Effect of differential gastric evacuation and multispecies prey items on estimates of daily energy intake in juvenile chinook salmon

Synopsis The caloric density of stomach contents in juvenile chinook salmon,Oncorhynchus tshawytscha, was not affected by gastric evacuation, suggesting a constant caloric density of stomach contents during evacuation. Differences in the caloric density of prey consumed did affect caloric density of stomach contents over a 24-h period. Consumption of the amphipodCorophium sp. was associated with reduced caloric densities of stomach contents. During periods whenCorophium contributed more than 4% of the stomach contents, average caloric density declined from 5.56 to 5.33 kcal g^–1. Despite this difference, estimates of daily energy intake of juvenile chinook salmon were only 3%, greater when developed from the mean caloric density of stomach contents excludingCorophium.     

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.     

Effect of temperature and salinity on the gastric evacuation of juvenile sole Solea solea and Solea senegalensis

The juveniles of Senegal sole, Solea senegalensis, Kaup 1858, and common sole, Solea solea (Linnaeus 1758) concentrate in estuarine and coastal nurseries of widely differing temperatures and salinities. Yet, little is known about the effect of these physiologically important variables on the gastric evacuation rates of these species. Gastric evacuation experiments were performed on juveniles of S. senegalensis and S. solea. Three temperatures were tested, 26, 20 and 14°C at a salinity of 35‰. A low salinity experiment was also carried out at 15‰, at 26°C. Experimental conditions intended to reflect conditions in estuarine and coastal nurseries where juveniles of these species spend their first years of life. The relation between stomach contents and time was best described by exponential regression models for both species. An analysis of covariance (ancova ) was performed in order to test differences in evacuation rate due to temperature and salinity (slope of evacuation time against stomach contents) for each species. While increasing temperature increased evacuation rates in both species (although not at 26°C in S. solea), the effect of low salinity differed among species, leading to a decrease in gastric evacuation rate in that of S. senegalensis and an increase in S. solea. Differences in gastric evacuation rate between species were related to its metabolic optimums and to its distribution in the nursery area where fish were captured. Implications for the habitat use of estuarine and coastal nurseries are discussed.     

Are the dynamics of gastric evacuation of natural prey in a piscivorous flatfish different from what is going on in a gadoid?

Despite the fact that the stomach of turbot Psetta maxima is a curved tube that forms a half circle, it was demonstrated that gastric evacuation in this predatory flatfish fed natural prey closely followed the surface‐dependent cylinder model developed from studies on gadoids with a straight stomach. Evacuation experiments were performed on two size groups of P. maxima fed sandeel Ammodytes tobianus as well as on P. maxima fed brown shrimp Crangon crangon at three different temperatures. This enabled the provision of a gastric evacuation model for studies on P. maxima, which takes into account the effects of the explanatory variables predator size, temperature, prey energy density and resistance of prey to the digestive processes in the stomach. Basically, the cylinder model predicts that a square‐root function accurately describes gastric evacuation, which is inconsistent with the conclusion of a previous study on P. maxima that evacuation of A. tobianus is essentially linear with time. Use of the cylinder model to the values of the explanatory variables presented in the latter study, however, gave accurate predictions of the actually acquired evacuation data, which points to the generic value of the model.     

Feeding chronology,daily ration,and the effects of temperature upon gastric evacuation in the pipefish,Syngnathus fuscus

Synopsis Feeding chronology, daily ration, and the effects of temperature upon gastric evacuation were examined in the pipefish,Syngnathus fuscus, from field and laboratory data.S. fuscus displayed a pattern of diurnal feeding, characteristic of syngnathids. Daily ration calculations yielded estimates of 4.0 and 4.4% body weight per day, which are comparable to estimates for other teleosts. Evacuation rate was found to be temperature dependent. with more rapid evacuation with increasing temperature. In addition, evacuation rate was found to be positively correlated with gut content. Slowing of evacuation rate with decreasing gut content may allow for increased assimilation efficiency during periods of low food availability. Daily ration, although controlled by the temperature dependence of evacuation rate, may also be controlled by prey abundance; fish maximize food intake during periods of high prey availability, and maximize upon assimilation during periods of low prey availability.Contribution number 1035 of the Virginia Institute of Marine Science of the College of William and Mary.     

