Trout Stomach Research Articles

Incorporating ecological heterogeneity and intraspecific variation differentiates complex interactions between apex predators and stocked fish

Ecological heterogeneity and intraspecific variation can impact energy flow from the base of the food web to top predators. We evaluated the effect of intraspecific variation on estimates of lake trout (Salvelinus namaycush) consumption of stocked kokanee (lacustrine sockeye salmon, Oncorhynchus nerka) in a 390 ha oligotrophic lake with two distinct basins. Lake trout stomach content analyses, stable isotope niche space, and catch rates indicated high intraspecific variation in resource use across habitats and basins. Intraspecific variation and ecological heterogeneity were incorporated into two bioenergetics modeling approaches; one lake-wide model, and one partitioned model that accounted for differences in lake trout diet composition and population size across habitats. The ecologically partitioned model highlighted that lake trout consumption was primarily in the epilimnion of one basin, while the lake-wide, unpartitioned model performed similarly but failed to provide ecological context for where that consumption occurred. Incorporating ecological heterogeneity and intraspecific variation into bioenergetic models can more accurately represent top predators foraging patterns across habitats and inform management actions to mitigate impacts to stocked fish.

Scanning Electron Microscopic Examination of Rainbow Trout Gastrointestinal Mucosa

Aim to study: The aim of this study was to determine the morphological characteristics of the gastrointestinal mucosa of the rainbow trout.
 Material and methods: This study was carried out on 10 rainbow trout's stomach and intestinal tissues using scanning electron microscope. In the study, 2.5% glutaraldehyde fixation and routine scanning electron microscopy procedures were applied.
 Results: The surface architecture of the gastrointestinal tract was examined in detail. It was observed that the mucosal folds in the stomach increased in number towards the pyloric region and were arranged in a configuration that allowed food to be directed to the intestine. The columnar cells of the digestive tract mucosa and the mucosal openings that allow mucus secretion were shown in detail.
 Conclusion: The gastrointestinal tract of trout, a carnivorous species, was studied in detail. The structure of the columnar cells, gastric glands and mucus openings of this system was revealed in three dimensions. The detailed anatomy of the mucosal surface, which is rapidly affected by food variation or pathological changes in fish, was revealed. This study will shed light on studies affecting the morphology of the digestive system.

Short‐term effects of wildfire on high elevation stream‐riparian food webs

Understanding how wildfires affect food web structure and function remains an important challenge, especially at high elevations that historically have burned infrequently. In particular, fires may alter the magnitude of reciprocal cross‐ecosystem subsidies, leading to indirect effects on aquatic and terrestrial consumers. We quantified characteristics of high‐elevation (2500–3000 m) stream‐riparian food webs at 10 locations in the southern Rocky Mountains less than one year following high‐intensity, stand‐replacing wildfires. Using a paired ‘burned−unburned' stream survey design, we assessed benthic periphyton, aquatic macroinvertebrate community structure, trout population characteristics, trout stomach contents, inputs and emergence of insects to and from streams, and abundance of predatory riparian spiders that consume aquatic insects. Benthic macroinvertebrate density, flux of emerging aquatic insects, and riparian spider abundances were lower at burned sites. Fluxes of insect inputs entering the stream did not differ with burn status, despite the loss of riparian vegetation due to fire. Trout were somewhat less abundant, but larger on average at burned sites and did not differ in body condition. These results suggest mortality of smaller trout from fire disturbance and/or recolonization of burned sites by larger individuals. Trout showed subtle changes in diet composition with burn status, but no change in biomass or number of prey consumed. In general, burned sites showed greater variation in community characteristics than unburned sites, which may reflect differences in the timing and magnitude of post‐fire flooding, erosion, and scouring of the stream bed. Taken together, our results suggest that short‐term effects of fire disturbance strongly altered some food web responses, but others appeared relatively resilient, which is notable given the high severity of the wildfires in the study area.

