A gastric evacuation model for three predatory gadoids and implications of using pooled field data of stomach contents to estimate food rations

The general applicability of the square root model to describe gastric evacuation in predatory gadoids independently of meal size was verified through experiments on whiting Merlangius merlangus , saithe Pollachius virens and cod Gadus morhua fed different fish and crustacean prey. Gastric evacuation rate was related by a negative power function to energy density of fish prey over an extended range from 3·4 to 11·5 kJ g^-1 of the effector variable. Gastric evacuation of crustacean prey seemed to depend on the characteristics of their exoskeleton. Gastric evacuation of mixed meals composed of fish prey with different energy densities could be described as a function of the overall energy density. The evacuation rate of each of the prey could be described directly by their share of the stomach content. A full gastric evacuation model including predator size, temperature and prey energy density was established for whiting, saithe and cod. It was demonstrated that estimates of food rations might be severely biased by use of mean values for prey composition and total mass of stomach contents.     

Food consumption and daily feeding periodicity: comparison between pelagic and demersal whiting in the North Sea

Pelagic North Sea whiting Merlagius merlangus fed at night, while demersal whiting fed by day. The estimated specific daily ration ranged from 4·38 to 7·84% in 1992 and from 3·99 to 10·31% in 1993 using the in situ rate of gastric evacuation. Using Andersen's evacuation model the specific daily ration ranged from 0·41 to 1·66% in 1992 and from 0·78 to 1·75% in 1993. The specific daily rations were significantly different where energy density of stomach content by length class of whiting was significantly different between the two layers and years. The fact that daily ration was related to prey composition and energy density of the prey and spatial distribution of the whiting, demonstrates the need for a sampling design that includes both pelagic and demersal layers when quantifying the food consumption of whiting.     

Effects of temperature, prey type and prey size on gastric evacuation in small cod and whiting

Gastric lavage was used to investigate the effects of temperature, prey type and prey size on gastric evacuation in small cod Gadus morhua and whiting Merlangius merlangus . The fish were fed to satiation and subsequently the stomach contents were sampled to determine the rate at which food was evacuated. Satiation meal size was positively related to temperature and differed between prey types. The gastric evacuation rate (GER) also tended to increase with temperature and varied with prey type. GER at temperatures of 11.3–12.7° C averaged 1.5–1.7 times higher than at 6–9.5° C. There was no significant difference (P>0.05) in the evacuation of lugworm Arenicola marina , sandeel Ammodytes spp., and herring Clupea harengus , but the GER of brown shrimp Crangon vulgaris was much slower (P<0.05). No significant changes in GER were observed when fish were fed on three different size groups of either herring or brown shrimp. In whiting, there was no significant difference in the GER of individual herring or brown shrimp when they were fed as single species meals or incorporated in meals containing a mixture of prey species.     

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 rate of burbot fed single-fish meals at different temperatures

The gastric evacuation rates of burbot Lota lota , fed a single meal of vendace, Coregonus albula , were measured in the laboratory at five temperatures (1·3, 2·6, 4·8, 9·4 and 12·6° C). Gastric evacuation rate increased exponentially with increasing temperatrure, but the results suggest that gastric evacuation rates of burbot at low temperatures are lower than those of other freshwater fish species. Temperature and the ratio of meal weight to burbot weight were the most important factors affecting gastric evacuation rate. There was no significant difference in gastric evacuation rate between three different prey species: vendace, perch Perca fluviatilis , and smelt Osmerus eperlanus .     

Diet comparison between pelagic and demersal whiting in the North Sea

Pelagic North Sea whiting Merlangius merlangus preyed upon (pelagic) sprat and sandeel, while demersal whiting preyed upon (demersal) Norway pout and herring. Values of diet breadth were low for both feeding groups using Levin's index. Diet overlaps within layers were low ( D <0·25), while the between layer food overlap was moderate ( D =0·25–0·74) to high ( D >0·74) using Schoener's index. Selection of prey was density dependent. However, prey size also played an important role. The diet of whiting shifted from amphipods and mysids to fish with increasing predator length, and the length of prey consumed increased significantly with length of whiting. The fact that the stomach contents differed between the feeding groups demonstrates the need for a sampling design that includes both pelagic and demersal habitats when trying to quantify the diet of whiting.     

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.     

