What do sea trout eat at sea? – Few reports are made on this subject and we have tried to piece together a interesting page on the subject, we aim to highlight the diverse array of aquatic lifeforms that sea trout feed on in marine and freshwater habitats. Similar to our Fishmadman pages on: What do salmon eat in the sea?, the text and images will serve as a guide for creating effective fishing flies tailored to the feeding habits of sea trout.
Three forms – one fish
The brown trout, scientifically known as Salmo trutta, is a widely distributed and highly adaptable species that originated in European waters. It has achieved global success due to its ability to thrive in diverse environments. Like many other members of the Salmonidae family, the brown trout exhibits significant variations in its life cycle.
Brown trout populations can be found ranging from 42°N to as far north as 71°N. There are three primary forms of brown trout:
The river-resident brown trout (Salmo trutta morpha fario) completes its entire life cycle within freshwater river habitats. Originated in European waters and boasts a widespread presence across the globe, thriving on most continents. These very brown trout from Europa serve as the distant ancestors of the immense Tierra del Fuego sea trout and the prized brown trout inhabiting the rivers of New Zealand.
The lake-resident brown trout (Salmo trutta morpha lacustris), starts its journey in rivers but migrates to freshwater lakes to mature. Here, a large specimen was caught from Lake Vänern in Sweden, which is famous for its landlocked Atlantic salmon and brown trout strains. In the 1960s, populations of these salmonid species were dwindling. However, due to a committed stocking program initiated since then, the fishery has not only been sustained but has also shown signs of improvement.
The trout pictured is one of the stocked and fin-clipped specimens. All wild fish are released back into the Vänern fishery.
The sea-migratory brown trout, (Salmo trutta morpha trutta) but commonly referred to as sea trout, starts its life as a river-resident trout. However, it eventually migrates downstream to the sea, where it spends a substantial portion of its life cycle.
These distinct forms of brown trout demonstrate the species’ remarkable adaptability and ability to thrive in various aquatic environments. On the following pages, we will be looking into some of the fascinating aspects of the form of brown trout we know as the sea trout.
Three significant Sea Trout Habitats
Sea trout are commonly associated with coastal fishing, and Scandinavian anglers pursue them during their migrations to and from river systems. Typically, anglers consider sea trout local fish and don’t expect them to venture further than 30 – 40 kilometres (18 – 25 miles) from their birth river. While this may be true for some sea trout, others may journey hundreds of kilometres from their ancestral home.
Here, we present three different trout caught in the northwestern part of Zealand, Denmark. The three fish are pinned on a Google Maps image for reference.
Living inside fjords and easturies
The fjords and estuaries can act as nurseries for juvenile trout before they embark on their journey into the open sea. However, some sea trout may opt to remain within the familiar and secure confines of these fjords and estuaries, evolving into sea trout that closely resemble the river brown trout. Some individuals may even retain the distinctive red spots and golden-green hues characteristic of river-dwelling trout. These well-camouflaged predators are adept at thriving in brackish waters’ shallow, weed-rich environments.
Living on the open coast
When sea trout leave the river as parr around April, they are already well-equipped for a life at sea. With their silver camouflage, some swiftly integrate into the coastal environment, capitalizing on available resources. In sustainable conditions, certain sea trout may remain close to the inner shoreline and local coastal reefs, seldom venturing far. They hunt near the shore when temperatures or darkness offer both food and safety from predators. During warmer periods and daylight hours, they may retreat to deeper waters and reefs with flowing currents to seek shelter and sustenance.
Sea trout living at sea
As sea trout transition from their juvenile freshwater habitat to saltwater, some quickly depart from the river and shoreline in search of larger prey, different sea temperatures, or salinity levels. These individuals often reach impressive sizes. In regions abundant with herring schools or sand eels, sea trout undergo accelerated growth, developing into formidable specimens compared to those that linger closer to the coastline or within estuaries.
Migrations along coastlines up to 580 km (360 miles) away from the natal river were observed in the Geo-Tag survey conducted in 2019.
A Danish studdy from 2019 on Migration routes and habitat use of Sea trout (Animal Biotelemetry 7(1):23) Authours: M. L. Kristensen, M. W. Pedersen, U. H. Thygesen, D. Del Villar-Guerra
The study was based on a total of 125 sea trout kelts that were tagged with data-transmitting geo-tags.
