Christian Möllmann

Hamburg University | UHH · Institute for Marine Ecosystem and Fisheries Science

Collapses and regime changes are pervasive in complex systems (such as marine ecosystems) governed by multiple stressors. The demise of Atlantic cod (Gadus morhua) stocks constitutes a text book example of the consequences of overexploiting marine living resources, yet the drivers of these nearly synchronous collapses are still debated. Moreover, i...

Understanding tipping point dynamics in harvested ecosystems is of crucial importance for sustainable resource management because ignoring their existence imperils social-ecological systems that depend on them. Fisheries collapses provide the best known examples for realizing tipping points with catastrophic ecological, economic and social conseque...

Fish represent a politically regulated, scientifically researched, industrially processed, commercially marketed and socially contested living marine resource. Related to this, the incorporation of resource users and stakeholders into fisheries management is particularly important. Such involvement has recently improved in terms of frequency, but i...

Understanding individual growth in commercially exploited fish populations is key to successful stock assessment and informed ecosystem‐based fisheries management. Traditionally, growth rates in marine fish are estimated using otolith age‐readings in combination with age‐length relationships from field samples, or tag‐recapture field experiments. H...

Zooplankton in estuaries encounter complex physical and biogeochemical processes that affect the quantity, quality, and origin of their food sources. The knowledge about how zooplankton deal with highly variable organic matter sources is sparse. Here, we investigated the spatial and temporal patterns of zooplankton trophic dynamics and carbon sourc...

Coastal marine and estuarine systems are subject to enormous endogenous and exogenous pressures, particularly climate change, while at the same time being highly productive sources and nurseries for fish populations. Interactions between host and microbiome are increasingly recognized for their importance for fish health, with growing evidence indi...

Fisheries worldwide face uncertain futures as climate change manifests in environmental effects of hitherto unseen strengths. Developing climate-ready management strategies traditionally requires a good mechanistic understanding of stock response to climate change in order to build projection models for testing different exploitation levels. Unfort...

Marine fisheries are increasingly impacted by climate change, affecting species distribution and productivity, and necessitating urgent adaptation efforts. Climate vulnerability assessments (CVA), integrating expert knowledge, are vital for identifying species that could thrive or suffer under changing environmental conditions. This study presents...

The Western Mediterranean fisheries significantly contribute to the regional blue economy, despite evidence of ongoing, widespread overexploitation of stocks. Understanding the spatial distribution and population dynamics of species is crucial for comprehending fisheries dynamics combining local and regional scales, although the underlying processe...

Coastal and estuarine environments are under endogenic and exogenic pressures jeopardizing survival and diversity of inhabiting biota. Information of possible synergistic effects of multiple (a)biotic stressors and holobiont interaction are largely missing in the Elbe estuary but are of importance to estimate unforeseen effects on animals’ physiolo...

Marine heatwaves (MHW) are globally occurring events observed with potentially long-lasting ecological effects and socio-economic repercussions. The Western Baltic Sea (WBS) is a transitional region between the North Sea and the Baltic Proper characterized by a unique physical oceanography. Warming of the area in response to climate change is signi...

Scientific monitoring is a fundamental basis of scientific advice. Among others, monitoring aims at contributing towards understanding the influence of anthropogenic use (e.g. fisheries), the health of a stock and individuum and effectiveness of management and conservation measures (e.g. MPAs). Monitoring of demersal and benthic fish communities is...

Recent scientific assessments have started to consider the presence of multiple populations of Atlantic cod (Gadus morhua) in the North Sea due to genetic evidence. The Atlantic cod stock as a whole has experienced a regime shift in the North Sea and is now considered to be in a persistent depleted state. Here we investigated if the regime shift de...

The North Sea (NS) is changing rapidly. Temporal variations in fishing intensity and eutrophic conditions, along with the ongoing impact of climate change, act in synergy resulting in modifications in marine communities. Although zooplankton has been extensively investigated, studies often ignore the large-sized meso-and macro-zooplankton (> 500 μm...

