The fish populations in inshore waters of the west coast of Scotland. The food and feeding of the whiting (Merlangius merlangus L,.)

The food and feeding of the whiting ( Merlangius merlangus ) of the inshore waters and sea lochs of the west of Scotland were studied from 1969–1973. Fish and Crustacea were the main food items and their occurrence in the stomachs was clearly related to the size of the whiting and also to their seasonal abundance. The diurnal changes in feeding were also investigated and the results indicated that the size of fish was again an important factor which could affect the interpretation of the seasonal data.     

Overwinter feeding in rainbow trout

The contents of the stomachs of 38 rainbow trout stocked in Llyn Alaw, Anglesey, in August 1969 and caught between October 1969 and February 1970 were analysed. The fish were actively feeding on the bottom fauna throughout the winter and 21 of the stomachs were full or distended. The mean volume of the contents of the stomachs was 2–8 times greater than that of the contents of stomachs of similarly sized brown trout caught at the same time.     

The food and feeding habits of the silver pomfret, Pampus argenteus (Euphrasen), in Kuwait waters

The food habits of the silver pomfret, Pampus argenteus (Euphrasen 1788), in Kuwait waters were investigated by examining the stomach contents of 738 specimens collected from May 1996 to April 1997. Their diet consisted of a broad spectrum of food types, but Crustacea were dominant, with copepods and their eggs constituting 39 % other non-copepod Crustacea constituted 16 %. The next major food group was Bacillariophyta (21 %), followed by Mollusca (11 %), fish scales (10 %) and, finally, fish eggs and larvae (3 %). In summer the species fed on a wider variety of food items than in winter. Copepods, other non-copepod crustaceans, and molluscs tended to occur in the stomachs in higher frequencies with an increase in P. argenteus size (up to 18.5–20.4 cm), while the bacillariophytes tended to increase in stomachs at fish sizes between 22.5–24.5 cm. Analysis of monthly variations in stomach fullness indicated that feeding intensity fluctuated throughout the year, with a low during August and September, corresponding to the spawning period.     

Rates of gastric evacuation in piscivorous brown trout, Salmo trutta

SUMMARY. 1. The dry weight of food remaining in the stomachs of piscivorous trout decreased exponentially with time. Gastric evacuation rates increased exponentially with increasing temperature but were unaffected by predator size, meal size or type of fish prey.
2. Mathematical models were developed to estimate both the rate and time for the gastric evacuation of different meal sizes (expressed as dry weight), and were applicable to piscivorous trout of different sizes (length range 10–32 cm) feeding on trout fry or sticklebacks at different temperatures (range 5–18°C).
3. The wet weight of food in the stomachs also decreased exponentially with time, but evacuation rates both increased with temperature and decreased with increasing meal size; the latter relationship occurred because relative rates of water loss from a meal also decreased with increasing meal size. Use of wet or dry weights can therefore lead to different conclusions about the effect of meal size on evacuation rates.
4. When piscivorous trout were fed three consecutive meals of varying size, the models predicted the total dry weight of food left in the stomach, but not the weight remaining for each individual meal. Interactions between meals led to an increase in evacuation rates for meals consumed early in the series and a decrease in evacuation rates for later meals.
5. Evacuation rates for piscivorous trout were compared with those for trout feeding on invertebrates in an earlier study, and were close to those for caddis larvae as prey, higher than those for mealworms and lower than those for a variety of invertebrate prey. Although a great deal is now known about the daily food intake and growth rates of trout feeding on invertebrates, there is little comparable information for piscivorous trout.     

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

The marine feeding pattern of anadromous brown trout (sea trout) Salmo trutta and Arctic charr Salvelinus alpinus was 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). Herring Clupea harengus dominated the total fish diet of both species, but the Arctic charr also fed significantly on gadoids and sandlance Ammodytes spp. While sea trout became virtually all piscivorous at fork lengths ( L _F) ≥250 mm, the Arctic charr was ≥400 mm L _F before 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.     

