European plaice. Photo: Dan Karlsson /Azote.

It is a typical summers day at one of the holiday resorts along the Polish coast. The sandy beaches are crowded. At noon, all the small bars on the boardwalk get busy as hordes of hungry sunbathers come to eat fish & chips.

Traditionally, the average Kowalski (the Polish equivalent to Smith) wants the dish made of “fresh flounder”. But in reality, whether or not it actually is flounder (Platichthys flesus), or perhaps plaice (Pleuronectes paltessa) or dab (Limanda limanda), doesn’t matter that much. From a fish and chips perspective, the target species for the hungry Kowalski has always been “flounder”.

This appetite would have been more in time 100 years ago, when flatfish actually was the main target species for the fishing industry, policy and management in the entire Baltic Sea. The modern fishing in the Baltic Sea started as a flatfish fishery. It was first after World War II, that cod, herring and sprat started to develop and grow, to gradually become todays main target species for Baltic Sea fisheries.

The longest consistent time series of historical catch data for the Baltic Sea dates back to 1906 (Hammer at al in Feistel et al., 2008), and shows an extraordinary fishing regime shift.

For herring, the catches increased five times during the 1950’s, to reach a maximum during the 1970’s. The catch levels of cod reached its maximum ten years later, in the 1980’s.

 For sprat, the maximum catch level was reached in the 1990’s –then ten (!) times higher than before World War II.

 And what about the flatfish?

First it was flatfish, then herring, then cod, and finally sprat

Maciej Tomzcak

Historically, plaice catches reached maximum levels in the 1920’s – about 40 000 tonnes per year. At that time, catch levels for cod and sprat was on low 3-5 000 tonnes  per year level.

Just after its peak, the plaice fisheries collapsed – while flounder catches reached their maximum (also around 60 000 tonnes). However, this flounder-bonanza lasted only a few years, before it too collapsed.

From the 1940s an onwards the flatfish fisheries saw some increasing trends but stayed at relatively low levels, while catches of other species exploded.

There is a puzzle. What happened to the flatfish stocks? And more importantly, what can we learn from it?

An obvious but important remark is that during the past hundred years or so, the catches of different species has most probably changed successively. First it was flatfish, then herring, then cod, and finally sprat.

We also have to realize that the Baltic Sea was a very different sea a hundred years ago; much less productive, thus no extensive dead zones as an effect of algal overgrowth, and much better oxygen conditions for flatfish along the bottoms.

But at the same time; less bottom living species as a food for flatfish and cod, less zooplankton and fish such as herring and sprat.

Furthermore, the fishing fleet has gradually transformed, from many smaller boats in the 1930’s to fewer but much bigger vessels in the 1990’s. During that time, the fishing gears also developed enormously and the ways to catch fish became much more effective.

Scientists still have various hypothesis on what actually happened to the flatfish stocks. It would of course be easy to just say that over-fishing caused the collapse of the flatfish. But that’s probably not entirely true.

More likely it was a combination of over-fishing, species interactions and environmental factors. The phosphorous and nitrogen input increased drastically from the 1950s and the first signs of eutrophication soon started to appear in the sea. The cod benefitted from various hydrological conditions and from this increased growth in the sea – while the flatfish did not.

The marine environment constantly change due to climate and human impact such as fisheries and nutrient use on land – and the animals in the sea change with it.

The modest eutrophication in the Baltic Sea during the 1950s, that worked favourably for the cod but not at all for the flounder, has since then increased enormously. Todays huge areas of hypoxic/anoxic bottoms now actually threatens the Baltic cod population.

At present, sprat is by far the major target species for fisheries in the Baltic Sea region. Sometime, maybe soon, and perhaps by pure necessity (people will always want to eat fish, right?), sprat will have to become a target species for the ordinary consumers as well.

The question is, will the Kowalskis at the Polish beach resorts be as keen to adapt as the sea has been, and start ordering fish & chips made of “fresh sprat”?

Text: Maciej Tomzcak