Comparison of field-based and indirect estimates of daily food consumption in larval perch and zander

Since bioenergetics models for 0+ fish have seldom been validated by field consumption estimates, field-based and indirectly estimated daily food rations were compared in larval perch Perca fluviatilis and zander Stizostedion lucioperca. Field-based estimates were calculated with linear and exponential evacuation rates based on gut fullness data during a 24-h cycle, with hourly field samplings instead of the normally recommended 3-h intervals. Indirect calculations used bioenergetics modelling with variable activity multipliers ( A ). Field-based estimates of daily rations ranged between 0·21 and 0·27 g g⁻¹ day⁻¹ in perch (mean L _T 13·1 mm) and 0·31–0·40 g g⁻¹ day⁻¹ in zander (mean L _T 10·6 mm). The higher values were calculated by using the exponential model. Daily rations calculated by bioenergetics modelling with A = 1 were only slightly higher than direct estimates in both species. However, if A values >1 were used, calculated daily rations were substantially higher than direct estimates. Estimates of daily ration based only on every third value ranged between 41 and 72% compared with 1-h intervals, mainly because of lower estimates of evacuation rate.     

Consumption and gut evacuation rate of laboratory‐reared spotted seatrout (Sciaenidae) larvae and juveniles

The temperature and mass dependence of maximum consumption rate was measured for larval and early juvenile spotted seatrout Cynoscion nebulosus . Maximum consumption ( C _MAX) estimates were obtained from feeding and gut evacuation experiments on larvae (3·8–19 mm standard length, L _S) at three temperatures (24, 28 and 32° C), and maximum consumption experiments on juveniles at three temperatures (20, 26 and 32° C). Feeding levels were determined for larvae fed live prey ( Brachionus plicatilis and Artemia salina ) ad libitum . The midgut and total evacuation times were estimated for fish feeding continuously and discontinuously using alternate meals of tagged and untagged live prey. Temperature and fish size had significant effects on gut evacuation and consumption. The gut evacuation time increased with increasing fish size, and decreased with increasing temperatures. Mass‐specific midgut contents increased for small larvae <0·156 mg dry mass ( M _D)( c . 4 mm L _S), and decreased for larger larvae and juveniles. Maximum consumption was modelled by fitting a polynomial function to a reduced dataset of individuals feeding at high levels. The C _MAX model predicted an initial increase in specific feeding rate from 70 to 155% M _D day⁻¹ for small larvae, before declining for larger larvae and juveniles.     

有氧运动训练对中华倒刺鲃幼鱼肠道排空及其数学模型选择的影响

为了考察有氧运动训练对中华倒刺鲃幼鱼肠道排空及其数学模型选择产生的影响, 在(250.5)℃条件下, 将108 尾大小相当的实验鱼(24.470.03) g, (10.900.02) cm随机分为对照组(CG), 1 BL/s 训练组(1TG)和2 BL/s 训练组(2TG), 在相应流速下运动训练8w 后, 轻度麻醉灌喂体重1.45%的饵料, 随后在不同时间点分别测定肠道内容物重量及其百分比, 用3 个常见数学模型对实验数据进行拟合并比较。结果发现, 1TG 和2TG 训练组的体重特定生长率(WSGR)显著高于CG 对照组 (P0.05);各实验组肠道内容物干重及其百分比随着摄食后时间的延长而显著降低;所有实验组肠道排空的最优数学模型均为平方根模型;训练组(1TG 和2TG)的肠道排空率(皆为0.49%/h)明显大于CG 对照组(0.41%/h)(P0.05), 1TG 和2TG 训练组的肠道排空时间(分别为 20.39h 和 20.33h)短于 CG 对照组(24.51h)。研究表明, 有氧运动训练没有对中华倒刺鲃幼鱼的肠道排空特征及其最优数学模型选择产生显著影响, 但明显提高了该种鱼的肠道排空率并缩短其肠道排空时间。       