Marine ecosystems worldwide experience abrupt changes and regime shifts in structure and functioning due to the interacting effects of multiple stressors. North Sea cod Gadus morhua is a key example of a species being strongly overexploited for decades, causing an abrupt stock decrease below scientifically advised sustainable levels. Despite reduct...

There is an increasing need for marine spatial planning in the North Sea given the multiple uses with competing objectives. Plans to increase renewable energy production by establishing offshore wind farms (OWFs) are likely to coincide with existing and planned marine protected areas (MPAs), with obvious incompatibilities relating to conservation g...

Worldwide, fisheries face the consequences of climate change and compete with expanding human activities at sea, which may trigger unforeseen reactions of fishers. Hence, knowledge on drivers of fishing behavior is crucial for management and needs to be integrated in resource management policies. In this study, we identify factors influencing fishi...

Recovery of depleted fish stocks is an important goal for fisheries management and crucial to sustain important ecosystem functions as well as global food security. Successful recovery requires adjusting fishing mortality to stock productivity but can be prevented or inhibited by additional anthropogenic impacts such as climate change. Despite mana...

Cumulative human pressures and climate change can induce nonlinear discontinuous dynamics in ecosystems, known as regime shifts. Regime shifts typically imply hysteresis, a lacking or delayed system response when pressures are reverted, which can frustrate restoration efforts. Here, we investigate whether the northern Adriatic Sea fish and macroinv...

Human impacts can induce ecosystems to cross tipping points and hence unexpected and sudden changes in ecosystem services that are difficult or impossible to reverse. The world´s oceans suffer from cumulative anthropogenic pressures like overexploitation and climate change and are especially vulnerable to such regime shifts. Yet an outstanding ques...

Recently, the rights of small-scale fishers have increasingly been acknowledged in ocean governance because coastal development and various maritime activities have reduced traditional fishing grounds. More specifically, small-scale fisheries (SSF) are increasingly being threatened by ocean grabbing, pollution, and a lack of inclusiveness in decisi...

The operational principle of offshore wind farms (OWF) is to extract kinetic energy from the atmosphere and convert it into electricity. Consequently, a region of reduced wind speed in the shadow zone of an OWF, the so-called wind-wake, is generated. As there is a horizontal wind speed deficit between the wind-wake and the undisturbed neighboring r...

Die westliche Ostsee steht unter massivem Druck. Bedingt durch negativen anthropogene Effekte wie Klimawandel, Eutrophierung und Überfischung folgt eine Veränderung der Ökosysteme entlang der Küste sowie der Fischerei und ihrer Kultur. Weitere Gründe für diesen Strukturwandel sind ein relativ sinkendes Einkommen in der Fischerei aufgrund starker Fa...

We present estimates of length–weight relationships (LWRs) of 55 mesopelagic fish species of 13 taxonomic families based on data collected in the eastern tropical North Atlantic (ETNA) in March/April 2015. Our data include novel records for 19 species, while for 25 species LWRs are based on the most robust sample sizes, and for 21 species they are...

Quantifying the morphology of organisms remains fundamental in ecology given the form-function relationship. Morphology is quantifiable in traits, landmarks, and outlines, and the choice of approach may influence ecological conclusions to an unknown extent. Here, we apply these three approaches to 111 individual coral reef fish of 40 species common...

With recent advances in Machine Learning techniques based on Deep Neural Networks (DNNs), automated plankton image classification is becoming increasingly popular within the marine ecological sciences. Yet, while the most advanced methods can achieve human-level performance on the classification of everyday images, plankton image data possess prope...

Background and aim Billfish are epipelagic marine predators facing increasing pressures such as overfishing and rising global temperatures. Overfishing is a major concern, as they are caught by industrial longline fishers targeting tuna. Billfish are targeted by multiple fishing sectors, which provides food, socio‐economic and cultural benefits. To...

