03

March 2021

By Martin Lykke Kristensen

Karup Å – the trout’s fantastic marine life

Phd. student Martin Lykke Kristensen, DTU Aqua gives here the latest insight into the fantastic marine life of the Karup Å sea trout.

They are adapted to the conditions in Karup Å and the Limfjord, and can break out of the fjord to the east in a few days. Along the way, they are exposed to great dangers that force them to have a very special behavior. Meet Karup Ås fast sea trout.

Phd. student Martin Lykke Kristensen, DTU Aqua gives here the latest insight into the fantastic marine life of the Karup Å sea trout.

They are adapted to the conditions in Karup Å and the Limfjord, and can break out of the fjord to the east in a few days. Along the way, they are exposed to great dangers that force them to have a very special behavior. Meet Karup Ås fast sea trout.

Karup Ås trout stock is in many ways legendary due to both the size of the stock and the individual fish. Many anglers therefore travel far to fish for river trout.

The latest research in the field now provides an insight into some of the things that make the sea trout from Karup Å special. The wild population of Karup trout has adapted to the conditions in the Limfjord over time, and therefore has some behavioral traits that impress and fascinate Å

We know this because at DTU Aqua we have tagged 80 declining trout (adult trout that have spawned) and 51 smolts (young fish leaving the stream for the first time) with electronic tags in 2017 and 2018 so we could study the fish's behavior. In this way, we have been able to compare the marine behavior of the Karup trout with that seen in sea trout stocks elsewhere in Denmark and the world.

This is a research area that is attracting a lot of attention at the moment. The research has only recently begun to get the technology to make detailed studies of fish behavior in fjords and seas. Where for several years it has been possible to study and document the fish's behavior while staying in the watercourses, it is only in these years that people have begun to understand how good sea trout stocks are to adapt to the conditions in the marine environment that awaits them outside the watercourses.

A smolt from Karup Å is measured before it is marked with an electronic mark. Subsequently, we can follow the fish's behavior in the river and the Limfjord.

A smolt from Karup Å is measured before it is marked with an electronic mark. Subsequently, we can follow the fish's behavior in the river and the Limfjord.

Examples of the type of equipment we use to study the behavior of fish. The fish are marked with 7 mm (smolt) or 9 mm (descendant) long marks that emit their ID once a minute, which is caught by the thermos-like hydrophone within a distance of approx. 200-500 meters. Thereby we can see if and when the fish has been in an area.

Examples of the type of equipment we use to study the behavior of fish. The fish are marked with 7 mm (smolt) or 9 mm (descendant) long marks that emit their ID once a minute, which is caught by the thermos-like hydrophone within a distance of approx. 200-500 meters. Thereby we can see if and when the fish has been in an area.

The unique Karup Å trout

Let us therefore take a closer look at some of the results from our studies of the Karup fish. Here, in this post, I choose to highlight the trout's behavior in the Limfjord, as their way of handling the fjord seems to be important for success in it. In other words, this is a rather special behavior, which has probably been adapted over generations.

When Karup descendants hike out into the Limfjord in winter and spring, however, they start out being a little bit Norwegian. In Norway, sea trout tend to stay close to the mouth of their domestic streams during their entire marine stay, and when examining descendants from a central Norwegian stock, 40% of the adult fish never got further than four km from the mouth of the stream. The Karup fish start in the same way by lurking close to the estuary in the winter and early spring, and 44% of the fish actually return to Karup Å on short visits several times. However, this slightly fickle behavior suddenly comes to an end.

The unique Karup Å trout

Let us therefore take a closer look at some of the results from our studies of the Karup fish. Here, in this post, I choose to highlight the trout's behavior in the Limfjord, as their way of handling the fjord seems to be important for success in it. In other words, this is a rather special behavior, which has probably been adapted over generations.

When Karup descendants hike out into the Limfjord in winter and spring, however, they start out being a little bit Norwegian. In Norway, sea trout tend to stay close to the mouth of their domestic streams during their entire marine stay, and when examining descendants from a central Norwegian stock, 40% of the adult fish never got further than four km from the mouth of the stream. The Karup fish start in the same way by lurking close to the estuary in the winter and early spring, and 44% of the fish actually return to Karup Å on short visits several times. However, this slightly fickle behavior suddenly comes to an end.

