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.     

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.     

鱼类的胃排空率及其影响因素

鱼类的胃排空率是研究鱼类能量学的重要参数,近年来国外有关研究结果表明,鱼类的胃排空率除了受鱼体自身生理状况和实验方法的影响以外,还受许多其它因素,如：鱼的种类,鱼体重,温度,食物,摄食频率以及饥饿时间等的影响。这些资料将为我国开展该方面的研究工作提供有价值的参考作用。     

Further Observations on the Blood Glucose Level in Channa punctatus (Bloch.)

There is a significant rise in the blood glucose level of Channa punctatus following surgical extirpation of the principal islets. The hyperglycemia persists for nearly 30 days in the isletectomized fish. A transient hyperglycemia is exhibited by the sham operated control fish also. If the isletectomized fish are force-fed, a further increase in the blood glucose level is obtained. The force feeding of normal, unoperated fish results in an alimentary hyperglycemia at about 10–24 hours post-prandial.     

东、黄海鳀鱼的胃排空率及其温度影响

在室内受控条件下,以卤虫幼体为饵料,测定了不同温度下鳀鱼的胃排空率,并比较了线性、指数和平方根3种常用数学模型对其排空曲线的拟合程度.统计结果表明,3种数学模型均可较好地描述鳀鱼的胃排空曲线 (df =10, r2=0.7820～0.9629, P<0.01),综合评价结果则进一步表明,指数模型最适于定量描述其胃排空曲线,平方根模型次之,直线模型较差.在研究温度范围内,鳀鱼的胃排空率随着温度的升高而加速增大,二者之间的定量关系可以用指数函数Rt=0.0354e0.0766T (R2=0.9770)加以定量描述.     

Thermal changes in the gastric evacuation rate of the freshwater sculpin Cottus nozawae Snyder

We experimentally measured thermal changes in the gastric evacuation rate (GER) of the freshwater sculpin Cottus nozawae Snyder under three water temperature regimes (2°, 7°, and 12°C). Laboratory experiments showed that the GER was accelerated with increasing water temperature. This result suggests that the daily food rations of fish are more likely to be underestimated at higher water temperatures if estimation is simply based on the stomach content weight alone. By comparing the GER for various fish species from subfrigid to temperate streams, we found a general pattern that the GER increases with water temperature, regardless of taxonomic group or foraging mode. However, the reaction norms of the GER against water temperature showed considerable interspecific variation. This means that stomach content weight is not comparable as a simple measure for determining the daily rations among fish species when water temperature regimes are different. To consider the temperature-dependent pattern of such a physiological phenomenon is important in understanding the feeding ecology of fishes and their roles in material cycles through food webs in aquatic ecosystems.     

The effect of dietary lipid content and stress on egg quality in farmed Atlantic cod Gadus morhua

The present study assessed differences in fecundity and egg quality from Atlantic cod Gadus morhua fed isoproteic diets containing 13% fat (low fat, LF) or 20% fat (high fat, HF) and either stressed or left unstressed as a control over the spawning season. Each diet was fed to triplicate groups of G. morhua from June 2009, through to first maturation and spawning. In January 2010 sub-groups of G. morhua were moved to land-based spawning tanks where the experimental trial was carried out. At the start of the experiment, G. morhua fed the high-fat diet were significantly larger than G. morhua fed low-fat diet. These differences were maintained through the spawning season, although with a loss of mass in both dietary groups. Relative fecundity through the season was significantly lower in stressed G. morhua fed LF compared to unstressed G. morhua fed the same diet. Stressed G. morhua had a higher variability in weekly amount of eggs spawned, spawning occurred more irregularly, and the spawning period lasted longer than in unstressed G. morhua. Several egg quality variables were also affected: eggs from G. morhua fed LF and exposed to stress had lower fertilization and hatching rates compared to the unstressed G. morhua fed the same diet as well as all G. morhua fed HF. Gadus morhua fed a low-fat diet appeared less tolerant to stress than fish fed a high-fat diet.     

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.     

Homing and group cohesion in Atlantic cod Gadus morhua revealed by tagging experiments

Homing behaviour and group cohesion in Atlantic cod Gadus morhua from the northern Gulf of St Lawrence were studied based on tagging-recapture data from two periods, the 1980s and a recent period from 1996 to 2008. Two or more tags from a single tagging experiment were frequently recovered together in subsequent years. The null hypothesis was tested that the frequency of matching tag recoveries occurred by chance only through random mixing of tagged G. morhua before their recapture by the commercial fishery. The alternative hypothesis was that non-random, positive association (group cohesion) existed among tagged individuals that persisted through time and during migrations. Results show that the G. morhua population exhibits a homing behaviour, with temporal stability across seasons and years: 50% of recaptured fish in the recent period were caught <34 km from their mark site, even 3 years after release. In the 1980s, G. morhua were located at <10 km from their release site 1 year after tagging during summer and at <16 km during spring and autumn combined. Despite the increasing distance between the mark and recapture sites over time, the difference was not significant. In addition, occurrences of two or more tagged fish from the same release event that were caught together indicated a non-random association among individual fish for periods of one to several years and through migrations over several hundred kilometres. Hence G. morhua showed group cohesion in addition to site fidelity. These two interacting behaviours may be fundamental for the rebuilding and conservation of depleted fish stocks.     

