Hidden fish populations protect us from ourselves!

Reference: Midway, S.R., White, J. W., and Scharf, F. S. 2018. The Potential for Cryptic Population Structure to
Sustain a Heavily Exploited Marine Flatfish Stock. Marine and Coastal Fisheries 10: 411-423. https://doi.org/10.1002/mcf2.10032.

The Last Wild Food

Most human cultures have transitioned from hunter-gatherer lifestyles to the domestication of animals and crops. Of the many mammals and birds that we ate in the past, we have chosen a select few to breed and raise for food. In the United States, these mainly include cows, pigs, chicken, and turkeys.

The exception to our tendency to simplify and domesticate animal protein? Seafood!

From trout, tuna, and tilapia to shrimp, shellfish, and squid, a shockingly diverse array of ocean wildlife across species, genus, and phylum are categorized under one label: seafood. However, as human populations grow, and fisheries technology advances, wild fish populations are now under threat. There is a need to understand these species and manage them carefully to ensure their populations remain stable.

The Study

Midway et. al. studied the spatial distribution and population structure of the Atlantic flounder (Paralichthys lethostigma). The species lives along the southeastern US coast, from northern Florida to North Carolina. In North Carolina, inshore fisheries remove a shocking 70-90% of the population; but somehow,  populations have mysteriously remained relatively stable! The study aimed to examine why populations have persisted at a sustainable level so that we can ensure they continue to do so.

Two potential hypotheses could explain the persistence of Atlantic flounder populations. The first was that populations had high steepness. This means that a high number of juvenile fish are born and survive to become adults even though fishing has drastically reduced the number of adult breeders.

The second hypothesis was that parts of the Atlantic flounder stock are unknown or inaccessible to commercial fishers. These “hidden” subpopulations are called cryptic biomass. Atlantic flounder are fished from inshore estuarine waters, where saltwater and freshwater mix and rivers transition to the sea. Adults migrate offshore to breed. Previously, we believed all fish return inshore after spawning, but the authors proposed that some adult fish may stay offshore, where they are inaccessible to fishers.

Divers have spotted Atlantic flounder adults on offshore reefs during times of the year that don’t correspond with spawning season: this may be more evidence that cryptic biomass plays a role in keeping fish populations at stable numbers.


Mathematical models combine known data and unknown data (represented as variables) to predict possible outcomes. To test their two hypotheses, the authors used a mathematical model to see how varying scenarios of fishing, steepness, and cryptic biomass affect population dynamics. The goal was to identify which scenarios would result in the maintenance of sustainable population levels.


The results of the model showed that the current levels of fish population persistence cannot be explained by high steepness alone. Therefore, it is likely that cryptic biomass plays a role in maintaining sustainable populations. Because Atlantic flounder migrate offshore for part of their life cycle, and because they have been seen on offshore reefs by divers, it is likely that these offshore reefs provide a refuge for some spawning adults.

So what?

The results of this study have important implications for fisheries management. Setting limits on fishing offshore habitats, perhaps through the establishment of marine protected areas, will ensure that cryptic biomass remains undisturbed and continues to contribute to population persistence. The more we know, the better we can protect our ecosystems and make sure we have enough fish to eat for generations to come!

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Sarah Shainker

Sarah is a Phd student at the University of Alabama in Birmingham interested in evolutionary ecology, population genetics, citizen science, and macroalgae. Before beginning grad school, she worked as an outdoor educator in the north Georgia mountains and as a coastal resource management volunteer for Peace Corps Philippines. Twitter: @SarahShainker

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