] and Barents Sea [39], spatial downscaling of population models relating environmental and predation dynamics on key species have illustrated that relationships such as the apparent increased role of predation at lower water temperatures, first reported in among-system comparisons [40], are mirrored at smaller spatial scale orders of magnitude (figures 2 and 3).…”

“…In the Black Sea, a pronounced regime switch from a predator-dominated to a planktivore-dominated system has been documented [52,53]. As described by Aristotle in his Historia Animalium [54], the main pelagic predators (bonito, bluefish and mackerel, but also bluefin tuna in the past) migrate in the spring from the southern Black Sea and Mediterranean to feed during the warm season along the most productive area of the basin-the large and shallow northwest shelf.…”

Seaweed invasions: conclusions and future directions

“…Quoy et J.P. Gaimard, A. Reid and C.R. Johnson, unpublished datax, and generalist predators in pelagic (e.g., the ctenophore Mnemiopsis leidyi, Shiganova 1998, Daskalov 2002, 2003 and benthic (e.g., Northern Pacific seastar, Asterias amurensis A. Agassiz; Ross et al 2003) habitats. All of these animals have the capacity to effect deleterious impact on richness, diversity and ecosystem functioning over large spatial scales.…”

History of expansion and anthropogenic collapse in a top marine predator of the Black Sea estimated from genetic data

“…Apex marine predators are a key component of this ecosystem, because they regulate the demography of species at lower trophic levels through a top-down regulation of the trophic food-web (4,7,8,11). Modeling studies have suggested that the recent decimation of these predators in the Black Sea was a key factor in reducing ecosystem resilience, inevitably leading to its reorganization (i.e., a regime shift) (7,8,10,11).…”