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  • Charlotte Pike

To sea or not to sea: Tracking sea trout on the Isle of Skye

Updated: Sep 27, 2022

Salmo trutta, a native species in the UK, has two ecotypes: the familiar brown trout that remains resident in freshwater throughout its life, and the marine migratory sea trout (Figure 1). Sea trout are anadromous which means they migrate to the ocean to feed, then return to freshwater to spawn. Before migrating to the ocean, juvenile trout must undergo ‘smoltification’, a process of physiological change which allows them to survive in the marine environment. Juveniles reside in freshwater for around 1 to 8 years before this process occurs, after which they become silvery in colour, making them easier to distinguish from the resident brown trout.

Fig 1. (a) sea trout vs (b) brown trout

But which environment is best?

Even amongst sea trout, there is large variation between where they spend their time. Some sea trout choose to remain closer to shores, whereas others will migrate larger distances. Some individuals make multiple migrations between fresh and salt water, whereas others remain in salt water for longer periods, ranging from a few months (finnock) to years (multi sea winter fish) before ever returning to freshwater. It is thought that both environmental and genetic factors play a role in this variation, but the specifics that drive these differences are unknown.

A trade-off exists between these two environments, with freshwater providing sufficient shelter from predators whilst being a less productive environment with less food resources. The marine environment is highly productive, providing ample food, however there is a higher chance of predation. These factors all contribute to a trout’s decision about when and how far to migrate, and thereby maximise their chances of survival and reproduction.

Across the UK, and similarly in Skye, S. trutta have undergone population declines. Threats to trout include parasites such as sea lice exacerbated by the salmon aquaculture industry (Figure 2), reduced prey species, and climate change. However, it is difficult to determine how much interaction and therefore impact any or all of these threats are having on specific populations. This is because we know very little about trout movements after leaving freshwater – which is where our study comes in.

Fig 2. Sea trout parasitised by sea lice (parasites indicated by red circles)

Tracking sea trout

Loch Snizort and Loch Greshornish, two sea lochs on the Isle of Skye, are the locations for our study. As part of a collaboration between the Zoological Society of London (ZSL), the Skye and Lochalsh Rivers Trust, and the University of Glasgow, we are conducting a tracking study to provide a better understanding of the key drivers for different patterns of behaviour, movement and survival of sea trout in freshwater and marine environments.

We are using acoustic telemetry to tag and track fish as they move through various environments. This involves using hydrophones, placed in a systematic array within our two study lochs (Figure 3). Trout are caught using fyke and seine nets (Figure 4) and a small transmitter is inserted into the peritoneal cavity; a t-bar anchor tag is also attached close to the dorsal fin to allow easy identification by fishers (Figure 5). Movement metrics gained through the use of acoustics will enable us to determine the migration routes, activity patterns and habitat use of wild sea trout.

Fig 3. (a) Acoustic hydrophones (b) placed in a systematic array throughout Loch Snizort and Loch Greshornish

Fig 4. (a) Fyke net and (b) seine netting methods to catch sea trout

Fig 5. T-bar tag attached to dorsal fin of sea trout

Alongside movement metrics, we also measure the length and weight of the fish, and sample scales and a small amount of tissue. These samples are used to determine nutritional state, for stable isotope analysis, and for population genetic analysis. We also examine the fish for indicators of migratory stage and damage and/or disease, looking for signs of bacterial or fungal infection and taking counts of parasites. Results from measurements, samples and visual examination can be used to determine a variety of factors, such as the age of the fish, physiology, genetic identity, and past life history.

The fish data will be analysed in the context of environmental data, such as water level and velocity, river discharge, currents, temperature, climate predictions and anthropogenic factors (e.g. barriers, fishing pressure, habitat disturbance, aquaculture), all of which will provide evidence to allow us to better understand why sea trout show such large levels of variation in their behaviour and life history strategies, as well as being able to quantify the risks posed to trout populations by both natural and anthropogenic environmental change. Ultimately, helping us to inform future management of these stocks.

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