Diel changes in invertebrate drift and the food of trout Salmo trutta L.

Diel changes in the stomach contents of 0+, 1 + and 2+ or older trout were compared with diel changes in invertebrate drift. Peaks in mean numbers and biomass of invertebrates per fish stomach occurred in the midday (10.00 to 14.00 h) and evening (18.00 to 22.00 h) samples. The major peak was usually in the evening sample, but the major peak in biomass was in the midday sample for 2+ fish. There was a good correlation between diel changes of benthic invertebrates in the diet and drift, with the major peak in the evening. A similar relationship did not always exist for emerging and terrestrial invertebrates. Known non-drifting benthic invertebrates (e.g. large caddis larvae) were excluded from the comparisons, and were only taken by 2+ or older trout in which they contributed a large biomass to the day food.     

Shifts of food habit and jaw position of white-spotted charr after damming

Stomach fullness of white-spotted charr, Salvelinus leucomaenis did not differ significantly between above-dam (allopatric with masu salmon) and below-dam (sympatric with masu salmon) areas, but stomach contents did. Juveniles (1+) below the dam consumed relatively more aquatic insects (benthic prey), and those above the dam consumed relatively more terrestrial insects (drifting prey), but there was no difference in the diet of >2+ fish between areas. The mouths of juveniles above the dam were more terminal than those of fish below the dam. The adaptive significance of these differences is discussed in relation to differences in fish assemblages. 1999 The Fisheries Society of the British Isles     

Piscivory by large eels on young-of-the-year fishes: its potential as a biomanipulation tool

Strong piscivory by eels Anguilla anguilla , which were regularly stocked in the Bautzen Reservoir, Germany, as a biomanipulation tool, was observed, with the number of fishes per stomach containing fish being 2· 2 ± 1·3 (mean ±  s . d .). The 0+ year perch Perca fluviatilis was the most frequent prey item (54%), and 0+ year pikeperch Sander lucioperca was the most important prey in terms of biomass (45%).     

The prevalence of Trypanosoma catostomi in white sucker (Catostomus commersoni) from southern Ontario.

During the summer of 1975, 285 white suckers (Catostomus commersoni) from 10 localities in southern Ontario were examined for trypanosomes. Trypanosoma catostomi Daly and DeGiusti, 1971 was found in the blood of 11.6% of the fish examined using the haematocrit centrifugation technique. Infected juvenile fish (33%) and infected adult fish (4%) were found in 6 of 10 locations.     

Shoaling behaviour of sticklebacks infected with the microsporidian parasite, <Emphasis Type="Italic">Glugea anomala</Emphasis>

We compared the shoaling behaviour of three-spined sticklebacks, Gasterosteus aculeatus, infected with the microsporidian, Glugea anomala, to that of non-infected conspecifics. Infected fish lost significantly more weight than non-infected fish during a period of food deprivation, suggesting a metabolic cost to parasitism. In binary shoal choice tests, non-infected test fish showed an association preference for a shoal of non-infected over a shoal of infected conspecifics; infected test fish displayed no preference. Infected fish, however, showed a higher overall tendency to shoal than non-parasitised fish. Furthermore, infected fish occupied front positions within a mixed school. We consider the behavioural differences between infected and uninfected fish in the context of their potential benefits to the fish hosts and the parasites.     

Evidence for negative impact of plerocercoid infection of Triaenophorus nodulosus on Perca fluviatilis L. stock in Upper Lake Constance, a water body undergoing rapid reoligotrophication

The macro-pathological impact of plerocercoid infection of perch Perca fluviatilis livers was quantitatively determined by an index of liver lesion ( I _LL) in fish from Upper Lake Constance (ULC). These results were compared to the I _LL values of perch from two other lakes. From February to November 1999, 2071 perch were examined from the three sites. Prevalence of infection in ULC was 13% in actively growing first year (0+ year) perch and 94% in older fish, whereas in the two lakes used for comparison the figures for 0+ year perch were 12 and 20%, and in older fish 62 and 60%, respectively. Infected livers from perch caught in ULC revealed significantly higher I _LL values than those of fish from the other two lakes. Perch livers from ULC containing more than three Triaenophorus nodulosus larvae also showed significantly more serious pathological alterations than uninfected livers or those with normal infection levels of one to three T. nodulosus . In all three lakes, excluding heavily infected perch from ULC, 0+ year perch livers revealed little or no damage, whereas in livers of older perch pathological alterations were apparent. In addition, analysis of age and total length data showed that perch infected with one to three parasites in ULC grew significantly slower than uninfected ones, and severely infected perch (greater than three parasites) grew significantly slower than those with a normal parasite load. With respect to mass, growth of ULC fish suffering normal infection (49% of all adults) was reduced by 9% and in those with severe infections (45% of all adults) growth was 16% below expected at normal harvesting age.     

