Niels Gerner Andersen

Life-history constraints on the success of the many small eggs reproductive strategy

The reproductive strategy of most fishes is to produce a large number of tiny eggs, leading to a huge difference between egg size and asymptotic body size. The viability of this strategy is examined by calculating the life-time reproductive success R(0) as a function of the asymptotic body size. A simple criterion for the optimality of producing small eggs is found, depending on the rate of predation relative to the specific rate of consumption. Secondly it is shown that the success of the reproductive strategy is increasing with asymptotic body size. Finally the existence of both upper and lower limits on the allowed asymptotic sizes is demonstrated. A metabolic upper limit to asymptotic body size for all higher animals is derived.

The effects of swimming pattern on the energy use of gilthead seabream (Sparus aurata L.)

Oxygen consumption ( ) was measured for gilthead seabream (Sparus aurata) during spontaneous and forced activities. During spontaneous activity, the swimming pattern was analysed for the effect on on the average speed (U), turning rate (θ) and change in speed (ΔU). All swimming characteristics contributed significantly to the source of spontaneous swimming costs, and the models explained up to 58% of the variation in Prediction of of fish in field studies can thereby be improved if changes in speed and direction are determined in addition to swimming speed. A relationship between swimming speed and during forced activity was also established. During spontaneous activity, 2.5 times more energy was used than in forced swimming at a speed of 0.5 BL s−1. This indicates that spontaneous swimming costs may be considerably higher compared with those of a fixed swimming speed. However, comparing at the respective optimum swimming speeds with the lowest cost of transport (U opt) resulted in similar values independent of swimming mode. This could be an important observation in estimating energetic costs of free-ranging fishes.

Influences of potential predictor variables on gastric evacuation in Atlantic cod Gadus morhua feeding on fish prey: parameterization of a generic model.

The parameter values of a generic model of gastric evacuation were estimated from evacuation data on Atlantic cod Gadus morhua fed meals of four fish prey: herring Clupea harengus, sprat Sprattus sprattus, lesser sandeel Ammodytes tobianus and dab Limanda limanda. The effects on evacuation of photoperiod and pre-experimental treatment of prey were also tested. Freshly killed A. tobianus were evacuated from the stomach of G. morhua at a rate similar to the value estimated from conspecifics kept deep-frozen and subsequently thawed prior to the evacuation experiment. The evacuation rate in G. morhua exposed to continuous light did not differ from the rate obtained from fish maintained under a 12L:12D photoperiod. The evacuation rates estimated from the latter fish in the dark and light periods, respectively, were likewise similar. These results indicate that the resistance of prey to the digestive processes is not altered significantly by the pre-experimental treatment of prey and that there is no diurnal variation per se in the rate of evacuation for G. morhua. Therefore, it is believed that the present parameterization of the evacuation model should prove especially useful for studying the role of G. morhua as a top predator in natural systems.

Does the cylinder model of gastric evacuation predict observed evacuation of mixed meals of prey of contrasting geometries in a piscivorous fish

The simple surface abstraction of the cylinder model (each prey as well as the total stomach contents is considered a cylinder that is gradually reduced by successive peeling off its curved side) was challenged by data on evacuation of a meal composed of three sandeels Ammodytes tobianus and a dab Limanda limanda fed to Atlantic cod Gadus morhua. While the body shape of A. tobianus comes close to that of a cylinder, the flatfish L. limanda takes a discoid form. As opposed to a modified form of the cylinder model, where the contrasting geometries of the fish prey were implemented, the simple, original cylinder model held the potential to predict evacuation of the individual prey types as well as the total stomach contents. Thus, the present study adds significantly to the increasing evidence that points to the generic nature of the model and its implicit square root function. Also, the present study corroborated a basic assumption that the variability of evacuation data not accounted for by the cylinder model primarily can be ascribed to the intraspecific variation in gastric performance of the predator.

Prey exoskeletons influence the course of gastric evacuation in Atlantic cod Gadus morhua

This study examined the effects of prey exoskeleton characteristics on gastric evacuation patterns in Atlantic cod Gadus morhua. Three distinct stages were highlighted in the gastric evacuation of crustacean prey characterized by a robust exoskeleton. The experiments confirmed that the three shrimp species, Pandalus borealis, Pandalus montagui and Eualus macilentus, and the crab Chionoecetes opilio, were evacuated from the stomach at different rates. The duration of all stages increased with increasing ash (and carbonate) content of the fresh prey. Thickness, chemical composition and morphology of the prey exoskeleton all affected gastric evacuation: duration of initial delay, overall evacuation rate and a decreased evacuation rate at the end of the process. The power exponential function (PEF), with its shape parameter, described the course of evacuation for these prey types well, especially the initial delay. The PEF does not, however, allow describing evacuation by the current stomach content mass independent of meal size, which limits its usefulness in estimating consumption rates of wild G. morhua. To predict and describe gastric evacuation of prey with a robust exoskeleton, it is therefore suggested that the square-root function be expanded with an initial lag phase, coupled to the mechanistically based cylinder model of gastric evacuation.

Modelling gastric evacuation in gadoids feeding on crustaceans.