Extracted pepsin of trout waste and ultrasound-promoted method for green recovery of fish collagen

Collagen type I is a prominent protein in animal tissues. This structural protein can be commercially derived from fish processing waste without any health risk. The current study aimed to investigate the efficacy of rainbow trout pepsin and the ultrasound-promoted method on collagen recovery. Pepsin extract was purified from rainbow trout stomach in order to use in collagen extraction from wasted yellowfin skin through ultrasound technique in five durations (0, 5, 10, 15, and 20 min). The 15 min ultrasound treated pepsin soluble collagen indicated the highest recovery yield (23.8%) as well as the highest imino acid content (18.2%). The structure of type I collagen was observed in the SDS-PAGE pattern of collagens and no structural change occurred during the extraction process. The presence of the helical structure in collagen samples was confirmed by UV and FTIR spectra. The ultrasound cavitation advanced the viscosity of collagens and resulted in excellent solubility in acidic environments and lower salt concentrations. Ultrasound-treated samples suggested superior water holding capacity along with emulsifying, and foam attributes. The improved gel strength of collagens was parallel with increasing the time of ultrasound up to 15 min. Free radicals scavenging ability and ferric reducing power of collagens were positively stimulated by increasing the time of the ultrasound up to 15 min. Therefore, this study contributes to the development of green technology and the promotion of by-product utilization for collagen recovery as a potential practical protein in biomaterial, wound dressing, drug delivery, food, and cosmetics products.

Predation By the Nonnative Rainbow Trout, Oncorhynchus Mykiss (Walbaum, 1792), on the Native Biota from Freshwater Environment of the Central Andes (Argentina)

Abstract This study aimed to examine the impact of the nonnative rainbow trout Oncorhynchus mykiss on the native biota that inhabits freshwater environments of the central Andes, Argentina. To assess the predation pressure on native Andean vertebrates and invertebrates, the stomach contents were taken from rainbow trout caught at three Andean sites. Results showed that the major prey items were the native torrent catfish Hatcheria macraei (100% in diet), Elmidae coleopterans (23%), and the native crab Aegla affinis (38%) in the rainbow trout stomachs collected from Cipolletti Dam (North zone), Yaucha River (Middle zone), and Vaina Stream (South zone), respectively. This preliminary study exhibits the predation pressure of the rainbow trout on native Andean species and suggests that policies are necessary to control the release of fry of this exotic salmonid in the Andes region.

A morphological, glycohistochemical and ultrastructural study on the stomach of adult Rainbow trout Oncorhynchus mykiss

In this study the morphology, the conventional glycohistochemistry and the ultrastructure of adult rainbow trout (Oncorhynchus mykiss) stomach is reported. For this purpose, stomach belonging to 10 adult female rainbow trout, weighting ~ 500 ± 20 gr and 32 ± 2.50 cm long, were collected and processed. Oncorhynchus mykiss has a siphonal stomach, with a J letter shape, showing a descending portion (cardiac region), an ascendant one (pyloric region), connected through the most represented middle portion (fundic region). Morphologically gastric wall shows an overlay of four typical vertebrate tunicae: mucosa, sub-mucosa, muscularis and serosa. Tunica mucosa, lacking of muscularis mucosae, is raised into longitudinal folds. The tunica propria submucosa is of considerable thickness and receives the bodies of many gastric glands whose lumen is lined with oxyntopeptic cells. Nervous cells, organized in ganglion, that can be compared to Mammals plexus submucosus, are noted. Glycohistochemistry was performed by staining with Periodic acid–Schiff (PAS), Alcian blue (AB) pH 2.5, AB pH 1 and AB-PAS. Histochemical analysis revealed a consistent presence of glycoconjugates produced by epithelial lining and gastric pit cells. Ultrastructural studies, performed at scanning electron microscopy, showed a mucosa that rises in folds with a geometrically well-defined shape and an abundance of gastric pits in the fundic region. Taken together our results integrate the anatomical description of rainbow trout stomach and will be helpful for future studies related to digestive physiology, development of appropriate feeding strategies and gastric disease control of this teleostean species.