Digestion rate, gut passage time and absorption efficiency in the Antarctic spiny plunderfish

The absolute gut evacuation rate (GER) (g day⁻¹) of Harpagifer antarcticus increased with increasing ration mass, fish mass only influenced the absolute GER at a daily ration level of 0·3% wet fish mass (approximately a maintenance ration). The relative GER (% of meal fed day⁻¹) was also affected differently by fish and ration mass depending on the relative ration level being fed; at rations of 0·7% wet fish mass or above the relative GER decreased with increasing fish or ration mass (in such a way that the absolute GER remained constant and unaffected by fish mass). At maintenance (0·3% wet fish mass) rations the relative GER was not affected by fish size or ration mass. Thus, there appears to be a ration threshold above which the digestion physiology alters. Mass-specific GER (% g fish⁻¹ day⁻¹) decreased with increasing fish mass. Within a set relative ration level (% wet fish mass) an increase in fish mass decreased the mass-specific GER. At a fixed ration mass, an increase in fish mass (i.e. a reduction in the ration expressed as % fish mass) resulted in a decrease in mass-specific GER. Gut evaluation time (GET) decreased and absorption efficiency (A) increased with increasing absolute GER. The effect of ration and fish mass on the absolute and relative GER followed the same pattern irrespective of the diet, however the A and GER (% day⁻¹ and g day⁻¹) were higher and the GET shorter when the fish were fed shelled krill rather than amphipods.     

Effects of surgically implanted transmitters on swimming performance, food consumption and growth of wild Atlantic salmon parr

Experiments were conducted on wild Atlantic salmon Salmo salar parr to determine the effect of surgically implanted dummy transmitters on swimming performance, food consumption and growth. Swimming performance of tagged fish (tag 1·7–3·7% of fish mass) was similar to that of control fish 1, 5 and 10 days after surgery. Negative effects on growth, however, were found up to day 36 of a 45 day experiment (tag 0·9–2·6% of fish mass). Consumption rates were similar between tagged and control fish and did not explain differences in growth.     

Electronic tags reveal behaviour of captured and discarded fish

Internally implanted data storage tags (DST) recording hydrostatic pressure (depth) and temperature in Atlantic cod Gadus morhua were used to track the behaviour of nine individuals during fishing operations. Gadus morhua showed identifiable and characteristic behavioural responses to different capture processes. In a unique observation, one fish survived being discarded and upon return to the seabed exhibited a cessation of activity comparable to observations on stressed fish under laboratory conditions.     

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.     

Effects of pre- and post-prandial starvation on meal size and evacuation rate of juvenile Atlantic salmon, Salmo salar L.

After a pre-prandial period of starvation or feeding with unlabelled food, 0+ salmon parr (0.8–11.7 g) were fed a test meal of iron particle labelled food and subsequently were again either starved or fed unlabelled food. The quantity of labelled food consumed and the evacuation rate was determined by serial radiographs. In fish of all sizes, pre-prandial starvation causes a larger test meal (as a percentage of body weight) to be consumed when compared to pre-prandially fed fish. In addition, pre-prandial starvation results in relatively larger meals as a percentage of body weight being taken by smaller compared to larger fish. This result was not evident for pre-prandially fed fish. Evacuation rate was unrelated to body size irrespective of feeding history. Post-prandial starvation decreased evacuation rate but this effect was inversely related to the quantity of food consumed. Larger meals were not evacuated differently from smaller meals if feeding occurred post-prandially, irrespective of pre-prandial starvation.     

Social interactions and cortisol treatment increase the production of aggressive electrocommunication signals in male electric fish,Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, continually emit a weakly electric discharge that serves as a communication signal and is sensitive to sex steroids. Males modulate this signal during bouts of aggression by briefly (approximately 15 ms) increasing the discharge frequency in signals termed "chirps." The present study examined the effects of short-term (1-7 days) and long-term (6-35 days) male-male interaction on the continuous electric organ discharge (EOD), chirping behavior, and plasma levels of cortisol and two androgens, 11-ketotestosterone (11KT) and testosterone. Males housed in isolation or in pairs were tested for short-term and long-term changes in their EOD frequency and chirping rate to standardized sinusoidal electrical stimuli. Within 1 week, chirp rate was significantly higher in paired fish than in isolated fish, but EOD frequency was equivalent in these two groups of fish. Plasma cortisol levels were significantly higher in paired fish than in isolated fish, but there was no difference between groups in plasma 11KT levels. Among paired fish, cortisol levels correlated positively with chirp rate. To determine whether elevated cortisol can cause changes in chirping behavior, isolated fish were implanted with cortisol-filled or empty Silastic tubes and tested for short-term and long-term changes in electrocommunication signals and steroid levels. After 2 weeks, fish that received cortisol implants showed higher chirp rates than blank-implanted fish; there were no difference between groups in EOD frequency. Cortisol implants significantly elevated plasma cortisol levels compared to blank implants but had no effect on plasma 11KT levels. These results suggest that male-male interaction increases chirp rate by elevating levels of plasma cortisol, which, in turn, acts to modify neural activity though an 11KT-independent mechanism.     