The results confirm the expectation that sea trout are more coastally oriented than other salmonids but also suggest that longer migrations occur in the seas surrounding Denmark compared to elsewhere.
Sea Trout: Investigating Their Diet
How did we determine what sea trout was eating at sea?
In the following sections and accompanying images, we present a compilation of observations gathered over the years by both ourselves and fellow anglers regarding the feeding habits of sea trout. These insights are drawn from specimens caught in various regions of Scandinavia and Scotland. Furthermore, in our endeavor to comprehend the marine diet of sea trout, we have referenced several marine biological surveys on the topic: “What do sea trout eat in the sea?” Throughout the text, we offer details and links to these surveys, should you wish to explore the subject further.
Finally, we have incorporated images from a.o: World Register of Marine Species and Dorris to illustrate the prey that sea trout consume in the most informative manner possible.
Both in freshwater and the sea, the brown trout show opportunistic feeding behaviours. The marine diet of sea trout is diverse and influenced by factors such as season, fish size, age and where they live. In the following text, we will explore the various forms of sea trout you could encounter and their feeding preferences.
Herring and sprat: Key food sources for sea trout
Baltic sea trout thrives on protein-rich diett
Baltic Sea trout follows a protein-rich diet, as observed in a study conducted by F. Chrzan at the Sea Fisheries Institute in Gdynia, Poland.
According to the study, sea trout experience significant weight gain during their first year of life, increasing by 24.5 times. In the second year, this growth slows to 3.5 times its original weight; in the third year, it approximately doubles in weight.
Most sea trout examined for stomach content were in their second year of marine life, with an average annual weight increase of 2,470 grams (2.47 kilo). They consumed approximately 14 grams of food per kilogram of body weight daily.
The investigation into the feeding behaviors of salmon and sea trout in the Baltic region spanned from 1959 to 1975, involving the examination of 252 sea trout. The primary food sources identified during this study were sprat and herring.
A European Sprat (Sprattus sprattus) as found in the Baltic. Photo by Hendron, C. Register of Marine Species. Shown here is the Common European Sprat (Sprattus sprattus), which serves as the main food source for Atlantic salmon inhabiting the Baltic Sea. These sprats usually reach a size of 14 to 15 centimeters.
Herring is a staple on the menu for sea trout
The Atlantic herring (Clupea harengus) belongs to the Clupeidae family and stands as one of the most abundant fish species globally. Found on both sides of the Atlantic Ocean, Atlantic herrings gather in vast schools, sometimes comprising several million individuals. Notably, the smaller-sized herring inhabiting the inner regions of the Baltic Sea is recognized as a distinct subspecies known as the “Baltic herring” (Clupea harengus membras).
Photo by Ueberschär, B. Register of Marine Species
Left Atlantic herring (Clupea harengus) as found in most European waters Photo Svensen, R. Register of Marine Species
The herring belonging to the family Clupeidae (herrings, shads, sardines, menhadens), which consists of some 200 species, A large family of fish of great importance to marine life also in the North Atlantic. It feeds on copepods, crustaceans and fish eggs. Prefer relatively shallow waters 1 – 200 meters.
Matching the hatch
The sea trout, much like its cousin the Atlantic Salmon, is an opportunistic feeder. The sight of a herring is often enough to trigger its hunting instincts. However, size and details in movement becomes crucial factors in the choice of artificial bait, a point well understood by anglers trolling for salmon and trout, especially in areas abundant with schools of herring of a specific size. If the bait is small, anglers won’t benefit from upsizing their artificial lures and plugs. Instead, it’s essential to match the hatch for optimal success.
Photo: A summer (June) sea trout caught using a live herring. Anglers using artificial lures are advised to fish early in the morning or late at night when herring are abundant, as sea trout tend to be more cautious during the daytime in the summer months.
Arctic charr and sea trout were seen feeding on small herrings in the high seas
Sand eels: another crucial food source for sea trout
The sand eel is perhaps the one fish most anglers associate with sea trout fishing. Thousands of lures, flies, and plugs have been designed to mimic its appearance.
