| Fly Fishing Devon | "The Heuristic Trout" | Is it a sea trout or brown trout?

Changing attitudes to sea trout  :   Salmo trutta's  lifestyle choices

It is now generally accepted that sea trout and brown trout are the same species   (Salmo trutta) . Brown trout that migrate to sea, return as sea trout to their river of birth..

But it was not always so. Writing in 1948 - about attitudes held by Devonshire anglers - Jeffery Bluett reported "There were for many years two distinct schools of thought - one claimed that the sea trout was a distinct species, and the other held that it was simply a variant of the brown trout"

For example: "One great evil is the erroneous belief that the salmon and salmon-peal are not identical. Let there be no salmon-peal killed for two years, and the abundance of salmon at the end of that time would carry conviction, we think , to every unpre judiced mind." (Pulman 1854 The Book of the Axe footnote on p 250)

"The valuable fish which is locally denominated the peal, as we have before observed, is undoubtedly the salmon after its first return from the sea to the river in which, a few months before, it was a smoult." (Pulman 1854 The Book of the Axe footnote on p 388)

Bluett expressed his views cautiously. I suspect he realised they might not be welcomed by some of his readers.

For example: "I may say straight away that I have for many years looked upon the sea trout as a brown trout, and I will go so far as to affirm that I consider that a brown trout may become a sea trout ; i.e. A fish hatched as a brown trout, of brown trout parents, may go to sea and return to the river as a sea trout. Whilst many may not be prepared to go as far as this, opinions of a somewhat similar nature have been advanced by those whose opportunity for investigation has placed them in the best position to judge." quotes from Bluett, pages 10-11

Perhaps it's not surprising that - in the first half of the last century - some influential Devonshire Avon anglers viewed sea trout as a piscivorous pest that should be removed to protect brown trout stocks. Sea trout in their spawning colours were labelled as 'cannibals' and removed from the river.

The Wild Trout Trust's Denise Ashton has published a   very useful article  on distinguishing between trout and sea trout. Their silvery appearance makes it easy to identify sea trout shortly after they have entered freshwater. But the silvery colour fades with time in the river, making it surprisingly difficult to distinguish between brown trout and sea trout, particularly in the autumn when they are close to spawning

Why is it important for anglers to conserve sea trout and brown trout?

Because of their larger size, female sea trout provide most of the trout eggs laid in a river. The case for sea trout conservation in South Devon rivers is made in   this article

The data in this histogram comes from a   fisheries survey   carried on the River Avon in South Devon

Nowadays if they are lucky and catch a large trout in this river, where a 12 inch fish is considered noteworthy, then thankfully, our members call for a photographer rather than a priest to administer the last rights. ! But it wasn't always so.

The Avon has a run of sea trout.
  • In 2019 the largest sea trout caught was 20 inches
  • In 2018 a sea trout of 27 inches was caught
  • Both fish were returned.

    But large South Devon sea trout have not always been so lucky

    This illustration, and comment, is from   A Trout Angler's Notebook

    published around 1900

    The author's comments on this - 27 inch, 7 1/2 lb - fish reflect how so-called 'cannibal' (sea?) trout were dealt with last century in ( an unnamed ) South Devon river!

    The word 'cannibal' is a pejorative term - with anthropomorhic connotaions - that can justify the eradication of larger than expected trout.

    'Cannibal' trout in the Devonshire Avon :

    In his   history  of fishing on the river Mr. Jack Notley (1893-1988) recorded that

    The river was restocked every spring with 5" to 6 " trout from the Exebridge hatcheries
    Then ...
    It was decided to restock with smaller fish 3 ins. to 4 ins length.
    This was a mistake as the wild trout in the river turned cannibal.

    Steps were then taken to deal with these 'cannibals'. At an Avon Fishing Association (AFA) committee meeting in July 1947: "The secretary was instruced to write to Mr. C.J. Hobbs, Builder, South Brent asking him to catch cannibal trout, but doing as little damage as possible, and at the end of the season to send in a report on the number of cannibal trout taken.". Mr. Hobbs was asked to continue this activity in 1948.