Scavenging on discarded saury by demersal fishes off Sendai Bay, northern Japan

Pacific saury Cololabis saira were found in 24 out of 1314 stomachs of demersal fishes collected by bottom trawling at 245–393 m depths. The same area was occupied by the stick-held dipnet fishery for saury, in which a landing limitation was enforced to avoid overfishing. The fish species containing saury were: Pacific cod Gadus macrocephalus , walleye pollock Theragra chalcogramma , and oilfish Ruvettus pretiosus . These demersal species would not encounter living saury naturally, which normally are distributed above thermoclines. The ingested saury were smaller than market size which suggests that the saury from fish stomachs were discarded by fishermen because of small sizes to maximize profits under the landing limitation.
The scavenged saury made up 41.5, 24.6 and 77.7% of the diets of large-sized (>30cm)Pacific cod, walleye pollock and oilfish, respectively in terms of DW composition. The extent to which discarded saury contributed to the total diet in the demersal fish assemblage, calculated by considering species composition and diets of bottom fishes, was 21.8% of the total diet. The discarded saury seemed to compensate the less productive feeding environment during autumn for the bottom fishes.     

Feeding habits of megrim, Lepidorhombus whiffiagonis (Walbaum, 1792), from the central Adriatic Sea

The feeding habits of the megrim, Lepidorhombus whiffiagonis , from the central Adriatic Sea were investigated with respect to fish size and seasons. Stomach contents of 510 specimens, 9.0–28.9 cm total length (TL), collected by bottom-trawl from 2004 to 2006 were analyzed. The percentage of 97 empty stomachs varied significantly with season (maximum during the winter, 32.1%; minimum during summer, 7.4%). The food composition suggests that the megrim is a carnivorous fish. Prey items identified in the stomachs belong to four major groups: Teleostei, Decapoda, Mysidacea and Cephalopoda. Teleosts were the most important (%IRI = 82.2), especially in fish larger than 14 cm TL. Mysidacea constituted the main prey for fish less than 14 cm TL. At the species level, the teleost Antonogadus megalokynodon (%IRI = 1.5) and decapod Processa canaliculata (%IRI = 0.9) were the most frequent prey. The mean weight of stomach contents increased significantly for fish larger than 14 cm TL, while the mean number of prey items did not differ significantly among size classes. Diet composition showed little seasonal variation; teleosts were the most important prey in all seasons, especially during spring. The lowest intensity of feeding recorded in winter could be related to the lower sea temperature and to the spawning period.     

Life-history effects of migratory costs in anadromous brown trout

Mean size of sexually mature anadromous brown trout (sea trout) Salmo trutta in south-east Norway increased significantly with migratory distance ( D ) between the feeding area at sea and the spawning area in fresh water, from 32 cm for those spawning close to the river mouth to 43 cm at the spawning grounds 40 km inland. This was largely due to an increased size of the smallest anadromous spawners with increasing D . The raised mean size of the long-distance migrants is paralleled by an increased age at sexual maturity. Body mass at the same length of sea trout decreased with D in fresh water, meaning that the fish moving far inland was slimmer than those spawning near the coast. Gonadal mass of first-time spawning anadromous males declined significantly with D , and the fecundity and the ratio of fecundity over mean mass of the individual eggs adjusted for variation in fish mass, increased with D . There was no clear relationship between the ratio of anadromous to resident fish and D , probably because more variables than D , influence this relationship in the study streams.     

The functional roles of different muscle fibre types identified in the myotomes of marine teleosts: a behavioural, anatomical and histochemical study

Norway pout ( Trisopterus esmarkii ) were collected during a trawling survey of inshore waters of the west coast of Scotland. 0–group fish were first taken by the bottom trawl in late summer and since there was no evidence of local spawning it was inferred that a migration from the open sea had occurred. The distribution and age composition of the stock was related to depth. The young fish migrated from the shallower stations during their first winter but persisted for a second winter at some of the deeper stations.
There was a marked seasonal variation in the quality of the diet and in the intensity of feeding. The most important food organisms were copepods, decapod larvae, mysids and euphausiids. Diurnal variations in the intensity of feeding were minimal in August but the quality of the food in the stomachs was dependent on the time of sampling. The converse was observed in January when feeding was at a maximum between dusk and dawn but the quality of the food remained fairly constant.     

