Where in the sea are sea trout?

As anglers, we often struggle to find fish in a stream, river or lake / loch, and we're generally seeking the bigger fish! Keeping track of the vulnerable juvenile life-stages is even more tricky, and then imagine translocating that problem to the sea.... OK, so with advances in acoustic telemetry, the boffins have a few tricks up their sleeves and are making some headway but the logistics of tracking in such a potentially vast environment are nonetheless challenging. Isabel Moore from the Scottish Centre for Ecology & The Natural Enviornment has risen to that challenge during her PhD and outlines one aspect below.

The brown trout is a remarkably diverse species; it can utilise multiple life-history strategies, ranging from freshwater residency through to migration into marine environments for a period of time before returning to freshwater to reproduce (i.e. anadromous sea trout). Unfortunately, this iconic species has been faced with significant population declines in recent decades across the UK and other parts of the world. A significant portion of the anadromous population decline is thought to occur in marine environments. However, the sheer areal extent of habitats utilised by sea trout makes the monitoring of their movements very difficult, leaving many unanswered questions about the types of challenges that sea trout face and how those challenges might affect the their survival rates. Both environmental (i.e. predation, climate change, etc.) and anthropogenic influences (i.e. overfishing, aquaculture, etc.) have been identified as potential sources of increased mortality, but further research is required to determine the effect of each on wild sea trout.

Resident brown trout (left) and anadromous sea trout with acoustic tag on the rule below (right)

High mortality is thought to occur during the initial marine phase of the smolt life stage. However, we actually know relatively little of their whereabouts during this period. Recent advances in acoustic telemetry equipment have created opportunities to observe the movement of sea trout in marine environments. Such studies have been conducted in Norway and have found several interesting trends, such as preference of young smolts to stay in coastal areas near to their natal streams, and the impacts of high salmon lice loads on the behaviour of sea trout (i.e. early returns of sea trout to freshwater in order to “de-louse” themselves).

One aspect of my PhD at the University of Glasgow is focused on the spatial movements and habitat use of young sea trout smolts as they first leave their natal rivers, and what level of interaction they might have with anthropogenic structures such as fish farm facilities. An acoustic telemetry project at this level of fine-scale movement of sea trout smolts has not been carried out in Scotland before and it is hoped that it will shed some light on the current problems facing our local wild fish.

For this project, we chose two adjacent sea lochs on the Isle of Skye in Scotland: Loch Snizort and Loch Greshornish. An active fish farm facility is located within Loch Greshornish. In April 2017, in time with the natural smolt run, 30 sea trout smolts were captured from rivers in both sea loch systems using a combination of fyke netting and electrofishing.

Fyke netting for smolts in the R Snizort (left) and some of the hardware to anchor my telemtry receivers into the lochs (right)

All 60 smolts were anesthetised then tagged with a small acoustic tag that was surgically implanted, before being released back into the site they were captured from. Each tag emits a unique acoustic “ping” that can be “heard” by an acoustic receiver up to ~200m away. These data can then be used to identify when and where a specific smolt was located during the course of the study. The information is stored in the receiver until it is downloaded onto a computer.

Locations of the acoustic telemetry receivers in Loch Greshornish (left) and Loch Snizort (right)

A total of 40 acoustic telemetry receivers were split between the two sea lochs and placed in strategic lines across the lochs. Several “double curtain” receiver arrays were also used to gather information about swimming speed and directionality.

The limiting factor to my study is the battery life of the tags put into the smolts. Unfortunately they only last for ~80 days, so the length of the study was constrained from the end of April until the end of July.

Although the data have not been fully analysed yet, an initial glimpse has identified at least one fish that was successfully recorded moving between the two monitored sea lochs, and several fish that were identified near the fish farm facility in Loch Greshornish. Once the data analysis has been completed, a paper will be published with the final results.

If you have any questions, please feel free to contact me directly.

Isabel Moore (i.moore.2@research.gla.ac.uk or @izzy_moore89)