Is this a sea or a ? This was the question that accompanied a photograph on social media recently, alongside a rather large trout. And the answer is... well, it’s complicated, but interesting. Denise Ashton tackles a tricky one

O start with what may not be lake) brown trout. Being brown and Tobvious: we think that sea spotty is a much better camouflage in trout and brown trout are the same the river than standing out, all bright species, trutta. A is and shiny silver. a brown trout that has decided to Once the obvious sliver colour go to sea and in order to do so, it has gone, there are some clues that has been through a process of will tell you it is a sea trout, but they ‘smoltification’. This process means are not absolutely reliable. The most trout change in some amazing ways: obvious is size. One reason that sea for example, they become silvery by trout follow the often-risky strategy producing guanine crystals on their of going to sea is because the food scales, their eyes enlarge and their supply at home is poor, but the internal organs adapt to cope with benefits must outweigh the costs. the moves between fresh and In order to grow big and produce seawater. It is the distinctive silvery lots of eggs to pass on genes to the colour that most people associate SHARMAN PAUL next generation (most sea trout are with sea trout, so a is female), it pays to go to sea to grow a sea trout and a brown trout is… It is the distinctive silvery large. The (often male) resident trout well, a ‘resident’ brownie. Isn’t it? colour that most people stay rather small in comparison. Certainly, a trout that is very However, there are sea trout in silvery and perhaps has a couple of associate with sea trout, chalk and limestone rivers where sea lice attached to it is very likely a so a silver trout is a sea there is plenty of food and in these sea trout (or maybe a salmon – but rivers, there may not be such a big that is another story). Fresh from the trout and a brown trout difference in size. Why would a sea, this is what you would expect is… well, a ‘resident’ trout go to sea, if there is plenty of of an adult sea trout. A very small brownie. Isn’t it? food at home? That is a question silvery trout, perhaps less than ten that still puzzles scientists. It’s clear inches or so, is most likely to be a places, sea trout ‘slob’ back and forth that anadromy (migrating to sea) smolt on its way downstream to the from river to estuary. And sea trout is a function of environment (e.g. sea. Smolts tend to migrate in shoals are great explorers. Vagrants may food availability) and genetics (all downstream at night in April /May have a nose around a few local rivers trout were once sea trout and retain and possibly also in the autumn. before returning to their natal river a propensity to migrate) but a recent Apparently, they swim backwards, to spawn. Some will stray to spawn – (2019) scientific paper by Prof Andy and hate going over weirs which this is how trout arrived in our Ferguson and co-authors suggests is why weirs can be a problem rivers after the last Ice Age and how that it’s an individual decision – a downstream as well as upstream Salmo trutta has become such a layman’s version of that paper will migration (again, another story). successful non-native invader in the appear in WTT’s 2020 journal. So, The story gets a little more southern hemisphere. During these genetics alone is not going to tell you complicated when sea trout return wanderings, and whilst holding up in if your trout is a sea trout or a river to fresh water. This may not be a the river waiting to spawn, sea trout ‘resident’ trout. simple upstream migration from start to lose their silvery colour and A rather more unreliable clue is sea to spawning gravels. In some revert to looking like in-river (or behaviour. Sea trout like to jump out

24 WILD TROUT TRUST NEWS AUTUMN 2019 A silver sea trout in the River Cothi. DAVID MILLER DAVID of the water especially at dusk and at (SIA). Different food webs have very but many trout are wanderers, sea night, they tend to move upstream at distinctive chemical signatures which trout perhaps most famously. ‘River’ night often quite noisily and are more stay in fish scales and this means that trout may well move huge distances likely to be in close proximity to one it is possible to trace where the fish at certain times of year (e.g. running another than wild resident brown has been feeding. Unfortunately, this upstream in the autumn or winter to trout. However, a trout caught on a technique isn’t readily available to spawn); some might nose into dry fly in the middle of the day might anglers! Our own Jonny Grey is an the estuary or an on-line lake for a still be a sea trout, especially if water expert in SIA and is supervising feed. ‘Lake’ trout might migrate big levels and light levels are low. students using this technique to distances within large lakes or into The next clue is not for the establish whether the surprisingly in- or out-flowing rivers, again riverbank and requires a modest large trout in some streams in eastern perhaps to spawn. So, mooching amount of science. A trout scale has England are sea trout or not. around a good bit is in a trout’s blood, growth rings in the same way as a tree One final note. Some trout in a big factor in why the species is so has growth rings. A sea trout scale freshwater seem to be real homies successful and incredibly interesting. should show a period of rapid growth due to the extra food it has found in saltwater. Sometimes it is obvious, sometimes not, and accurate scale reading needs a good level of skill and lots of experience. So finally, the most likely source of ALAN KETTLE-WHITE a definitive answer is proper science For more information on the topics covered in this article, go to the Wild Trout – and one powerful technique which Trust website. Use the ‘search’ box (top right) to find articles, web pages and blogs can be applied non-destructively on topics such a scale reading, genetics, sea trout and stable isotope analysis. See to the fish is stable isotope analysis also our Trout Facts and Sea Trout pages in the ‘About Trout’ section.

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