Dag L. Aksnes

A theoretical model of aquatic visual feeding

Abstract A model for visual feeding by aquatic predators is derived. The predators visual range, which depends on its visual capability, surface light, water clarity, and size and contrast of the prey, is emphasised. Central to the model is the assumption that a prey may be recognized only if the difference in retinal flux, with and without the prey image, exceeds a threshold. This assumption is equivalent to requiring that the product of apparent contrast at retina, retinal background irradiance and area of prey image must exceed a threshold. Visual range ( r ) is found from the equation r 2 exp ( cr + Kz ) = ϱE 0 | C 0 | πs 2 ΔS e −1 , where c is beam attenuation coefficient, z is depth, K is diffuse attenuation coefficient, ϱ is light loss through the surface, E 0 is surface light intensity, C 0 is inherent contrast of prey, β is prey radius and ΔS e is sensitivity threshold of the eye for detection of changes in irradiance. The model predicts that visual range increases non-linearly with increasing predator size and ambient light. Visual range also increases almost linearly with increasing prey size and decreases non-linearly with increasing turbidity. These predictions are compared with experimental data. It is shown that characteristic fluctuations in light regime may be more important to feeding than characteristic variations in prey abundance in aquatic environments. Due to the direct impact of light on the feeding process of several predators (and thereby on the mortality process of prey), we conclude that light should be considered an important top-down control in aquatic ecosystems in addition to the bottom-up control exerted through primary production. Finally, the model is testable, and should stimulate a stronger interaction between theory and experiments in aquatic feeding ecology of visual predators.

Large mesopelagic fishes biomass and trophic efficiency in the open ocean

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

A revised model of visual range in fish

Abstract Models of visual range and location distances are crucial for quantification of vision based feeding opportunities and predation risk in the pelagic habitat. We compare an earlier published model with measurements of the reactive distance of Gobiusculus flavescens relative to two species of copepods. Although this model gave reasonable predictions at low light intensities, the measurements of reactive distance at higher intensities were much lower than those predicted by the model. We modified the model to account for saturation at high light intensities. With this additional feature, the correspondence with the G.flavescens observations was significantly improved. Furthermore, the revised model is consistent with earlier published data on fish contrast thresholds obtained over a wide range of target sizes and irradiance levels. Given the values of only two parameters, one sensitivity threshold and one saturation parameter, the model is capable of predicting visual ranges for relatively large int...

Modelling the primary production in the North Sea using a coupled three-dimensional physical-chemical-biological ocean model

A coupled three-dimensional physical-chemical-biological model system has been implemented, and applied to study mass and volume transports and primary production throughout the North Sea. The model was run twice for the year 1985 with specified (for the North Sea Task Force) time series of riverine and atmospheric inputs of nutrients, and also with these nutrient inputs reduced by 40 and 50%, respectively. In particular, the evolution of the chemical and biological variables in the two situations was studied. The model output agreed quite well with the general quantitative and qualitative knowledge of the total yearly production. The intercomparison with some salinity profiles also indicated that the model handles the large-scale circulation and vertical mixing fairly well. Estimates for the transport of excess nutrients to Skagerrak and Kattegat in the highly pulsating Jutland coastal current are given. The estimates demonstrate the need for such models for calculating transport of matter from one area to another. Significant reductions in both primary production and transport of matter were seen from comparisons between the two runs.

Vertical distribution and trophic interactions of zooplankton and fish in Masfjorden, Norway

Abstract The distribution, biomass, and predator-prey relationships of the pelagic assemblage in Masfjorden, western Norway, was studied in January 1989. The pelagic biomass was dominated by particulate organic matter. Biomasses of copepods, macroplankton, and mesopelagic fishes were of the same order of magnitude, while the biomass of larger pelagic fishes were one order less. Predator-prey relationships seemed most important at intermediate and higher trophic levels. Two sound-scattering layers, consisting of adult Maurolicus muelleri (lower layer) and juvenile M. muelleri (upper layer) performed instantaneous lightdependent vertical migration. Vertical distributions are explained in terms of balancing food demands against predation risk.