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 effects of food consumption rate, body size and temperature on net food conversion efficiency in saithe and whiting

Net food conversion efficiency κ was estimated from growth experiments on saithe Pollachius virens and whiting Merlangius merlangus that were fed natural prey at a range of ration levels including satiation rations. The conversion efficiency, which specifies the net energy fraction of ingested energy C , was described appropriately by a simple power function of food consumption rate as κ  = 0·272 C ^0·18 and κ  = 0·426 C ^0·11 for saithe and whiting. This functional relationship was supported by the patterns of accretion of lipids and proteins in saithe. No significant effects of temperature and body size on κ was, however, demonstrated in this study except for the indirect influence using feeding levels (rations expressed relatively to satiation rations) in bioenergetics models.     

Seasonal variations in the energy density of fishes in the North Sea

The energy density ( E _D, kJ g^-1 wet mass) of saithe Pollachius virens , haddock Melanogrammus aeglefinus , whiting Merlangius merlangus , Norway pout Trisopterus esmarki , herring Clupea harengus , sprat Sprattus sprattus , sandeel Ammodytes marinus and pearlsides Maurolicus Muelleri , from the North Sea, increased with total length, L _T. However, there was not always a significant ( P> 0·05) linear relationship between L _T and E _D. Seasonal differences in E _D were obvious in mature fish, while geographical differences were insignificant. For all species there was a highly significant correlation ( P< 0·0001) between the percent dry mass of the fish ( D S) and E _D. A general relationship was established for gadoids and sandeel E _D=–3·1492+0·3459 D _S and herring E _D=–4·6395+0·4170 D _S. Thus seasonal and size-specific data on E _D needed for bioenergetics and gastric evacuation models can be determined simply from D _S, which is considerably less costly and time consuming than calorimetry or proximate analysis.     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

Gastric evacuation rates and daily ration of piscivorous coho salmon, Oncorhynchus kisutch Walbaum

Effects of temperature and meal size on gastric evacuation rates of juvenile coho salmon, Oncorhynchus kisutch , consuming sockeye salmon, O. nerka , fry were examined and used in the estimation of daily meal, daily ration and number of fry consumed by coho in Chignik Lake, Alaska. Evacuation of fry by coho was best described by a negative exponential model (average R² = 0.93). A square root model also provided a good fit (average R² = 0·93), but the y-intercepts deviated more from the expected value than did the y-intercepts of the exponential model. The effect of temperature ( T , 5–13° C) and meal size (MS, 0·166–0·367 g) on the exponential evacuation rate (r_e, h^-1) could be described as
In the lake, coho fed continuously during the 24-h period in early June 1986 and 1987. Estimates of daily meal and ration of coho calculated by the Eggers method and the geometric mean of prey weight ranged from 0·224 to 0·435 g (2.1–4.4% body wt) depending on location and year. The Elliott & Persson method provided similar estimates of food consumption, whereas estimates based on the Pennington method and square root evacuation of prey differed from the exponential models. Sockeye fry represented 93% of the total prey weight. The average number of sockeye fry consumed per coho per 24 h, based on the arithmetic mean of prey weight, was 3·0–3·9 fry.     

Gastric evacuation of mixed stomach contents in predatory gadoids: an expanded application of the square root model to estimate food rations

A mechanistic gastric evacuation model of mixed stomach contents was established. Its simple generic nature should prove useful for studying predator‐prey interactions in fish communities. Only prey lengths were required as additional input. Surface dependent digestive processes were considered to act on equal fractions of individual prey‐cylinder surfaces. Primary and interactive effects of size, energy density and resistance to digestion of individual prey in a stomach were described by the model. Model predictions of results from experiments on gastric evacuation of meals composed of different prey types demonstrated the capability of this model, as opposed to previously applied model principles, to predict evacuation of mixed stomach contents involving the three above‐mentioned prey characteristics. The study furthermore illustrated that estimates of food ration might be severely biased by using improperly formulated effects of prey characters on gastric evacuation, and demonstrated that the new model holds the potential to predict food ration and diet composition for wild populations of predatory fishes.     