Gut evacuation and absorption efficiency of walleye larvae

Gut evacuation and absorption efficiency in zooplanktivorous walleye Stizostedion vitreum larvae (10–19 mm; 0.53–6.8 mg dry weight) were examined in the laboratory. The decline in dry weight of whole digestive tract contents was examined at 15, 20, and 25) C in discontinuous feeding larvae. The observed relationship between short-term evacuation rate and gut fullness indicated that evacuation approximated most closely an exponential decline. When gut evacuation rate, R , was calculated as the slope of loge gut fullness v . time there were significant effects of temperature and walleye size on R . Gut evacuation rate was higher at 20° C than at 15 or 25° C and declined with increasing walleye dry weight. Absorption efficiency at 20° C was examined by qualitative analysis of food and faeces. Apparent digestibility, D _a, of crustacean zooplankton increased with gut retention time and declined with walleye dry weight. Maximum D _a was estimated to be 79% of organic matter from food retained in the gut for 6 h.     

Evacuation of pelleted feed and the suitability of titanium(IV) oxide as a feed marker for gut kinetics in Nile tilapia

The present study assessed the suitability of titanium(IV) oxide, TiO₂, as a digesta passage marker in Nile tilapia Oreochromis niloticus and studied the shape of the evacuation curve in this species. In three separate trials, fish were given one dose of either 0·5, 0·25 or 0·1% of their body mass (% BME) of feed marked with 1% TiO₂ or 0·5% BME of the same feed without marker. The fish were serially slaughtered at intervals after feeding and the stomach contents analysed for dry mass and marker content. The data for individual trials were analysed with the linear, square root, surface area and exponential evacuation models and parameter comparisons showed that, although the marker interfered slightly with the evacuation process, true meal size could be predicted more accurately from the marker data. The results of an analysis of the combined data sets suggested that stomach evacuation in this species is dependent more on food particle surface area (surface area model) than on stomach content mass (exponential model) as is generally assumed. On the basis of these results, it was concluded that TiO₂ at an inclusion level of 1% is an acceptable marker for quantifying evacuation with a view to predicting food consumption but should be used with caution in digestibility studies.     

A first test of a new modelling approach to estimate food consumption in particle-feeding fish

Models for estimating food consumption in fish by analysing changes in stomach fullness over time are invariably based on a stomach evacuation rate obtained when the fish is fasting, on the assumption that this rate also applies to when the fish is feeding. However, this often is not the case in fish that feed on small particles. A new modelling approach was therefore tested, which is based not only on stomach fullness but also on gut contents. To eliminate errors arising from assimilation in the gut, titanium(IV) oxide (TiO₂) was used as an indigestible marker. When applied to a dataset obtained from tilapia given several equal doses of pelleted feed over a 2.5‐h period, the new approach gave a closer true consumption estimate than a conventional model. The evacuation rate proved to be a more sensitive parameter than the ingestion rate, but the former was no longer required by the new approach for estimating ingestion, thus liberating the food consumption estimate from any errors and dependencies inherent in the evacuation rate. The new approach assumes that the digesta of previous feedings can be distinguished from those of the feeding phase being analysed and therefore needs further refinement for those cases when this does not apply. Suggestions for such refinements are also given. This new approach is expected to be equally suitable for estimating consumption in stomachless 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.     

The feeding ecology of Morone americana larvae in the Chesapeake Bay estuarine turbidity maximum: the influence of physical conditions and prey concentrations

Feeding ecology of white perch Morone americana larvae, a major component of the ichthyoplankton community in the estuarine turbidity maximum (ETM) region of upper Chesapeake Bay, was evaluated in years of contrasting physical conditions. In 1998, high river flow led to an ETM with high turbidity and pronounced salinity stratification. In 1999, under low river flow conditions, turbidities in the ETM region were lower and the salt front was located 15 km up‐stream of its location in 1998. Copepodites and adults of Eurytemora affinis were the predominant prey in guts of all length classes of larvae (3·2–9·8 mm standard length, L _S) that were examined. Repeated‐measures, multiple regression analyses were conducted to evaluate the influence of several factors on feeding success by determining if physical conditions (temperature, current velocity, salinity, turbidity and light) and prey concentrations explained variability in mean gut fullness (gut contents mass per body mass) of small (<5 mm L _S) and large (>5 mm L _S) white perch larvae. Eurytemora affinis concentrations were significant in the statistical models for both small and large larvae. High concentrations of E. affinis , which enhanced encounter rates of white perch larvae with prey, may have been the most important factor determining feeding success in the ETM region. Larval feeding incidence (percentage of guts with food) was higher in 1998, suggesting that annual variability in river flow influenced larval feeding success by controlling physical and biological conditions in the ETM region.     