Feeding activity and food habits of the shad, Ethmalosa fimbriata (Bowdich), in the coastal waters of Cape Coast, Ghana

The monthly feeding activity and food habits of populations of Ethmalosa fimbriata occurring in the sea and an estuary in Cape Coast, Ghana district were investigated during a 14-month period. The juvenile fish, whose total length (T.L.) ranged from 5.3 to 14.7 cm, occur in the estuary and appeared to be more active feeders than the marine samples which measured 15.0–30.4 cm. The peak feeding activity in the marine population coincided with the onset of'upwelling' in the sea, which period is characterized by high salinities, decreasing temperatures and high zooplankton production. The species is primarily a plankton feeder. The stomachs of the marine samples contained mainly protozoa, crustacea, molluscan larvae and a considerable amount of detritus while the stomachs of the estuarine fish contained mainly phytoplankton, protozoa, sand grains and organic detritus. These observations have been discussed in relation to other reports on populations occurring along the West African coast.     

Prey orientation in piscivorous brown trout

Piscivorous brown trout Salmo trutta change their feeding behaviour depending on prey species, prey size and number of prey eaten. In trout which had eaten fish recently, most had one fish in their stomach, but up to 16 prey fish were found. Individuals of the small-sized minnow Phoxinus phoxinus were swallowed chiefly tail first, whereas individuals of the larger Arctic charr Salvelinus alpinus were taken both head and tail first. The largest charr were swallowed head first. In stomachs containing more than one fish prey, prey orientation was likely to be mixed. For all three types of prey orientation (i.e. tail first, head first and mixed), significant and positive correlations existed between prey length and predator length. The maximum prey size eaten tail first or in mixed orientation was about 70–85% of the size of prey eaten head first, indicating morphological advantages in eating the prey head first.     

Continuous variation in the pattern of marine v. freshwater foraging in brown trout Salmo trutta L. from Loch Lomond, Scotland

Carbon stable-isotope analysis showed that individual brown trout Salmo trutta in Loch Lomond adopted strategies intermediate to that of freshwater residency or anadromy, suggesting either repeated movement between freshwater and marine environments, or estuarine residency. Carbon stable-isotope (δ¹³C) values from Loch Lomond brown trout muscle tissue ranged from those indicative of assimilation of purely freshwater-derived carbon to those reflecting significant utilization of marine-derived carbon. A single isotope, two-source mixing model indicated that, on average, marine C made a 33% contribution to the muscle tissue C of Loch Lomond brown trout. Nitrogen stable isotope, δ¹⁵N, but not δ¹³C was correlated with fork length suggesting that larger fish were feeding at a higher trophic level but that marine feeding was not indicated by larger body size. These results are discussed with reference to migration patterns in other species.     

Comparison of growth, diet and food consumption of sea-run and lake-dwelling Arctic charr

Sea-run post-smolt Arctic charr Salvelinus alpinus , (15–26 cm) from Storvatn, northern Norway (70°39'48"N) had significantly higher average specific growth rates in two years (1·64 and 1·66) than the corresponding lake-dwelling charr (0·53 and 1·20). The post-smolts displayed fast compensatory growth in the first 2–3 weeks of their sea residency, but then almost stopped growing prior to their return to fresh water. Lake-dwelling charr grew more evenly during the same time period. Thus, the anadromous charr may return to the lake after only 5–6 weeks in the sea, because the potential to maintain a high growth rate in the sea is reduced. The marine diet consisted mainly of the two crustacean plankton species Calanus finmarchicus , and Thysanoëssa , sp. (88%), and less of fish (6%), insects (4%) and benthos (2%). The diet of lake-dwelling charr consisted mainly of insects (58%, mostly chironomid pupae) and zoobenthos (29%), and less of zooplankton (13%) during the same time period. Although post-smolts had the highest growth rates, they had significantly lower food consumption rates and higher frequencies of empty stomachs than the corresponding lake-dwelling fish. Possible explanations for this paradox are discussed in relation to stomach evacuation rates, water temperature, feeding behaviour and the energy content of the food in the two environments.     