Application of Aurantiochytrium sp. L3W for food-processing wastewater treatment in combination with polyunsaturated fatty acids production for fish aquaculture

Thraustochytrids such as Aurantiochytrium are heterotrophic microorganisms that are known to produce valuable polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). In this study, Aurantiochytrium sp. strain L3W was used to remove dissolved organic carbon (DOC) and dissolved nitrogen (DN) from bean-boiling (BB) and miso-processing (MP) wastewater and to simultaneously produce PUFAs. Strain L3W removed 52% of the DOC and 37% of the DN from sterilized BB wastewater and produced biomass that contained 137 mg/g of fatty acids (FAs), including 96.2 mg/g of DHA. Growth of strain L3W in sterilized MP wastewater resulted in the production of biomass containing 147.6 mg/g of FAs, including 97.8 mg/g of DHA, and removal of 47% of the DOC and 55% of the DN from the wastewater. The biomass of strain L3W was digestible by the enzymes extracted from the stomachs of rainbow trout. These results confirmed the potential for use of strain L3W to remove DOC and DN from food processing wastewater and to produce PUFAs. This study also provided the first evidence that the raw biomass of Aurantiochytrium sp. can be used as a fish feed additive.

Growth, Condition, and Trophic Relations of Stocked Trout in Southern Appalachian Mountain Streams

AbstractStream trout fisheries are among the most popular and valuable in the United States, but many are dependent on hatcheries to sustain fishing and harvest. Thus, understanding the ecology of hatchery‐reared trout stocked in natural environments is fundamental to management. We evaluated the growth, condition, and trophic relations of Brook Trout Salvelinus fontinalis, Brown Trout Salmo trutta, and Rainbow Trout Oncorhynchus mykiss that were stocked in southern Appalachian Mountain streams in western North Carolina. Stocked and wild (naturalized) trout were sampled over time (monthly; September 2012–June 2013) to compare condition and diet composition and to evaluate temporal dynamics of trophic position with stable isotope analysis. Relative weights (Wr) of stocked trout were inversely associated with their stream residence time but were consistently higher than those of wild trout. Weight loss of harvested stocked trout was similar among species and sizes, but fish stocked earlier lost more weight. Overall, 40% of 141 stomachs from stocked trout were empty compared to 15% of wild trout stomachs (N = 26). We identified a much higher rate of piscivory in wild trout (18 times that of stocked trout), and wild trout were 4.3 times more likely to consume gastropods relative to stocked trout. Hatchery‐reared trout were isotopically similar to co‐occurring wild fish for both δ13C and δ15N values but were less variable than wild trout. Differences in sulfur isotope ratios (δ34S) between wild and hatchery‐reared trout indicated that the diets of wild fish were enriched in δ34S relative to the diets of hatchery‐reared fish. Although hatchery‐reared trout consumed prey items similar to those of wild fish, differences in consumption or behavior (e.g., reduced feeding) may have resulted in lower condition and negative growth. These findings provide critical insight on the trophic dynamics of stocked trout and may assist in developing and enhancing stream trout fisheries.

Effects of Wintertime Stocking of Rainbow Trout on the Forage Community of an Oklahoma Impoundment

AbstractRainbow Trout Oncorhynchus mykiss are sometimes stocked to create put‐and‐take fisheries in small impoundments. Information about the effects of these stockings on existing piscivores is poorly understood. Therefore, we performed a study to evaluate the forage consumption by Rainbow Trout stocked in Lake Carl Etling, Oklahoma. A creel survey during the 2014–2015 trout season determined that angling pressure was low, which left the majority of stocked trout in the lake until their thermal maximum was exceeded in late May–June. Rainbow Trout stomach contents were collected monthly from November through April annually (2015–2018) via gastric lavage. Gizzard Shad Dorosoma cepedianum were consumed during most months, but their occurrence in Rainbow Trout stomachs was highest in January and December. A sharp decline in Gizzard Shad consumption was followed by an increase in detritus consumption each year. Using either Gizzard Shad numbers observed in monthly Rainbow Trout diets or the estimated monthly abundance of Rainbow Trout, we postulated that Rainbow Trout substantially affected age‐0 Gizzard Shad biomass (reducing it by 66.7 or 54.6 kg/ha, respectively), potentially negatively affecting growth rates of other piscivores in the system. This study demonstrates the unintended consequences of stocking an additional predatory fish into small impoundments and the importance of evaluating angler harvest of these stocking programs to ensure the stocked species receives sufficient angler harvest to match stocking rates and prevent negative impacts on the fish community (both forage and other piscivorous species).