A famous bait, such as the Rapala Original Floater 11cm shown in the image, is one of those artificial baits sea trout anglers turn to when they need a bait that is true to the real thing.
Image top left: A haul of sand eels from the stomach of a sea trout weighing approximately 3 kilograms (6 lb) All sand eels are very fresh and were likely consumed within 10 – 20 minutes of the sea trout being caught.
Above: Photographed here is Hyperoplus immaculatus, commonly referred to as the sand eel or sand lancer, names used for numerous species of fish. Sand eels predominantly inhabit areas near the seabed floor, especially regions with smooth gravelly sand bottoms. They play a crucial role as a primary food source for various fish, birds, and whales.
Above: Pictured is a typical late summer sea trout weighing around 1 kilogram or 2 pounds. As the spawning season approaches, the sea trout’s appearance undergoes a transformation. Once silver, its colouration has now taken on a golden hue while its skin and fins toughen in preparation for the journey upstream. This small female is gearing up for the upcoming spawning migration in the river system where it once was born.
Sea trout are surface-oriented, greedy fish
The first wake fly I encountered was during my summer holidays in Wales in the early 80s, where seasoned sea trout anglers introduced me to this ingenious creation. I vividly remember an old “sewin” angler (Welsh for sea trout) pulling out a large and weathered wake fly from his fishing bag. It was a simple yet effective design, consisting of half a wine cork attached to a large single hook, with additional hooks protruding from the cork on either side. The angler shared with me that this fly was so potent that it had been banned on numerous rivers in Wales. Variations of these wake flies were crafted in various shades and shapes, tailored to different fishing conditions.
Photo: The rare Hardy No. 2 wake lure is made of wood with small protruding treble hooks. Today most wake lures will be made with closed-cell foam – simple, efficient flies.
Some sea trout eat in the river
While mature Atlantic salmon may occasionally consume food in rivers, the same holds true for mature sea trout entering river systems. Scientific research, such as that conducted by J. M. Elliott in April 2005 for the Journal of Fish Biology, indicates that female sea trout, in particular, demonstrate some level of food intake within rivers.
Left: Depicted is a late summer male sea trout displaying distinct signs of spawning colors. These mature sea trout may begin entering river systems as early as May and continue to do so as late as November. Some individuals will linger outside river mouths for extended periods, while others will transition directly from their life in the open sea to the river system.
Sea Trout: A Study in Greed and Discernment
Sea trout have truly enriched my life as a fly fisher, and I’ve been fortunate to pursue them as often as my heart desired. Growing up near the sea with access to several small rivers within biking distance, I eagerly awaited the thawing of ice on the rivers to cast my line for sea trout. From the earliest days of the season to its very last, spanning all 365 days of the calendar, I indulged in this pursuit. Along the way, I encountered sea trout in various states of voracity, devouring frogs, other trout, and even a bat. One memorable catch was a 43 cm sea trout that had tried to swallow a 25 cm Rock Gunnel. Moreover, I’ve dedicated countless days to casting for discerning autumn sea trout, which occasionally succumbed to a mere #20 offering.
Attached is a photo from the 80s, capturing a much younger version of myself proudly displaying a typical sea trout caught in March.
What do salmon eat in the sea?
Shrimps: a sought-after meal
* Studies into salmon feeding at high seas in the North-East Atlantic by biologists Jacobsen J. A. and Hansen L. P. (2001) show that shrimps accounted for 95% of the food in number but only about 30% by weight.
By weight, 66% of the stomach content was fish, particularly mesopelagic fish. Some larger pelagic fish, such as herring, blue whiting, and mackerel, were also part of the diet.
Fish and crustacean prey accounted for 96% of the weight of all prey taken by salmon, but
the report also mentions a small percentage of birds and bird remains… being part of the salmon stomach content !!
Right: The Frances fly is one of the most successful salmon flies ever made -most likely interpreted by salmon as a shrimp.
