Out with the new, in with the old: can removing Asian carp benefit native fish populations?
Featured Image: Underwater photo of an adult bighead carp. Photo by Ryan Hagerty/USFWS.
Reference:
Love, S. A., Lederman, N. J., Anderson, R. L., DeBoer, J. A., & Casper, A. F. (2018). Does aquatic invasive species removal benefit native fish? The response of gizzard shad (Dorosoma cepedianum) to commercial harvest of bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix). Hydrobiologia, 817(1), 403-412. https://doi.org/10.1007/s10750-017-3439-1
What’s the deal with carp?
Bighead and silver carp, often referred to in tandem as Asian carp, are arguably one of the most notorious groups of aquatic invasive species in the United States. Imported to the United States from China in the mid-1970s, the fish escaped confinement and made their way to the Mississippi River Basin where they soon established a healthy population. Asian carp are extremely fecund, have fast growth rates, and mature rapidly. They also have a large and rather indiscriminate appetite, feeding on detritus, phytoplankton, and zooplankton.
As Asian carp have taken up residency in U.S. rivers, they have become a threat to aquatic ecosystems both directly, through competition with native species, and indirectly, by decimating the food supply for fish that supplement commercial and recreational fisheries.
In response to the Asian carp invasion, many management plans have been developed and implemented to curb the spread of these fish. A famous example is an electric fish barrier installed in the upper Illinois River in an effort to prevent the carp from entering Lake Michigan. Another example is the employment of contractual commercial harvest of bighead and silver carp, which is when fishermen are paid to harvest Asian carp from certain areas of the Illinois River.
Since scientists are well aware of the effects that Asian carp have on native populations and habitat, S.A. Love and coauthors conducted a study looking to see if the measures used to control carp populations can also benefit native fish populations. That is, if the control efforts work, will native populations be able to thrive again?
The gizzard shad: a native species indicator
The authors chose to use the gizzard shad as the focus species of this study because adult gizzard shad compete with Asian carp for food, and there are lots of data describing shad health and abundance in the Illinois River. The sampling data indicate that there was a decline in both abundance and body condition of gizzard shad since the introduction of Asian carp, suggesting that competition is likely affecting the health of the shad population. Using this dataset, the authors aimed to find out if the body condition and abundance of the gizzard shad would be able to rebound after the implementation of contractual commercial harvest to control Asian carp.
After the contractual harvest was underway, the authors found that the health of gizzard shad increased so much that it was similar to the health of shad before Asian carp were introduced. Interestingly, the number of gizzard shad did not change when Asian carp were first introduced, but the population did increase beyond initial amounts when the contractual harvest was in place.
Is the proof in the poop?
The increase in shad health during the harvest period suggests that the contractual harvest may have decreased competition for food between gizzard shad and Asian carp. However, there is a twist! This change could also be accounted for by something else: a shift in gizzard shad diet. A previous study showed that Asian carp cannot fully digest certain phytoplankton commonly found in the Illinois River. This means that Asian carp poop may be a nourishing food option for other fish, including gizzard shad. So, ironically, the influx of Asian carp might actually be helping the gizzard shad population.
Take Home Message
Although it’s difficult to pinpoint the cause for improved gizzard shad health and increased abundance, the authors do present some convincing evidence to suggest that harvesting Asian carp may be somewhat responsible for these changes. Continued monitoring of gizzard shad abundance and health paired with Asian carp control methods may show that in addition to the removal of these nuisance species, native species could recover.
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