Temperature and state‐dependence of feeding and gastric evacuation in juvenile Pacific halibut

Relationships between nutritional state, behavioural response to prey and gastric evacuation rates were examined in juvenile Pacific halibut Hippoglossus stenolepis feeding on squid. Pacific halibut reared at 2, 6 and 10° C were fasted for 1 or 7 days to generate variation in energetic state. The 7 day fast resulted in measurable declines in condition indices at 10 and 6° C but not at 2° C. At 10° C, all Pacific halibut consumed the first meal offered, but fish previously fasted for 7 days took significantly longer to locate and consume the meal than fish fasted for only 1 day. At 2° C, Pacific halibut fasted for 7 days did not generally consume the first meal offered, but resumed feeding 2·1 days sooner, on average, than fish fasted for only 1 day. The gastric evacuation rate of the squid meal was best described by a power model with near‐exponential curvature ( a  = 1·011). The evacuation rate was strongly temperature‐dependent ( Q ₁₀ = 3·65) but displayed the same degree of variability at each temperature. The evacuation rate in Pacific halibut was not affected by feeding history, body size or energetic state. Furthermore, individual variation in gastric evacuation rate was not correlated with feeding responsiveness at any temperature. These results indicate a general plasticity in the behavioural but not physiological aspects of energy acquisition.     

Gastric evacuation of three food types in the black and yellow rockfish Sebastes chrysomelas (Jordan and Gilbert)

The evacuation patterns of shrimp, crab and fish from the stomachs of black and yellow rockfish, Sebastes chrysomelas , were examined by feeding meals of known size and measuring the amount remaining after various post-prandial intervals. Linear, square-root, exponential, power exponential, logistic and Gompertz models (the latter two with unrestricted lower asymptotes, or with lower asymptotes restricted to 0% food remaining) were fitted to the wet weight, dry weight and volume of food remaining in the stomach as a function of post-prandial time. Evacuation patterns ranged from steeply concave (fish wet weight, dry weight, volume, shrimp dry weight), to linear (shrimp wet weight, crab dry weight), to highly convex with lag phases of up to 30 h (crab wet weight, crab and shrimp volume). Friability, the ease with which a food item is fragmented in the stomach, may be an important factor in determining evacuation patterns. The evacuation of a crab meal by tagged, free-ranging S. chrysomelas in the field was not significantly different from that of S. chrysomelas held in the laboratory.     

In situ estimates of gut evacuation and its dependence on temperature in five cyprinids

Gut evacuation and its dependence on the temperature in bream Abramis brama , silver bream Blicca bjoerkna , roach Rutilus rutilus , gibel Carassius aurtaus gibelio and carp Cyprinus carpio , held in mesh cages in Lake Balaton, was best fitted by an exponential model in 18 of 36 trials, while in 14 and four trials, a linear and a square-root model, respectively, gave a better fit. Adjusted r ² values, however, often did not differ markedly between the three models. The shape parameter, B , was 0.36 for carp and ranged from 0.81 to 1.24 for the other four cyprinids, according to the general evacuation model expanded with the temperature variable, and fitted to whole data sets. Relationships between the food evacuation rates obtained from the exponential models and the temperature were described by exponential functions for bream, silver bream, roach and gibel.     

Gastric digestion and evacuation in whiting, Merlangius merlangus (L.)