Factors affecting gastric evacuation in cod, Gadus morhua L., fed single-meals of natural prey

Functional relationships were derived in order to describe the effects of fish weight (262–2066 g), temperature (1–14° C), prey size and ration size (0.4–8.4% body weight) on gastric evacuation in cod fed four natural prey types. Interindividual (unexplained) differences among voluntary fed cod were relatively large. Power functions were appropriate for describing the effects of fish weight and ration size on the half-time of evacuation. Effects of meal and/or prey size appeared to counterbalance the effects of cod size. It is suggested that when cod of different sizes are fed fixed proportions of their body weight evacuation time is constant and independent of body size. There was found to be an exponential relationship between temperature and gastric evacuation time but, observations made at similar temperatures at different times of the year suggested that seasonal effects were of minor importance.     

Daily ration and prey size of juvenile piscivore Japanese Spanish mackerel

Daily ration of juvenile Japanese Spanish mackerel Scomberomorus niphonius (32·1–33·1 mm standard length, L _S) was estimated at three temperatures (18·6, 19·5 and 21·2° C) in the laboratory. Gastric evacuation rate ranged between 0·398 (18·6° C) and 0·431 (21·2° C). Diel change in instantaneous consumption rate indicated that juvenile Japanese Spanish mackerel are daylight feeders. The estimated values of the daily ration ranged between 66·1%(18·6° C) and 82·6%(21·2° C) of body mass. These per cent values of daily ration were converted to daily consumption in mg (28 mg at 18·6° C to 34 mg at 21·2° C) using the mean dry body mass of juvenile Japanese Spanish mackerel of 30 mm (42·1 mg). Stomach content analysis of wild specimens collected in the Seto Inland Sea, south‐western Japan, revealed that the majority of wild Japanese Spanish mackerel larvae and juveniles ingested fish larvae with a body size >50% of the predator L _S. Based on the predator‐prey size relationship, the daily consumption of a Japanese Spanish mackerel juvenile of 30 mm was equivalent to 5·1 (18·6° C) to 6·4 (21·2° C) Japanese anchovy Engraulis japonicus larvae which was the dominant prey organism in stomachs of the wild Japanese Spanish mackerel.     

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.     

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.     

Spring cannibalism on 1 year walleye pollock in the Doto area,northern Japan: is it density dependent?

Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island, Japan was important only during spring (April to June), and its importance increased from 0% in dry mass for <200 mm L _S fish to 48·9% for >400 mm L _S fish. Most of the prey was represented by age 1 year fish, showing a unimodal body size distribution with a mode at 121–130 mm. Although cannibal body size was larger in deeper (>150 m) water, there was no difference in prey size by depth, suggesting impingement of the predators inhabiting deeper water into the shallow areas to cannibalize 1 year fish. The minimum ratio cannibal: prey size was 1·74. There was a positive but non-significant correlation between the contribution of cannibalism to a potential predator's (>300 mm) diet and an estimate of the previous year's recruitment. This was due to an extremely high contribution of cannibalism during 1992, when a distinctly larger size of predators seemed to bias the contribution. When the 1992 data were removed from the analysis, a significant correlation was obtained ( r ²=0·77, P <0·01), showing that Pollock cannibalism is rather density dependent. Based on the results, it is hypothesized that the'overflow' of 1 year fish from the shelf waters due to their high abundance and the weak stratification in the spring water column results in increased co-occurrence with adult fish and consequent cannibalism.     

Gastric evacuation in horse mackerel. I. The effects of meal size, temperature and predator weight

Gastric evacuation experiments were performed on horse mackerel Trachurus trachurus. A nearly full matrix experimental design with respect to the variables predator weight (<10–400 g) meal size (up to 7·8% body weight) and temperature (10–20°) was covered with 0-group smelt Osmerus eperlanus as prey. A general evacuation model without meal size as a variable was fitted to the data on wet weights as well as on dry weights by means of non-linear regression technique. Two methods of data transformation, relative data and square root transformation, were applied to improve variance homogeneity. The most reliable model fit was achieved on dry weight data applying the square root transformation technique: where S_t=stomach content (g wet weight) at time t after ingestion, S₀=the initial meal size, W =predator (g wet weight), and T =temperature. The estimated coefficient of the exponential temperature function, δ=00·032, corresponds to a Q ₁₀ value of 1·4 which is outstandingly low in comparison with results on other species. However additional experiments to determine maximum daily food rations indicated that appetite in contrast to gastric evacuation is strongly temperature dependent.