Fisheries can have a large impact on marine ecosystems, because the effects of removing large predatory fish may cascade down the food web. The implications of these cascading processes on system functioning and resilience remain a source of intense scientific debate. By using field data covering a 30-year period, we show for the Baltic Sea that the underlying mechanisms of trophic cascades produced a shift in ecosystem functioning after the collapse of the top predator cod. We identified an ecological threshold, corresponding to a planktivore abundance of Ϸ17 ؋ 10 10 individuals, that separates 2 ecosystem configurations in which zooplankton dynamics are driven by either hydroclimatic forces or predation pressure. Abundances of the planktivore sprat above the threshold decouple zooplankton dynamics from hydrological circumstances. The current strong regulation by sprat of the feeding resources for larval cod may hinder cod recovery and the return of the ecosystem to a prior state. This calls for the inclusion of a food web perspective in management decisions.alternative dynamics ͉ ecological thresholds ͉ ecosystem resilience ͉ Baltic Sea ͉ climate versus top-down control

Anthropogenic disturbances intertwined with climatic changes can have a large impact on the upper trophic levels of marine ecosystems, which may cascade down the food web. So far it has been difficult to demonstrate multi-level trophic cascades in pelagic marine environments. Using field data collected during a 33-year period, we show for the first time a four-level community-wide trophic cascade in the open Baltic Sea. The dramatic reduction of the cod (Gadus morhua) population directly affected its main prey, the zooplanktivorous sprat (Sprattus sprattus), and indirectly the summer biomass of zooplankton and phytoplankton (top-down processes). Bottom-up processes and climate-hydrological forces had a weaker influence on sprat and zooplankton, whereas phytoplankton variation was explained solely by top-down mechanisms. Our results suggest that in order to dampen the occasionally harmful algal blooms of the Baltic, effort should be addressed not only to control anthropogenic nutrient inputs but also to preserve structure and functioning of higher trophic levels.

Despite the common occurrence of ontogenetic niche shifts, their consequences for morphological adaptations have been little studied. To address this question, we studied morphological adaptations related to ontogenetic niche shifts in Eurasian perch (Perca fluviatilis) in eight lakes that varied in density of benthic resources and planktivorous fish biomass. Perch start to feed on pelagic zooplankton, then shift to benthic resources at intermediate sizes, and finally, when large enough, mainly feed on fish. These three functional niches over ontogeny are expected to set constraints on the morphology and size-specific growth of perch. The growth of perch was negatively related to planktivorous fish biomass in the zooplanktivorous niche, but positively related to planktivorous fish biomass in the piscivorous niche. The number of gill rakers of perch was negatively related to the biomass of planktivorous fish, providing evidence for the occurrence of character displacement as a consequence of competition in the zooplanktivorous niche. Perch in lakes with low densities of predator-sensitive macroinvertebrates had greater body height measurements and a larger mouth early during ontogeny. This pattern is suggested to be a result of a selection for increased efficiency in the benthic niche when the availability of benthic resources is low. Perch in lakes with a high biomass of planktivorous fishes had fusiform body morphology, a thicker tail and a larger mouth then the average piscivorous perch. The different responses of perch morphology to variation in the availability of benthic resources compared to variation in planktivore biomass are suggested to be partly because the availability of the former resource to a larger extent is set by abiotic conditions (humic content). We suggest that the key factors affecting size-specific growth and body morphology of perch in the system studied are the availability of resources in the benthivorous and piscivorous niches. We also provide evidence for morphological trade-offs, especially between the benthivorous and the piscivorous ontogenetic niches.

Investigating the factors regulating fish condition is crucial in ecology and the management of exploited fish populations. The body condition of cod (Gadus morhua) in the Baltic Sea has dramatically decreased during the past two decades, with large implications for the fishery relying on this resource. Here, we statistically investigated the potential drivers of the Baltic cod condition during the past 40 years using newly compiled fishery-independent biological data and hydrological observations. We evidenced a combination of different factors operating before and after the ecological regime shift that occurred in the Baltic Sea in the early 1990s. The changes in cod condition related to feeding opportunities, driven either by density-dependence or food limitation, along the whole period investigated and to the fivefold increase in the extent of hypoxic areas in the most recent 20 years. Hypoxic areas can act on cod condition through different mechanisms related directly to species physiology, or indirectly to behaviour and trophic interactions. Our analyses found statistical evidence for an effect of the hypoxia-induced habitat compression on cod condition possibly operating via crowding and density-dependent processes. These results furnish novel insights into the population dynamics of Baltic Sea cod that can aid the management of this currently threatened population.

The eastern Baltic (EB) cod (Gadus morhua) stock was depleted and overexploited for decades until the mid-2000s, when fishing mortality rapidly declined and biomass started to increase, as shown by stock assessments. These positive developments were partly assigned to effective management measures, and the EB cod was considered one of the most successful stock recoveries in recent times. In contrast to this optimistic view, the analytical stock assessment failed in 2014, leaving the present stock status unclear. Deteriorated quality of some basic input data for stock assessment in combination with changes in environmental and ecological conditions has led to an unusual situation for cod in the Baltic Sea, which poses new challenges for stock assessment and management advice. A number of adverse developments such as low nutritional condition and disappearance of larger individuals indicate that the stock is in distress. In this study, we (i) summarize the knowledge of recent changes in cod biology and ecosystem conditions, (ii) describe the subsequent challenges for stock assessment, and (iii) highlight the key questions where answers are urgently needed to understand the present stock status and provide scientifically solid support for cod management in the Baltic Sea.