Three meal sizes of sandeels were fed to whiting in order to monitor the evacuation of food out of the stomach. The stomach contents were sampled at intervals after feeding, using a stomach pump. In such experiments, the proportion of fish with empty stomachs tends to increase with time and, since stomach contents are limited to zero or positive values, the variance of the stomach contents becomes censored at zero. This tends to produce a curved relationship between mean stomach content and time, which gives the impression that evacuation rate slows down at low levels of stomach fullness. By taking account of censoring, it was shown that the evacuation curve generated for whiting was consistent with and could be generated from a linear model in which the rate of gastric evacuation exhibited by the fish was constant and independent of meal size, level of stomach fullness and time after feeding. The parameters of the linear model were estimated by maximum likelihood and then applied in a second model to predict the observed mean stomach content. The average gastric evacuation rate of whiting of mean weight 268 g at 10°C was 0.31 g h⁻¹.     

Rates of gastric evacuation in piscivorous brown trout, Salmo trutta

SUMMARY. 1. The dry weight of food remaining in the stomachs of piscivorous trout decreased exponentially with time. Gastric evacuation rates increased exponentially with increasing temperature but were unaffected by predator size, meal size or type of fish prey.
2. Mathematical models were developed to estimate both the rate and time for the gastric evacuation of different meal sizes (expressed as dry weight), and were applicable to piscivorous trout of different sizes (length range 10–32 cm) feeding on trout fry or sticklebacks at different temperatures (range 5–18°C).
3. The wet weight of food in the stomachs also decreased exponentially with time, but evacuation rates both increased with temperature and decreased with increasing meal size; the latter relationship occurred because relative rates of water loss from a meal also decreased with increasing meal size. Use of wet or dry weights can therefore lead to different conclusions about the effect of meal size on evacuation rates.
4. When piscivorous trout were fed three consecutive meals of varying size, the models predicted the total dry weight of food left in the stomach, but not the weight remaining for each individual meal. Interactions between meals led to an increase in evacuation rates for meals consumed early in the series and a decrease in evacuation rates for later meals.
5. Evacuation rates for piscivorous trout were compared with those for trout feeding on invertebrates in an earlier study, and were close to those for caddis larvae as prey, higher than those for mealworms and lower than those for a variety of invertebrate prey. Although a great deal is now known about the daily food intake and growth rates of trout feeding on invertebrates, there is little comparable information for piscivorous trout.     

Rotifers as food in aquaculture

The rotifer Brachionus plicatilis (O.F. Muller) can be mass cultivated in large quantities and is an important live feed in aquaculture. This rotifer is commonly offered to larvae during the first 7–30 days of exogenous feeding. Variation in prey density affects larval fish feeding rates, rations, activity, evacuation time, growth rates and growth efficiencies. B. plicatilis can be supplied at the food concentrations required for meeting larval metabolic demands and yielding high survival rates. Live food may enhance the digestive processes of larval predators. A large range of genetically distinct B. plicatilis strains with a wide range of body size permit larval rearing of many fish species. Larvae are first fed on a small strain of rotifers, and as larvae increase in size, a larger strain of rotifers is introduced. Rotifers are regarded as living food capsules for transferring nutrients to fish larvae. These nutrients include highly unsaturated fatty acids (mainly 20: 5 n–3 and 22: 6 n–3) essential for survival of marine fish larvae. In addition, rotifers treated with antibiotics may promote higher survival rates. The possibility of preserving live rotifers at low temperatures or through their resting eggs has been investigated.     

Gut evacuation of walleye pollock larvae in response to feeding conditions

Gut residence times of first-feeding walleye pollock larvae were measured at 6°C in continuous and discontinuous feeding regimes. Larvae fed tagged copepod prey evacuated their guts more quickly in continuous feeding as compared to the discontinuous feeding treatment. The mean gut residence time was estimated to be 5.0 h for larvae feeding continuously. Gut evacuation by larvae fed tagged prey and then isolated without food (discontinuous treatment) was slower and more variable, with an estimated gut residence time exceeding 8.0 h. Field and laboratory observations suggest that the larval fish gut may be modeled as an intermittent plug-flow reactor (PFR) in response to diel feeding patterns. The cyclical nature of gut dynamics has implications for gut content analyses and the estimation of daily food rations.     

