Øyvind Fiksen

THE EVOLUTION OF SPAWNING MIGRATIONS: STATE DEPENDENCE AND FISHING‐INDUCED CHANGES

Individuals migrate to exploit heterogeneities between spatially separated environments to modulate growth, survival, or reproduction. We devised a bioenergetics model to investigate the evolution of migration distance and its dependence on individual states. Atlantic cod Gadus morhua ranges from sedentary populations to stocks that migrate several thousand kilometers annually. We focused on the Northeast Arctic cod stock, which migrates south to spawn. A linear relationship between migration distance and the expected survival of offspring was assumed, here understood as the prospects for future survival and development that a fertilized egg faces at a particular spawning location. Reasons for why it may increase southward include warmer water that increases development rates, and thereby survival, along the pelagic drift trajectory. In the model, ingested energy can either be allocated to growth or stored for migration and reproduction. When migrating, individuals forgo foraging opportunities and expend energy. Optimal energy allocation and migration strategies were found using state-dependent optimization, with body length, age, condition, and current food availability as individual states. For both a historical and contemporary fishing regime we modeled two behaviors: (1) homing cod returning to the same spawning site each year and (2) roaming cod with no such constraints. The model predicted distinct regions of locally high spawning stock biomass. Large individuals in good condition migrated farthest, and these also tended to mature later in life. The roaming cod spread farther south as they grew larger and older. Homing cod did not have this freedom, and spawning was generally concentrated along a narrower stretch of the coastline. Under contemporary fishing, individuals matured earlier at a smaller size, had shorter migrations, spawned over a contracted geographical range, and tended to be in poorer condition. The effects were most pronounced for the homing behavior.

A MODEL OF OPTIMAL LIFE HISTORY AND DIEL VERTICAL MIGRATION IN CALANUS FINMARCHICUS

Abstract A copepod in a seasonal environment continuously faces the trade-offs among allocating surplus growth to a storage compartment (lipids), to somatic growth or to reproduction. By building up lipids, it can survive periods with low food, and gain reproductive success the following season by transforming fat to eggs. Also, copepods face a trade-off between survival and growth, as surface waters generally are more risky and productive than the dark refuges at greater depths, both in the diel and annual temporal scale. Implicit in these trade-offs are the numbers of generations, population dynamics and productive potential of the copepod. The authors develop a model which is used to investigate (1) how optimal diel arid ontogenetic vertical migration might vary with season, individual state (size and lipid reserves) and growth conditions; (2) whether fat is mainly used for overwintering or for fueling reproduction in early spring; and (3) the adaptive value of behavioural flexibility in migration patt...

Size-selective fishing gear and life history evolution in the Northeast Arctic cod.

Industrial fishing has been identified as a cause for life history changes in many harvested stocks, mainly because of the intense fishing mortality and its size‐selectivity. Because these changes are potentially evolutionary, we investigate evolutionarily stable life‐histories and yield in an energy‐allocation state‐dependent model for Northeast Arctic cod Gadus morhua. We focus on the evolutionary effects of size‐selective fishing because regulation of gear selectivity may be an efficient management tool. Trawling, which harvests fish above a certain size, leads to early maturation except when fishing is low and confined to mature fish. Gillnets, where small and large fish escape, lead to late maturation for low to moderate harvest rates, but when harvest rates increase maturation age suddenly drops. This is because bell‐shaped selectivity has two size‐refuges, for fish that are below and above the harvestable size‐classes. Depending on the harvest rate it either pays to grow through the harvestable slot and mature above it, or mature small below it. Sustainable yield on the evolutionary time‐scale is highest when fishing is done by trawling, but only for a small parameter region. Fishing with gillnets is better able to withstand life‐history evolution, and maintains yield over a wider range of fishing intensities.

