Small Fish Turns Tables on Larger Predators

[BillTre]

TL;DR

Small stickleback fish can reduce the population of larger predators by eating the predator's young.

Cycles of predator-prey population size changes have been described where the predator eats so many of its primary prey that the reduced prey population can no longer support the predator population. In these conditions, the predator population crashes, which allows the prey population to recover, and the cycle continues.

Another kind of predator prey interaction is described in a news article in Science, here.
The research article is here.

The research is based upon 13,000 fish surveys from 1979 to 2017.
In the Baltic Sea, sticklebacks (~3-4 inches long) can live in sea water as well as freshwater.
Freshwater dwelling pike and perch can move out toward the sea as long as outflowing freshwater dilutes the sea water enough for them to survive.

Instead, complex ecological factors appear to have first worked against pike and perch: Beginning in the 1990s, gray seals became more common, thanks to better water quality and an end to bounty hunting. The seals, along with cormorants, began to eat more pike and perch. Meanwhile, sticklebacks were thriving in the rapidly warming seas. And overfishing of cod, the top predator, and large herring translated into fewer predators on sticklebacks.

Once the sticklebacks numbers increased, the sticklebacks become major predators on the juvenile perch and pike, thus reducing their numbers below what they might normally have been.

The work “clearly shows that the [disappearance] of larger predators can cause cascading effects all the way down to algae, and that these changes can unfold over vast spatial scales like falling dominoes,” says Boris Worm, a marine biologist at Dalhousie University. Worm worked in the Baltic Sea as a Ph.D. student, and he mourns the change, calling it “a slow-motion disaster through the Baltic Sea.”

Among other things, they are proposing to stock bays with juvenile pike and perch too big for stickleback to eat.

 

[jim mcnamara]

@Dr. Courtney may have an interest in this and a comment as well.

 

[Dr. Courtney]

@Dr. Courtney may have an interest in this and a comment as well.

I think this is a new development in this ecosystem, but there are at least a few examples where this is well established in freshwater ecosystems. The blueback herring is a voracious predator on the eggs of some of its freshwater predators, and quickly throws systems out of balance in so doing when it is introduced (often illegally) as an invasive species. For example, numbers of largemouth bass almost always decline when blueback herring are introduced.

As mentioned above, one solution is to stock predators too big for the "prey" to eat, but in most fisheries, this needs to be a short term approach to restoring balance, as there aren't many fisheries where this is a sustainable approach for government agencies (except where subsidized with fishing license sales.)

Food webs are often robust, but examples like these shows some fragility. But how much of the fragility is more humans having favorite species. "Genetic diversity" is a buzz phrase in many management scenarios, but I often wonder if many systems might be better off if we simply let natural selection do its thing. Especially when the alternative is "old lady who swallowed a fly" proposals.