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Knowing what fish eat can help us make smarter choices about the fish we eat

Written by Clint Leach, SoGES 2013-2014 Sustainability Leadership Fellow, and PhD Candidate in the Department of Biology and Graduate Degree Program in Ecology

It is sometimes startling for me to think how many fish interiors I have seen. Summing over three tours on the annual Gulf of Alaska/Aleutian Islands survey run by NOAA's Alaska Fisheries Science Center (AFSC), the number is surely in the thousands. For someone with no fewer than three blood-induced fainting incidents on his resume, there has to be a pretty compelling reason to willfully look at the insides of that many fish. Fortunately, there is – to find out what they eat. Though I was only a small cog in a much larger data-collecting machine, I was out there because I am fascinated by food webs – networks that map who eats whom in an ecosystem. Assembling these food webs means identifying what all the species are eating, and in the case of marine fish, this means identifying what's in their stomachs. Collecting these data requires a great deal of effort – AFSC scientists have peered into hundreds of thousands of stomachs over the last thirty years – but offers powerful insights in return.

Despite the visual tangle of an assembled food web (network diagrams like the one shown here are sometimes unaffectionately referred to as hairballs), there are patterns – nonrandom structures – that can be extracted. For instance, many food webs, like the Chesapeake Bay marine food web, can be broken into a few tightly knit groups that only loosely interact with one-another (Krause et al. 2003)⁠. In the case of the Chesapeake Bay, this means that the food web separates into two groups: a benthic group (bottom-dwelling species), and a pelagic group (water-column species), with a few key species connecting the two. 

Identifying such structures in the tangle of a food web can tell us a great deal about how energy moves through an ecosystem (flowing from plants on up) and how it might respond if one or more species are lost. When a species is removed, or its abundance substantially reduced, the effects can cascade through the food web, affecting many other species.  Knowing the structure of the food web allows us to predict what other species will be affected and how severely. In the case of the Chesapeake Bay food web, the effects of the loss of a benthic species are more likely to be contained within the benthic group, without affecting the pelagic species.  Knowing how species break into groups allows us to identify the major avenues through which energy flows and how the loss of different species will disrupt that flow.

Such tools are especially useful in fisheries management (hence the interest from AFSC) as they allow us to explore how the harvest of a particular species of fish will affect all of the others in a community.  For instance, the collapse of the Atlantic cod population on the Scotian Shelf in the early 1990's created a cascade that affected the whole community (Frank et al., 2011)⁠. Without the cod there to eat them, the fish that had previously been the cod's prey – herring, capelin, and sandlance – exploded in population. Because of this boom, they in turn drove down the populations of their prey, and so on through the food chain.  Understanding such chains of events, and how they are governed by the layout of the food web, can help us to better manage how we harvest fish so that we can keep the whole community stable.

Acknowledging and studying such interconnections highlights the fact that we participate in, and exert a large influence over, these marine food webs. Before it made it to your plate, that fish was part of a community, part of a food web, where it acquired its own dinners and might have provided dinner for something else had it not made it to you first. The energy that reaches our plate must first pass through the complex interactions between myriad organisms, and understanding how and where exactly that energy flows is of critical importance if we are to continue to safely and sustainably enjoy the products of the sea. 

References:

Frank, K. T., Petrie, B., Fisher, J. a D., & Leggett, W. C. (2011). Transient dynamics of an altered large marine ecosystem. Nature, 477(7362), 86–9.

Krause, A. E., Frank, K. a, Mason, D. M., Ulanowicz, R. E., & Taylor, W. W. (2003). Compartments revealed in food-web structure. Nature, 426(6964), 282–5.

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