Circulation patterns in the North Atlantic and possible impact on population dynamics of Calanus finmarchicus

Abstract Due to advective impact few consistent time-series describing the population development of the copepod C.finmarchicus exist. Quantitatively, the spring generation seems to be the most important, and especially in the northern areas one generation per year seems to prevail. A fecundity of 102-103 eggs female-1 and a sex ratio 1:1 then give a maximal reproductive rate of 3.9-6.2 year-1. This low rate seems to be compensated by low mortality due to effective predator avoidance in terms of diurnal and seasonal vertical migrations. We have calculated that the advective renewal of the habitats of C.finmarchicus in the subpolar gyre and in the Nordic Seas are 0.13 and 0.29 year-1 respectively, while the birth and death rates of C.finmarchicus typically are above 3.9 year-1.This means that the biological rates are likely to dominate over the advective rates in the two ocean areas, and that the C.finmarchicus development is facilitated by local production in both areas. Rough calculations for the Nordic ...

Ecological modelling in coastal waters: Towards predictive physical-chemical-biological simulation models

Abstract A simple, but general, simulation model is specified according to the state-of-the-art within phytoplankton modelling: Process representations are based upon prevailing theoretical and empirical representations given in the literature, and a set of earlier published values of model coefficients that have demonstrated good fit to reliable observations was selected. The emerging phytoplankton model was then validated against data obtained from enclosure experiments with light-, N-, P- and Si-limitations. We applied no tuning of the coefficients as the purpose of this test was to estimate the predictive power of the proposed model. The general standard deviations between model predictions and observations were on the range 0.04–0.36 and 0.13–0.42 for the nutrient and phytoplankton state variables respectively. Not surprisingly, these values are higher than those obtained in tuned simulations. Nevertheless, several characteristics, such as the balance between diatoms and flagellates, were predicted b...

Estimation techniques used in studies of copepod population dynamics — A review of underlying assumptions

Abstract The literature on zooplankton population dynamics provides more estimation techniques than reliable estimates of population parameters. In this review we show how different techniques relate to each other in terms of underlying models and assumptions. There are two main routes to parameter estimates. The vertical approaches utilize the stage-structure of samples taken at the same point in time. They require assumptions about the constancy in the parameters, but relax assumptions concerning advective influence. The horizontal approaches utilize information provided by the stage structure within samples, as well as information on temporal changes in abundance. They relax assumptions about the constancy of parameters, but advective influences may be introduced. Of the horizontal methods, variants of what are commonly termed cohort methods have been widely used. These provide mathematical simplicity, but are based on more restrictive assumptions than methods fitting predescribed models (delay differe...

Ontogeny, season and trade-offs: Vertical distribution of the mesopelagic fish Maurolicus muelleri

Abstract To investigate the validity of static optimization models, the vertical distributions of two age groups of Maurolicus muelleri is compared to the optimal annual ontogenetic growth rate: mortality risk trade-offs as determined from generation-time based life-history equations. Calculations indicate that juvenile feeding rate was near the maximum for efficient conversion to growth, and at times constrained by digestion rate. Feeding rate of adults seems not to have been high enough to sustain body mass. Juveniles seem to follow the static ontogenetic trade-off between growth and survival, while adults in winter emphasize feeding far less than predicted from static optimization. Static trade-offs are thus inadequate in predicting their distributions, and models accounting for time- and state-dependencies are required. The difference between the two age groups with respect to following the annual trade-off, is explained by their different feeding-to-fitness functions: the relation between adult feedi...

A coupled physical-biological pelagic model of a shallow sill fjord

Abstract A vertically resolved model for the land-locked fjord Lindaspollene, western Norway is presented. Salinity, temperature, oxygen, nitrogen-nutrients, silicate, and two groups of phytoplankton and herbivores are represented as dynamic variables. From ‘below’ the model is driven by solar radiation, precipitation, wind and tidal exchange and from ‘above’ by herbivore mortality. Simulation results are presented and discussed together with actual observations from Lindaspollene. The main seasonal and vertical characteristics of the phytoplankton and herbivore dynamics seem to be well reflected by the model, and realistic seasonal patterns may be produced for several successive years. The most characteristic vertical features are the formation of a summer surface production maximum and a deep chlorophyll maximum. Furthermore, a herbivore biomass which develops in the surface layer divides into a shallow and a deep component during summer and becomes concentrated in the surface layer again in the autumn. The nutricline and the pycnocline develop independently of one another, with consequences for the supply of nutrients to the upper euphotic zone. The bottom-up control exerted by the meteorological forcing, especially the freshwater runoff, seems to be of paramount significance for the observed vertical structure and seasonality of the present fjord system.