    In 1954, the committee heard that: "26 Cannibals [trout] were caught in the upper reaches of the Avon, 8 of which were over 1lb. It was unanimously agreed that Mr. Wallis should again arrange for the destruction of Cannibals." There was no news :"concerning destruction of Cannibals in the lower reaches."  (Minutes of AFA committee meeting on 18th November 1954). in 1955, Mr. Wallis "reported that 22 cannibals had been taken out of the upper Avon - the largest weighing 4 lbs 2 ozs. " 

    The November 1962 committee meeting recorded that: "The presence of a large number of cannibals above Avonwick was no doubt the cause of the shortage of small fish in that area."

    "The Avon used to be stocked with little 6 inch trout, I’m talking about 55 years ago [1948] and Sergeant Hawkins, he used to allow me to worm the river between Stabbs cottage and Avonwick station and I used to take out all the big cannibal trout before they stocked it.I had the most fantastic 6 weeks "
    From   transcript of audio recording  made by North Huish and Avonwick History Society (Speaker unknown, Mr. Hobbs?)

    Stabb's cottage is opposite Kerrydown Bridge.

    Sergeant Hawkins was probably a bailiff appointed by the water authority (SWWA)

    The Wild Trout Trust offer up-to-date   advice  on trout stocking.

    As sea trout runs increased on the Avon, they were blamed for the deterioration in brown trout stocks. This accounts for the cavalier approach to removing anything that interfered with brown trout fishing. For example there was a   proposal   from the local Fishery Board to remove salmon because "they are useless, and the young consume food which would be better employed in feeding the brown trout"

  • According to Jack Notley's   history  of the Avon, before 1914 sea trout were practically unknown on the river. Up to 1918 they were conspicuous by their absence, a very few did come into the lower reaches and a few small ones ran up to Brent
  • By 1940, Sea Trout were now running the river in fair numbers, the larger ones up to 7 or 8 lbs coming into the lower reaches the end of April and May followed by the "School" fish later in the season.
  • By 1980, Sea trout were increasing in numbers and the Trout fishing was deteriorating each season, chiefly owing to the influx of the Sea Trout which spawned on the same beds as the trout, but unfortunately after the latter had spawned, and so most of the trout ova was disturbed and floated away, to be devoured by other fish etc.
  • Jack Notley concluded that "The influx of Sea Trout and the abstraction of water from the river at source were the two main reasons for the deterioration of the Trout fishing. "

    I knew, and have great admiration for Jack Notley, particularly for his introduction - a hundred years ago - of Single-Handed Spey Cssting on South Devon rivers. He was years ahead of his time as an innovative   fly fishing instructor.  Yes, we disagreed over his view that "The Avon never was, nor never will be a salmon river." The root causes, and consequences, of that prejudice have still not been resolved.

    I think that, if alive today, Jack would agree that removing 'cannibal' trout from any river that has a run of sea trout would damage the brown trout fishing.

    Distinguishing between large brown trout and sea trout is not straightforward

    The fish in the next picture was caught just below the Glazebrook ( a spawning tributary ) on the Devonshire Avon 16th November 2019. Length estimated at 49.5cm 19.5 inches

    Is it a potential 'cannibal', brown trout or a sea trout?
    Here is a   table  to calculate an estimate of weight from a trout's length

    Can you distinguish between a sea trout and a brown trout ?

    In 2020 The Tweed Foundation posted this invitation on their Facebook Page   because they were aware of how difficult it is for anglers to distinguish brown from sea trout as the season progresses.

    They asked anglers to express an opinion, and a few days later The Tweed Foundation identified each of the 12 pictures as either brown trout or sea trout.

    Here are the 12 pictures

    Clicking the images below reveals a larger picture, in a new tab in most browsers , and the identification of each fish as sea trout or brown trout.

    When do sea trout return to sea after spawning?

    Sea trout do not usually die after spawning. Around 75% of sea trout are repeat spawners.

    Devonshire Avon sea trout hold an interesting record: a 15 lb fish from the Avon (Devon) spawned eight times ... this appears to be the greatest number of repeat spawnings recorded for English ... sea trout." ( Harris & Morgan "Successful Sea Trout Angling", 1996 )

    After a period of residence in freshwater and before returning to sea,  the surviving sea trout kelts regain a silvery sheen. But the return date varies: Some return shortly after spawning, the majority may overwinter in freshwater and migrate to sea in March and April. A few may stay in the river until May. (Menzies,   Sea Trout and Trout.  page 129).

