The Sea Trout Enigma

August 2012, Alistair Maltby, Director North, The Rivers Trust

964coloured-seatroutSea trout are perhaps the fish most important to the majority of people who have a strong interest in the River Lune. It is therefore frustrating that these fish that drive so much passion for the river fall completely through the conservation net. The native trout is considered amongst the most abundant in Europe and therefore not worthy of the protection and funding afforded to rarer species of fish, despite their popularity with the people. This leaves the conservation of trout very much in our hands as custodians of the river. This article hopes to update you on the current state of knowledge for this enigmatic fish, and suggests what we need to do to improve its population so that it can capture the imagination of future generations, in the same way that it has captured many of you.

The first thing we need to appreciate in the ecology of sea trout is that sea trout and brown trout are the same species. This has been known for quite some time now, but the suspicion that sea trout are different has led to speculation that it must be a sub species or have some other distinction from resident brown trout. In fact, it has emerged that in most rivers sea trout are a female component of the trout population. Only where environmental conditions in the river are not favourable do a high proportion of male trout appear to migrate to sea (steep, rocky and nutrient poor rivers). The evolutionary advantages of this are obvious. A proportion of your population are always at sea guarding against any catastrophe that might hit the river, fish feeding at sea get larger and produce more eggs, and are able to distribute more extensively and lay them in a wider range of stone sizes, and the inevitable wandering that occurs enables populations to mix. Female trout that don’t go to sea are the other part of the safety net, not breeding every year, and putting a much smaller proportion of their body weight into eggs. They still continue a base level of reproduction that ensures survival if separated from the sea by calamity or change.

One of those recurring questions that I have had over my career has been, ‘what makes a trout go to sea?’, and honestly, for every reason given, there is a river somewhere with an exception. I have come to look at it in a different way – what makes a sea trout not go to sea? The reasons for this are much easier to understand, and help to explain many of the exceptions that added confusion to the debate. The difference in these two outlooks may be subtle, but I think are important. All the rivers of Northern England were covered in ice during the ice age, and the fish populations that returned were all migrants. This makes any trout a potential sea trout, with a proportion staying perhaps because of an abundance of habitat or food, or migrating only as far as needed to find these things, perhaps in a large lake or lower reaches of a large lowland river. This also explains how moving trout around can dramatically alter their behaviour, with many examples of brown trout turning to sea trout when moved from one river or lake to another, one of the most famous of which being the sea trout of Tierre del Fuego, in Argentina, which were stocked from a supposed brown trout population.

What does the current knowledge about sea trout tell us about how to increase their abundance? Firstly, if you want to look after sea trout, you need to look after brown trout. They are the majority of male spawners, and their numbers are probably significant in determining the number of trout that go to sea. Remove man-made barriers to migration, even small ones. Fish passage is often determined by the size of fish: large female sea trout may be able to get over, but can the smaller male trout? Many trout populations are depleted above migratory barriers so restoration of connectivity for the whole trout population is important. Very high proportions of sea trout smolts have recently been shown to be lost to predation in the impounded water associated with dams and weirs, with figures of 50% to 80% being consistently quoted FOR EACH BARRIER, particularly in times of lower flows. Fish passes just don’t cut the mustard and should be for last resort situations. There is no win-win from hydropower generation for trout. Avoid stocking. There is every likelihood that this could cause harm to the whole trout population, and do we really want put and take fisheries in our rivers? Restore habitats and connectivity to the very smallest of streams. Evidence shows disproportionate value for juvenile trout of streams smaller than one metre in width, the streams which are more sensitive to pollution, and are more likely to be damaged by land drainage. Invest in learning more about your trout population. Knowledge is key to management. What size of brown trout are spawning, and how does this correlate with size limits of fish taken from the river? What proportion of sea trout are male? Which tributaries are they spawning in? How far are sea trout migrating and how long are they at sea? There are sound ways of collecting all of this information and more, critical to developing a conservation plan for sea trout, many people able and willing to help, but very few rivers that actually do it. Be different, know more about your trout population than anybody else, and people will unite behind your knowledge.