Trout Feed Research Articles

Trophic relationships of exotic anadromous salmonids in the southern Patagonian Shelf as inferred from stable isotopes

On the basis of their thermal preferences, salmon introduced in South America can distribute over the whole Southern Patagonian Shelf Ecosystem (SPSE). Stable isotope (δ13C andδ15N) data show that the SPSE is dominated by zooplanktivorous species (15 of 37), and its community is distributed over six trophic levels (TL), with salmonids occupying levels 4.2 to 4.9. A dual isotope mixing model was used to estimate diet proportions of adult salmonids (validated with stomach content analyses) and showed that chinook salmon (TL 4.9) and brown trout (TL 4.7) feed largely on sprats (96% and 79% predicted by the model) and were clustered together with intermediate size fish and cephalopod predators (TL 4.6‐5), a diet similar to that in their native ranges (North Pacific and North Atlantic Oceans). Adult anadromous rainbow trout clustered together with zooplanktivorous species (TL 3.2‐4.4), and the model predicted a diet dominated by planktonic crustaceans (55%), very different from the diet dominated by fish and squids characteristic of adult steelhead in the North Pacific Ocean. Diet predictions based on stable isotopes for Chinook salmon were indistinguishable from direct assessments of stomach contents of fish captured by bottom trawlers and overlapped widely with the diet of Magellanic penguins. The lack of reports on captures of anadromous rainbow trout by bottom trawlers suggests that this species has a shallower distribution. Stable isotope data for rainbow trout are very similar to those of rockhopper penguins, which feed on planktonic crustaceans, fish larvae, and juveniles.

The effects of the acetic acid “pain” test on feeding, swimming, and respiratory responses of rainbow trout ( Oncorhynchus mykiss)

The acetic acid test is commonly used in conscious mammals to determine nociceptive thresholds and efficacy of analgesics. The purpose of the present study was to examine the response of rainbow trout that were not anaesthetised during the acetic acid test. Rainbow trout ( n = 31) were trained to eat in response to a light cue and assigned to one of four treatments: (i) control (handled only), (ii) saline subcutaneous (SC) injection in the lip region, (iii) 2% acetic acid (AA) SC injection in the lip region, and (iv) 5% AA SC injection in the lip region. Swimming behaviour and respiratory frequency were monitored prior to, and 5, 30, 60, 90, 120, 240, and 360 min after treatment. Nine of the 16 fish from both acetic acid treated groups lost equilibrium for 1.7 ± 0.6 min before returning to an upright position swimming normally in the current. The respiratory frequency of all fish significantly increased by 69% ( P < 0.001) after treatment; the saline and control group returned to their resting levels after 120 min while the acetic acid groups were 12% higher than resting levels 120 min after treatment until the experiment was terminated at 360 min. Food was presented 15 min after treatment and every fish ate immediately. Compared with two previous studies that used anaesthetised rainbow trout, the acetic acid test in the current study negatively affected the swimming behavioural response for a much shorter duration and did not affect the feeding behavioural response. However, results for respiratory frequency were comparable to those of anaesthetised rainbow trout in the other work.

Seasonal change in the gastric evacuation rate of rainbow trout feeding on natural prey

Seasonal changes in the gastric evacuation rate (R) and gut contents of a wild population of rainbow trout Oncorhynchus mykiss feeding on natural prey at four water temperatures (2, 7, 9 and 12° C) were measured. The R and mass of the gut contents increased with water temperature, and prey items changed seasonally. These results suggest that the R of fish feeding on natural food depends primarily on water temperature, with their consumed prey being a secondary factor.