Read the interesting report by Mr. Jan Arge Jacobsen and Lars Petter HansenImportant crustaceans from the Jacobsen and Hansen survey
Studies into salmon feeding at high seas in the North-East Atlantic show that shrimps accounted for 95% of the food in number, but only about 30% by weight
Above: Crustaceans of the genus Thermistor libellula one of the crustaceans salmon, had eaten in the sea north of the Faeroe Islands. Along with other crustaceans, this food source was found in high numbers during the survey conducted during late autumn. Photo Mr Claude Nozères, World Register Of Marine Species
Above: Thermisto libellula in comparison to its cousin: Themisto abyssorum. Photo Mr Kwasniewski Slawomir, World Register Of Marine Species
Above: Meganyctiphanes norvegica swimming with a Thermisto libellula (see above) – Both important animals in the zooplankton food chain. Both are 30 – 45 millimetres big. Photos Mr Claude Nozères, World Register Of Marine Species
Above: Sergestes arcticus (top of photo), another type of shrimp. Below in the same photo is a Nothern krill, Meganyctiphanes norvegica. Photo Mr Claude Nozères, World Register Of Marine Species
Above: Paraeuchaeta norvegica. A marine planktonic copepod 6 – 7 millimetres long – The blue sack at the rear is eggs. Photo © ®Fisheries and Oceans Canada, Jean-François St-Pierre, 2011
Above: Crustaceans of the genus Themisto abyssorum – another important food source found during the survey. Photo Mrs Joanna Legeżyńska, World Register Of Marine Species
Above: From top to bottom: Thermisto libellula, Themisto abyssorum and Thermisto compressa. Three crustaceans were found in great numbers in the survey conducted in the North-East Atlantic. Photo Mr A. Kraft, Alfred-Wegener-Institut
Above: Meganyctiphanes norvegica A krill commonly known as the Northern Krill. A crustacean that is a central figure in the zooplankton mass and an important course of food for whales, birds and fish in the North Atlantic. Photo: © & ® Øystein Paulsen
Above: Hymenodora glacialis Brilliant blood-red shrimp growing to 19 – 20-millimetre. Photo Mr Russ Hopcroft, World Register Of Marine Species
Above: Eusirus holmi – A 40 – 50-millimetre big Gammaridea. Photo Mr Russ Hopcroft, World Register Of Marine Species
Important fish and squid from the Jacobsen and Hansen survey
By weight, 66% of the stomach content was fish, particularly mesopelagic fish. Some larger pelagic fish such as herring, blue whiting and mackerel were also part of the diet.
Above: Maurolicus muelleri: Pearlside. A small silver fish with pigment spots. Size: 40 – 80 mm – a species that lives (mesopelagic) from 20 – 400 meters deep. It lives at greater depth during the daytime and rises toward the surface when it gets dark. Photo Mr Jim Ellis, World Register Of Marine Species
Above: lampanyctus crocodilus or Jewel lanternfish. A small fish that feeds on zooplankton found from 45 – 4000 meters depth – fully grown it will stay between 700-1,000 meters during the day and 4000-1,000 meters at night. Photo Mr Costa, F, Discoverlife
Above: Myctophum punctatum. Its common name is spotted lanternfish. Just like Notoscopelus kroeyeri it lives to the depth of 1000 meters during the daytime but may come to the surface during nighttime Photo Mrs Daphne Themelis, World Register Of Marine
Above: Gaidropsarus argentatus. Commonly known as Arctic rockling, it is a genus of lotid fishes. (Here, a juvenile specimen). Found offshore at depths of 150- 2000 meters on soft bottoms. Photo Mr Claude Nozères, World Register Of Marine Species
Left: Scomber scombrus. The Atlantic mackerel is a pelagic schooling species on both sides of the North Atlantic Ocean. The species is also called Boston mackerel or just mackerel. The mackerel is, in its own right, a ferocious predator that can grow to a length of 50 centimetres – but like most fish, they start out being sizable prey to salmon that roam the same waters as the mackerel. The mackerel strain is growing strong these years, and the species is migrating further north into the Arctic Ocean – Some say that they have become a potent adversary to the Atlantic salmon, who will have to fight for the same food as the Atlantic mackerel – New surveys on this hot topic will surely be worth reading.
