West Coast Groundfish Diet Research
Fishes’ food habits along the U.S. West Coast.
What Do Fish Eat?
Knowing the relationship between predators and prey is fundamental to understanding ecosystems. Teams within the Groundfish Ecology Program collect information about the diets of a variety of marine fishes that live along the U.S. West Coast. Stomach contents provide a snapshot of a fish’s diet at a specific place and time. We either record fish stomach contents at sea or preserve whole stomachs to examine back in a lab.
Analysis of stomach contents begins with the identification of specific prey items. Skilled scientists pull apart the ball of partially digested prey from the stomach, called the ‘bolus.’ We then sort the prey into the lowest possible taxonomic groups. We note the number of individuals, weight, volume, and state of digestion for each prey group found in a stomach.
What About Long-Term Diets?
Stomach contents give us useful information about what a fish was eating when we captured it. They don’t tell us whether something is being eaten often or infrequently. Some prey gets digested more quickly, and we may never observe it in a stomach at all. This uncertainty can result in over- or underestimating the importance of prey. Collecting stomachs over a longer period can help, but we have an additional tool we use to address these kinds of questions.
Stable isotopes occur naturally and are incorporated into the tissues of all living things from their diets. In other words, you are what you eat. This process happens gradually over time, making stable isotopes effective tracers of long-term diets. After a stomach is collected, we take and freeze a small sample of muscle tissue. We process the tissue back at the lab and then run it through a special kind of machine called a mass spectrometer. This machine tells us the stable isotope composition of the muscle tissue. By comparing this to the stable isotopes of prey, we gain an improved understanding of the long-term importance of different parts of a fish’s diet.
What Species Do We Look At?
Our teams target a variety of fish, including:
What Do We See in Diet Data?
There are many different ways to look at and summarize diet data. Here is an example where we grouped the contents into larger categories for Greenstriped rockfish (Sebastes elongatus). We took a total of 78 stomachs between 2008 and 2011. This type of analysis is a simple presence/absence, also called frequency of occurrence, and shows the percentage of total stomachs that contain each prey item.
- The Pacific hake survey takes diet information during every survey for Pacific hake, but also has taken diets for Widow rockfish.
- The West Coast Bottom Trawl Survey collects diet information for a variety of species, including lingcod, sablefish, petrale sole, arrowtooth flounder, Pacific sanddab, and many rockfish species
- Alaska Fisheries Science Center Stomach Examiner’s Tool is a great resource for a variety of information for performing prey analysis.
- Northeast Fisheries Science Center Food Web Dynamics maintains a food habits database for Northeast Atlantic fishes.
- Stable isotopes in fish ecology.