Alternatives to Freeze‐Dried Krill in the Feed Training Phase of Largemouth Bass

AbstractFry of largemouth bass Micropterus salmoides are typically pond reared on natural foods until they reach approximately 4–6 cm. Fish that are to be raised on pellets are typically feed trained in tanks using freeze‐dried krill (FDK), which is gradually replaced with prepared diet. The cost of FDK (retail &gt;US$40 per kg) can be a significant expense in the feed training phase. An experiment was conducted to determine whether FDK could be eliminated by simple diet amendment. The largemouth bass fingerlings (mean weight ± SD = 2.2 ± 0.3 g) were stocked in 190‐L tanks at 150 fish/tank. Four treatments with four replicates each were compared: (1) fingerlings were initially fed FDK and then were gradually weaned to a commercial pellet (1.5‐mm floating trout feed; control [CTL], current practice); (2) fish were offered only the dry commercial pellet throughout (DRY); (3) commercial pellets were moistened with water before use (MST); and (4) commercial pellets were top‐dressed with menhaden fish oil at 10% of diet by weight initially and at a gradually reduced percentage over time (OIL). After 18 d, there was no significant difference (P &gt; 0.05) in the average weights of fish. However, in terms of percentage of fish that were successfully habituated to the final dry pellets, the DRY treatment resulted in a significantly lower (P &lt; 0.05) success rate (82%) than other treatments. Training success was not significantly different (P &gt; 0.05) among the CTL (98%), MST (93%), and OIL (92%) treatments. The best diet for feed training of largemouth bass will vary depending on the cost and availability of FDK and the economic importance of production maximization.

Winter Feeding Success of Stream Trout under Different Streamflow and Turbidity Conditions

AbstractTo investigate the relationship between turbidity and trout feeding success in natural systems, we sampled the stomach contents of resident rainbow trout Oncorhynchus mykiss and coastal cutthroat trout O. clarkii clarkii under different streamflow and turbidity conditions during winter in two northwestern California streams (total sample size = 161). Feeding success generally did not differ sharply for fish collected under low versus high streamflow and turbidity conditions. Bioenergetics modeling suggested that feeding success on average did not reach maintenance levels for seven of eight sets of samples, but extensive feeding on oligochaetes during one period of elevated streamflow resulted in positive energy balances. Occasional peaks in food intake could greatly influence the energy budgets and growth of trout in mild‐winter systems. Under some natural conditions, trout appear able to achieve relatively high feeding success when turbidity limits visibility.

INTRODUCED TROUT SEVER TROPHIC CONNECTIONS IN WATERSHEDS: CONSEQUENCES FOR A DECLINING AMPHIBIAN

Trophic linkages between terrestrial and aquatic ecosystems are increasingly recognized as important yet poorly known features of food webs. Here we describe research to understand the dynamics of lake food webs in relation to a native riparian amphibian and its interaction with introduced trout. The mountain yellow-legged frog Rana muscosa is endemic to alpine watersheds of the Sierra Nevada Mountains and the Transverse Ranges of California, but it has declined to a small fraction of its historical distribution and abundance. Although remaining frogs and introduced trout feed in different habitats of alpine lakes, our stable-isotope analyses clearly show that the same resource base of benthic invertebrates sustains their growth. During one period, insect emergence from naturally fishless lakes was nearly 20-fold higher compared to adjacent lakes with trout, showing that fish reduce availability of aquatic prey to amphibious and terrestrial consumers. Although trout cannot prey on adult frogs due to gape limitation, foraging post-metamorphic frogs are 10 times more abundant in the absence of trout, suggesting an important role for competition for prey by trout in highly unproductive alpine watersheds. Most Sierran lakes contain fish, and those that do not are usually small isolated ponds; in our study, these two lake types supported the lowest densities of post-metamorphic frogs, and these frogs were less reliant on local, benthic sources of productivity. Since Rana muscosa was formerly the most abundant vertebrate in the Sierra Nevada, the reduction in energy flow from lake benthos to this consumer due to fish introductions may have had negative consequences for its numerous terrestrial predators, many of which have also declined. We suggest that disruptions of trophic connections between aquatic and terrestrial food webs are an important but poorly understood consequence of fish introduction to many thousands of montane lakes and streams worldwide and may contribute to declines of native consumers in riparian habitats.

