Technology helps in trout count

They might not look very different to the naked eye, but counting brown trout effectively can be key to preserving stock.

Friday, 29th April 2016, 12:38 pm

And on the River Tweed, technology is providing a helping hand when it comes to logging numbers.

Until recently, there had been no practical way of recording recapture rates of trout on the river.

A number of external tags are available and could be used for this, but they are a snag risk and not that suited to a fish which likes to squeeze under tree roots and overhung banks.

Now, however, river managers have pattern recognition software which allows them to recognise individual trout from the complex spot patterns found on their gill covers – the spotting is even different on either side of the same fish.

Over the two previous trout fishing seasons (in 2014 and 2015), Jim Cairns from the Coldstream and District Angling Association had been collecting scale samples from the brown trout that he caught (originally as part of a different project to establish the male to female ratios of Tweed brown trout).

Along with each scale sample, Jim took a picture of the trout which was later run through the new software.

After this was done, it appeared that most trout caught weren’t recaptured (by Jim, at least), although there were six recaptures picked up.

The recaptures occurred anything from three to 313 days after Jim initially landed the fish, with two brown trout even being caught twice in a week.

How often fish are caught is one of the most important pieces of information that is required in managing fish stock.

After all, if a brown trout has the potential to be caught multiple times over, in the space of a single season, then killing a trout can potentially reduce the catches of the following anglers.

This is particularly the case for larger and older trout where compensatory mortality may no longer be in effect.

In effect, the losses of younger trout from the river can be compensated for by better survival of others.

River Tweed commissioners can now log trout into their database and check for recaptures from pictures taken by anglers on mobile phones or digital cameras. And they are currently putting together a simple trout photographing protocol and which, once completed, will be put out in an appeal to anglers to take photos of the trout they catch.

As of yet, there is no discernible pattern in the recaptures and they appear very variable. It may be that trout recaptures are highly variable based on differences in levels of aggression or feeding risk-taking between individual fish.

In fact, given the recaptures are of fish already caught on rod and line, the samples may already consist of fish which are more likely to be caught.

However, it was interesting to see that although Jim generally fishes in the same area there was little localised movement of trout and it was particularly surprising to see two trout caught almost a year later in the same spot.

Had they gone upstream to spawn then returned to the exact same beat, or had they not spawned for some reason and overwintered in their summer feeding grounds?

Hopefully, fishing photos will clear up the mysteries of the River Tweed’s fish.