Count grey seal pups to estimate the quality of the Baltic Sea’s food web

Did you know that you could assess the health of the Baltic sea by simply counting the number of grey seal pups? We explain to you how it works in this post.

The Baltic Sea, between salt and freshwater

The Baltic Sea is a little arm of the North Atlantic Ocean tucked away in northeastern Europe. It is one of the largest brackish water bodies globally, meaning the salinity (amount of dissolved salt) is much lower than regular oceanic water. In some parts of this sea, the surface water has so little salt; it can be considered fresh. This makes the Baltic Sea unique as it can sustain organisms from both worlds, for example, a cod (a saltwater fish) and a perch – a freshwater fish.

Everything in the environment is interconnected

As all ecosystems we know of, the Baltic Sea one is an interconnected network of organisms relying on each other to thrive. From microscopic algae to crustaceans, small fish, big fish, and then to apex predators, it creates a food web. Each level regulates and influences the next and the previous, ensuring the ecosystem is healthy and maintaining a delicate balance. This intricate network has created a wealth of food sources from which we humans benefited throughout the centuries. Currently, however, the Baltic Sea is facing the same deadly problem as other seas, and that is overfishing and the genuine prospect of the whole food web collapse.

A push to save the Sea

Therefore, experts from countries encircling the sea founded the Helsinki Commission (aka HELCOM) to appraise the sea’s health and find ways to protect its environment. One of the most remarkable outcomes of this organization’s work is “State of the Baltic Sea holistic assessment 2011-2016”. Critical to this endeavor is the ability to assess the status of the sea’s food web. There are several methods you can use. Most of them come down to estimating the number of animals like fish or crustaceans, etc. But these organisms are complicated to count. Wouldn’t it be nice to have a species that is easy to count and that gives a decent approximation of the health of the rest of the food web?

Grey seals are apex predators in The Baltic Sea

Grey seals come to the rescue! These magnificent animals are the largest predator naturally living in the Baltic Sea. They are also the apex predator meaning they sit at the top of the food chain and have no natural predators in this sea. The major part of their diet is fish like herring, cod, perch, etc. They will also eat larger crustaceans, and they could potentially attack other, smaller marine mammals living in the Baltic Sea. This apex predator’s position means that grey seals are affected by any changes to their prey – fish, which are affected by their prey – small crustaceans, which are affected by their prey – even smaller crustaceans and algae.

Apex predators like grey seals shape their environment

This connection, though, goes both ways. Grey seals being apex predators can actually shape their environment. One of the most famous examples of the apex predator reshaping the environment is the case of wolves in Yellowstone Park. In short, you can conjure this very simplified relation. If the apex predator population is healthy, then their food base needs to be healthy, and their food source needs to be healthy, etc., all the way down the food web. There we have it. Just assess the condition of the grey seal population, and you can estimate the condition of the entire ecosystem. Job is done! But there might be an even simpler way.

The count of grey seal pups reflect the condition of the females

The rate at which they reproduce is affected by the females’ body condition. The better the situation, i.e., health, the higher the birth rate usually is. And the body condition is related to food availability. In this study, researchers found out that: 1) the birth rate of grey seals was related to changes in the quality of Herrings, and 2) the quality of herrings reflected changes in the Baltic food web. Thus, the birth rate of the grey seals can be used as an indicator of the status of the Baltic food web.

How a number of grey seals pups reflect the quality of the Baltic Sea

In short, the more seal pups are born each year, the better the body condition of the females. The better the female grey seals’ body condition, the better the health of herring in the Baltic Sea. The better the herring condition is, the better the condition of the Baltic Sea food web. This is obviously a massive simplification, but it does mean that all you need to do to learn about the condition of the entire sea’s food web is to count the grey seal pups each year. And that is easy enough as grey seals usually give birth in a few well-known locations, on land in a particular time of year.

A simple but powerful indicator

The birth rate of grey seals is even easier to measure than the population size. Moreover, we can spot any changes easily from year to year, so we have a much greater chance of spotting potential problems quickly. These results are fascinating and are truly making estimating the health of the entire sea relatively easy. However, it is only a rough estimation. You can think of it as an indicator. If you find that the birth rate is decreasing, you can treat it as a sort of warning light, that something is going on, and you should take a closer look. At least we now have this early, easily available, and simply achievable warning sign.

Sources & further reading

• Helsinki Commission website
• HELCOME report: State of the Baltic Sea holistic assessment 2011-2016.
• Changes to the environment done by wolves in Yellowstone Park
• Scientific article: Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction
• Scientific article: Grey seal, as an indicator of the changes in the Baltic food web

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Seals “see” and hunt fish with their whiskers