Gastric evacuation in plaice, Pleuronectes platessa L.: effects of temperature and fish size

Using a volume dependent model of gastric evacuation, the effects of temperature and fish size were examined. Rates of gastric evacuation were unaffected by fish size but increased with increasing temperature. The relationship between maximum stomach volume and fish weight was found to be a linear one. From information of gastric evacuation rates and stomach volume, the amount of food evacuated from the stomach per day was calculated for different size classes of fish. Daily food evacuation increased in proportion to body weight to the power 0·68. Assuming these methods give a crude estimate of daily food intake, the results are discussed in relation to published work on food intake in fishes.     

Estimation of food consumption from field observations of fish feeding cycles

The available methods of estimating food consumption by fish require that experiments be performed on confined animals and that experimental results may be validly applied to free fish. A method is described by which food consumption in periodically feeding fish may be estimated without performing laboratory experiments. A relatively simple input–output model of stomach contents is fitted to the observed time trajectory of stomach fullness, and food consumption is calculated from the estimated model parameters. Feeding is considered to be restricted to a distinct feeding period, and the rate of feeding during that period can be either constant or linearly decreased with the quantity of food already present in the stomach.
The method is applied to three examples. The model appears robust, and generally provides very similar food consumption estimates to those obtained from methods requiring an independent estimate of gastric evacuation rate. However, the described method is sensitive to violation of the assumption that feeding occurs only during a discrete feeding period.     

Effects of delayed first feeding on the nutritional condition and mortality of California halibut larvae

The effect of the timing of first feeding (3, 4, 5, 6, 7 and 8 days post‐hatch, dph) on laboratory‐reared California halibut Paralichthys californicus larvae was evaluated by means of morphologic, morphometric and histological criteria. Larvae began to feed exogenously at 3 dph (2·7 ± 0·01 mm standard length, L _S) at 18° C. Eye pigmentation, rather than mouth opening was the most distinctive trait of California halibut larvae at first feeding. Larval growth was significantly affected by the time of first exogenous feeding. At notochord flexion (21 dph), the L _S of larvae fed for the first time at 3 dph was significantly larger (5·1 ± 0·1 mm) than that of those fed at 4 and 5 dph (4·9 ± 0·1 mm), although the latter fish had a more uniform size distribution. The point of no return was reached at 7 dph. Survival of larvae initially fed at 3, 4 and 5 dph was similar (58·4–60%), while no larvae were able to survive when food was offered for the first time between 6 and 8 dph. Food deprivation resulted in a progressive deterioration of the larval digestive system and atrophy of skeletal muscle fibres. Significant changes in the anterior and posterior enterocyte height were detected after 2 days of food deprivation. Similarly, tail height: L _S and trunk length: L _S ratios were the most sensitive morphometric indices to detect the effect of fasting on larval condition. Present results show that a combination of morphometric and histological variables can be used to evaluate the nutritional condition of California halibut larvae.     

Food consumption of vendace Coregonus albula larvae in Lake Lentua, Finland

The daily ration of vendace larvae (mean TL 10·1–12·8 mm) was assessed after a time-intensive field survey in Lake Lentua. Ten larvae were sampled every 2 h throughout the 10-day sampling period and their alimentary tract contents analysed. The gut was cut into four quarters of equal length and each quarter was analysed separately. This procedure enabled the assessment of the proportions of different zooplankton taxa in the diet based on the least degraded first-quarter contents and to reconstruct the entire gut contents. Secondly, the gut passage time of ingested zooplankton was evaluated by comparing the degradation rate of zooplankton in the guts of experimentally fasted larvae to the degradation of zooplankton in the gradient from pharynx to anus among the larvae sampled from the lake. Finally, food consumption was calculated as zooplankton individuals and carbon biomass based on the two variables, gut passage time of constantly feeding larvae and reconstructed gut contents. The daily ration of vendace larvae varied between 14 and 450 zooplankters in the monitored period.