Stick or twist? The brown trout’s migratory dilemma Facultative migration, where some individuals migrate while others from the same population do not, is common across several different species, yet the causes and consequences of this intriguing phenomenon remain poorly understood. Researchers in the ALH project are investigating the factors that influence whether brown trout choose to migrate, as Dr Thomas Reed explains. A type of salmonid fish, brown trout can be found in a variety of different habitats, including the sea, lakes and river systems. While some brown trout in rivers migrate to the sea before returning to freshwater, others choose to stay where they are, a topic of great interest to Dr Thomas Reed. “A complex interaction between genes and environment determines whether a given fish decides to migrate or to remain resident in their natal stream,” he outlines. As Principal Investigator of the ALH project, Dr Reed and his team are investigating these interactions underpinning choice of migratory tactics. “It seems to be closely related to the energy balance of the fish,” he says. “If a fish is not getting enough energy from its environment, it might decide to go to a place with more feeding opportunities. That could be in a lake downstream, or an anadromous trout might go to sea.”
Photograph by Ross Finlay
Waterfall study The project brings together researchers from several different disciplines, including genetics, physiology and ecology, to build a deeper picture of the differences between these migratory and resident fish. One strand of the project’s research centres around comparing trout from above and below different waterfalls across Ireland, which offers a good opportunity for researchers to
If a fish is not
getting enough energy from its environment, it might decide to go to a place with more feeding opportunities. That could be in a lake downstream, or an anadromous trout might go to sea. There are boundless feeding opportunities in the oceans but also more predators. While there are different factors at play here, such as the age of the fish and the nature of the surrounding environment, essentially the decision on whether to migrate comes down to binary choices. “Do I migrate or not? If I do, do I go to the sea or a lake? Those are the major binary choices that we’re interested in, they’re what we call the alternative life history tactics,” says Dr Reed. The decision is typically made months before it actually happens, giving time for the migratory fish to prepare for the marine environment, while the resident fish pursue a different strategy. “In this preparatory phase the fish that are migrating start to invest more energy in attaining a critical body size to ensure better survival prospects at sea, whereas those that stay behind maybe invest more in storing energy as fat to fuel earlier reproduction” explains Dr Reed.
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investigate genetic differences. “Sea trout returning from the sea can’t get above these waterfalls, they’re just too big. But the same species of brown trout also live above these falls, and that’s a purely resident population,” outlines Dr Reed. Researchers are sampling individual fish from above and below these waterfalls and comparing their genomes. “We would expect to see genetic differences across the whole genome,” says Dr Reed. Researchers can identify those parts of the genome where differences are particularly pronounced, however, from which Dr Reed and his colleagues can look to gain deeper insights. “Some parts of the genome show exaggerated differences and might point us towards the genes that are involved in these alternative life histories, of anadromous migration vs residency,” he explains. Another topic on
the project’s agenda is sexual conflict, where the evolutionary interests of males and females diverge. “Males might have different strategies from females to maximise their evolutionary success,” says Dr Reed. “In the case of trout, the benefits of going to sea tend to be higher for females.” The reason for this lies in the abundant feeding opportunities available at sea, which means migratory fish are often much bigger than resident fish when they return to freshwater to spawn. This is very important to their chances of reproductive success, as bigger females produce more and larger eggs, yet body size doesn’t really affect males in the same way. “Males may prefer to become residents, because the advantages of going to sea and getting bigger are not as pronounced,” explains Dr Reed. This sets up the potential for sexual conflict, a topic that Dr Reed and his team plans to investigate further. “It’s an interesting area that is relevant not just to fish but many other species including humans, yet about which we know rather little at a genetic level,” he says.
ALH Alternative life histories: linking genes to phenotypes to demography. Dr Thomas Reed, Senior Lecturer BSc, PhD Distillery Fields, North Mall Campus University College Cork Cork, Ireland T: +353 (0)21 4904661 E: treed@ucc.ie W: http://fisheye.ucc.ie/ W: http://research.ucc. ie/profiles/D026/treed Dr Thomas Reed is Senior Lecturer in Zoology at University College Cork, a position he has held since 2017, while he has also worked in institutions in Europe, the US and Africa. He is interested in how organisms respond - and populations adapt to variable and changing environments.
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