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.
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.
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.
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
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