Guar gum in rainbow trout ( Oncorhynchus mykiss) feed: The influence of quality and dose on stabilisation of faecal solids

In a dose–response study to identify optimal binder inclusion parameters, a control diet and five diets incorporating binder (0.2, 0.3 and 0.4% of a mid-viscosity guar gum (MV) and 0.2%, and 0.3% of a high-viscosity guar gum (HV)) were fed to triplicate groups of rainbow trout at a ration of 1.2% BW d − 1 . Feed conversion, growth, and with one exception, apparent digestibility of macronutrients were not significantly effected by the addition of binder. All binder containing diets improved stability of fish faeces significantly with the most effective (HV 0.3%) leading to increases in viscosity and elastic modulus of up to 266 and 209%, respectively. The high-viscosity guar gum (HV) performed significantly better than the mid-viscosity (MV), and improvements were positively dose- and quality dependent, except in the case of MV 0.4%, which yielded a slightly less marked improvement than MV 0.3%. The possibility of inactivation by microbial fermentation is discussed. Furthermore, compared to the control, suspended particles from binder-stabilized faeces remained significantly larger and therefore removable by mechanical means after exposure to defined hydrodynamic stress. According to the stability data recorded, the best result was achieved by supplementing feed with 0.3% high-viscosity guar gum. This resulted in a reduction in post-microscreening residual load of about 52% for a 100 μm gauze. The improvement correlated strongly ( r 2 = 0.95) with measured faecal stability data, suggesting that final particle size in self-cleaning trout farms is in effect a measure of mechanical stability of faeces in the face of given turbulences. It is suggested that the economic and ecological advances deemed essential in fish farm process management may be achieved through systematic manipulation of the physicochemical properties of faeces by certain NSPs.

Temporal variability in marine feeding of sympatric Arctic charr and sea trout

The marine feeding pattern of anadromous brown trout (sea trout)Salmo truttaand Arctic charrSalvelinus alpinuswas studied during June to August in 1992–1993 and 2000–2004 in a fjord in northern Norway. In general sea trout fed proportionally more on fishes than on crustaceans and insects (81, 1 and 18% by mass, respectively) by comparison with Arctic charr (52, 25 and 22% by mass, respectively). HerringClupea harengusdominated the total fish diet of both species, but the Arctic charr also fed significantly on gadoids and sandlanceAmmodytesspp. While sea trout became virtually all piscivorous at fork lengths (LF) ≥250 mm, the Arctic charr was ≥400 mmLFbefore shifting totally to a fish diet. Despite annual variation in diet and forage ratios, there was a clear shift in diet from 1992–1993 to 2000–2004. Sandlance and different crustaceans constituted most of the diet during the initial period with a shift towards gadoids and especially herring during the latter period. This shift seemed to be associated with a high abundance of herring larvae during the latter sampling period, indicating a preferential selection on herring when present, particularly by sea trout. Furthermore, an index indicated dietary overlap in years with intensive feeding on herring of both species, and usually differences in the trophic ecology during years feeding mostly on other prey species. In combination, it was hypothesized that the two species reflect the type of marine prey present within a fjord system over time, and therefore provide an index of variation in the production and biological diversity of their potential prey within fjords.

Early feeding of carnivorous rainbow trout (Oncorhynchus mykiss) with a hyperglucidic diet during a short period: effect on dietary glucose utilization in juveniles

Based on the concept of nutritional programming in higher vertebrates, we tested whether an acute hyperglucidic stimulus during early life could induce a long-lasting effect on carbohydrate utilization in carnivorous rainbow trout. The trout were fed a hyperglucidic diet (60% dextrin) at two early stages of development: either at first feeding (3 days, stimulus 1) or after yolk absorption (5 days, stimulus 2). Before and after the hyperglucidic stimulus, they received a commercial diet until juvenile stage (>10 g). Fish that did not experience the hyperglucidic stimuli served as controls. The short- and long-term effects of the stimuli were evaluated by measuring the expression of five key genes involved in carbohydrate utilization: alpha-amylase, maltase (digestion), sodium-dependent glucose cotransporter (SGLT1; intestinal glucose transport), and glucokinase and glucose-6-phosphatase, involved in the utilization and production of glucose, respectively. The hyperglucidic diet rapidly increased expressions of maltase, alpha-amylase, and glucokinase in stimulus 1 fish and only of maltase in stimulus 2 fish, probably because of a lower plasticity at this later stage of development. In the final challenge test with juveniles fed a 25% dextrin diet, both digestive enzymes were upregulated in fish that had experienced the hyperglucidic stimulus at first feeding, confirming the possibility of modification of some long-term physiological functions in rainbow trout. In contrast, no persistent molecular adaptations were found for the genes involved in glucose transport or metabolism. In addition, growth and postprandial glycemia were unaffected by the stimuli. In summary, our data show that a short hyperglucidic stimulus during early trout life may permanently influence carbohydrate digestion.