Vertical distribution and population dynamics of copepods by dynamic optimization

Vertical distribution and population dynamics of copepods are viewed as a consequence of individual maximization of reproductive value (RV). RV for all individuals of all size classes (stages) and conditions (physiological states) is found by dynamic programming, and specifies a trajectory of optimal habitats in time and space. The optimal habitat is found by balancing the risk of predation and growth. Predation risk from visually searching planktivores is included as a mechanistic submodel, and growth is a function of individual size, food concentration, temperature, and energetic costs of migratory behaviour. The optimal policy followed by single individuals eventually gives rise to the population dynamics, based on individual mortality and reproduction rates. The model focuses on the role of temperature, predators, and food resources on dynamics and distribution, and shows that food can affect predation risk through both physiological and physical mechanisms. In fact, increased food concentration may influence dynamics more through reduced predation than through increased growth, because the planktivores’ searching efficiency is very sensitive to increased turbidity. This effect is suggested as a potentially important factor in the survival of planktonic organisms susceptible to visually searching predators, and may be most beneficial to macrozooplankton and fish larvae. The optimal copepod response (vertical migration) to increased density of planktivores is to seek less risky habitats, and therefore the predation risk of copepods is a non-linear function of planktivore density. The model suggests that optimal die1 migration intensity is changed with food density from no migration at low food levels, reaches a maximum at intermediate levels, but is reduced again at high algal concentrations.

Using an individual-based model for assessment of sea turtle population viability

Marine turtle species have a complex life history characterized by interannual variability in reproductive performance and a long life span. These ecological features in combination with the animals’ highly migratory nature create numerous difficulties when trying to assess population dynamics. This study attempts to couple existing information on species demographics and behavioral strategies with simple energetic rules in a theoretical framework. We study sea turtle population dynamics using an individual-based model that incorporates known behavioral-ecological characteristics of the species. Methodology used to design the model was based on the superindividual approach (Scheffer et al. Ecol Model 80:161–170, 1995). We constructed our simulation experiment on a virtual sea turtle population, which was parameterized by using recent literature reviews with emphasis on reproductive parameters of the Mediterranean loggerhead sea turtle population. Switching rules describing critical processes of reproductive performances were established as theoretical functions of efficiency of energy transfer. In order to explore the significance of variable reproductive patterns upon population dynamics and persistence, a series of simulations was performed. The model was also run under fluctuated demographic variables to perform a sensitivity analysis of critical parameters and life-history stages. Based on the specific model parameterization, simulation results show that population persistence was most sensitive to fecundity and to survival at the pelagic juvenile stage. Additionally a surprising finding is the relatively high importance of egg survival in terms of both hatching and hatchling success. We conclude that enhancing the population with new individuals by increasing survival in the early life stages could compensate for additional losses in other age classes. The need for further research regarding biological and behavioral features as well as basic demographic insights into the endangered loggerhead sea turtle is also highlighted.

ALLOCATION PATTERNS AND DIEL VERTICAL MIGRATION: MODELING THE OPTIMAL DAPHNIA

I present a model which at the same time evaluates optimal diel vertical migration (DVM) and optimal allocation .patterns to growth or reproduction in the cladoc- eran Daphnia magna. The combined policies that maximize the intrinsic rate of increase r are found using the Perron Frobenius Theorem and dynamic programming. The model attempts to incorporate two traditions in aquatic behavioral ecology within the same model framework: variations in vertical migrations (short time behavior) and age or size at maturity (life history models). The predictions from the model are compared with observations from an experiment in thermally stratified flow-through tubes containing various concentrations of fish exudates. The model is able to reproduce many of the observations of both DVM and allocation, though there are some interesting deviations. I look at theoretical behavior and life history consequences of various size dependencies in mortality. Finally, I do a comparative analysis of two fitness measures, the intrinsic rate of increase r, and the net lifetime reproduction, R0, where it turns out that the use of r makes predictions in better agreement with the observations.