Gastric evacuation in mackerel: the effects of meal size, prey type and temperature

Gastric evacuation experiments, performed with groups of Atlantic mackerel Scomber scombrus and three different prey species: sprat Sprattus sprattus , sandeel Ammodytes marinus and krill Meganectyphanes norwegicus , revealed a strong temperature effect on gastric evacuation ( Q ₁₀=4.l) and a curvilinear evacuation with a shape intermediate between the square root and the exponential model. From the experiments with krill, where no internal tagging was possible, the medians and upper quartiles of the stomach content distributions were analysed instead of individual data. From these experiments the evacuation rate of krill was 60% higher than that of sandeel. An additional simulation study confirmed that the use of medians results in more accurate estimates of the evacuation rates when compared to the arithmetic means.     

The predation impact of juvenile herring Clupea harengusand sprat Sprattus sprattus on estuarine zooplankton

The consumption of estuarine copepods by juvenile herring and sprat during estuarine residency was estimated using fish biomass data and daily rations calculated from two models of feeding in fish: a bioenergetic model and a gastric evacuation model. The bioenergetic model predicted daily rations that were, on average, three times higher than those estimated by a model based on field records of stomach contents. The biomass of herring and sprat in the estuary was negatively correlated with the daily ration suggesting that the clupeid fish populations were resource-limited. Copepod production decreased towards the winter and peaked in spring and summer. The relative importance of predation changed seasonally in function of the migration pattern of herring and sprat. In the spring and the summer, in situ production ofcopepod biomass was higher than the in situ consumption by fish. During the fall and the winter, consumption exceeded production. This suggests that top–down control exerted by marine pelagic fish may be an important force structuring estuarine copepod populations.     

Gastric evacuation of single and multiple meals by piscivorous coho salmon,Oncorhynchus kisutch

Synopsis Gastric evacuation of multiple meals of recently emerged sockeye salmon, Oncorhynchus nerka, consumed by juvenile coho salmon, O. kisutch, were compared with that expected from a single meal evacuation rate model developed for the estimation of food consumption by coho in the field. Significant interaction occurred between meals consumed two hours apart (p<0.001). Evacuation of the first of two meals (29.9% of initial weight remaining) was significantly faster (p<0.05) than that of a single meal (36.1% of initial weight remaining) after four hours, whereas evacuation of the second meal (72.2% of initial weight remaining) was significantly slower (p<0.001) than that of single meals (50.6% of initial weight remaining) after two hours. The total weight of the multiple meal remaining after four (0.191 g), six (0.138 g) and eight (0.070 g) hours averaged within 6% of that predicted by the single meal evacuation model (0.186 g, 0.115 g and 0.073 g, respectively). These data suggest that the stomach evacuation model based on single meals is adequate for estimating the evacuation of prey consumed by continuously feeding coho salmon.     

Relationship between size of parent at cell division and relative size of its progeny in Escherichia coli

This article examines the empirical basis for the assumption of independence between the relative size (length or surface area) of a newborn cell w and the absolute size of its mother at cell division. Random samples from two strains of Escherichia coli B/r cells in steady-state exponential growth, covering a range of doubling times, were fixed in osmium tetroxide and prepared for electron microscopy by agar filtration. Length and diameter of over 3000 constricted cells were measured from the electron micrographs and cell surface area computed by assuming an idealized geometry of right circular cylinders with hemispherical polar caps. In general, these strains were found to divide into two daughter cells with a precision that is independent of the size of the mother. In addition, both a normal and a symmetrical beta-distribution were shown to fit the observed size distributions of w rather well; theoretical grounds for preferring the latter are discussed.     

Gastric evacuation in little skate

The effects of prey type and prey preparation on the mathematical forms and rates describing gastric evacuation in little skate Raja erinacea were examined. Linear and square–root models best described the gastric evacuation of whole, thin–shelled krill Meganyctiphanes norvegica , clam feet/muscle Spisula solidissima/Placopecten magellanicus , polychaetes Glycera spp. and sand lance Ammodytes dubius . Evacuation of krill and clams was faster than polychaetes and sand lance. A logistic model best described the evacuation data of thick–shelled benthic shrimp Crangon septemspinosa/Palaemonetes spp. Cut polychaetes Nereis spp. were digested at an exponential rate and were evacuated faster than would be predicted based upon comparison with live polychaetes and previously published evacuation–temperature relationships. The results of this study suggest that a single equation does not describe the evacuation process for all prey, and that whole prey should be used if laboratory–derived rates of gastric evacuation are to reflect what might occur in wild fishes.     

