Scientists group fishes to improve prediction

Brandon Peoples

Most scientists share the gripping compulsion to place things into tidy categories. Fish ecologists are no exception.

Why group?

Grouping species helps improve our understanding and management of aquatic ecosystems. Because local fish diversity can be quite high, predicting responses of individual species to environmental change or nonnative introductions can be difficult. Grouping fishes based on biological or ecological similarities can help reduce that complexity because species in the same group often respond similarly to same threats.

Didn’t we already have groups?

By the time we finish elementary school, we’re all familiar with the hierarchical Linnaean classification system that groups organisms by their common ancestry. Of course, this system is one of the greatest contributions to the natural sciences, and is the basis for phylogenetic analysis.

However, closely-related species often perform starkly different ecological functions. For example, darters are much more closely related to walleye (they’re in the same family, Percidae) than they are to sculpins. Yet, darters eat, behave and function much more similarly to sculpins than to walleye.

Thus, in ecological analyses, phylogenetic groupings may not always be as effective as other types of groups.  

Redfin darter (left) and banded sculpin (right) are small benthic insectivores that inhabit riffle habitats; walleye (center) are large pelagic predators. Source

What types of groups?

Fishes are often grouped according to guilds—species that exploit the same resources typically in similar ways. Guilds can be based on any ecological requirement (some of these include flow, habitat type, etc…). The two most commonly applied guilds are trophic (feeding) and reproductive.   

Trophic. Trophic guilds are classified by what fishes eat and are subdivided by how they eat it (modes of feeding). A well-accepted classification system* of freshwater fishes recognizes five trophic guilds and 26 feeding modes. For example, there are 10 different feeding modes that invertivores use. A few of these include surface drift feeding, grazing and digging.

Carmine shiners are drift-feeding insectivores, and can be affected by riparian deforestation. Source

Reproductive. Reproductive guilds are firstly classified by what is done after eggs are laid—they are either guarded, unguarded, or born on/inside the fish. Within this scheme, classifications are subdivided based on the substrate needed for spawning**. A few examples include lithophils (requiring gravel), psammophils (requiring sand), and speleophils (requiring cavities).

How do fish ecologists use groups?

Human impacts. Groupings have greatly improved our understanding of the impacts of human development on fishes. In fact, groupings are the basis of community-level bioassessment. For instance, abundances of lithophilic fishes predictably decrease with increasing sedimentation from human development.

Simple lithophils like this blacktip jumprock are vulnerable to sedimentation. Photo by Brandon Peoples. 

Indirect effects.  Groupings help us to conceptually simplify food webs to improve fisheries management. Many fisheries-related problems, especially in smaller systems, are related to unbalanced food webs. Understanding which group’s biomass needs to be increased or decreased can help us solve the problem.  

Improving our groups

Groupings are only as good as the information used to build them. Oftentimes information on a species’ feeding or reproduction only comes from one study. However, species traits are plastic—they can vary through space and time. For widely-distributed species, scant life history information may lead to poor classifications.

As we continue to learn more about the basic biology of fishes, our groupings will become more precise so that we can better predict fishes’ response to environmental change and nonnative introductions.  

*Goldstein, R. M. and T. P. Simon. 1999. Toward a united definition of guild structure for feeding ecology of North American freshwater fishes. Pages 123-220 in T. P. Simon, editor. Assessing the Sustainability and Biological Integrity of Water Resources using Fish Communities. CRC Press, Boca Raton.

**Balon, E. K. 1975. Reproductive guilds of fishes - proposal and definition. Journal of the Fisheries Research Board of Canada 32:821-864.