Hematologic responses in captive white-winged doves (Zenaida asiatica), induced by various radiotransmitter attachments

White blood cell counts, heterophil-lymphocyte ratios, and leukocyte differentials of captive white-winged doves (Zenaida asiatica) from Texas equipped with different radiotransmitter attachment packages were monitored. Doves were segregated by gender and age by males, females, and hatching year; individuals housed in 30 large outdoor pens in groups of seven. Treatments consisted of controls, glue-on transmitters, body loop harnesses, surgically implanted intracoelomic transmitters, surgically implanted subcutaneous transmitters, intracoelomic surgery without implants, and subcutaneous surgery without implants. We used multivariate analysis of variance with pen as a blocking variable and gender nested and repeated measures analysis of variance to identify differences among any of the transmitter attachment techniques and the control for dependent variables. We found no difference in blood parameters between transmitter attachment technique versus a control.     

How are prey fishes of multiple meals evacuated from the stomach of a piscivorous fish?

The observations that the gastric evacuation of a meal was accelerated after ingestion of an additional meal, whereas the evacuation of the latter meal was slowed down were compared to the predictions of mass-dependent models derived from experiments on evacuation of single meals. In contrast, the present study on cod Gadus morhua fed identical double meals of sandeel Ammodytes tobianus demonstrated that the evacuation of each meal was slowed down relative to the evacuation of a single meal. This fact could be explained by the exposed meal surface area being reduced while the meals were together in the stomach as it was expected by a surface-dependent cylinder interpretation of the square-root model. The cylinder model forecasted accurately the observed evacuation of each meal in the double-meal situation. The simple surface considerations of the model further explained the discrepancies between observed and expected evacuation rates using mass-dependent models, which suggested that the above finding of the previous studies was an artefact. The ability of the cylinder model to predict properly also the evacuation of multiple meals in juvenile coho salmon Oncorhynchus kisutch indicated that the model would apply to gastric evacuation in small-sized predatory fishes as well. This should increase considerably its value as a generic tool for studies on the feeding biology of fishes in the wild.     

Time of start of spawning in Atlantic cod (Gadus morhua) females in relation to vitellogenic oocyte diameter, temperature, fish length and condition

Spawning time of female Atlantic cod ( Gadus morhua L.) can be predicted to within 3 days using vitellogenic oocyte diameters measured over the last month before spawning. Simulations show that a l°C drop in temperature during vitellogenesis delays spawning by about 8–10 days. Similar results were obtained with fish on low ration for 4–8 months before spawning. Moderately fed females held over two to four consecutive spawning seasons demonstrated a significant delay (⋍3–40 days) in the date of first spawning as the fish grew older and more fecund. Forecasting studies on spawning migrating Arcto-Norwegian cod gave no evidence of a significant effect of fish length on the time of spawning. Warmer water temperature during vitellogenesis for the larger fish appears to compensate for the negative effect of fecundity On spawning time.     

A radiographic estimation of the effect of temperature on gastric emptying time in Sarotherodon niloticus (L) × S. aureus (Steindachner) hybrids

A radiographic technique was used to determine temperature effects on gastric emptying time in S. niloticus/aureus hybrids. Stomach evacuation times for 30 g fish fed 3% of their body weight were 8.5 h at 30° C, 10.8 h at 25° C and 16.4 h at 20° C. Hence, there was a negative correlation between stomach evacuation time and temperature. The Q ₁₀ between 20 and 30° C was calculated to be 1.92. Similar relationships were found for intestinal evacuation and total evacuation.
The data were used to estimate maximum daily feed intake levels of fish within the size range studied. Levels for 30 g fish were 8.5% body weight at 30° C, 6.6% at 25° C and 4.4% at 20° C. However, the optimum feeding regime should take account of the feeding behaviour of the animals.     

The effects of food type, meal size and body weight on digestion and gastric evacuation in turbot, Scophthalmus maximus L.

Experiments were conducted on a size range of turbot using whole prey organisms. Stomach contents were serially sampled at intervals after feeding on a range of meal sizes of sandeels and sprats. Gastric evacuation was essentially linear with time, though some fluctuations were observed. Control over evacuation appeared to be by regulation of the processes leading to chyme production. Close agreement between evacuation rate and the satiation feeding rate of fish in laboratory experiments was observed. Implications of these findings for techniques of estimating the feeding rates of natural fish populations are discussed.