Feeding habits of ocellate spot skate,Okamejei kenojei (Müller & Henle, 1841), in coastal waters of Taean,Korea

The feeding habits of Okamejei kenojei were studied using 592 specimens collected in the coastal waters of Taean, Korea from April 2008 to March 2009. O. kenojei is a bottom‐feeding carnivore that consumes mainly shrimp, fishes, and crabs. Its diet also includes small quantities of amphipods, mysids, cephalopods, euphausiids, copepods, isopods, and polychaetes. The total length (TL) of individuals in this study ranged from 8.2 to 49.0 cm. Cluster analysis based on %IRI (index of relative importance) identified three size classes. Group A (< 20 cm TL) ate primarily caridean shrimp and amphipods; group B (20–30 cm TL) ate exclusively shrimp; and group C (> 30 cm TL) ate penaeoidean shrimp, fishes, and crabs. O. kenojei showed ontogenetic changes in feeding habits. Although shrimps were the primary food consumed by all size groups, the proportion of shrimp in the total diet decreased and the consumption of fishes and crabs gradually increased with the body size of O. kenojei. Size of the prey organisms also increased. Smaller individuals fed mainly on small prey, such as amphipods, mysids, and small shrimp, whereas larger individuals preferred larger prey, such as larger shrimp, fishes, and crabs. The size‐related diet breadth and the percentage of empty stomachs were significant; the diet breadth gradually increased with body size, whereas the percentage of empty stomachs decreased. Seasonal changes in the O. kenojei diet were not significant, but shrimp constituted 97.3% of the summer diet by %IRI. Seasonal changes in diet breadth and the percentage of empty stomachs were not significant.     

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.     

Anaemia and salmonid swimming performance: the potential effects of sub‐lethal sea lice infection

The effect of two known rates of repeated blood loss on rainbow trout Oncorhynchus mykiss swimming performance was measured and blood‐feeding rates of sea lice Lepeophtheirus salmonis were calculated to predict the point at which blood ingestion causes anaemia in infected fish. Known quantities of blood were sampled from rainbow trout over a 5 day period followed by critical swimming performance ( U _crit) testing. A predictive equation was developed using masses of blood‐feeding sea lice and host blood loss calculated for increasing levels of sea lice infection. Blood loss of 8% total blood volume caused a decrease in U _crit for rainbow trout. Total blood volume losses of 3·2% reduced erythrocyte stores, but did not affect fish swimming performance. The predictive feeding rate model suggests that 15–25% of the tissue consumed by sea lice is blood. This consumption of blood at higher sub‐lethal infection levels (≥0·5 sea lice g⁻¹) may cause anaemia and a further decrease in swimming performance. Anaemia would compound the osmotic balance problems due to infection and potentially precipitate the morbidity seen at lethal sea lice levels (0·75–1·0 lice g⁻¹).     

