Ken H Andersen

Technical University of Denmark | DTU

The FishErIes Size and functional TYpe model (FEISTY) is a mechanistic ecosystem model that fully integrates ecosystem structure across trophic levels through functional types. We present an R package that enables users to run simulations ranging from a 0D chemostat to full global scales. The library is written in Fortran90 with an R interface and...

Robust projections of future trends in global fish biomass, production and catches are needed for informed fisheries policy in a changing climate. Trust in future projections, however, relies on establishing that models can accurately simulate past relationships between exploitation rates and ecosystem states. In addition, historical simulations ar...

Trait-based models founded on biophysical principles are becoming popular in planktonic ecological modeling, and justifiably so. They allow for slim, efficient models with a significant reduction in parameters, well suited for modeling the past and future climate changes. In their simplest form, trait-based models describe the ecosystem in one set...

Fish and other metazoans play a major role in long-term sequestration of carbon in the oceans through the biological carbon pump. Recent studies estimate that fish can release about 1,200 to 1,500 million metric tons of carbon per year (MtC year-1) in the oceans through feces production, respiration, and deadfalls, with mesopelagic fish playing a m...

Robust projections of future trends in global fish biomass, production and catches under different fishing scenarios are needed to inform fisheries policy in a changing climate. Trust in future projections, however, relies on establishing that the models used can accurately simulate past relationships between exploitation rates, catches and ecosyst...

Aim We evaluate whether the biomass and trait biogeography of cephalopods follow the distribution expected by metabolic theory for ectotherms with rapid growth and high metabolic rate. Location Continental shelves of the North Atlantic and Northeast Pacific oceans; global marine ecoregions. Time Period 1968–2020. Major Taxa Studied Cephalopods a...

Motivated by the global rise in demand for marine products, there is increased interest in exploitation of the mesopelagic zone. However, the feasibility of this potential fishery remains uncertain, partly due to limited biological data and knowledge on sustainability of exploitation, and due to questions related to its economic viability. Conseque...

Modeling fish community responses to dam removal is an emerging field of study as dam removals become more common, but uncertainties concerning recovery time and community stability remain. In Europe, an EU-wide biodiversity strategy plans to restore around 25,000 km of rivers to free-flowing status, which emphasizes the importance of being able to...

Squid differ from fish by their high growth rate, short life span and feeding behaviour. Their fast life strategy is thought to impose a high predation pressure on zooplankton, fish and other squid preys, and a rapid transfer of energy to upper trophic-levels of marine food webs. However, there is a lack of understanding of how squid's fast life cy...

Aim Theory predicts fish community biomass to decline with increasing temperature due to higher metabolic losses resulting in less efficient energy transfer in warm‐water food webs. However, whether these metabolic predictions explain observed macroecological patterns in fish community biomass is virtually unknown. Here, we test these predictions b...

Phago-mixotrophy, the combination of photoautotrophy and phagotrophy in mixoplankton, organisms that can combine both trophic strategies, have gained increasing attention over the past decade. It is now recognized that a substantial number of protistan plankton species engage in phago-mixotrophy to obtain nutrients for growth and reproduction under...

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate near-global (global ocean minus coastal areas and high latitudes) carbo...

Fish community biomass is generally thought to decline with increasing temperature due to higher metabolic losses resulting in less efficient energy transfer in warm-water food webs. However, whether these metabolic predictions explain observed macroecological patterns in fish community biomass is virtually unknown. Here we test these predictions b...

While behavioral responses of individual organisms can be predicted with optimal foraging theory, the theory of how individual behavior feeds back to population and ecosystem dynamics has not been fully explored. Ecological models of trophic interactions incorporating behavior of entire populations commonly assume either that populations act as one...

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate global carbon fluxes and sequestration by fish and zooplankton due to r...

The microfossil record demonstrates the presence of eukaryotic organisms in the marine ecosystem by about 1,700 million years ago (Ma). Despite this, steranes, a biomarker indicator of eukaryotic organisms, do not appear in the rock record until about 780 Ma in what is known as the “rise of algae.” Before this, it is argued that eukaryotes were min...

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We connect cell-level processes to cell size and to first principles of diffusion, geometry, and fluid mechanics. We scale cell-level processes up to the plankton community size distribution to show that first principles determine ov...

The magnitude and efficiency of particulate carbon export from the ocean surface depends not only on net primary production (NPP) but also on how carbon is consumed, respired, and repackaged by organisms. We contend that several of these processes can be captured by the size spectrum of the plankton community. However, most global models have relat...