bove: gonatus fabricii or Boreoatlantic gonate squid or boreoatlantic arm hook squid, is a squid in the family Gonatidae. (Here is a juvenile specimen) It occurs in the northern Atlantic Ocean from Canada to the Barents Sea. Grows to 30 cm in length. This tiny beautiful squid was well represented in the Norwegian team’s late autumn and winter surveys. Photo Mr Claude Nozères, World Register Of Marine Species
Above: Arctozenus risso – A long fish of the baracudinas family, occurring singly or in small schools mainly at 200-1000 meters depth. Grows to 29-centimetre. Photo Mr Henk Heessen, World Register Of Marine Species
Above: Notoscopelus kroeyeri, Another deep sea fish that lives to the depth of 1000 meters during the daytime but may come to the surface during nighttime – Grow to 17 centimetres. It has glowing dots along the body that help disguise the fish’s outline when seen from below. Photo Mr Henrik Carl, Fiskeatlas –
Above: Benthosema glaciale. Commonly known as Glacier lanternfish. Also, a small migrating deep-sea fish that surges to 700 – 1000 meters at day time to return close to the surface at night time Photo Fisheries and Oceans Canada, World Register Of Marine
Above: mallotus villosus capelin: The capelin or caplin is a small fish of the smelt family found in the Atlantic and Arctic oceans – A important food source for the Atlantic salmon. Photo Mr Claude Nozères, World Register Of Marine Species
Above: Micromestistius poutassou. Commonly known as Blue Whiting, belonging to the cod family. Found over the continental slope and shelf to more than 1000 meters, but more common at 300-400 meters. Blue whiting can grow to a length of more than 40 centimetres. Photo Mr Henk Heessen, World Register Of Marine Species
Above: Belone belone. Commonly known as garfish. Yet another ferocious predator that the Atlantic salmon will feed on when they find them in the correct size. The garfish, or sea needle, is a pelagic, oceanodromous needlefish found in brackish and marine waters of the Eastern Atlantic, the Mediterranean Sea and the Baltic Sea. The garfish is a long and slender fish that grows to about 50 to 75 centimetres – They hunt in packs near the surface, Photo Mr Pillon Roberto, World Register Of Marine Species
Above: Gasterosteus aculeatus or three-spined stickleback. Lives in marine, brackish, fresh. Most populations are anadromous (they live in seawater but breed in fresh or salty water). Among the larger sticklebacks, it is usually 5 cm (2 in) long.
What do salmon eat in the sea? - Facts from other surveys on salmon feeding
Science will tell us that adult salmon will prey on whatever organisms are present
86 % of the food was Caplin
A survey from 1952 done at the Faroese Islands showed that pre-grilse had eaten amphipods (thermistor gaudichaudi) – and squid (Brachioteuthis riisei), while another study from 1967 done by Shearer and Balmain off Greenland showed that salmon had been taking mainly 86 % Capelin mallets Villousus
Right: mallotus villosus capelin: The capelin or caplin is a small fish of the smelt family found in the Atlantic and Arctic oceans – An important food source for the Atlantic salmon. Photo Mr Claude Nozères, World Register Of Marine Species
Salmon inhabiting the coastal waters of Greenland predominantly feed on fish.
According to Reddin’s research in 1988, salmon off West Greenland primarily consume Capelin and sand eels (also known as sand lance) in coastal waters. In the Labrador Sea, their diet includes herring, barracuda (Parallels coregonides borealis), amphipods, euphausiids, and squid, which collectively form a significant portion of their food intake.
Left: Pictured is Hyperoplus immaculatus, commonly known as the Sand eel or sand lancer, names used for numerous species of fish. Sand eels prefer habitats near the seabed floor, particularly areas with smooth gravelly sand bottoms. They serve as a vital food source for many fish, birds, and whales.
Salmon in the Grand Banks region predominantly feed on Capelin and sand eels.
According to Templeman (1968), Lear (1972), and Lear and Christensen (1980), as cited by Redding (1988), salmon caught over the Grand Banks were observed feeding on Capelin and sand lance (sand eel). Additionally, it was noted that salmon in the oceanic depths to the east of the Grand Banks were consuming barracuda, black smelt, and amphipods.
Right: Pictured is Hyperoplus immaculatus, commonly known as the Sand eel or sandeel.