Climatic effects on a cold stenotherm species Lepidurus arcticus (Branchiopoda, Notostraca) on the southern outreach of its distribution range

On the Norwegian mainland, the Arctic tadpole shrimp Lepidurus arcticus is a typically alpine species mainly co-occurring with fish. The Hardangervidda mountain plateau is the main area of the southernmost L. arcticus populations. Here L. arcticus is widely distributed, especially in the central and eastern parts of the plateau. Lepidurus arcticus has a univoltine life cycle emerging from resting eggs at ice break-up and fulfil development during the short mountain summer, before they reproduce and die. The main goal in this study was to investigate the thermal thresholds for L. arcticus. Timing of ice break-up and the proceeding water temperature are hypothesized to constitute the primary drivers of L. arcticus's development and growth. Stomach analyses of brown trout is the most sensitive method for sampling L. arcticus, especially at low densities. Analyses of 4 460 brown trout stomachs revealed that at least 400 degree-days are needed for development and growth from hatching in to adults. It is a significant gradient in climatic conditions on Hardangervidda with a coastal impact in the western part of the plateau with a three - to fourfold winter deposition compared to a dryer inland region in the east. The snow deposits have increased since the 1980s and snow rich winters have been more frequent. An additional increase is projected. This may give lower water temperature and shorter ice-free seasons. Thus, the cold western lakes could be even more hostile habitats for L. arcticus. During the same period, an abrupt increase in spring-summer temperatures was detected. This has improved the conditions for L. arcticus in many lakes. However, as a cold stenotherm species a further increase in temperature may be detrimental. Especially in shallow lakes in the central and eastern parts this may be a problem even without reaching critical levels by inducing life cycle mismatches.

A reassessment of the carnivorous status of salmonids: Hepatic glucokinase is expressed in wild fish in Kerguelen Islands

Salmonids belong to a high trophic level and are thus considered as strictly carnivorous species, metabolically adapted for high catabolism of proteins and low utilisation of dietary carbohydrates. However they conserved a “mammalian-type” nutritional regulation of glucokinase encoding gene and its enzymatic activity by dietary carbohydrates which remains puzzling regarding their dietary regime. The present study investigates the hypothesis that this conservation could be linked to a real consumption by trout of this nutrient in their natural habitat. To do so, brown trout were sampled in the sub-Antarctic Kerguelen Islands, a site presenting oligotrophic hydrosystems and no local freshwater fish fauna prior the introduction of salmonids fifty years ago. Qualitative and quantitative analysis of carbohydrate content within Kerguelen trout stomachs demonstrate that these animals are fed on food resources containing digestible carbohydrates. Additionally, glycaemia and more particularly gck mRNA level and gck enzymatic activity prove that Kerguelen trout digest and metabolise dietary carbohydrates. Physiological and molecular analyses performed in the present study thus strongly evidence for consumption of dietary carbohydrates by wild trout in natural environments. Investigating differences between Kerguelen individuals, we found that smaller individuals presented higher glycaemia, as well as higher carbohydrates contents in stomach. However no relationship between scaled mass index and any physiological indicator was found. Thus it appears that Kerguelen trout do not turn to carbohydrate diet because of a different condition index, or that the consumption of carbohydrates does not lead to a generally degraded physiological status.As a conclusion, our findings may explain the evolutionary conservation of a “mammalian-type” nutritional regulation of gck by dietary carbohydrates in these carnivorous fish.

Mass aggregations of Bibio pomonae (Insecta: Diptera: Bibionidae), an indication of climate change?