In situ estimation of gastric evacuation and consumption rates of burbot (Lota lota) in a summer‐warm lowland river

The burbot (Lota lota) is the only cold‐stenothermal gadoid inhabiting freshwaters with high temperature amplitudes. Summer temperatures up to 25°C have been reported as being far above the thermal preferendum of this species. Thus this study aimed to determine gastric evacuation, daily food consumption and energy uptake of burbot at high temperatures under in situ conditions. These data are prerequisites for bioenergetics modelling of the adaptive physiological behaviour of burbot in summer‐warm freshwaters. The study was conducted in the lower Oder River, Germany between May and October 2003 covering a temperature range between 4.6 and 23.4°C. A total of 1683 burbot was caught in five successive 24‐h fisheries. The mean index of stomach fullness showed a significant decrease with rising temperature. Highest stomach fullness values were reached at 4.6°C in October. A negative correlation was observed between gastric evacuation and temperature. In contrast to the data reported in the literature gastric evacuation was found to be high at 4.6°C, which may indicate an ongoing compensatory adaptation to the long period of high temperatures in July and August, when a significant reduction of the mean daily ration was observed. Balancing the energy flux probably resulted in a decrease in the hepatosomatic index at high temperatures and an increase during the cool period, when the liver energy was restored. The endogenous energy pool of the liver may determine the survival of burbot during summer months. The ability of burbot to actively forage during winter and to rapidly digest high amounts of food at cold temperatures was considered an adaptation to increase survival chances in an environment with high environmental, especially thermal, heterogeneity as is typical for a summer‐warm lowland river. Limitations of the energy budget have to be expected with extended periods of high temperatures.     

The effects of temperature and meal size on the rate of gastric evacuation in perch (Perca fluviatilis) fed on fish larvae

SUMMARY. The rate of gastric evacuation in perch (Perca fluviatilis) (89–170 mm length) fed on fish larvae was studied at temperatures between 12.0 and 21.7°C. Gastric evacuation rates were usually described by an exponential function. The instantaneous rate of gastric evacuation ( R ) was constant for a large number of different meal sizes. At higher food rations, a lag phase in digestion was found during the first part of digestion, and this ration size was smaller for smaller perch (89–110 mm) than for bigger perch (120–170 mm). Below these larger meal sizes, gastric evacuation was similar for the different size classes studied. The relation between R and temperature was described by an exponential function. The effects of meal size, number of food items, fish size and temperature on the rate of gastric evacuation are discussed.     

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.     

Egg distributions and the information a solitary parasitoid has and uses for its oviposition decisions

Approximately three decades ago the question was first answered whether parasitoids are able to assess the number or origin of eggs in a host for a solitary parasitoid, Leptopilina heterotoma, by fitting theoretically derived distributions to empirical ones. We extend the set of different theoretically postulated distributions of eggs among hosts by combining searching modes and abilities in assessing host quality. In the models, parasitoids search either randomly (Poisson) (1) or by vibrotaxis (Negative Binomial) (2). Parasitoids are: (a) assumed to treat all hosts equally, (b) able to distinguish them in unparasitised and parasitised hosts only, (c) able to distinguish them by the number of eggs they contained, or (d) able to recognise their own eggs. Mathematically tractable combinations of searching mode (1 and 2) and abilities (a,b,c,d) result in seven different models (M1a, M1b, M1c, M1d, M2a, M2b and M2c). These models have been simulated for a varying number of searching parasitoids and various mean numbers of eggs per host. Each resulting distribution is fitted to all theoretical models. The model with the minimum Akaike's information criterion (AIC) is chosen as the best fitting for each simulated distribution. We thus investigate the power of the AIC and for each distribution with a specified mean number of eggs per host we derive a frequency distribution for classification.Firstly, we discuss the simulations of models including random search (M1a, M1b, M1c and M1d). For M1a, M1c and M1d the simulated distributions are correctly classified in at least 70% of all cases. However, in a few cases model M1b is only properly classified for intermediate mean values of eggs per host. The models including vibrotaxis as searching behaviour (M2a, M2b and M2c) cannot be distinguished from those with random search if the mean number of eggs per host is low. Among the models incorporating vibrotaxis the three abilities are detected analogously as in models with random search.Experiments with two species of solitary parasitoids (L. heterotoma and Asobara tabida) are conducted. All theoretically postulated distributions are separately fitted to the resulting experimental egg distributions. The AIC criterion is used to choose the best fitting theoretical distribution. For both parasitoid species the frequency distribution of best fitting models for experimental data is compared to the classification of distributions generated by simulations. This leads to the conclusion that both L. heterotoma and A. tabida are able to distinguish between parasitised and unparasitised hosts. For L. heterotoma the results point to an ability to assess the number of eggs in a host, whereas A. tabida does not seem to have this ability. This difference suggests that an egg is more valuable for L. heterotoma than for A. tabida.     

