State-dependent Energy Allocation in Cod (Gadus Morhua)

Growth and maturation are processes that are tuned to the external environment an individual is likely to experience, where food availability, the mortality regime, and events necessary to complete the life cycle are of special importance. Understanding what influences life history strategies and how changes in life history in turn influence population dynamics and ecological interactions are crucial to our understanding of marine ecology and contemporary anthropogenic induced change. We present a state-dependent model that optimises life-long energy allocation in iteroparous fish. Energy can be allocated to growth or reproduction, and depends in the individuals age, body length, stored energy, and the state of the environment. Allocation and the physiological processes of growth, storing energy, and reproduction are modelled mechanistically. The model is parameterised for Atlantic cod (Gadus morhua); more specifically for the Northeast Arctic cod stock. Growth and maturation predicted by the model fit well with field observations, and based on a further investigation of cod reproduction in the model we conclude that the model has the ability to recapture complex life history phenomena, e.g. indeterminate growth and skipped spawning, and therefore provides an important tool that can improve our understanding of life history strategies in fish

Atlantic bluefin tuna spawn early to avoid metabolic meltdown in larvae

To predict shifts in phenology and distribution of organisms we need to understand how survival through early life stages depends on environmental conditions. Here, we present a mechanistic model of development, feeding and bioenergetics of early life stages in bluefin tuna and predict the optimal time of the year for them to be born. We find that the availability of prey, particularly nauplii, is sufficient for fast growth in tuna larvae while temperature is moderate during midsummer, but not when temperatures increase later in summer. High temperatures benefit egg and yolk-sac stages, but the metabolic needs of feeding larvae are hard to sustain during the warmest periods. Heatwaves, such as the one in 2003, increase larval survival potential, but shorten the viable part of the season for the larvae. Atlantic bluefin tuna is a large, highly migratory marine top predator that spawns while temperatures are rising, but before the heat leads to a metabolic meltdown in larvae. This means that food resources modulate how temperature change shifts optimal phenology.En prensa

Foraging success in planktivorous fish increases with topographic blockage of prey distributions

Banks and shelves are productive zones of the ocean, and often home to large fish stocks. Can shallower bottom topographies improve foraging opportunities for pelagic fish by blocking zooplankton from hiding in deep, darker water? We use mechanistic principles of visual prey search and an extensive dataset of zooplankton depth distributions to model foraging success in planktivorous fish across a large marine ecosystem. Our results show that zooplankton distribute deeper with deeper bathymetry, and that fish find exponentially less food with increasing bottom depths. Over shallow banks, zooplankton are forced into higher light exposure, providing higher prey encounter rates for fish despite lower abundance of prey. Stomach data analyses from a key planktivore support these predictions and suggest that fish foraging on copepods are more successful over shallower grounds. Our study demonstrates that prey availability for planktivorous fish is not proportional to zooplankton abundance, while the bottom depth is an important factor in fish foraging success and zooplankton mortality rates.publishedVersio

Survey report from the CODFUN 2016114 Barents Sea survey 5.-11. oktober 2016

The survey is part of the project “The cod – capelin interaction in the Barents Sea: spatial dynamics in predator prey overlap and functional response (CODFUN)” funded by the Norwegian Research Council (NRC project number 243676/E40). The CODFUN project was motivated by the need to update and improve the current formulation of the functional response of cod used in the capelin assessment model. The functional response is the relationship between the feeding rate of individual predators and density of prey. Functional response, together with density of predators and prey is used to calculate natural mortality. In all natural environments, there is spatial heterogeneity in densities of prey and predators and their overlap. This heterogeneity becomes a very important determinant of natural mortality due to predation. The aim of the CODFUN project is to quantify and explain spatial heterogeneity in the capelin-cod interaction at different spatial scales in the Barents Sea. The aim of the survey was to study spatial variation at the scale of individual cod and capelin schools. We aimed at quantifying attack rates of cod on capelin using acoustics, and to estimate proportion of successful attacks using stomach samples from trawl catches and handline. Furthermore, we aimed to relate variation in attack rates to variation in capelin densities and light conditionspublishedVersio

Body size adaptions under climate change: Zooplankton community more important than temperature or food abundance in model of a zooplanktivorous fish

One of the most well-studied biogeographic patterns is increasing body size with latitude, and recent body size declines in marine and terrestrial organisms have received growing attention. Spatial and temporal variation in temperature is the generally invoked driver but food abundance and quality are also emphasized. However, the underlying mechanisms are not clear and the actual cause is likely to differ both within and among species. Here, we focused our attention on drivers of body size in planktivorous fish that forage through vision. This group of organisms plays a central role in marine ecosystems by linking the energy flow from lower to higher trophic levels. Using a model that incorporates explicit mechanisms for vision-based feeding and physiology, we investigated the influence on optimal body size of several biotic (prey size, prey energy content, and prey biomass concentration) and abiotic (temperature, latitude, and water clarity) factors known to affect foraging rates and bioenergetics. We found prey accessibility to be the most influential factor for body size, determined primarily by prey size but also by water clarity, imposing visual constraints on prey encounters and thereby limiting feeding rates. Hence, for planktivores that forage through vision, an altered composition of the prey field could have important implications for body size and for the energy available for reproduction and other fitness-related tasks. Understanding the complicated effects of climate change on zooplankton communities is thus crucial for predicting impacts on planktivorous fish, as well as consequences for energy flows and body sizes in marine systems.publishedVersio