Examination of stomach contents of the brown trout is a sensitive method to evaluate the animals occurring in a lake and its surroundings. Yearly test fishing from Lake Sandvatn at the Hardangervidda Mountain plateau in August has shown regular occurrence of Bibio pomonae in the brown trout stomachs every third year since 2001. The stomach fullness and selectivity for other food items is influenced by the local weather conditions, but in calm and sunny weather, mass aggregation of B. pomonae are reflected by high stomach fullness and the brown trout predate B. pomonae selectively. The stomach fullness with B. pomonae may give a significant measure of the mass aggregation phenomena in such periods. On the Hardangervidda Mountain plateau mass aggregation have been observed regularly every third year since 2001. Mass aggregations elsewhere on the southern part of the Scandinavian Peninsula seem to follow the same pattern, but with a one-year delay in the northern part. Our results substantiate that mass aggregations of B. pomonae seem to be an irregular phenomenon in the previous century. In 2016 mass aggregation of B. pomonae was observed in the southern part of the Norwegian mountain range with its maximum abundance in the lower alpine zone. The abundance seems to have extended its maximum from the subalpine zone into the lower alpine zone. Mass aggregations were observed 1678 m a.s.l., 318 m higher up than previously recorded. In 2016 B. pomonae was also recorded earlier than ever before. Climate change may be a driver of the regular mass aggregations, the apparently increase in strength, the elevation of the abundance maximum, the new extreme altitude swarming, and the early hatchings seen after 2000.

Purification and characterization of pepsinogen and pepsin from the stomach of rainbow trout (Oncorhynchus mykiss)

Three pepsinogen isolates (PG-I till III) from the stomach of rainbow trout (Oncorhynchus mykiss) were purified by using ammonium sulfate precipitation, ion exchange chromatography, and two subsequent gel filtrations containing seven pepsinogen isoforms. SDS-PAGE revealed that pepsinogen isolate I contained two isoforms with molecular masses of 45 kDa (PG-I a) and 44 kDa (PG-I b) and the molecular masses of the PG isolates PG-II and PG-III were 42 kDa, respectively. P-I till P-III converted into the corresponding pepsins (P) at pH 2.0 with molecular masses of 37 kDa (P-I a, b, P-III) and 35 kDa (P-II). The isoelectric points were 4.0 (PG-I a, b), 5.9 (PG-II), and pepsinogen isolate III exhibited four isoforms with isoelectric points of 3.73 (PG-III a), 3.78 (PG-III b), 4.0 (PG-III c) and 4.15 (PG-III d). After conversion from pepsinogen, trout pepsins exhibited optimal activity at pH 3.0 and 40 °C (P-I) and pH 2.5 and 30 °C (P-II, P-III). The N-terminal amino acid sequences of the three pepsin isolates (P-I till III) were determined up to 20 amino acids. P-II showed 100 % identity (ID) to pepsin A of Oncorhynchus keta, but P-I and P-III revealed high similarity to chitinases (85 % ID). Catalytic constants KM and kcat for proteolysis of acid-denatured hemoglobin were determined as 2.8 × 10−8 M (P-I), 1.3 × 10−8 M (P-II) and 7.9 × 10−9 M (P-III), and 5.43–19.1 S−1, respectively.

The orexinergic system in rainbow trout Oncorhynchus mykiss and its regulation by dietary lipids.

In this study, we report the distribution of orexin A (OXA), orexin B (OXB), and orexin receptor (OX2R) immunoreactive (ir) cells in the hypothalamus and gastrointestinal tract of Oncorhynchus mykiss fed diets with different dietary fatty acid compositions. Trout were fed five iso-energetic experimental diets containing fish oil, or one of four different vegetable oils (olive, sunflower, linseed, and palm oils) as the added dietary lipid source for 12 weeks. OXA, OXB, and OX2R immunoreactive neurons and nervous fibers were identified in the lateral and ventro-medial hypothalamus. OXA, OXB, and OX2R ir cells were found in the mucosa and glands of the stomach and in the mucosa of both the pyloric cecae and intestine. OX2R ir cells were localized in the mucosa layer of both the pyloric cecae and intestine. These immunohistochemical (IHC) results were confirmed via Western blotting. Antibodies against preproorexin (PPO) crossreacted with a band of ∼16 kDa in the hypothalamus, stomach, pyloric cecae, and intestine. Antibodies against OX2R crossreacted with a band of ∼38 kDa in the hypothalamus, pyloric cecae, and intestine. The presence and distribution of OXA, OXB, and OX2R ir cells in the hypothalamus and gastrointestinal tract did not appear to be affected by dietary oils. The presence of orexin system immunoreactive cells in the stomach, pyloric cecae, and intestine of rainbow trout, but not in the enteric nervous system, could suggest a possible role of these peptides as signaling of gastric emptying or endocrine modulation, implying a main local action played by orexins.

