A New Perspective On An Old Issue

Carp get fingered for a lot of negative aquatic issues, but a new study about to be published takes a new look at one particular aspect of an old issue. "Effects of Common Carp on Reproduction, Growth, and Survival of Largemouth Bass and Bluegills" by Wolfe, et. al. (see related studies) took a look a what happens in these interactions on a larger mesocosm scale (600 m>2). As expected, turbidity was substantially higher in enclosures containing common carp than those without. What was unexpected though was successful spawning by both bluegill and bass at rates equivalent to non-carp enclosures. In fact, survival of juvenile largemouth bass was higher in enclosures with carp than in those without, and larval abundance of both bass and bluegill were equivalent to non-carp enclosures. This suggested little if any feeding by carp upon the eggs or nests of both species, a commonly accusatory phenomenon. Higher survival rates of larval and juvenile largemouth bass were theorized as possibly attributable to the increase in turbidity making young fish less susceptable to predation as they might have used that turbidity as a form of cover or camouflage.

That said, the news wasn't all good. While survival of larvals was equal to or greater than non-carp enclosures, and growth was also equivalent in that age group, growth of older juveniles suffered strongly. While they couldn't pinpoint a specific reason, theories included either elimination of a specific food source or type required at a critical stage of development due to turbidity or vegetative removal, or possible intraspecific competition between juveniles due to an increased survival rate and a potentially limited food supply.

Ultimately, researchers summarized, "Although common carp did not directly reduce reproductive output or initial survival of largemouth bass and bluegills, the reductions in observed growth of juveniles may have long-term implications for centrarchid population abundance and size structure", and that "negative effects of common carp on centrarchid populations may persist over several generations, eventually impairing sport fisheries and making improvements to population size structure difficult."

Comments

[richard ziert]

"While they couldn't pinpoint a specific reason, theories included either elimination of a specific food source or type required at a critical stage of development due to turbidity or vegetative removal, or possible intra-specific competition between juveniles due to an increased survival rate and a potentially limited food supply."

I wonder how the birth-bloom, growth, and spread of plankton plays into all of this ? I feel that is exactly what they are talking about or should be looking at. Carp are bottom feeders, or stirrers in most cases. Plankton is born of the bottom ooze where light penetration is sufficient to cause photosynthesis. Bass spawn in the historical path of plankton movements to take best advantage of the food source; the most productive spawning grounds are found in the most productive areas. Turbidity caused by carp reduces depth at which photosynthesis can occur. Carp eat or kill plankton by their feeding process as well. Plankton blooms which make it through carp predation is insufficient to build on normalcy, and is too soon moved by current and wind to "other" locales not conducive to critical timing of bass fry benefit. This occurrence applies to other creatures that depend on plankton at this critical stage; setting up a situation where juvenile and adult predators of all kinds including bass will eat what is available. . . . the young of the year of any species including cannibalism.

[Tim Smith]

Juvenile predator growth in many species has been shown to rely on "ontogenetic shifts" from one prey type to another.

Early juvenile bass growth depends on the availability of larger prey such as larval fish or macroinvertbrates (dragonfly larvae etc). High productivity and high zooplankton densities due to nutrient enrichment of the water column by carp might well help early survival of bass (and it's no surprise that the carp don't eat eggs, nesting bluegill drive away even snapping turtles and bass are even tougher than that). The lack of late growth is probably due to these missing items. It could even be direct competition with carp, which selectively remove macroinvertebrates and repress growth of aquatic plants that promote macroinvertbrate survival.

Remember, thought, that this is just a mesocosm. Larger scale experiements or field conditions with more complex environments could tell a different story.

[Big Indiana Bass]

Thanks for the details Tim! Interpretation and scaling up is always the real issue it seems. Reminds me of the piece I wrote a couple years ago about the applied biology piece out of California (Scripps?).