Doppelgangers: How Fish Connect Us
words & photos :: Leslie Anthony
On a warm, sunny autumn day enlivened by a stiff wind, I cast a rubber lure into a weedy bay. As I reel in, letting the lure falter here and there as a real-life wounded minnow might, I feel the heavy tug of a big fish.
Eventually I net a feisty northern pike of some 5 kg. After a perfunctory photo of the fish and release back to its watery umbra, I relax to watch my companions battle similar piscine phantoms (much smaller than my own, I note with satisfaction). It's a fun day of catch-and release, aided in no small way by stellar surroundings.
Christian Jansson hefts the author's northern pike before releasing it back to the Baltic. Photo: Les Anthony
From Canadian Freshwater to The Baltic: A Fish Tale
The spot where Christian and Ulrika keep us "geo-anchored" (via remote-controlled, GPS-enabled electric motor that obviates the need to throw out a bottom-damaging chunk of metal) is one of surpassing boreal beauty—a constellation of pine-picketed islands, limned by a scatter of polished, bare-granite islets that seem to dolphin toward a big-water horizon. Many of the forested islands feature docks with well-worn paths leading up from the water to tidy cottages, shuttered now that summer has passed. As we fish, kayakers we observed packing up camp on a gorgeous point that morning pass us by with a friendly wave, bound for another idyll on which to spend the night.
Looking a northern pike in the eye. Photo: Kelly Sikkema via Unsplash
As an Ontarian whose youth was spent canoeing the iconic landscapes of Algonquin, Haliburton, Muskoka and Georgian Bay, this familiar terraqueous existence speaks to both heart and soul. But though it may softly whisper home, it's actually some 7,000 kilometres removed. And when I mention this to my friends, they can only smile and say Ja? because comparable as it may be, this is their home—the Stockholm Archipelago, a glory of 30,000 islands and rock skerries arcing along the Baltic coast of southeast Sweden. And yet there's a definite connection, in both aesthetic and ecological terms.
You'd have to go back some 50 million years to the Eocene to pinpoint the divergence of these fishes…
The northern pike, Esox lucius, is a good starting point, as it's the same species caught in Canada. But the aquatic pairings go deeper (or shallower, as you'll see): the Baltic's shoreline wetlands also abound with European perch, sister species to our own yellow perch, as well as the much-sought-after zander, similarly related to walleye. The latter two grow much larger here than in Canada, and though pike maintain a similar size range throughout their impressive circumpolar distribution, the world's all-tackle record of 25 kg comes from Germany. This would all be little more than a natural-history footnote were we discussing freshwater ecosystems that were recently connected across the Northern Hemisphere, but that isn't the case.
In fact, you'd have to go back some 50 million years to the Eocene to pinpoint the divergence of these fishes—a long period of separation over which to maintain such doppelganger similarities. Beyond deep time, however, the virtual aquatic-fauna mirror poses a greater quandary: what are fish that inhabit only freshwater in North America doing in the Baltic Sea?
The Stockholm Archipelago. Photo: Les Anthony
Millennia of Adaptation
To begin, the Baltic was created by successive continental glaciations that scoured out its two main branches. With the melting of the final Pleistocene ice sheets some 10,000 years ago, the area filled with freshwater. As the land rebounded and drainages shifted over the next few thousand years, the Baltic alternated between non-saline and saline several times—similar to what happened with Lake Ontario and the St. Lawrence/Ottawa River Valleys. Today the waters of the Baltic are technically "in between"—the largest area of brackish water on the planet.
Because the relatively narrow Danish Straits are its only saltwater ingress from the North Atlantic, salinity in the Baltic is significantly lower than the open ocean's average of 3.5 per cent, and steadily decreases as you go north and east, varying from 0.6 – 0.8 per cent in its centre to 0.1 per cent in the north. Thousands of freshwater streams and rivers entering from Norway, Sweden, Finland, Russia, Estonia, Lithuania, Latvia, Poland, Germany and Denmark contribute 1/40th of the Baltic's volume and overlie the denser saltwater flowing in from the Danish Straits.
Kayak guide David Kvart maps out a day's paddle through the archipelago from Fejan Island. Photo: Les Anthony
Surface water at the lower end of the Gulf of Bothnia, for instance, has no salt taste. As a result, the Baltic features flora and fauna adapted from both regimes to this middle ground (so to speak)—slightly divergent forms of plankton, seaweed, shoreline vegetation, invertebrates, and, of course, fishes such as the fresh-to-brackish-water shifting of European perch and zander, and salt-to-brackish-water shifting of Baltic herring, a smaller variant of Atlantic herring that once formed the basis of the region's food and trading commerce. The salinity gradient also combines with a parallel North-South temperature gradient to limit many species to relatively narrow regions. In addition, other than storm surges, the Baltic has no tide to speak of, and this lack of an intertidal ecological zone has affected which marine species can adapt to the Baltic's low salinity surface waters.
Admittedly this isn't relevant to most clients enjoying a day out with Christian and Ulrika, whose Sjö&Hav not only guides all over the archipelago, but engages in significant conservation efforts for these same fish species—just like fish guides do in Canada. For a biogeography and evolution nerd like me, however, bobbing amongst familiar glacier-polished rock adorned in familiar boreal plants catching familiar boreal fishes is more than just dust in the geological wind; it is, I muse, as we discuss a shoreline lunch of fresh perch, food for thought.
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