Seasonal plankton–fish interactions: light regime, prey phenology, and herring foraging

When prey and predator are seasonal migrants, encounters depend on migration phenologies and environmental constraints on predation. Here we investigate the relative contribution of seasonality in irradiance and prey abundance in shaping the rapid seasonal body condition increase of a migrating predator searching visually for its prey: the Norwegian spring-spawning herring, Clupea harengus, feeding on the copepod Calanus finmarchicus. Two main seasonal pulses of prey are available to herring: (1) the parent generation of C. finmarchicus, with peak abundance in March-April, which appear too early to cause the main increase in herring condition; and (2) the abundant offspring generation of C. finmarchicus, with peak abundance in June-July, too late to explain the main increase in body condition. However, a mechanistic model of ingestion rate, including both solar irradiance and prey abundance, predicted seasonal food intake in good accordance with observed herring body condition. This suggests that the seasonality in herring foraging and energy storage is closely linked to the return of longer days in spring, and less dependent on a match or mismatch with seasonal peaks in abundance of their zooplankton prey. Consequently, light related constraints on foraging may make visually searching predators at high latitudes resilient to changes and fluctuations in prey phenology and abundance, but vulnerable to changes in the light regime, such as water clarity.

Ideal free distribution of copepods under predation risk

Abstract Optimal vertical distribution of a copepod population of equal competitors under predation hazard is modelled by ideal free distribution (IFD). The foragers may be limited by both depletable (food) and non-depletable (temperature) resources. Individuals are assumed to maximize growth rate per mortality risk (g/M). Mortality risk is assumed density-dependent whenever the copepod concentration is high enough to satiate predators. The growth rate depends upon temperature or food concentration in absence of competition, and is density-dependent under competition. These relationships may yield peaked habitat profitability curves. For L depths with peaked profitability curves, the computational complexity scales to 3 L . Simplifying restrictions to allow numerical solutions when a large number of depths are available are presented and discussed. At moderate and high copepod stock size, the restrictions find the optimal distribution much faster, but at low stock sizes they may predict suboptimal distributions. The model predicts that individuals shall be more sensitive to predation risk at low and moderate competitor abundance and more sensitive to resource input rate at higher competitor abundances. Deviations from a food-based IFD are therefore most pronounced at low copepod population size. The IFDs are compared with predictions from a dynamic programming model with state- and time-resolved motivation of the copepods.

Effects of the Emotion System on Adaptive Behavior

A central simplifying assumption in evolutionary behavioral ecology has been that optimal behavior is unaffected by genetic or proximate constraints. Observations and experiments show otherwise, so that attention to decision architecture and mechanisms is needed. In psychology, the proximate constraints on decision making and the processes from perception to behavior are collectively described as the emotion system. We specify a model of the emotion system in fish that includes sensory input, neuronal computation, developmental modulation, and a global organismic state and restricts attention during decision making for behavioral outcomes. The model further includes food competition, safety in numbers, and a fluctuating environment. We find that emergent strategies in evolved populations include common emotional appraisal of sensory input related to fear and hunger and also include frequency-dependent rules for behavioral responses. Focused attention is at times more important than spatial behavior for growth and survival. Spatial segregation of the population is driven by personality differences. By coupling proximate and immediate influences on behavior with ultimate fitness consequences through the emotion system, this approach contributes to a unified perspective on the phenotype, by integrating effects of the environment, genetics, development, physiology, behavior, life history, and evolution.

The emotion system promotes diversity and evolvability

Studies on the relationship between the optimal phenotype and its environment have had limited focus on genotype-to-phenotype pathways and their evolutionary consequences. Here, we study how multi-layered trait architecture and its associated constraints prescribe diversity. Using an idealized model of the emotion system in fish, we find that trait architecture yields genetic and phenotypic diversity even in absence of frequency-dependent selection or environmental variation. That is, for a given environment, phenotype frequency distributions are predictable while gene pools are not. The conservation of phenotypic traits among these genetically different populations is due to the multi-layered trait architecture, in which one adaptation at a higher architectural level can be achieved by several different adaptations at a lower level. Our results emphasize the role of convergent evolution and the organismal level of selection. While trait architecture makes individuals more constrained than what has been assumed in optimization theory, the resulting populations are genetically more diverse and adaptable. The emotion system in animals may thus have evolved by natural selection because it simultaneously enhances three important functions, the behavioural robustness of individuals, the evolvability of gene pools and the rate of evolutionary innovation at several architectural levels.