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.     

Epizootiology of Eustrongylides ignotus in Florida: distribution,density, and natural infections in intermediate hosts

A total of 63,451 fish, representing 39 species, was collected from 176 foraging sites used by ciconiiform wading birds in peninsular Florida (USA) and examined for larvae of Eustrongylides ignotus. Infected fish were identified from 30 (17%) of the sites, all of which had been altered by human disturbance such as removal of sediment to construct ditches and dikes, improve water flow, or increase storage capacity and had a history of receiving anthropogenic nutrients such as sewage effluent, urban runoff, or agricultural runoff. The mosquitofish (Gambusia holbrooki) and several species of sunfish (Centrarchidae) were the most important intermediate hosts. Infected fish were not collected at any of the unaltered sites. A total of 10,508 oligochaetes (representing 36 species) was identified from 22 sites that had fish infected with E. ignotus and 36 sites where no infected fish were collected. None of the oligochaetes was infected with larvae of E. ignotus. Immature tubificids without hair setae (probably Limnodrilus sp.), Dero digitata, and L. hoffmeisteri were the most abundant oligochaetes at sites where infected fish occurred, making up 78% of the total collected. Compared to unaltered sites, altered sites were characterized by higher mean densities of fish and oligochaetes; surface waters with decreased dissolved oxygen and increased total nitrogen, total phosphorus, and chlorophyll-a; sediments with higher soil oxygen demand and total phosphorus; larger grain sizes; and higher percentage emergent vegetation and grasses.     

野生与养殖黄鳍鲷消化道形态组织结构

采用常规石蜡组织切片的方法对野生和养殖黄鳍鲷(Sparus latus)消化道的形态组织结构进行了比较观察。结果表明,野生和养殖黄鳍鲷的消化道存在一定差异。(1)形态学研究表明,食道粗而短,胃呈V形,分为贲门部、胃体部和幽门部,胃与肠的连接处有4条幽门盲囊,肠道在体腔内迂回两个回折。野生黄鳍鲷牙齿更为坚硬锋利,体腔中脂肪较少,消化道更为粗短。野生和养殖黄鳍鲷的肠道系数分别为0.71±0.03和0.94±0.12。(2)组织学研究表明,食道黏膜上皮由扁平细胞层和杯状细胞层组成,杯状细胞发达。胃黏膜由单层柱状上皮组成,无杯状细胞,贲门部和胃体部胃腺发达。幽门盲囊组织学特征与肠相似,上皮为柱状上皮,其中的杯状细胞少于肠。肠中,前肠杯状细胞最多,中肠次之,后肠最少。直肠杯状细胞多于肠。野生与养殖黄鳍鲷组织学的区别在于,消化道相同部位养殖鱼的杯状细胞多于野生鱼,野生鱼的肌层厚度大于养殖鱼。黄鳍鲷消化道的形态组织结构与其生活环境和食物是相关的。     

Effects of copper, cadmium and chromium cations on the freshwater fish Clarias batrachus L

The data on the effects of cations such as Cu2+, Cd2+ and Cr6+ on the changes in the biochemical parameters in a freshwater fish, Clarias batrachus L., showed an increase of the protein content in the liver, kidney, stomach, intestine, testis and ovary, and a decrease in the muscle after Cu2+ and Cd2+ treatment as compared with control data; but the Cr6+ did not cause any changes of protein concentration in the kidney and testis. The administration of Cu2+ and Cd2+ increased the concentration of free amino acids in all the fish organs, whereas the Cr6+ did not changes this concentration in the muscle. A decrease in dry weight, and an increase in tissue permeability after these treatments were recorded in all the organs studied. In general, the above biochemical parameters of the organs were affected by treatments of the above cations in the following order: Cd greater than Cu greater than Cr over control values of C. batrachus, and their effects were markedly pronounced in the liver and kidney, followed by the intestine, stomach, muscle, testis and ovary in this species.     