Young cod is on the menu in Labrador.
In the coastal waters of Newfoundland, salmon feed mainly on herring, capelin and sand eels, while in Labrador; pteropods, sand eels, young cod and capelin are the primary food (Lear, 1972b.). Reddit (1988) concludes from this wide variety of prey species that adult salmon are opportunistic feeders and prey on whatever organisms are present. Similar conclusions could also be made from examining salmon stomachs from fish caught in the North-east Atlantic.
Above left: Gadus morhua. Common name cod. Here is a trio of tiny cod. Photo Mr Claude Nozères, World Register Of Marine Species
Above right: Gadus morhua (juvenile) – A cod about 10 cm or four in here. Cod is found from the shoreline to the edge of the continental slope. Photo Mr Claude Nozères, World Register Of Marine Species
Caught on sprats, but their primary food sources were small krill and tiny crustaceans.
Research by Strtuhers (1970-71) highlighted sea animals such as amphipods and euphausiids as the main dietary components.
Fish caught on longlines baited with sprats supported these findings. Another survey conducted at the Faroese Islands also utilized longlines and discovered amphipods (parathemisto spp.), euphausiids, myctophidae (Lantern fish), capelin, and Maurolicus muelleri (pearlsides).
Right: Pictured are myctophidae, fish of the Lanternfishes family, typically found offshore at depths ranging from 300 to 1200 meters during the day and between 10 and 100 meters at night. Photo credit: Mr. Henk Heessen. World Register Of Marine Species
Squid served as a significant food source in the deep sea.
A separate survey conducted in 1985, examining 1145 salmon stomachs, was carried out by Hanson and Pethon off the shelf of Helgeland/Trøndelag (Norway) and in the oceanic waters of Andenes (Norway). The results revealed that essential food items varied between locations.
At the shelf of Helgeland/Trøndelag, euphausiids and hyperid amphipods were predominant, whereas in Andenes, myctophid Benthosema glacial and the squid Gonatus fabric, along with euphausiids, were the most frequently found food items in the stomachs of caught salmon.
Scottish salmon have been observed feeding on ragworms.
Some Scottish salmon, caught in drift nets, were found to have consumed polychaete worms (Nereis spp.), amphipods, euphausiids (Meganyctiphanes norvegica), as well as herring, sprats, whiting, and sand eels (Fraser, 1987).
Photo on the right: Polychaete worms, commonly known as Ragworms (Nereis spp.), are inhabitants of soft and muddy bottoms, where they dwell in a network of burrows. They emerge from their hiding places to spawn and are sought after as prey by various marine animals. Photograph Copyright © 2008 Field Studies Council.
How do salmon get to the food out at sea ?
Flowing with the currents
Sea currents, including gyres, play a crucial role in transporting small salmon parr from rivers and fjords to their feeding grounds. These young salmon appear to migrate towards waters with optimal temperatures for growth, adjusting their movements to follow the cyclical patterns of their prey within the vast water column as sea temperatures fluctuate throughout the seasons. (Image source: sciencelearn.org)
Spanning the oceans
Atlantic salmon exhibit extensive oceanic migrations, seeking out areas where oceanographic conditions foster the distribution of zooplankton, a primary food source for smaller fish, shrimps, and crustaceans. Remarkably, tagged salmon have been documented traveling on intercontinental journeys, with individuals tagged in the United Kingdom later being recaptured in North America, while others tagged in North America have been caught in Norway.
Climate changes may alter the life at sea – and stocks of Atlantic salmon
Certain scientists have proposed that the notable shifts observed in sea-surface temperatures across the North Atlantic might have played a role in heightened mortality rates among young salmon, often referred to as post-smolt. It is theorized that these temperature variations could be linked to the pronounced decline witnessed in Atlantic salmon populations over recent decades.
Studies by Cushing (1983) and Dunbar (1981) have demonstrated that climate changes can exert significant impacts on the distribution and abundance of various marine fish species. Alterations in climate can affect surface water temperatures, the intensity of ocean currents, and, by extension, the salinity of seawater due to increased ice melting. Any of these factors have the potential to influence food availability, survival rates, growth patterns, maturation processes, and overall activity levels within marine ecosystems.