Individual metabolism generally scales with body mass with an exponent around 3/4. From dimensional arguments it follows that maximum population growth rate (rmax) scales with a −1/4 exponent. However, the dimensional argument implicitly assumes that offspring size is proportional to adult size. Here, we calculate rmax from metabolic scaling at the...

The biological carbon pump transports photosynthetically fixed carbon from surface waters to depths. It removes carbon from the atmosphere and sequesters it in the deep ocean, playing an important role in global climate regulation. As the biological carbon pump is directly related to biological processes, it is heavily influenced by the biomass and...

Marine predatory fish face unpredictable prey environments, ranging from abundance to scarcity of food. Dimensioning their assimilative system to accommodate gorging and fasting is therefore a central life history choice. Assimilative capacity experiments typically operate with sustained feeding to satiation, and therefore ignore the fluctuations i...

Nitrogen (N2) fixation by heterotrophic bacteria associated with sinking particles contributes to marine N cycling, but a mechanistic understanding of its regulation and significance are not available. Here we develop a mathematical model for unicellular heterotrophic bacteria growing on sinking marine particles. These bacteria can fix N2 under sui...

Aim Understanding how fish food webs emerge from planktonic and benthic energy pathways that sustain them is an important challenge for predicting fisheries production under climate change and quantifying the role of fish in carbon and nutrient cycling. We examine if a trait-based fish community model using the fish traits of maximum body weight an...

Individuals of different interacting populations often adjust to prevailing conditions by changing their behavior simultaneously, with consequences for trophic relationships throughout the system. While we now have a good theoretical understanding of how individuals adjust their behavior, the population dynamical consequences of co-adaptive behavio...

Diel vertical migration of fish and other metazoans actively transports organic carbon from the ocean surface to depth, contributing to the biological carbon pump. Here, we use a global vertical migration model to estimate global carbon fluxes and sequestration by fish and metazoans due to respiration, fecal pellets, and deadfalls. We estimate that...

The flux of detrital particles produced by plankton is an important component of the biological carbon pump. We investigate how food web structure and organisms’ size regulate particulate carbon export efficiency (the fraction of primary production that is exported via detrital particles at a given depth). We use the Nutrient-Unicellular-Multicellu...

Size-spectrum models are a recent class of models describing the dynamics of a whole community based on a description of individual organisms. The models are motivated by marine ecosystems where they cover the size range from multicellular plankton to the largest fish. We propose to extend the size-spectrum model with spatial components. The spatia...

Managing salmon louse Lepeophtheirus salmonis outbreaks is a crucial part of salmon aquaculture in sea cages. Treatment management strategies can be optimized with the aid of salmon-louse population dynamic models. These models, however, need to be calibrated and validated with biologically meaningful parameters. Here, based on a time-series of lic...

Multicellular zooplankton, such as copepods, are the main link between primary producers and fish. Most models of plankton communities, such as NPZ-type models, ignore the life-cycle (ontogeny) of multicellular zooplankton. Ontogeny has profound implications on population dynamics and community structure. Our aim is to provide a generic food-web fr...

Modified fish behaviors in response to anthropogenic stressors, such as chemicals, microplastics, acoustic emissions and fisheries, are a debated driver of change in freshwater ecosystems and oceans. Our ability to judge the severity of observed behavioral responses is hampered by limited knowledge regarding how subtle behavior modifications in pre...

Global climate change is expected to impact ocean ecosystems through increases in temperature, decreases in pH and oxygen, increased stratification, with subsequent declines in primary productivity. These impacts propagate through the food chain leading to amplified effects on secondary producers and higher trophic levels. Similarly, climate change...

Fisheries science and management is founded upon the Beverton–Holt theory of fish stock demography. The theory uses age as the structuring variable; however, there are several reasons to use body size as the structuring variable. Most of the processes that affect a fish are determined by its body size rather than its age: consumption, mortality, ma...

Nitrogen (N 2 ) fixation by heterotrophic bacteria associated with sinking particles contributes to marine N cycling, but a mechanistic understanding of its regulation and significance are not available. Here we develop a mathematical model for unicellular heterotrophic bacteria growing on sinking marine particles and can fix N 2 under suitable env...