Diel variation in feeding rate and prey composition of herring and mackerel in the southern Gulf of St Lawrence

Diel feeding patterns of herring Clupea harengus and mackerel Scomber scombrus in the southern Gulf of St Lawrence were examined based on samples obtained by midwater trawling between 19 and 26 June 2001. Within 3 h time periods, stomach contents tended to be more similar between fish from the same tow than between fish from different tows. Thus, in contrast to previous diet studies, which have used individual fish stomachs as independent observations, tow was used as the experimental unit in statistical analyses in this study. Diel patterns in stomach fullness were identified using generalized additive models. Two peaks in stomach fullness occurred for herring, one in the morning and the other in the evening. Mackerel showed an increase in feeding intensity throughout the day with a peak in mid‐afternoon. The diel changes in stomach contents suggested rapid gastric evacuation rates for both species, especially for herring. The estimate of the instantaneous evacuation rate for herring was twice that for mackerel. Calanus copepods (mainly C. hyperboreus ), fishes (mainly capelin Mallotus villosus ) and euphausiids were the main prey found in the stomachs of both species. Calanus copepods dominated the diet of herring regardless of time period. They also dominated the diet of mackerel during the late afternoon, evening and night while fishes and euphausiids were dominant during the morning and early afternoon. These diel patterns emphasize the need for sampling throughout the day and night in order to estimate ration and diet composition for bioenergetic and ecosystem models.     

The duration of sea and freshwater residence of the sea trout,Salmo trutta,from the Vardnes River in northern Norway

Synopsis The sea trout,Salmo trutta, from the Vardnes River generally stays in freshwater during winter. The results from 11 years of tagging experiments showed that the mean annual duration of the feeding migration in the sea was 68 ± 21 days (yearly means ranging between 54 and 88 days). A pronounced variation was found with size, sex and time of migration of the fish and between the different years. The males stayed for a shorter time in the sea (mean 66 days) than the females (mean 69 days) and they usually descended later and ascended earlier. Those fish that descended first stayed for the longest period in the sea. The mean annual duration of stay in the sea for first and second time migrants to the sea was positively correlated with the mean sea temperature in May (r² > 0.5). A positive correlation was also found between the annual duration of stay in the sea for the sea trout above 33 cm and the mean temperature in the river in July (r² > 0. 6). Prolonged mean yearly sea sojourn was generally correlated with higher sea temperatures. Water temperature-salinity effects are discussed.     

Long‐term changes in the feeding of Pollachius virens on the Scotian Shelf: responses to a dynamic ecosystem

The diet and feeding ecology of pollock Pollachius virens from the Scotian Shelf and Bay of Fundy in the north‐west Atlantic changed over the last few decades, which was associated with decreases in euphausiid abundance. Stomach contents data for 2078 pollock collected during the 1958–1967 period and for 1230 pollock collected during the 1996–2002 period indicated that pollock diet contained fewer euphausiids and feeding activity decreased. During the early period, euphausiids were present in 65% of the pollock stomachs that contained food and only 9% of these stomachs in the recent period. The decrease of euphausiids was not wholly offset by an increase in piscivory, since there was little increase in the frequency of fish prey in the diet or in the fullness index for this prey type. Empty stomachs were significantly more common in the recent period during both the winter and summer. The decreased occurrence of euphausiids in stomach samples coincided with a significantly decreased abundance of this prey, suggesting that the near‐absence of euphausiids in recently collected pollock stomachs reflected prey abundance. Concurrent with changes in diet and feeding intensity, the condition or 'plumpness' of pollock significantly declined from the early to the late sampling periods.     

Impact of live food on survival and growth of hatchery‐reared sea trout (Salmo trutta trutta L.) parr in the wild

Survival rates and growth parameters of hatchery‐reared sea trout (Salmo trutta trutta L.) fry were determined after stocking in the wild. The larvae were hatchery‐reared for 12 weeks in two groups: fry were fed either on live zooplankton and live chironomidae larvae (LFG), or fed a pellet diet (PFG). The survival rate and specific growth rates were higher in the LFG than in the PFG group. Most effective for hatchery‐reared fish intended for stocking was the natural, live feed. The mean number of chironomid larvae found in the stomachs of fish that were initially captured in the wild was significantly higher in the LFG than in the PFG group. The live diet supplied in the rearing period had a positive impact on the foraging skills of the sea trout fry and their survival in the wild after their release on 24 April 2010.