Effects of different diets on growth performance and body composition of Russian sturgeon (Acipenser gueldenstaedtii, Brandt &amp; Ratzenburg, 1833)

Summary The effects of a commercial trout feed (45% crude protein, 12% crude fat) and a carp diet (35% crude protein, 10% crude fat) on growth performance of Russian sturgeon (Acipenser gueldenstaedtii) was studied and expressed as weight gain (WG), specific growth rate (SGR), feed conversion ratio (FCR) and body composition (protein, lipid and dry matter).). 23-months old sturgeons were hand-fed (pelleted diets) twice daily throughout the 330-day experimental period. Fish were grown at ambient temperatures (seasonal range: 6.2-26.3 oC). Initial mean weight in the two groups was 399 ± 12 g and 404 ± 13.1 g, respectively. Mean weight gain in the two groups was calculated to be 95.3% in fish fed the trout pellets (Group 1) and 77.3% in fish fed the carp pellets (Group 2). Mean specific growth rate (SGR) in sturgeons fed the trout and carp diets was found to be 0.19 and 0.16, respectively. Mean feed conversion ratio (FCR) for both test groups was 4.9 and 5.2, respectively. Corresponding crude protein values, were 19.04% and 18.21% in muscle tissue, respectively. The results indicated that growth performance of Russian sturgeons using trout diets seem to be somewhat better than that of the fish fed the carp feed. However, growth performance (WG, SGR and FCR) and body composition were not statistically different in two groups.

Influence of dietary linseed and sunflower oil on sensory characteristics of rainbow trout (Oncorhynchus mykiss)

This study describes the effects of dietary linseed and sunflower oil on the sensory characteristics of rainbow trout (Oncorhynchus mykiss) fillets. Rainbow trout were fed a diet containing either 2.5% or 5% of linseed or sunflower oil or 5% mixture of both oils (2.5% of each). Control group received a commercial feed mixture. A selected and trained panel carried out descriptive sensory tests on fish fillet cross-sections. The addition of vegetable oil into fish feed did not affect firmness, juiciness or intensity of total taste but had a significant impact on colour, intensity of odour and fish oily taste. A higher content of n-3 polyunsaturated fatty acids (PUFA) in linseed oil, in comparison with sunflower oil, is often responsible for the development of an off-flavour. No trout fillet was rejected owing to the off-flavour presence. Both concentrations of linseed oil decreased the intensity of fish oily taste. Therefore, for the partial replacement of fish oil in rainbow trout feed it is recommended the use linseed oil, which is a good source of n-3 PUFA. &amp;nbsp;

Availability of genetically modified soybean meal in rainbow trout Oncorhynchus mykiss diets

The use of genetically modified defatted soybean meal (GM SBM) as rainbow trout feed was investigated, in comparison with non-GM SBM. Both SBMs were included at levels of approximately 15 and 30% in four diets (42% protein). The diets were fed to juvenile fish (48.3 g average weight) for 12 weeks. The nutrient use showed that there was no significant difference in growth and feed performance between GM and non-GM SBM groups at both inclusion levels at the end of 12th week. The cauliflower mosaic virus 35S promoter fragment (220 bp) of the GM SBM was detected in the muscle of fish receiving both levels of GM SBM diet by nested PCR, but the frequency of detection was greater at the higher inclusion level. Additionally, the promoter fragment was not detected by the fifth day after changing the diet to non-GM. Conversely, the promoter fragment was not detected from fish fed with the non-GM SBM diet. The results demonstrated that the availability of protein in GM SBM was similar to that of non-GM SBM, and the promoter fragments found in the muscle of fish were not detectable after changing the diet to non-GM, verifying the availability of the GM SBM in rainbow trout feed.

