Predatory fish follow their food, despite climate change

The scientific study of time may primarily be the domain of physicists, but biologists are well aware that time is fundamental to a species’ survival, and it’s a concept that climate change brings into sharp relief. Phenology is the timing of life-cycle events in biological organisms like plants and animals. The synchronization of those events, like the breeding, foraging, or migration seasons with the environment may be disrupted thanks to climate change.

For example, the annual migration of monarch butterflies evolved to coincide with the availability of milkweed. The humpback whales return each year at the same time to feed in the nutrient-rich polar waters. If climate change results in changes in the availability of those resources, shifting their peak availability to earlier or later in the year, the mismatch can result in precipitous population declines. That’s especially problematic for migratory species, which rely on cues like temperature or the length of the day (or night) to know when it’s time to move.

And indeed, research efforts have yielded important information on how the changing climate may affect migrants and the resources on which they rely. But the migrants themselves are also a resource, for their predators. And according to National Park Service researcher Christopher J. Sergeant, “virtually no studies have explored how climate change affects the synchronisation of these predator–prey migrations.”

That is what Sergeant and his colleague set out to change. They report their findings, which focus on the relationship between Dolly Varden, a species of Alaskan char, and their migrating food source, salmon eggs, in the journal Freshwater Biology.

Salmon are known for migrating each year, moving nutrients from marine waters into freshwater streams, rivers, and lakes. They deposit those nutrients in the form of eggs, when they spawn. As the planet’s higher latitudes have warmed, the salmon have altered the timing of their spawning season. What Sergeant and his group wanted to know was how the altered salmon migration affected the animals who rely on salmon – and their eggs – for survival. While Dolly Varden can acquire most of the yearly energy intake by feasting on salmon eggs, they must do so during a very narrow window in which the salmon spawn. That window can be as short as two to six weeks.

If the char are even a little bit early or late, relative to the salmon, they could find themselves without enough to eat. Can the char adjust their own migration to match that of the salmon? Sergeant focused on Auke Creek, near the southeastern coast of Alaska.

The researchers found a rare bit of good news when it comes to climate change. The Dolly Varden have managed to stay in sync with the salmon, despite the salmons’ shifting spawning season. By adjusting the timing of their migration from the ocean to freshwater, the char have retained access to their eggy feasts.

But how? Instead of taking migration cues from environmental variables like water temperature, the researchers suspect that the Dolly Varden simply wait around near the stream entrance for the salmon to begin moving upstream. “Dolly Varden in south-eastern Alaska, which typically do not migrate long distances offshore and often remain near stream entrances,” they write, “are likely to either see adult salmon entering a catchment or smell the production of salmon eggs.” In other words, the Dolly Varden never travel far enough from the streams to miss noticing the salmon returning to spawn.

What this all suggests is that changing phenology may not necessarily sentence predators to their doom. The dumb system of evolution has done something exceedingly clever, for some species at least. It’s set their migration to cue to the presence of their food itself, rather than some additional environmental variable. “Some predators,” the researchers conclude, “may be more resilient to trophic mismatch than ecologists often assume.” – Jason G. Goldman | 02 January 2015

Source: Christopher J. Sergeant, Jonathan B. Armstrong, & Eric J. Ward (2014). Predator-prey migration phenologies remain synchronised in a warming catchment, Freshwater Biology, DOI: http://dx.doi.org/10.1111/fwb.12524

Header image: Top, sockeye salmon via Wikimedia Commons/cacophony; bottom, Dolly Varden via J. Armstrong, used with permission.

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