Throughout much of the world's oceans, life is organized around seasonal cycles of feast and famine. Here we seek to understand the life-history strategies by which marine organisms contend with seasonal variations through a range of adaptations and traits, including overwintering stages, dormancy, investment in reserves, and migration. Our perspec...

Aim Higher temperatures increase the metabolic rate of ectothermic organisms up to a certain level and make them grow faster. This temperature‐sensitivity of growth is frequently used to predict the long‐term effects of climate warming on ectotherms. Yet, realized growth also depends on ecological factors and evolutionary adaptation. Here we study...

Planktonic ecosystems are usually modeled in terms of autotrophic and heterotrophic compartments. However, the trophic strategy of unicellular organisms can take a range of mixotrophic strategies with both autotrophic and heterotrophic contributions. The dominant emerging strategy found in nature depends on the environment (both biotic and abiotic...

Most animals face seasonal fluctuations in food availability and need to develop an annual routine that maximizes their lifetime reproductive success. Two particularly common strategies are reducing energy expenditure and building storage to sustain the animal in meager periods (winters). Here, we pose a simple and generic model for an animal that...

Trait-based ecology allows much of the complexity of ecosystems to be projected onto a low-dimensional trait space. We conjecture that three key traits capture the main aspects of diversity of unicellular planktonic organisms: cell size, trophic strategies (relative investment in photosynthesis and phagotrophy) and vacuolation. The three selected t...

The concept of biodiversity–ecosystem functioning (BEF) has been studied over the last three decades using experiments, theoretical models and more recently observational data. While theoretical models revealed that species richness is the best metric summarizing ecosystem functioning, it is clear that ecosystem function is explained by other varia...

Five decades of stomach content data allowed insight into the development of consumption, diet composition, and resulting somatic growth of Gadus morhua (Atlantic cod) in the eastern Baltic Sea. We show a recent reversal in feeding level over body length. Present feeding levels of small cod indicate severe growth limitation and increased starvation...

Five decades of stomach content data allowed insight into the development of consumption, diet composition, and resulting somatic growth of Gadus morhua (Atlantic cod) in the eastern Baltic Sea. We show a recent reversal in feeding level over body length. Present feeding levels of small cod indicate severe growth limitation and increased starvation...

The recruitment and biomass of a fish stock are influenced by their environmental conditions and anthropogenic pressures such as fishing. The variability in the environment often translates into fluctuations in recruitment, which then propagate throughout the stock biomass. In order to manage fish stocks sustainably, it is necessary to understand t...

Biomass distribution among size classes follows a power law where the Log-abundance of taxa scales to Log-size with a slope that responds to environmental abiotic and biotic conditions. The interactions between ecological mechanisms controlling the slope of locally realized size-abundance relationships (SAR) are however not well understood. Here we...

Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C∶N and C∶P ratios than photoautotrophic phytoplankton, whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, b...

Variability in community composition is often attributed to underlying differences in physical environments. However, predator–prey interactions can play an equally important role in structuring communities. Although environmental differences select for different species assemblages, less is known about their impacts on trait compositions. We devel...

This chapter looks into the differences and similarities between the two groups of fish: the teleosts and the elasmobranchs. In the data analyses done so far in this volume, the fish most considered were the teleosts ( Teleostei ), which represent by far the dominant group, in terms of both biomass and living number of species. Second in line comes...

This chapter develops descriptions of how individuals grow and reproduce. More specifically, the chapter seeks to determine the growth and reproduction rates from the consumption rate, by developing an energy budget of the individual as a function of size. To that end, the chapter addresses the question of how an individual makes use of the energy...

This chapter calculates the abundance (or biomass) of all species in a community as a function of their asymptotic size. It develops a purely analytical theory of the asymptotic size trait distribution in a fish community. The theory is based upon the Sheldon community spectrum developed in Chapter 2, and the new theory is used here to formulate an...

Fish are one of the most important global food sources, supplying a significant share of the world's protein consumption. From stocks of wild Alaskan salmon and North Sea cod to entire fish communities with myriad species, fisheries require careful management to ensure that stocks remain productive, and mathematical models are essential tools for d...

This chapter outlines four future research questions where the size- and trait-based theory can be applied: stochasticity, behavioral ecology, coupling to primary production, and thermal ecology and climate change. The chapter first argues that differences in growth can be modeled with the size-based framework by introducing stochasticity into the...

This chapter develops a basic evolutionary impact assessment of fishing. It does so by combining the size-based theory developed in chapters 3 and 4 with classic quantitative genetics. The impact assessment estimated the selection responses resulting from size-selective fishing on three main life-history traits: size at maturation, growth rate, and...