Optimal reproductive phenology under size‐dependent cannibalism

Intra-cohort cannibalism is an example of a size-mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade-off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game-theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age-structured game-theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size-dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter-cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions

¿Qué pueden decirnos las distribuciones de talla dentro de cohortes sobre los procesos ecológicos en larvas de peces?

Marine fish larvae are subject to variable environments, which is probably reflected in their growth and survival rates. Mortality rates are generally high and size-dependent. At the species level, these mortality rates are usually accompanied by correspondingly high growth rates. Here we provide examples from experimental studies with Atlantic cod (Gadus morhua) and Atlantic herring (Clupea harengus) larvae, in which multiple cohorts were followed over time. Body size, prey concentrations, and temperature are shown to influence growth rates. We present a method based on cumulative size distributions (CSDs) for visualizing variability of sizes within cohorts over time. Analysis of CSDs revealed size-selective mortality and variations among populations in size- and temperature-dependent growth throughout ontogeny. We found that cod larvae consistently exhibit higher growth rates than herring larvae. While cod larvae may have an advantage over herring larvae when food availability is high, herring were more able to survive at low food concentrations than cod. Cod and herring seem to represent two growth strategies: cod larvae are relatively small at hatching and a high growth rate appears to be a prerequisite for success, whereas herring larvae are initially large, but grow more slowly.Las larvas de peces marinos están sujetas a ambientes variables que probablemente se reflejan en sus tasas de crecimiento y supervivencia. las tasas de mortalidad son generalmente altas y dependientes de la talla. A nivel de especies, estas tasas de mortalidad están usualmente acompañadas de tasas de crecimiento altas. en este trabajo mostramos ejemplos a partir de estudios experimentales con larvas de bacalao atlántico (Gadus morhua) y arenque atlántico (Clupea harengus), en los que se siguieron cohortes múltiples a lo largo del tiempo. Se muestra como la talla del cuerpo, la concentración de presas y la temperatura influyen en la tasa de crecimiento. Presentamos un método basado en distribuciones de frecuencias de talla acumuladas (DTAs) para visualizar la variabilidad en tallas dentro de las cohortes a lo largo del tiempo. el análisis de DTAs reveló mortalidad selectiva por talla, y variaciones entre poblaciones en el crecimiento dependiente de la talla y la temperatura a través de la ontogenia. encontramos que las larvas de bacalao mostraron consistentemente mayores tasas de crecimiento que las de arenque. Mientras las larvas de bacalao pueden tener una ventaja sobre las de arenque cuando la disponibilidad de presas es alta, las de arenque son más capaces de sobrevivir a bajas concentraciones de comida. Bacalao y arenque parecen representar dos estrategias de crecimiento; las larvas de bacalao son relativamente pequeñas a la eclosión y una alta tasa de crecimiento parece un prerrequisito para el éxito, mientras que las de arenque son inicialmente más largas, pero crecen más lentamente

Predator–prey overlap in three dimensions: cod benefit from capelin coming near the seafloor

Spatial overlap between predator and prey is a prerequisite for predation, but the degree of overlap is not necessarily proportional to prey consumption. This is because many of the behavioural processes that precede ingestion are non-linear and depend on local prey densities. In aquatic environments, predators and prey distribute not only across a surface, but also vertically in the water column, adding another dimension to the interaction. Integrating and simplifying behavioural processes across space and time can lead to systematic biases in our inference about interaction strength. To recognise situations when this may occur, we must first understand processes underlying variation in prey consumption by individuals. Here we analysed the diet of a major predator in the Barents Sea, the Atlantic cod Gadus morhua, aiming to understand drivers of variation in cod's feeding on its main prey capelin Mallotus villosus. Cod and capelin only partly share habitats, as cod mainly reside near the seafloor and capelin inhabit the free water masses. We used data on stomach contents from ~2000 cod individuals and their surrounding environment collected over 12 years, testing hypotheses on biological and physical drivers of variation in cod's consumption of capelin, using generalized additive models. Specifically, effects of capelin abundance, capelin depth distribution, bottom depth and cod abundance on capelin consumption were evaluated at a resolution scale of 2 km. We found no indication of food competition as cod abundance had no effect on capelin consumption. Capelin abundance had small effects on consumption, while capelin depth distribution was important. Cod fed more intensively on capelin when capelin came close to the seafloor, especially at shallow banks and bank edges. Spatial overlap as an indicator for interaction strength needs to be evaluated in three dimensions instead of the conventional two when species are partly separated in the water column.publishedVersio