Stomach pH,feeding rhythm and ingestion rate in Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Awasa,Ethiopia

Fish were caught in February and October, 1985 with gill and trawl nets at different time intervals. The length and weight of the fish, the stomach pH and the weight of the stomach contents were determined. The feeding pattern of O. niloticus was observed to have a diel rhythm. The stomach became more acidic towards the afternoon and most of the evening. The pH of the stomach had a significant regression on stomach fullness. The feeding rate of O. niloticus in Lake Awasa was estimated from daily feeding periodicity and rate of gastric evacuation. The fish ingests 11.5% of their wet body weight daily at an average water temperature of 21.5 °C. It is believed that the fish ingests and assimilates adequate supply of energy (9.1 KJ/day) for growth, maintenance and other energy requiring activities of the fish.     

Differential digestion and evacuation rates of prey in a warm-temperate grouper, gag Mycteroperca microlepis (Goode & Bean)

Prey-specific gastric evacuation rates and digestion state indices were modelled for gag Mycteroperca microlepis , a large warm-temperate grouper, consuming meals of either baitfish (scaled sardine Harengula jaguana ) or crab (purple swimmer crab Portunus gibbesii ). Power exponential models best fit the wet and dry mass gastric evacuation data and the average digestion indices over post-prandial time (PPT), regardless of prey type or gag size (Adjusted R² ≥ 0·79). Gag mass ( M ) or total length ( L _T) incorporated into an expanded power exponential model, along with exponential scalars, resulted in highly predictive ( R² ≥ 0·87) gastric evacuation and average digestion state models. The expanded power exponential models fit to the baitfish and crab wet mass gastric evacuation data differed significantly (Kimura's likelihood ratio test (LRT), both P < 0·001). Gag consuming crab showed a digestive lag period of at least 4 h (wet mass) and took a longer time to complete digestion relative to gag consuming baitfish. Gag, as well as many other warm-temperate and tropical groupers, consume a mixture of fish and crab prey and they will therefore require the development of a consumption model that incorporates mixed-prey gastric evacuation models.     

Gastric evacuation in the sandbar shark, Carcharhinus plumbeus

Sandbar sharks maintained in an enclosure in the natural environment were fed meals of soft blue crab, Callinectes sapidus , or menhaden, Brevoortia lyrannus , and were sacrificed at intervals after feeding to measure rates of gastric emptying. Regression analysis was used to evaluate the adequacy of various models in describing the decrease in stomach contents with time after feeding. A Gompertz growth curve provided the best fit to the data for both food types. This model suggests that gastric emptying is characterized by an initial lag phase during which evacuation rate increases, a period of maximal evacuation, and a decreasing evacuation rate during the later stages of digestion. Passage of food from the stomach was slow relative to other species of fish studied. The time required to evacuate 98% of a meal was 92–3 and 70–7 h for sharks fed menhaden and blue crab respectively. Evacuation rates did not differ between day and night periods.     

Mechanistic modelling of gastric evacuation in predatory gadoids applying the square root model to describe surface‐dependent evacuation

A new model approach to gastric evacuation in predatory fishes was shown to give accurate and reliable estimates of evacuation. It is believed that such a model would prove particularly useful in understanding predator‐prey interactions in natural systems. The model is a simple, geometric abstraction of the square root model predicting that evacuation time is proportional to the square root of meal size in accordance with extensive empirical evidence. Digestive processes are assumed to be restricted to an outer surface of total stomach content that is represented by the curved side of a cylinder. This way, total stomach content is considered a cylinder of constant length the mass of which is gradually reduced by successive peeling off its side leaving the ends unaffected. The fundamental rate δ describes mean thickness of the layer that is peeled off the cylinder per time unit. In the model, δ is constant independent of cylinder thickness but inversely proportional to the square root of cylinder length. Anatomical and dynamic characteristics of the stomach render this formulation of δ plausible. Using mean evacuation rate over time during evacuation of a meal, the model disregards prey heterogeneities that influence instantaneous evacuation rate. Relationships between prey heterogeneities and evacuation patterns were indicated by application of a general power model to evacuation data, and it was suggested how the effects of prey characteristics might be incorporated into the square root model to produce a generic model of gastric evacuation.     

The effects of predator size, temperature, and prey characteristics on gastric evacuation in whiting

No effect of prey size on gastric evacuation rate was found in whiting Merlangius merlangus .Prey energy density explained most of the data variation among fish prey, and evacuation time increased by a factor 1·6–1·7 with prey energy density increasing from 3·4 to 5·6 kJ g⁻¹. The power model expanded to include predator size, temperature, and prey energy density could describe gastric evacuation in whiting fed fish prey. Krill Meganyctiphanes norvegica was evacuated at a rate similar to fish prey of equal energy density, while the evacuation of brown shrimp Crangon crangon took almost twice as long.