This chapter uses the community model to repeat many of the classic impact calculations of a single stock on the entire community. Here, a focus is the appearance of trophic cascades initiated by the removal of large predators. When a component of an ecosystem is perturbed, the effects are not isolated to the component itself but cascade through th...

This chapter follows the size-structure of the entire marine ecosystem. It shows how the Sheldon spectrum emerges from predator–prey interactions and the limitations that physics and physiology place on individual organisms. How predator–prey interactions and physiological limitations scale with body size are the central assumptions in size spectru...

This chapter proposes a shortlist of fish “master” traits and connects these traits to classic life-history strategy thinking. First, it sets the historical background for the current state-of-the-art thinking about fish life history strategies. From there, the chapter explains that the main axes of variation between fish species can be captured by...

This chapter focuses on a generalization of a classic consumer-resource model with a single population embedded in a community. It develops this physiologically structured consumer-resource model by extending the static model in Chapter 4. The chapter then studies how density dependence emerges in the model, and how it changes the population size s...

This chapter provides some context on the overall themes and theory of this volume. Throughout, the theory is applied to relevant problems in fisheries science: impact of fishing on demography, fisheries reference points, evolutionary impact assessments, stock recovery, ecosystem-based fisheries management, and so on, as well as to basic ecological...

This chapter considers population dynamics where the population changes over time, owing to environmental noise, fishing, or both. It first derives the population growth rate with various analytic and numeric approximations. Next, the chapter develops a full numerical solution to the McKendrick–von Foerster equations and uses it to create stylized...

This chapter exploits the previous chapter's demographic model to make impact assessment of fishing and calculate fisheries reference points for fish stocks with asymptotic sizes of 10 g, 333 g, and 10 kg. The three asymptotic sizes span the variation in fish life histories from small and short-lived forage fish species, such as sardine or sprat; t...

The ectoparasite Lepeophtheirus salmonis has for decades plagued salmon aquaculture by decreasing profits and impacting wild salmon stocks. To protect migrating wild salmon stocks and avoid excessive cross-farm infections, authorities require treatments when sea lice level reach a given threshold. The treatment threshold is set to protect wild salm...

Increasing temperatures under climate change are thought to affect individual physiology of fish and other ectotherms through increases in metabolic demands, leading to changes in species performance with concomitant effects on species ecology. Although intuitively appealing, the driving mechanism behind thermal performance is contested; thermal pe...

Ectotherms typically increase growth and reduce body size when temperature increases. This physiological response to temperature, termed the temperature-size rule (TSR), is often used to predict how rising temperatures with climate change will affect higher levels of organization, i.e. guilds, communities and ecosystems. Here we study whether faste...

Large-scale spatial heterogeneity in fisheries production is predominantly controlled by the availability of zooplankton and benthic organisms, which have a complex relationship with primary production. To investigate how cross-ecosystem differences in these drivers determine fish assemblages and productivity, we constructed a spatially explicit me...

A warmer ocean will change plankton physiological rates, alter plankton community composition, and in turn affect ecosystem functions, such as primary production, recycling, and carbon export. To predict how temperature changes affect plankton community dynamics and function, we developed a mechanistic trait‐based model of unicellular plankton (aut...

The competition–defense tradeoff is a significant source of functional diversity in ecological communities. Here, we present a theoretical framework to describe the competition–defense tradeoff and apply it to a size‐based model of a unicellular plankton community. Specifically, we investigate how the emergent community structure depends on the sha...

The two parameters of the Michaelis–Menten model, the maximum uptake rate and the half‐saturation constant, are not stochastically independent, and the half‐saturation constant is not a measure of nutrient affinity, as commonly assumed. Failure to realise their interdependence and mechanistic interpretation may lead to the emergence of false trade‐...

A recent meta-analysis by Barneche et al. (2018; Science, 360(6389): 642–645) shows that fish reproductive output scales hypergeometrically with female weight. This result challenges the common assumption that reproductive output is proportional to weight. The implication made is that current theory and practice severely underestimate the importanc...

The trait‐based approach is increasingly used in plankton ecology to understand diversity, community dynamics, and biogeography. While on the global scale phytoplankton traits are fairly well established, zooplankton traits are only beginning to be understood. One taxa‐transcending aspect of zooplankton diversity is the distinction between ambush a...