06/01378 Power strategies for maximum control structure of a wind energy conversion system with a synchronous machine: Bouscayrol, A. et al. Renewable Energy, 2005, 30, (15), 2273–2288

This study examined the current energy balance, energy conversion efficiency, and farm-level efficiency of trout and sea bass production in the Black Sea of Turkey. Using a structured survey, we collected data from nine monoculture trout farms and five polyculture trout and sea bass farms during the 2005–2006 production season. Energy values were calculated using energy equivalents of the inputs and outputs for the farms. Data envelopment analysis was used to calculate farm-level efficiency. The total energy use per cubic meter was 46.57 MJ for trout production and 87.13 MJ for sea bass production. The main energy inputs were feed and diesel oil for both trout and sea bass production. Indirect energy use was dominant in trout production, while direct energy use was more common in sea bass production. The energy input-to-output ratio of trout production was higher than that of sea bass production. Sea bass production was more economically energy efficient compared to trout production. The allocative and economic energy efficiencies for trout and sea bass production were 0.788 and 0.881, respectively. For both types of farms, the most discriminative variable affecting economic efficiency was operator experience. Increasing focus on product distribution costs could decrease total energy costs by approximately 12%.

Comparison of white flakes and toasted soybean meal partly replacing fish meal as protein source in extruded feed for rainbow trout ( Oncorhynchus mykiss)

An experiment was carried out to study the nutritional value of solvent extracted, toasted soybean meal (SBM) and solvent extracted, untoasted soy “white flakes” (WF) partly replacing high-quality fish meal (FM) in extruded diets for rainbow trout. Particularly, we wanted to find out if feed extrusion provides sufficient heat to reduce the soybean trypsin inhibitor activity (TIA) to a tolerable level in diets for trout. Three diets were produced: a control diet with 490 g FM kg − 1 (FM diet), a diet with 290 g FM and 250 g SBM kg − 1 (SBM diet), and a diet with 290 g FM and 250 g WF kg − 1 (WF diet). Each diet was fed to three groups of trout for 63 days. The TIA levels in the SBM and WF diets after extrusion were 0.6 and 2.1 mg (g dry matter) − 1 , respectively. Feed intake was lower ( P < 0.05) in trout fed the SBM and WF diets than in those fed the FM diet, resulting in reduced growth and poorer feed conversion. The diets did not significantly affect whole-body content of dry matter, protein and fat, or carcass yield, total gut weight, or weight of pyloric, mid and distal intestine. Trout fed the WF diet had higher stomach weight than trout fed the FM diet. Liver weight, plasma cholesterol and triacylglycerol were lower in trout fed the SBM and WF diets than in those fed the FM diet. Apparent digestibility of nitrogen was lower for the WF diet than for the FM diet and intermediate for the SBM diet. The SBM and WF diets caused lower digestibility of fat and most amino acids and lower nitrogen retention than the FM diet. Moderate morphological abnormalities were found in the distal intestine of trout fed the SBM and WF diets. Bile acid concentration in chyme was lower in fish fed the SBM and WF diets than in fish fed the FM diet, except for the proximal pyloric region and the posterior distal intestine. Trypsin activity in chyme of the distal intestine was higher and leucine amino peptidase activity in intestinal wall tissue was lower in trout fed the SBM and WF diets than in trout fed the FM diet. In conclusion, both SBM and WF had a negative influence on growth and feed conversion compared with the FM diet. Extrusion did not influence soy-related challenges such as reduced fat digestibility, and morphological and enzymatic changes. However, extrusion was sufficient to lower TIA in the diets to an acceptable level for rainbow trout.