In April, something fascinating happens. Even though the water is still cold, the Karup descendants unfold their tail fins and move them 120 km towards the eastern exit of the Limfjord at some speeds not previously seen for sea trout. Thus, it only takes a few days from the time the fish decide to leave the fjord until they have actually done so.

At the same time, the timing of the decision is almost as fascinating as the speed at which it is executed. The fish agree astonishingly on when to hike, and in the course of 14 days, virtually all of our tagged fish went from a position in Skive Fjord to having left the Limfjord completely. We can not say with certainty what it is that starts the migration of the fish, but it seems that a rapid rise in the temperature of the fjord plays a big role.

On the way out of the fjord, they swam really hard. On the stretch from Aggersund to Aalborg, for example, this year we have registered one of our tagged sea trout of 71 cm that swam at an average speed corresponding to 83 km / day on a day with co-current. If we look at the fastest fish in more neutral current conditions, we have a fish of 61 cm that swims the distance at a speed of 44 km / day, while the average speed for all the tagged fish on this stretch was 31 km / day.

Even on the stretch from Hvalpsund to Aggersund where the fjord is open and the current conditions are more stable, the fish kept an average speed of 13 km / day, while the fastest fish on the stretch came through with an average speed of 29 km / day.

Here it is worth noting that the fjord was still 8-10 degrees warm, which is below the fish's optimal swimming temperature. When the fish returned home from the sea during the hot summer, they therefore had even more speed, and the fastest fish swam the 120 km from the fjord's eastern mouth at Hals to the river in 2.2 days or with the equivalent of 55 km / day .

Example of the walking behavior of a 53 cm Karup descendant. The fish leaves the river on 3 March and stays in Skive Fjord until 19 April, after which it migrates quickly out of the Limfjord. The trip from Hvalpsund to Hals took the fish 6.3 days, which is at the high end for the descendants from the river.

Example of the walking behavior of a 53 cm Karup descendant. The fish leaves the river on 3 March and stays in Skive Fjord until 19 April, after which it migrates quickly out of the Limfjord. The trip from Hvalpsund to Hals took the fish 6.3 days, which is at the high end for the descendants from the river.

In comparison, descending trout migrated from the river Gudenåen with average speeds of 1.9 km / day

out through Randers Fjord when DTU Aqua examined them, and the fastest fish in that study walked at 7.5 km / day. A little faster it went in another study with adult sea trout on the way back through Randers Fjord towards Gudenåen, where they found maximum swimming speeds of 12.8 km / day. This used to be relatively fast migration rates for sea trout.

In comparison, descending trout migrated from the river Gudenåen with average speeds of 1.9 km / day

out through Randers Fjord when DTU Aqua examined them, and the fastest fish in that study walked at 7.5 km / day. A little faster it went in another study with adult sea trout on the way back through Randers Fjord towards Gudenåen, where they found maximum swimming speeds of 12.8 km / day. This used to be relatively fast migration rates for sea trout.

However, we are going to the UK to find the fastest single measurement of a sea trout so far, although this is a bit thin due to the current conditions on site. At the river Fowey, the migration of descending trout through the tidal zone was studied, and out of 45 fish, one was observed to make the trip at a speed corresponding to 32 km / day over a distance of eight km. However, the high speed occurred primarily because the fish managed to complete the entire trip through the tide zone with the tide in the back. To illustrate how much co-current this fish had, some of the other fish in the studio were pushed backwards towards the river when the tide turned. The other fish in the study therefore had to spend several tidal cycles to climb the eight km.

Although the research in this area is still fairly new, it seems that the Karup trout are able to migrate very quickly and purposefully when they decide to do so compared to other trout stocks.

We must therefore get hold of big brother to find studies that have measured speeds of the caliber of Karup trout in salmonids. In Pacific salmon, speeds of 36-72 km / day have been measured in the open ocean while the fish were on their way home to spawn. This was measured with an accelerometer, where deviations from the direct line also count, which according to the same study gives the fish an advantage of 24 - 1900% compared to when measured in the direct line as it does for our sea trout.