Experimental evaluation of rainbow trout Oncorhynchus mykiss predation on longnose dace Rhinichthys cataractae

AbstractLaboratory and in‐stream enclosure experiments were used to determine whether rainbow trout Oncorhynchus mykiss influence survival of longnose dace Rhinichthys cataractae. In the laboratory, adult rainbow trout preyed on longnose dace in 42% of trials and juvenile rainbow trout did not prey on longnose dace during the first 6 h after rainbow trout introduction. Survival of longnose dace did not differ in the presence of adult rainbow trout previously exposed to active prey and those not previously exposed to active prey ( = 0.28, P = 0.60). In field enclosures, the number of longnose dace decreased at a faster rate in the presence of rainbow trout relative to controls within the first 72 h, but did not differ between moderate and high densities of rainbow trout (F2,258.9 = 3.73, P = 0.03). Additionally, longnose dace were found in 7% of rainbow trout stomachs after 72 h in enclosures. Rainbow trout acclimated to the stream for longer periods had a greater initial influence on the number of longnose dace remaining in enclosures relative to those acclimated for shorter periods regardless of rainbow trout density treatment (F4,148.5 = 2.50, P = 0.04). More research is needed to determine how predation rates will change in natural environments, under differing amounts of habitat and food resources and in the context of whole assemblages. However, if rainbow trout are introduced into the habitat of longnose dace, some predation on longnose dace is expected, even when rainbow trout have no previous experience with active prey.

Can translocated native fishes retain their trophic niche when confronted with a resident invasive?

AbstractDiet interactions between native and non‐native fishes may influence the establishment of native species within their historical range (i.e., reintroduction). Therefore, we illustrated the food web structure of and followed the transition of the federally endangered humpback chub Gila cypha into a novel food web following translocation and determined the potential for a non‐native species, rainbow trout Oncorhynchus mykiss, to influence translocation success. Humpback chub and rainbow trout used resources high in the food web and assimilated similar proportions of native fishes, suggesting non‐native rainbow trout may occupy an ecological role similar to humpback chub. Subsequently, humpback chub may be well suited to colonise tributaries because of their ability to consume resources high in the food web. Additionally, diet partitioning may occur between all members of the fish community as indicated by separation in trophic niche space and little trophic overlap; although all species, particularly bluehead sucker Catostomus discobolus, used a broad range of food resources. Rainbow trout stomach content analysis corroborated stable isotope analysis and suggested rainbow trout diet consisted of aquatic and terrestrial macroinvertebrates, while larger rainbow trout (>120 mm total length) consumed a greater proportion of fish (incidence of piscivory = 5.3%). Trophic interactions may reveal an underutilized niche space or biotic resistance to the establishment of translocated native fishes. Continued translocation of humpback chub into tributaries appears to be one option for conservation. However, successful establishment of humpback chub may depend on continued removal of non‐native trout, increasing availability of diet sources at higher trophic levels.

A Single Rainbow Trout Cobalamin-binding Protein Stands in for Three Human Binders

Cobalamin uptake and transport in mammals are mediated by three cobalamin-binding proteins: haptocorrin, intrinsic factor, and transcobalamin. The nature of cobalamin-binding proteins in lower vertebrates remains to be elucidated. The aim of this study was to characterize the cobalamin-binding proteins of the rainbow trout (Oncorhynchus mykiss) and to compare their properties with those of the three human cobalamin-binding proteins. High cobalamin-binding capacity was found in trout stomach (210 pmol/g), roe (400 pmol/g), roe fluid (390 nmol/liter), and plasma (2500 nmol/liter). In all cases, it appeared to be the same protein based on analysis of partial sequences and immunological responses. The trout cobalamin-binding protein was purified from roe fluid, sequenced, and further characterized. Like haptocorrin, the trout cobalamin-binding protein was stable at low pH and had a high binding affinity for the cobalamin analog cobinamide. Like haptocorrin and transcobalamin, the trout cobalamin-binding protein was present in plasma and recognized ligands with altered nucleotide moiety. Like intrinsic factors, the trout cobalamin-binding protein was present in the stomach and resisted degradation by trypsin and chymotrypsin. It also resembled intrinsic factor in the composition of conserved residues in the primary cobalamin-binding site in the C terminus. The trout cobalamin-binding protein was glycosylated and displayed spectral properties comparable with those of haptocorrin and intrinsic factor. In conclusion, only one soluble cobalamin-binding protein was identified in the rainbow trout, a protein that structurally behaves like an intermediate between the three human cobalamin-binding proteins.

