European Wilderness Society

The impact of the otter on the fish population

This post is written by Aurelien Rinaudo.

The European otter is a small aquatic mammal who lives in Europe but also in Asia and in a part of Morocco, Tunisia and Algeria. This species is on the red list of the IUCN as a “Near-threatened” species. After a period of decline and following their protection, European populations were able to begin to recover. But the otter still has problems with the population for his diet. We will see what is its habitat, the diet and what is its relationship with fish populations. 

The otter habitat

The European otter lives in almost all types of aquatic environments. However, the pollution of its habitat is a negative factor for its presence. The otter is a territorial animal, so a single otter will live on a portion of a river. The otter’s territory can be between 10 and 40 km, depending on the quantity of fish in the river.  

The otter diet 

In the otter diet, we can find fish (like trout, perch,…)  which are present in the river but also crustaceans such as crayfish and amphibians. Small mammals such as muskrats, birdies and insects may also form part of their diet, although this is rarer. The otter eats about 1 kg of fish every day (10% of its weight). 

The relation between the otter and the fish population 

The relationship that otters have with fish in a river is the same as that of any predator with its prey. Otters will never exploit the entire resource because, as soon as they have less food, various regulatory mechanisms will come into play. These mechanisms are multiple. For example, there is a decrease in their reproduction rate and an increase in their fishing territories. Dominant mothers may also kill the offspring of others to limit the population increase and thus reduce the pressure on their food supply. However, when the fish stock replenishes, the otter population will increase simultaneously. The old adage that if there are many fish, there are many otters aptly illustrates this phenomenon.

But sometimes humans impact these natural regulatory dynamics. When humans release fish into the river to ensure there are fish for fishing, they introduce many fish that become easy prey. Otters then find enough food and can reproduce successfully. This can lead to conflicts with fishermen because the fish stocks will diminish significantly. These conflicts also occur when otters come into contact with ponds where fish are farmed. They can cause significant damage and decimate the fish populations in these ponds.


Otters are small mammals that inhabit our waterways. But as otters have an impact on fish populations in our waterways, man has begun to hunt them. This hunting has led to a decline in otter populations and their disappearance from certain areas of Europe. Habitat destruction and pollution also contributed to the decline. Today, thanks to the protection of the species and their habitats, these populations are beginning to grow again.

Leave a Reply

Your email address will not be published. Required fields are marked *