Animals that catch fish!

What has otter, mink, heron and cormorant in common? The question can any angler answer.

The fisherman is not the only one who is interested in catching fish. Otter, mink, herons and cormorants have the same desire, and that does not make the fishermen happy.
But how big is the problem really with these fish robbers? DTU Aqua has investigated the case, and particularly they have investigated the damaging animals´ impact on freshwater fish. The results of the study were published in 2014. The referenced here is very concise:



The otter was totally protected in Denmark in 1967, because at that time it was a rare and endangered animal. This led to a large increase in the Jutland otter population. This fact has been considered a contributing factor to the decline in the number of large brown trout and grayling in several streams. However, there is not a clear overlap between the streams that have had many otters and the streams that have recorded significant declines in stocks of large trout and grayling. It is therefore difficult to conclude overly rigid at the otter’s impact on fish stocks.


The mink does not naturally live in the Danish countryside. The mink we encounter in the open landscape comes from mink farms.
Unlike the otter are the minks regarded as pests, and can be hunted throughout the year. There is no doubt that the mink eat trout, but it cannot be demonstrated that it happens to such an extent that it affects the fish population.




The heron standing at the rivers and lakes lurking on whether a fish had to pass, is a familiar sight to anyone who walks in nature. The heron is a frequent but not welcome guest at fish farms, because it does justice to the food.


With good appetite the heron is hunting sea trout and especially smolt (common name for young salmon or sea trout, which are silvery and wandering towards the sea). The overall assessment is that the heron’s influence on Danish stocks is limited compared to the damage that cormorants are causing.




30-40 years ago, the cormorant was not a bird that attracted much attention in Denmark. We did not have very many of them, but we have got many, and that does not arouse enthusiasm among fishermen for compared to other fish-eating birds, are cormorants the most effective.


Cormorants can fish in open water along the shore and dive 30 meters down for prey. It can also go on shallow bottom in rivers and pick the fish up between the stones. Cormorants can be seen fishing in large and small groups or alone, in flowing water, in stagnant clear water and muddy lakes with very low visibility depth. It eats small fish, but it also enjoys swallowing some of the big ones.

The increasing invasion of cormorants during the eighties, had great consequences for net fishermen. Their catch was destroyed by cormorants. They devoured and damaged fish in the nets. It also had consequences in relation to freshwater fish because smolt from trout and salmon as well as undersized-trout that goes in the nets also were eaten or damaged and they can not be put out again, as it otherwise was practiced.
In 2010 sport fishermen struck alarm because cormorants had started going in search of prey far up the rivers where they had never been before. Especially many were observed in Skjern Å, Varde Å, Storå and Gudenåen.

Especially in autumn are larger Danish lakes a pantry for cormorants. Studies of migratory behavior of fish in and out of Danish lakes indicate that the cormorant’s impact on the fish population can vary considerably between years, species and lakes. It is a relationship that DTU Aqua estimates that will be particularly relevant to illuminate in the future.

We must hope that in time we will find a balance between fishermen and cormorants and other fish robbers. We would ideally could be here all together in a reasonable number.


Kategorier: Naturvidenskab, Fisk