A mechanistic, prey surface-dependent model was expanded to describe the course and rate of gastric evacuation in predatory fishes feeding on crustacean prey with robust exoskeletons. This was accomplished by adding a layer of higher resistance to the digestive processes outside the inner softer parts of a prey cylinder abstraction and splitting up the prey evacuation into two stages: an initial stage where the exoskeleton is cracked and a second where the prey remains are digested and evacuated. The model was parameterized for crustaceans with different levels of armour fed to Atlantic cod Gadus morhua or whiting Merlangius merlangus and recovered from the stomachs at different post-prandial times. The prey species were krill Meganyctiphanes norvegica; shrimps and prawns Crangon crangon, Pandalus borealis, Pandalus montagui and Eualus macilentus; crabs Liocarcinus depurator and Chionoecetes opilio. In accordance with the apparent intraspecific isometric relationship between exoskeleton mass and total body mass, the model described stage duration and rate of evacuation of the crustacean prey independently of meal and prey sizes. The duration of the first stage increased (0-33 h) and the evacuation rate of both stages decreased (by a half) with increasing level of the crustacean armament in terms of chitin and ash. A common, interspecific parameterization of the model within each of the categories krill, shrimp and crab can probably be used if the contents of chitin and ash are similar among prey species per prey category. The model offers a simple way for estimating evacuation rates from stomach content data in order to obtain food consumption rates of wild fishes, provided that information about digestion stage of crustacean prey is available.

Recovery of gastric evacuation rate in Atlantic cod Gadus morhua L surgically implanted with a dummy telemetry device

The current study investigated how the gastric evacuation rate (GER) was affected after surgically introducing dummies of a blood flow biotelemetry system into the abdominal cavity of Atlantic cod, Gadus morhua. Gastric evacuation experiments were performed two and 10 days postsurgery on surgically implanted and control G. morhua force-fed sandeel, Ammodytes tobianus. The results were compared with previously obtained estimates from unstressed conspecifics voluntarily feeding on a similar diet. After two days, GER was significantly lower in the group of fish with the dummy implants compared with the control group, but following 10 days of recovery no significant difference was seen between the two groups. The difference between implanted and control fish observed two days postsurgery may have resulted either from surgery, postsurgical stress and/or the presence of the implant. The conclusion is that 10 days of postsurgical recovery will stabilize GER in G. morhua, thus indicating that at this point the implant per se did not affect GER. Both the fish with surgical implants and controls in this study evacuated their stomachs much slower and with much higher interindividual variation compared with G. morhua feeding voluntarily on similar prey items. The lower GER and higher interindividual variation for force-fed fish indicate that handling, anaesthetization and force-feeding impair GER and that individual fish respond differently to the suppressing effects.

The diet of whiting Merlangius merlangus in the western Baltic Sea.

The diet of whiting Merlangius merlangus in the western Baltic Sea was investigated and compared to the diet in the southern North Sea. Clupeids were important prey in both areas, but especially in the western Baltic Sea where they constituted up to 90% of the diet of larger individuals. Gobies, brown shrimps and polychaetes were the main prey of juveniles in the western Baltic Sea, while a wider range of species were consumed in the North Sea. The shift to piscivory occurred at smaller sizes in the western Baltic Sea and the fish prey consumed was proportionately larger than in the southern North Sea. Estimates of prey abundance and food intake of M. merlangus are required to evaluate its predatory significance in the western Baltic Sea, but its diet suggests that it could be just as significant a fish predator here as in the southern North Sea.

Growth and food consumption of whiting Merlangius merlangus

In the western Baltic Sea (WBS), whiting Merlangius merlangus is the main piscivorous fish together with cod Gadus morhua. In the present study, we investigate the growth and food consumption rates of WBS M. merlangus and compare the growth rates of males and females with those of M. merlangus in the North Sea (NS). Food consumption rates are estimated directly from sampled stomach contents in the WBS using a gastric evacuation rate model and indirectly by using a static energy-budget model together with the growth rates. The results indicate that male and female M. merlangus in the WBS have similar feeding and growth strategies, while in the NS M. merlangus show more pronounced differences in food consumption and growth dynamics between the sexes. Female WBS M. merlangus grow significantly slower than their conspecifics in the NS, but there is no significant difference for males. Sexual size dimorphism is seen in both areas, but for M. merlangus in the WBS the difference is less pronounced. Food consumption rates in the WBS differ between seasons, with the lowest food intake in the first 2 quarters of the year and the highest in the 3rd quarter. No differences in consumption rates were seen between males and females, which could be related to the more similar growth pattern seen for M. merlangus in the WBS.

Editorial on writing equations and mathematical expressions: some basic principles

2. Vectors should be written in lower case bold font, e.g . a and matrices in upper case bold, e.g . A. Other algebraic symbols (except Greek uppercase letters) including parameters and scalar variables should be written in italics. Operators such as log, e, cos and d should be written in regular (and not in italics unlike their associated variables x and y : log x , ex, cos x , dy dx ). Note that e x should normally be used unless the exponent is a complicated function when exp(x ) is acceptable.