Immunohistochemical and immunological detection of ghrelin and leptin in rainbow trout Oncorhynchus mykiss and murray cod Maccullochella peelii peelii as affected by different dietary fatty acids

In this study, we report ghrelin and leptin immunoreactive (ir) cells distribution in the gastrointestinal tract and blood ghrelin and leptin levels in rainbow trout (Oncorhynchus mykiss) and Murray cod (Maccullochella peelii peelii) fed diets with different fatty acid compositions. Juvenile rainbow trout and Murray cod were fed five iso-energetic experimental diets containing fish oil (FO) or one of the following vegetable oils (VO): olive oil (OO), sunflower oil (SO), linseed oil (LO), and palm oil (PO); as the added dietary lipid source. The presence and distribution of both ghrelin and leptin ir cells in the gastrointestinal tract were affected by the inclusion of VO. Ghrelin ir cells were found in the gastric glands of rainbow trout and in the mioenteric plexuses of the stomach of Murray cod fed FO. Ghrelin ir cells were localized in the mucosa of the intestine of rainbow trout and Murray cod fed VO. Leptin ir cells were more abundant in the epithelial lining of the mucosa folds and in the glands of the stomach of rainbow trout fed VO. Leptin immunoreactivity was detected in the gastric mioenteric plexus of Murray cod fed FO. No differences were found both in ghrelin and leptin levels in blood plasma or in the growth rates of rainbow trout and Murray cod fed the different experimental diets. These observations suggest that dietary fatty acids play a role in the peripheral feeding regulation.

Do exotic salmonids feed on native fauna after escaping from aquaculture cages in Tasmania, Australia?

Atlantic salmon, Salmo salar , and rainbow trout, Oncorhynchus mykiss , are farmed in Tasmania, Australia, where fish sometimes escape into the natural environment. If escapees are able to survive and feed on native fauna, it is likely that they will have ecosystem impacts. Stomach content, body condition (muscle lipid content and Fulton’ K), stable isotope, and fatty acid analysis were used to determine if escaped salmonids feed on native fauna. Results indicate that, in general, escaped salmonids do not feed on native fauna. Salmonids loose condition after escaping, and escapee stomachs were mostly empty or contained non-nutritious material or feed pellets. Nevertheless, almost a quarter of rainbow trout stomachs contained native fauna. The majority of escapees had biochemical composition similar to caged animals, indicating that these fish had not switched to feed on local food sources. However, a small fraction of escapees conclusively showed changes in biochemical parameters indicative of a shift to feeding on native fauna. Given the numbers and frequency of escapes, this can have an important impact on native species and on the ecology of Macquarie Harbour.

Nutritional responses in rainbow trout (Oncorhynchus mykiss) fed diets with different physical qualities at stable or variable environmental conditions

Rainbow trout with initial body weight 1144 g were fed two diets with high (feed A) or low (feed B) water stability for 6 weeks. During the last 2 weeks either stable or fluctuating oxygen saturation, salinity and temperature was introduced. High water stability of the feed was associated with harder pellets, less dust formation and less broken pellets compared to feed with low water stability. During the first 4 weeks, feed intake was 23% higher in trout fed diet B than in those fed diet A. Fluctuating environment resulted in a severe drop in feed intake for both dietary groups. At termination of the trial, stomachs of trout fed feed B contained mashed pellets and free water and oil. Stomachs of trout fed feed A contained more intact pellets and little fluid. The apparent digestibility of protein, starch, dry matter and energy was highest in feed A. The apparent digestibility of P and Zn was highest at stable environment, and the absorption of P was highest from feed A. In conclusion, physical quality affected the nutritional value of the feeds, and differences in nutritional value at stable or fluctuating environment appeared to be related to feed intake.