Our Karup trout therefore seem to have an ability to migrate both quickly and purposefully, and together with the close relatives in Simested Å, they are the fastest sea trout research has studied so far.

At the same time, it is worth noting that the Karup trout's migration out of the fjord takes place within such a short time frame and in such a uniform way in both 2017 and 2018 that it is probably an important behavior to succeed in the area. This makes sense, as we in the Limfjord have also observed the clearly highest mortality rates for adult trout in any fjord anywhere in Denmark and the world. 64% of the adult trout and 80% of the smolts that migrate into the Limfjord thus end up dying in it, which is very high. The trout are thus pressured to migrate quickly through the dangerous fjord, where seals appear to pose a great danger to the adult fish along the way, while cormorants appear to be hard at the smolts.

The smolts are also unique

The behavior of leaving the fjord quickly also seems to be inherited, as the smolts also have it. Unlike the descendants who have experience, this is the first time the smolts have seen the fjord. When observing a widespread and relatively uniform behavior of the smolts, it is therefore reasonable to assume that it is an inherited adaptation that is important for survival and success.

The smolts also seem to have a strong preference for leaving the fjord quickly, and after a few weeks of acclimatization in the salt water, they swim quickly towards the exit. We do not have many data points on the smolts, but the fastest Karup smolt was measured at a speed of 17.5 km / day over a distance of 91 km from Skive Fjord to Aalborg. This is still a very impressive migration speed for a fish of about 20 cm, because the fish does not have to just swim 17.5 km / day straight ahead. It must also be able to find its way through a fjord it has not been in before. That it succeeds so elegantly is one of the things that makes sea trout so fascinating.

The smolts are also unique

The behavior of leaving the fjord quickly also seems to be inherited, as the smolts also have it. Unlike the descendants who have experience, this is the first time the smolts have seen the fjord. When observing a widespread and relatively uniform behavior of the smolts, it is therefore reasonable to assume that it is an inherited adaptation that is important for survival and success.

The smolts also seem to have a strong preference for leaving the fjord quickly, and after a few weeks of acclimatization in the salt water, they swim quickly towards the exit. We do not have many data points on the smolts, but the fastest Karup smolt was measured at a speed of 17.5 km / day over a distance of 91 km from Skive Fjord to Aalborg. This is still a very impressive migration speed for a fish of about 20 cm, because the fish does not have to just swim 17.5 km / day straight ahead. It must also be able to find its way through a fjord it has not been in before. That it succeeds so elegantly is one of the things that makes sea trout so fascinating.

The Karup trout have therefore surprised us in the two years we have studied them. Both smolts and downhills swim with speeds and a focus that has not been documented elsewhere. At the same time, this behavior is so pronounced that it is probably important for success as a Karup fish, which makes sense in light of the very high mortality rates we saw in the fjord for both adult and young fish.

The next time you catch a sea trout in the river, you can therefore rejoice that it is hardy and has lived a fascinating life where it has several times defied the dangers and managed the needle's eye through one of the most difficult fjords to survive in. Only the elite there returns home to Karup, and perhaps that is, in fact, what makes fishing in the river so attractive.

You can find more information about DTU Aqua's surveys in Karup Å and other places at www.fiskepleje.dk and www.ørreder.dk

The Karup trout have therefore surprised us in the two years we have studied them. Both smolts and downhills swim with speeds and a focus that has not been documented elsewhere. At the same time, this behavior is so pronounced that it is probably important for success as a Karup fish, which makes sense in light of the very high mortality rates we saw in the fjord for both adult and young fish.

The next time you catch a sea trout in the river, you can therefore rejoice that it is hardy and has lived a fascinating life where it has several times defied the dangers and managed the needle's eye through one of the most difficult fjords to survive in. Only the elite there returns home to Karup, and perhaps that is, in fact, what makes fishing in the river so attractive.

You can find more information about DTU Aqua's surveys in Karup Å and other places at www.fiskepleje.dk and www.ørreder.dk

Literature

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