Studies show how habitat changes affect different fish populations
When I taught fisheries management at Mississippi State, I used what I called “fish stories” to convey fundamental principles of fisheries management.
The practice of fisheries management — and thus the teaching — is based on the outcomes of well-designed research studies. My so-called “fish stories” were research studies, but they had a little flair or maybe a surprise twist like a detective story.
Little Rock Lake
One of my favorite fish stories is a study conducted by University of Wisconsin researchers to evaluate the importance of shoreline woody cover to a simple fishery consisting primarily of largemouth bass and yellow perch.
Little Rock Lake is shaped like a Figure-8, with two basins joined by a narrow connection. The lake has no shoreline development and is closed to public access and private development. The two basins were isolated by a barrier installed at the narrow connection that prevented any fish movement between basins. Seventy-three percent of logs greater than 4 inches in diameter were removed per mile of shoreline in the treatment basin. No logs were removed from the other (reference) basin.
In the two years before wood removal, bass fed primarily on yellow perch, and yellow perch fed on aquatic invertebrates such as insect larvae, snails and crayfish living on the bottom or on the woody substrates in both basins. Forage of terrestrial origin was less than 12% of bass diets in both basins.
The food chain
Biologically, Little Rock Lake is a northern version of a Mississippi farm pond stocked with bass and bream; the bream feed on invertebrates, bass feed on the bream.
In the two years after wood removal, perch were less than 14% of the bass diet, and terrestrial-origin prey increased to 51% to 55% percent of the bass diet in the treatment basin. In the reference basin, the proportion of yellow perch in the bass diet decreased in the first year after wood removal and then increased. Consumption of terrestrial prey by bass did not change.
The abundance of perch declined sharply in the treatment basin but increased in the reference basin after wood removal.
Growth of bass was slightly greater in the treatment basin than the reference basin before wood removal. After wood removal, growth of bass in the treatment basin declined to less than in the reference basin.
The simple conclusion from this well-designed study: when you remove the habitat that provides both food (the invertebrates living on the woody material) and shelter for the primary prey, both the prey (yellow perch) and predator (bass) collapse.
Why did the researchers remove 73% of large wood from the reference basin? Ah, the twist: this removal reduced the amount of shoreline woody cover to the average amount of wood that remains on the shorelines of similar Wisconsin lakes that have moderate shoreline development.
The alternate fact
A similar study was recently conducted in five small lakes in Ontario, Canada, to evaluate the effect of wood removal on simple fish assemblages dominated by brook trout. The lakes were protected from fishing and had similar amounts of shoreline woody cover before the study.
All wood was removed from 50% of the shoreline in three lakes (treatment lakes) and two lakes were left unaltered (reference lakes). The fish communities were monitored for two years before wood removal and 10 years after wood removal.
Fish biomass and production did not differ between the treatment and reference lakes before and after wood removal, suggesting removing a large amount of woody cover had little effect on the fish communities and clearly refuting the results of the Wisconsin study
Although, like all field experiments, many uncontrolled variables are in play and can influence outcomes. But the big difference between the studies: Little Rock Lake had only a small amount of aquatic vegetation, whereas the lakes in the Ontario study had abundant aquatic vegetation that would provide substrate for attachment for invertebrates and shelter for small fishes. Indeed, aquatic macrophytes tended to be a more preferred habitat than the woody habitat. Removing the wood had a negligible effect on the habitat.
The fundamental principle conveyed by these two “fish stories” is that habitat matters. What ecologists call “complex habitat” — habitats with a lot of surface area and interstices — benefits fish production. The increased surface area provides abundant substrate for growth of algae and the aquatic invertebrates that feed on it. The interstices provide refuge from predation for forage fish.
Both wood and aquatic vegetation provide complex habitat, but aquatic vegetation does not grow in all lakes. In these lakes, woody material is especially important to sustain fish populations and fish production.
The source of woody material in lakes is trees on the shoreline that die and fall into the water. Thus, the health of fisheries depends on landowners to leave shoreline woody vegetation intact.
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