Seasonal marine feeding and body condition of sea trout (Salmo trutta) at its northern distribution

AbstractThe seasonal marine feeding pattern of sea trout was studied from March to December 2001 in two large fjords situated at latitudes 69°N and 66°N in the northern part of Norway. Despite low sea temperatures and high salinity, sea running trout were caught during all sampling occasions in both fjords. The trout had fed extensively on marine crustaceans (shrimps, amphipods, and krill) and polychaetes during early and late winter, and had a stable or increasing condition factor during this period. In summer and autumn, the trout fed predominantly on fish, mainly juvenile herring. Food consumption rates were lowest during late autumn and early winter (October–December) in both fjords, while trout in the southernmost fjord had the highest consumption rates from April to August and trout in the northernmost fjord from May to September. These patterns in both fjords matched the seasonal variations in condition factor and relative lipid content of the fish during the same periods. The marine winter migration of these northern sea trout populations appears to be a feeding migration in which the fish maintain or increase their body condition, representing a previously undocumented alternative to the more common life history strategy of over-wintering and starvation in freshwater at the northern distribution of this species.

Changing of fat content and fatty acid profile of reared pike (Esox lucius) fed two different diets

Fat content and fatty acid profile of two pike yearling groups grown on two different diets were compared. The groups originated from culture on artificial feed. One group (cultured pike fed first pellet then prey-fish (PF)) was fed with natural food, live prey-fish, while the other one (cultured pike fed exclusively pellet (PP)) by trout feed through a 3-month experimental period. Growth of pike was lower with PP than with PF. The trout feed resulted in an increase of the fat content of fillet and the formation of abdominal fat depots. Feeding of natural food decreased the fat content. The proportion of the saturated fatty acids in fillet was higher in the (PF) group. The n-6 fatty acids (arachidonic C20:4n-6 and docosatetraenic C22:4n-6 acids) were lowest in PP-fed pike. Regarding total n-3 fatty acids ratio there was no significant difference between the groups, but the level of α-linolenic (C:18:3n-3) acid showed significant difference among groups.

Growth, survival and fillet composition of paddlefish, Polyodon spathula (Walbaum) fed commercial trout or catfish feeds

Paddlefish are gaining increasing acceptance as an aquaculture species worldwide. Commercial trout feeds, containing high protein and lipid levels, are currently used in intensive culture; however, nutritional requirements of paddlefish are not currently known. A study was conducted examining the effects on growth, survival and fillet composition of juvenile paddlefish when fed commercial feeds differing in protein and lipid levels. Paddlefish larvae were first stocked in 14.0 m3 round tanks and fed trout starter feeds for 43 days until trained to accept a 1.6 mm pellet. Paddlefish juveniles of mean weight (±SE) 20±0.27 g were randomly stocked into six0.02 ha ponds at 12 500 ha−1 and fed floating commercial trout or catfish (lower protein and lipid) feeds, twice daily (08:00 and 15:30 hours) for 92–97 days. At harvest, there were no significant differences in final weight, percent survival, specific growth rate , relative growth and feed conversion ratio between treatments, which averaged 223.6 g, 96.2%, 2.5% day−1, 10.2 and 1.98 respectively. Surface feeding activity index was significantly higher in ponds supplied with catfish feed than in ponds supplied with trout feeds. Relative pellet buoyancy was not a factor in feeding activity. Fulton's condition factor averaged0.238, was not significantly different, and was similar to a reported value for extensively cultured paddlefish (zooplanktivore). There was no significant difference in liver somatic index between treatments, which averaged 1.91%. Percent protein and moisture of fillets averaged 14.9% and 80.9%, respectively, and were not significantly different between treatments. However, lipid content of fillets was significantly higher in paddlefish fed the trout feed (4.45%), compared with paddlefish fed the catfish feed (2.42%). Fillet lipid content for both treatments was higher than reported values for extensively cultured paddlefish. Percent abdominal fat was significantly higher (0.82%) in paddlefish fed the trout feed compared with paddlefish fed the catfish feed (0.52%). Results from this study indicate that paddlefish can be fed a commercial catfish feed labeled to contain 32% protein and 4.5% lipid without adverse effects on growth, survival and fillet composition, lowering production costs.