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.     

The physiological effects of salmon lice infection on post-smolt of Atlantic salmon

The physiological effects of salmon lice infections on post-smolt of Atlantic salmon were examined by experimentally infecting hatchery reared post-smolts with infective copepodids. Even at high infection intensities, ranging from 30–250 lice per fish, early chalimus stages did not have severe, physiological effects on the fish. There was a sudden increase in fish mortality after the appearance of preadult I stages. Infected fish were then suffering due to lesions and osmoregulatory failure. Plasma chloride level increased significantly and total protein, albumin and haematocrit decreased significantly in infected compared to uninfected fish. All infected fish became moribund before adult lice appeared. Infection intensities above 30 salmon lice larvae per fish thus appear to cause death of Atlantic salmon post-smolt soon after the lice reach their pre-adult stage.     

Marine feeding of anadromous Salmo trutta during winter

Juvenile and adult anadromous trout Salmo trutta utilize the sea for feeding during the winter in the Skagerrak. This finding conflicts with the traditional view that anadromous trout overwinter in fresh water. Adults, just prior to spawning, were captured at sea in October to December, and spent fish were caught at sea from October to April, showing that the fish may leave the stream and move to sea just after spawning and spend the winter there. During mid‐winter (January to February), the feeding probability (chance of finding a fish with food in its stomach) increased markedly with increasing body length, with no similar effect during early and late winter (October to December and March to April). Among individuals with food in their stomach (72·5%), there was no evidence for variation in feeding intensity [stomach fullness = (mass of stomach content)(fish body mass)⁻¹] among early, mid‐, and late winter.     

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.     

Morphological changes during metamorphosis in cod (Gadus morhua L.), with particular reference to the development of the stomach and pyloric caeca

Metamorphosis in cod, Gadus morhua L., was investigated with respect to morphological changes in the alimentary tract (mouth, stomach and pyloric caeca) and external characters (finfold and fin development). Morphometric measurements and histological investigations were made on both larvae and juveniles and all fish lengths are given as standard length. Median unpaired fins started to develop when the fish were 9–10 mm in length. The larval median finfold was resorbed when fish reached a size of 11–15 mm. The vertebrae were formed in fish of 11–20 mm. The supraterminal mouth, seen in larvae <12 mm, changed gradually to a sub-terminal position in juveniles larger than 65 mm. The development of the stomach and pyloric caeca started at 15 mm and the juvenile form was seen in fish of 40 mm. The ability to store and grind food particles in the stomach developed gradually as fish increased in size from 20 to 40 mm. Compared to many other teleost species, cod juveniles are relatively large when the stomach and pyloric caeca become fully developed. These findings may have important implications for both start-feeding of cod larvae and the weaning of juveniles on to artificial diets.     

Feeding by fish visiting inundated subtropical saltmarsh

Australian saltmarshes are inundated less frequently and for shorter periods than most northern hemisphere marshes, and when inundated provide transient fish a diverse prey assemblage. We determined the extent of feeding on saltmarsh by examining stomach contents of a common marsh transient, glassfish (Ambassis jacksoniensis), in the Coombabah estuary in subtropical Queensland. We tested the hypotheses that fish caught after visiting the marsh (after (M)) would have similar quantities of food (stomach fullness index, SFI) but different prey composition (abundance, weight) both to fish collected before (Before) saltmarsh inundation and to fish that had not visited the marsh but were caught after marsh inundation (After (NM)). Sampling was done on multiple nights over 3 months in winter, when the marsh is inundated on spring tides at night only. SFI values of After (M) fish were significantly higher (SFI ≈ 12%) than those of Before and After (NM) fish (SFI ≈ 0-1%). After (M) fish also had very different prey composition, eating more crab zoea (> 100 zoea fish^− 1) than Before fish (10) and After (NM) fish (0). After (M) fish showed a consistent pattern in zoea abundances among sampling nights, in all months, with lower zoea abundances on the first night that the marsh was inundated than on subsequent nights. This is attributed to the synchronized spawning of crabs resident on the marsh, releasing their zoea on the ebb of the second inundating tide of the month. Fish stomach contents did not differ before and after smaller high tides not inundating the marsh (SFI ≈ 0-1%). Experimental evidence showed that glassfish evacuate their stomach contents in about an hour under starvation conditions, further strengthening our contention that the stomach contents of After (M) fish represent prey ingested on the marsh. The demonstration of intensive feeding by fish visiting this marsh points to a potentially important role of saltmarsh in the trophodynamics of subtropical Australian estuaries.     