    Overwintering sea trout could be mistaken for brown trout during their period of residency.

    Bluett (page 17) explains the implication thus:"It is my opinion that some of the so-called brown trout, of two pounds and more, which are occasionally taken in May and June, are really sea trout which ran up nearly twelve months previously and which have wintered in what was their nursery."

    Could it be that some so-called 'cannibal' trout in our river had spawned, overwintered in the river, and would have returned to sea sometime between March and April or even as late as May the next year?

    Cannibal trout were removed from the Avon in 1948 over a 6 week period ( perhaps between 15th March and 30th April ) before stocking with 6 inch trout.

    A unusually large   Salmo trutta  for the Devonshire Avon

    On 22nd May 2019 two AFA member were watching 5 big trout feeding at Knapp Mill. One angler managed to catch this one on a Pheasant Tail Nymph at the top of Hedge Pool, it measured 16.25 inches.

    Here is a   table  to calculate an estimate of weight from a trout's length

    Trout lead an interesting and mysterious life. We certainly shouldn't label any of it as cannabilist.

    A Sea Trout's Lifestyle: Decisions, Decisions, Decisions !

    The simplified figures below are based on a diagram of the sea trout's lifecycle in   a scientific paper  authored by Andrew Ferguson et al (2019)

    Professor Ferguson uses a 'threshold trait model' to explain two decision points in the early life of trout before they migrate to become sea trout. For clarity, I have extracted and expanded the 'threshold trait model' element from the larger diagram. And removed elements in the original diagram that relate to potamodromy - migrations that occur entirely in freshwater.

    Andy Ferguson has provided   a user-friendly version   of his scientific paper.

    Decision points in the sea trout's lifecycle

    Decision 1 at Fry stage

    Decision 2 at Parr stage

    Ferguson et al use the 'threshold trait model' to explain the 2 decisions that are made before a trout migrates to sea for the first time.

    1. If the fry's energy (nutritional) status is above the threshold it will remain as a resident brown trout.
      If the fry's energy (nutritional) status is below the threshold it is on track to migrate, and may become a sea trout depending upon the result of Decision 2 taken later at the parr stage
    2. This decision is only made by a parr that is on track to migrate as a result of Decision 1 taken as a fry. If the parr passes the size threshold it will migrate that year.If the parr is smaller than the threshold, it will defer taking Decision 2 until the following year(s).

    Remaining mysteries in Salmo trutta's lifestory 
    To borrow from John Gierach 'Let me introduce some ideas, just some things to kick around.'

    What controls the length of overwintering?

    It might involve adding a third Decision point to Professor Ferguson's threshold trait model:
  • After spawning: If the energy (nutritional) status is above the threshold that trout will remain as a resident.
  • After spawning: When the energy (nutritional) status falls below the threshold that trout will migrate to sea.

  • Why do some sea trout not spawn when they return to freshwater ?

    Based on his experience, Malloch concluded that finnock (young sea trout) do not spawn on their first return to freshwater. He wrote: "My opinion is that they do not spawn. I have seen them at all times and have looked for one with ova, but never found one." He was careful to acknowledge that this was ".. a point on which there is great difference of opinion.." (Malloch, page 132)

    Malloch's opinion is supported by a   report   written by Dr A F Walker that reexamined collections of sea trout scales from surveys conducted from 1928 to 2009.

    Only one in Walker's collection had any scales from finnock that spawned after only a few months at sea: Nall (1928) states that 315 of his fish had one or more spawning marks on their scales and, of these fish, 12.7% had matured as finnock (0+ sea winters), 68.6% at 1+ sea winters and 18.7% at 2+ sea winters.

    Walker commented: So a large majority of the sea trout did not mature until they were in their second year in the sea.

    Menzies commented   from eight to twelve percent ripen and proceed to the spawning-grounds ...

    These findings confirm my opinion that young sea trout (called 'school peal' on the Avon) should be treated as an older type of smolt, and carefully returned to enable them to reach their reproductive potential.