Zusammenfassung Dem Dorsch der Westlichen Ostsee geht es schlecht und die von ihm abhängige Fischerei ist in Gefahr. Neueste Abschätzungen des Internationalen Rates für Mee-resforschung (ICES) zeigen einen katastrophalen Zustand des Bestands. In einem wissenschaftlichen Artikel haben wir mit weiteren Kolleg:innen die historische Ent-wicklung des Be...

Die westliche Ostsee und ihre Fischerei durchleben aktuell eine umfassende Transformation. Sinkende Fangquoten für die Hauptzielfischarten Dorsch und Hering als Folge von Überfischung und Klimawandel führen zu einem Rückgang der Beschäftigung in der Fischerei. Hinzu komm das generelle Fehlen von Nachwuchs in der Fischerei. Ein transdisziplinärer Bl...

Sustainable environmental management needs to consider multiple ecological and societal objectives simultaneously while accounting for the many uncertainties arising from natural variability, insufficient knowledge about the system's behaviour leading to diverging model projections, and changing ecosystem. In this paper we demonstrate how a Bayesia...

Mesopelagic sound scattering layers (SSLs) are predominantly associated with mesopelagic fish taxa with gas-inflated swimbladders that perform active nocturnal diel vertical migration (DVM), like species of the families Myctophidae, Gonostomatidae, Sternoptychidae and Phosichthyidae. Larger-sized species with limited, partial, asynchronous and non-...

Spatially explicit records of fishing activities’ distribution are fundamental for effective marine spatial planning (MSP) because they can help to identify principal fishing areas. However, in numerous case studies, MSP has ignored fishing activities due to data scarcity. The vessel monitoring system (VMS) and the automatic identification system (...

The concept of social–ecological knowledge diversity (SEKD) provides a novel way of examining coupled human–environment interactions—it acknowledges differences in knowledge, values, and beliefs of stakeholder groups within social–ecological systems (SES). Thus, understanding and measuring SEKD is an essential component of sustainable management wi...

Winter has long been regarded as a period of minor importance in marine zooplankton ecology with static, low concentrations and growth rates of organisms. Yet, there is growing evidence that winter conditions influence spring bloom strength. With rising water temperatures, growing importance of fish larvae survival during winter and the lack of dat...

Improving the health of coastal and open sea marine ecosystems represents a substantial challenge for sustainable marine resource management, since it requires balancing human benefits and impacts on the ocean. This challenge is often exacerbated by incomplete knowledge and lack of tools that measure ocean and coastal ecosystem health in a way that...

Global environmental changes have accelerated at an unprecedented rate in recent decades due to human activities. As a consequence, the incidence of novel abiotic conditions and biotic communities, which have been continuously emerging in the Earth system, has rapidly risen. Despite growing attention to the incidence and challenges posed by novelty...

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year‐round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio‐temporal coverage of cod habitats. Cod diet composition in shallow areas (<20 m depth) was dominated by benthic invertebrate species, ma...

Conflict is a common feature in conservation and resource management. Environmental conflicts are frequently attributed to differences in values; however, variability in the perception of facts, rooted in social and cultural differences also underlies conflicts. Such differences in perception have been termed the Rashomon effect after the Kurosawa...

Predators not only have direct impact on biomass but also indirect, non-consumptive effects on the behavior their prey organisms. A characteristic response of zooplankton in aquatic ecosystems is predator avoidance by diel vertical migration (DVM), a behavior which is well studied on the population level. A wide range of behavioral diversity and pl...

One of the most applied tools to create ecosystem models to support management decisions in the light of ecosystem-based fisheries management is Ecopath with Ecosim (EwE). Recently, its spatial routine Ecospace has evolved due to the addition of the Habitat Foraging Capacity Model (HFCM), a spatial-temporal dynamic niche model to drive the foraging...

