Blue Catfish: Invasive?

How do we define an invasive species?

Next to habitat degradation, invasive species are the greatest threat to global biodiversity (Light and Marchetti 2007), yet definitions of “invasive species” vary broadly (Lockwood et al. 2013). It is important to remember that introduced species are not necessarily invasive, and many are beneficial (Gozlan 2008; Gozlan et al. 2010). Many terms are used to describe organisms that have been transplanted into a new environment. These species are commonly referred to as “non-native”, “non-indigenous”, or “introduced”, but invasive has a different meaning. Some define an invasive species as a non-native species that has been demonstrated to cause ecological or economic harm. This is problematic because it requires human assessment of harm, which is prone to human bias (Lockwood et al. 2013). It also ignores the possibility that non-native species can be beneficial economically and ecologically (Davis et al. 2011). Many ecologists define an invasive species as a species that is self-sustaining and expanding in its new environment, regardless of impact, yet “expanding” can have subjective definitions too (Lockwood et al. 2011).

The federal government has its own definition for invasive species. The National Invasive Species Council defines an invasive species as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” They also state that for a species to be “invasive”, its negative impacts must outweigh any benefit that it provides, and they recognize that many “alien species are non-invasive and support human livelihoods or a preferred quality of life” (ISAC 2006).

Characteristics of invasive species in aquatic environments

Moyle and Light (1996) were able to make several generalizations about invasive species in aquatic environments. First, most invasions are unsuccessful; however, when they occur, most do not result in major ecological changes. Second, invasions are more successful in systems that are polluted or otherwise altered by human activity. Third, predators are more likely to alter the communities than omnivores.

Blue catfish biology and life history

Blue catfish were introduced into Virginia’s tidal rivers in the 1970s and 1980s, and populations have expanded. Blue catfish now occupy every major drainage of the Chesapeake Bay. We have collected diet contents from over 15,000 catfish, and we have found that blue catfish are omnivorous, feeding mostly on vegetation, Asiatic clams, and other aquatic invertebrates. Only the largest catfish eat other fish, and these large fish make up a small percentage of the population.

Trophic levels are often used to understand how an organism fits into the food web. Trophic level one is composed of autotrophs or organisms that make their own food, usually plants or plant-like organisms. Trophic level two feeds on vegetation, trophic level three is omnivorous, and the upper trophic levels are carnivorous (see image below).

We calculated trophic levels for blue catfish based on their diet. Trophic levels for blue catfish ranged from 2.72 to 3.55, indicating an omnivorous feeding strategy. We also identified the length(s) at which blue catfish began to eat other fish, and calculated trophic levels for these “fish eating” catfish. Blue catfish began to eat significantly more fish when they reached lengths of 20 – 28”; however, even these larger catfish were omnivorous, feeding on vegetation, mollusks, and crustaceans. Trophic level calculations for these large blue catfish were not indicative of an “apex predator” (<3.55). Big, predatory fish in the system like striped bass and flathead catfish occupy higher trophic levels (4.70 and 4.33, respectively). Blue catfish occupy similar trophic levels to non-native common carp, which is not that surprising as both feed heavily on aquatic vegetation.

There has also been some concern that blue catfish are feeding heavily on imperiled species such as river herring and American shad, which swim up Virginia’s tidal rivers every spring to spawn. We collected stomachs from thousands of catfish during the spring spawning migration, and American shad and river herring were found in less than 1% of blue catfish stomachs.

Interactions between blue catfish and blue crab have also been a concern, as blue crab support one of the most lucrative commercial fisheries within the Chesapeake Bay. Blue catfish do eat blue crabs, and predation increases in brackish areas. Yet predation of blue crab is still relatively uncommon (< 15% of stomachs).

Generalizations about freshwater invasions (Moyle and Light 1996) provide valuable insight into the “invasiveness” of blue catfish in the Chesapeake Bay. The authors concluded that invasion is more likely in areas disturbed by human activities. Much of the Chesapeake Bay is far from pristine, and has a long history of pollution and/or disturbance.  Agricultural runoff, raw sewage overflows, and chemical spills have degraded water quality in the Bay over the last century. The Bay now has seasonal “dead zones” due to low oxygen, and many of the fish from the Chesapeake Bay are contaminated with toxins (PCBs and methylmercury).

It’s not all bad news, as most invasions do not result in major community changes. Our data shows that blue catfish are omnivorous, and even the largest fish still occupy lower trophic levels than other predators in the system. Omnivores are less likely than predators to cause major ecological changes (Moyle and Light 1996).

Are blue catfish invasive?

This ultimately depends on your definition of invasive. If you adopt the definition suggested by many ecologists, blue catfish are invasive. With this definition, any species is invasive if it is “self-sustaining” and “expanding geographically”.  After introduction, blue catfish populations expanded downriver and into neighboring systems, and they now occupy every major drainage of the Chesapeake Bay.

If you adopt an impact-based definition, like the definition used by the National Invasive Species Council, blue catfish may not be invasive after all. Here, for a species to be invasive, its negative impacts must outweigh any benefit it provides. Blue catfish in the James River support a spectacular trophy fishery. This trophy fishery employs full time guides and provides a financial boost to the local economy. People travel from around the U.S. to experience trophy catfish on the James River, which regularly exceed 50 lbs in weight, and numerous articles about this fishery appear in Field and Stream and In Fisherman. In addition to recreational fishing opportunities, blue catfish support a growing commercial fishery that financially supports commercial fisherman and fish processing facilities.

Our research has shown that blue catfish are not apex predators, but are omnivores with trophic levels ranging from 2.72 – 3.55. They do not regularly consume American shad or river herring. The one concern is that blue catfish eat blue crabs in brackish areas, yet blue crabs were still fairly uncommon. While we can’t estimate impacts on blue crab populations without knowing how many blue catfish there are, it is doubtful that their impact on this resource financially exceeds the benefits that they provide.

Ultimately, before an agency or researcher labels a species as “invasive” they should first provide their definition for an invasive species. Future work is still needed to assess impacts of blue catfish in the Chesapeake Bay. Population estimates are needed, so that they can be integrated with our diet information and our estimates of consumption rates. This will allow us to estimate blue catfish consumption of blue crabs (lbs/year), which can be compared to other sources of mortality.

References

Davis, M. A., M. K. Chew, R. J. Hobbs, A. E. Lugo, J. J. Ewel, G. J. Vermeij, and K. Thompson. 2011. Don’t judge species on their origins. Nature 474(7350):153-154.

Gozlan, R. E. 2008. Introduction of non‐native freshwater fish: is it all bad? Fish and Fisheries 9(1):106-115.

Gozlan, R. E., Britton, J. R., Cowx, I., and G. H. Copp. 2010. Current knowledge on non‐native freshwater fish introductions. Journal of Fish Biology 76(4):751-786.

Jenkins, R. E., and N. M. Burkhead. 1994. Freshwater Fishes of Virginia. American Fisheries Society, Bethesda, Maryland.

(ISAC) Invasive Species Advisory Committee. 2006. Invasive species definition clarification and guidance white paper. National Invasive Species Council.US Department of Agriculture, National Agricultural Library. Washington, DC. Available at http://www.invasivespeciesinfo.gov/docs/council/isacdef.pdf. (Oct 2016).

Lockwood, J. L., Hoopes, M. F., and M. P. Marchetti. 2013. Invasion Ecology. John Wiley & Sons, Hoboken, New Jersey.

Moyle, P. B., and T. Light. 1996. Biological invasions of fresh water: empirical rules and assembly theory. Biological Conservation 78(1):149-161.