Effect of structured lipids based on fish oil on the growth and fatty acid composition in rainbow trout ( Oncorhynchus mykiss)

The aim of the study was to investigate whether it was possible a) to increase the relative incorporation of n − 3 very long chain polyunsaturated fatty acids (VLCPUFA) in a low VLCPUFA diet by feeding trout structured triacylglycerols and b) to reduce fat accumulation by feeding trout a diet containing DAG. A feeding experiment where groups of rainbow trout were fed six diets containing different types of oils for 61 days was performed. The lipid fraction of the six diets was as follows: 1) Fish oil and rapeseed oil (FO diet), 2) Specific structured lipid and rapeseed oil (SL diet), 3) Randomised structured lipids and rapeseed oil (RL diet), 4) Medium chain triglyceride and fish oil (MCT diet), 5) Diacylglycerol and fish oil (DAG diet), 6) Fish oil (FO max diet). Five of the diets (1-5) contained mixed oils blended to contain the same amount of EPA and DHA. Three of these diets (2,3 and 4) contained medium chain fatty acids incorporated in TAG to be positioned either intentionally as specific (SL), by chance as randomised (RL) or added as medium chain TAG (MCT). Diet 1 contained fish oil (FO) in order to investigate the effect of MCFA and diet 4 contained diacylglycerol (DAG). Diet 6) was a reference diet containing pure fish oil (FO max). After the feeding period, FO and FO max fed fish were significantly larger than SL fed fish. Digestibility, measured by adding yttrium oxide as inert marker was significantly lowest for the MCT diet. Fish on the RL and MCT diets had significantly higher protein contents than fish fed FO and FO max diets. The total fatty acid compositions of the fillet, liver, carcass and viscera were similar and reflected closely that of the diet, also in the sn-2 position. In conclusion, addition of MCT to the diet increased protein content of the fish. There was no additional effect of incorporating the medium chain fatty acids in specific positions and no weight reducing effect of adding DAG to the diets.

Resource use by salmonids in riverine, lacustrine and marine environments: Evidence from stable isotope analysis

We measured stable isotope ratios (δ13C and δ15N) of invertebrates, Atlantic salmon, Salmo salar, and brook trout, Salvelinus fontinalis, in three distinct freshwater environments (headwater tributary, ultra-oligotrophic lake, and main-stem river) in the Western Brook system, Newfoundland, Canada. Large differences in the stable carbon signatures of invertebrates allowed the identification of organic matter assimilation from each environment by resident parr and migrating smolts. Brook trout captured in the headwater tributary in June had a carbon signature characteristic of the tributary, while those collected in August had enriched 13C (maximum = −15.6‰) and 15N (maximum = 12.8‰) values. These enriched carbon and nitrogen signatures were indicative of foraging at sea. There was a low correlation between δ13C and δ15N (r2 = 0.198) for individual fish that was likely due to the confounding influence of trout feeding in the lake and the lower main-stem of the river, where δ13C of food sources was high but δ15N was low. Smolts emigrating from Western Brook Pond where they had been foraging (based on lacustrine carbon signatures) were significantly larger than those emigrating from a nursery brook and the main river in the same basin, despite having the same median age. These results suggest better growth opportunities in the lake environment. Trout fork length was positively correlated with δ13C and δ15N, demonstrating that larger individuals had been feeding outside the brook. These results support previous studies that found increased growth potential for salmonids in lacustrine and marine environments, and further, indicate possible adaptive advantages for salmonid movement away from natal brooks.

Pelagic marine feeding of Arctic charr and sea trout

In a fjord in northern Norway, both immature and maturing individuals (170–500 mm fork length) of anadromous Arctic charr Salvelinus alpinus and anadromous brown trout (sea trout)Salmo trutta were captured up to 5000 m offshore. Both species had a high feeding intensity and an almost exclusive piscivorous diet, with herring Clupea harengus as the dominant prey, an apparently opportunistic response to a high density of small‐sized herring in the pelagic zone.