Identifying key indicator species, their life cycle dynamics and the multiple driving forces they are affected by is an important step in ecosystem-based management. Similarly important is understanding how environmental changes and trophic interactions shape future trajectories of key species with potential implications for ecosystem state and ser...

Practical and applied knowledge of local fishers can help to improve our understanding of target species ecology and fisheries management decisions. In the Western Baltic Sea (WBS), the spatio-temporal distribution of cod is still largely unknown despite decades of research. We studied changes in cod distribution by obtaining information on tempora...

The ecology of vertically migrating mesopelagic micronekton is affected by physical properties of their environment. Increased light attenuation in particle-rich productive waters, as well as low oxygen conditions decrease the migration amplitude. This likely has implications on the trophic organisation of micronekton communities, which are predomi...

Structured, systematic processes for decision-making can facilitate implementation of ecosystem-based fisheries management (EBFM). In US fisheries management, existing fishery ecosystem plans (FEPs) are primarily descriptive documents — not action-oriented planning processes. “Next-generation” FEPs extend existing FEPs by translating ecosystem prin...

Sustainability in the provision of ecosystem services requires understanding of the vulnerability of social-ecological systems (SES) to tipping points (TPs). Assessing SES vulnerability to abrupt ecosystem state changes remains challenging, however, because frameworks do not operationally link ecological, socio-economic and cultural elements of the...

A fundamental challenge in ecology is to understand why species are found where they are and predict where they are likely to occur in the future. Trait-based approaches may provide such understanding, because it is the traits and adaptations of species that determine which environments they can inhabit. It is therefore important to identify key tr...

The ability of organisms to adapt their foraging behaviour to spatial variations in food availability and habitat quality is crucial to maximize energy intake and hence fitness. Under ideal conditions, habitat selection should result in a spatial distribution of individuals such that their fitness (energy reserves or condition) is roughly equal acr...

The ICES Working Group on comparative analyses between European Atlantic and Mediterra-nean Ecosystems to move towards an Ecosystem-based Approach to Fisheries (WGCOMEDA) recently completed its second three-year cycle. WGCOMEDA was established in 2014 and works in cooperation with other groups within the ICES Integrated Ecosystem Assessments Steeri...

Ecological communities are constantly being reshaped in the face of environmental change and anthropogenic pressures. Yet, how food webs change over time remains poorly understood. Food web science is characterized by a trade-off between complexity (in terms of the number of species and feeding links) and dynamics. Topological analysis can use comp...

linearity in stock-recruitment relationships of Atlantic cod: insights from a multi-model approach. The stock-recruitment relationship is the basis of any stock prediction and thus fundamental for fishery management. Traditional parametric stock-recruitment models often poorly fit empirical data, nevertheless they are still the rule in fish stock a...

The Eastern Mediterranean Sea (EMS) is an ultraoligotrophic semi-enclosed sea with low nutrient levels, low primary production rate, impoverished phytoplankton populations and low zooplankton standing stocks. The Cretan Passage in the western Levantine Sea is one of the least explored areas of the EMS. We measured the mesozooplankton biomass, abund...

Combined analyses of Fatty Acid Trophic Markers (FATM) and Stable Isotopes (SI) were used to characterize food preferences among copepod species/taxa and to trace their food sources in the ultra-oligotrophic Cretan Passage of the Eastern Mediterranean Sea (EMS). FATMs are based on the conservation and transfer of specific source Fatty Acids (FAs) t...

Marine social-ecological systems (SES) are vulnerable to regime shifts, triggered at multiple scales by a combination of anthropogenic stressors, e.g. climate change, overfishing. These abrupt and unexpected changes threaten living marine resource populations and their sustainable exploitation with adverse social and economic consequences for human...

Overfishing and global warming are threatening the sustainability of marine social-ecological systems (SES) worldwide. In this study, we focused on the particular case of Western Baltic cod fishery which has shown a decreasing trend over the last fifteen years inducing substantial social and economic consequences. To understand the conflicts arose...