    Here is a   brief description  of how scales are used to construct a trout's life history

    Over the last three years,  the Westcountry Rivers Trust funded by South West Water has added 700 tonnes of granite gravel to a 2.5 mile section of the River Avon below the Avon Dam.

    The importance of good spawning sites

    Jack Notley may have been right when he blamed the decline in Avon trout stocks to the abstraction of water from the river at source.

    The Avon Dam - completed in 1957 - created a 50 acre reservoir to supply water to the South Hams. The dam prevents natural downstream replenishment of spawning gravel.

    Good opportunities for sea trout to spawn was one of the first thngs Menzies examined when looking for reasons for a decline in a sea trout fishery.

    "In the well populated rivers spawning ground is plentiful and of good quality , while in the others it is deficient in various degrees up to almost absence" (Menzies, p 206)

    A SHRIMP project aims to restore salmonid spawning and rearing habitat below the Avon Dam.

    About the author

    Paul guiding ITV News reporter in June 2019

    with sea trout in camera range ...

    Paul Kenyon lives in Ivybridge on the southern edge of Dartmoor about 6 miles from the Upper Yealm Fishery.

    Paul devotes more time than is reasonable to his love of all things associated with fish, fishing, instruction and guiding on Dartmoor rivers.

    He retired in 2006 from the Department of Psychology, University of Plymouth where he lectured in behavioural neuroscience and evolutionary psychology.

    email paul@flyfishingdevon.co.uk


  • to   The Wild Trout Trust's Denise Ashton for bringing science to a wider audience
  • to West Country guide  Dominic Garnett  for bringing the   Biodiversity Heritage Library  to my attention in his blog
  • to The Tweed Foundation for their Brown Trout or Sea Trout Quiz
  • to Lyn and Brian Dent for preserving the oral historyof the Devonshire Avon
  • to members of the   Avon Fishing Association for capturing a photographic record of fish caught on their river
  • to Professor Andy Ferguson and his team for their work on sea trout biology, and artist Robin Ade for his drawings that illustrate their science

  • References

  • Lamond, Henry The sea-trout. A study in natural history
  • J. Bluett, Sea Trout and Occasional Salmon, Cassell, 1948.
  • J M Elliott et al, National Rivers Authority Fisheries Technical Report No.3 Sea Trout Literature Review And Bibliography
  • Gilvear et al (circa 1998) Dispersal Of Augmented Gravel, To Increase Salmonid Spawning Habitat, In A Boulder-bed River Draining Dartmoor
  • Harris & Morgan "Successful Sea Trout Angling: The Practical Guide" Coch-y-Bonddu Books, 1996
  • P.D.Malloch   Life History and Habits of the Salmon Sea Trout Trout and Other Freshwater Fish   Published by Adam and Charles Black (1910)
  • WJM Menzies,   Sea Trout and Trout. Edward Arnold & Co, 1936
  • South Hams River Improvement Projects (SHRIMP) Restoration of the Upper Avon:Gravel Augmentation Monitoring Study
  • Andrew Ferguson et al (2019)   Anadromy, potamodromy and residency in brown trout Salmo trutta: the role of genes and the environment 
  • Westcountry Rivers Trust Gravel bringing salmon back to Devon river
  • Brown Trout Weight Length Conversion
  • Institute of Freshwater Ecology A Guide to the Interpretation of Sea Trout Scales

  • Illustration from Institute of Freshwater Ecology A Guide to the Interpretation of Sea Trout Scales

    This scale is from a trout that spent 3 years in a river, and just over 2 years at sea.

    The "River Zone" on the scale corresponds to the period spent in freshwater before the smolts migrate. The "Sea Zone" corresponds to the period spent at sea after the smolt migration, including any time spent in freshwater as an adult.

    The notation used to record years spent in the River Zone and Sea Zone is discussed at length in the guide. A decimal point is used to separate the years spent in each Zone.

    This scale would be recorded as: 3.2+

    The annotations on this photomicrograph show all the features used to construct the lifestory of a sea trout.

    Scale reading is time consuming and requires experienced staff. The task is an obvious candidate for a computer-based approach using artificial intelligence

    See for example: Automatic interpretation of otoliths using deep learning