Gastrointestinal responses of rainbow trout to dry pellet and low-fat herring diets

Long-term voluntary-feeding experiments were carried out on farmed, 2-year-old rainbow trout offered a commercial dry feed, or chopped low-fat Baltic herring. Despite large differences in dietary water, protein and lipid content, the trout adjusted their intake to consume similar amounts of dry matter. After an 18-week trial, the stomach volumes of the herring-fed trout were significantly larger (30–35%) than those fed on the dry diet. Greatest differences were observed when fish were fed one meal per day; increasing the number of daily feeding opportunities reduced these expected stomach volumes on each diet by 15–20%. The relative increase in stomach volume was shown to be due to growth of the cardiac stomach region (corpus) rather than the pyloric region, and not to muscle relaxation; the change was completed within 10 weeks. Data were collected in a separate study to investigate stomach size in fish (age 0+, 1+, 2+) of similar genetic backgrounds which had been grown using dry pelleted diets. Despite considerable variation between populations, stomach volume to body weight relationship was allometric ( S = a W ^b) with the exponent in the range of 0.3–0.4.     

The intestinal parasite Pomphorhynchus laevis (Acanthocephala) interferes with the uptake and accumulation of lead (210Pb) in its fish host chub (Leuciscus cephalus)

Uninfected chub as well as fish experimentally infected with the acanthocephalan parasite Pomphorhynchus laevis were exposed to ²¹⁰Pb²⁺ for up to 38 days and the uptake and distribution of lead within different fish organs and the parasites was determined at various time points. Highest metal concentrations were detected in the acanthocephalans, followed by intestine, bile, liver, gill and muscle of the fish host. Infected chub had significantly lower ²¹⁰Pb levels in the gills on day 17 (P≤0.01), in the bile on day 24 (P≤0.05) and in the gills as well as in the intestine on day 38 compared with uninfected fish. A subsequent polynomial regression revealed that lead levels for the infected fish ranged below the levels determined for uninfected fish during most of the exposure period. This is the first proof that P. laevis reduces lead levels in the bile thereby diminishing or even impeding the hepatic intestinal cycling of lead, which may reduce the amount of metals available for the fish organs. This is especially important for ecotoxicological research. For example, organisms used as accumulation indicators may erroneously indicate low levels of pollution if they are infected with parasites which alter their pollutant uptake mechanisms. Additionally, the results gave further experimental evidence for acanthocephalans as accumulation indicators for metals.     

Digestibility of chitin in cod,Gadus morhua,in vivo

Sixteen cod,Gadus morhua (L.), were individually fed a single ration of shrimps,Crangon allmanni. Four fish were killed and examined 6, 12, 24 and 48 h after the fish had been fed. Chitinase activities were measured in the extracts of stomach contents, stomach tissue, pyloric caecae, intestinal contents and intestinal tissue. The level of enzyme activity in different parts of the digestive tract was shown to be dependent on the phase of the digestive process. High concentrations of the chitin degradation product N-acetyl-D-glucosamine were determined in the stomach and in the intestinal contents. Based on the chitin concentration in the food organisms and the individual food uptake, the amount of chitin consumed by each fish could be calculated. Only up to 9% of the ingested chitin was recovered from the intestinal contents of the fish at any given time after feeding (6, 12, 24 and 48 h). In addition, only 2.4% of the chitin consumed with the food could be recovered in the collected faeces of the fish. The 4 cod killed 48 h after feeding had completely emptied their stomach. Chitin digestion in these fish was calculated to have been 90%.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.