Req 7c — Fish Stomach Contents

Every fish you’ve ever seen in a lake or stream is either hunting or being hunted. The stomach contents of a fish are a snapshot of that fish’s role in the food web — what it eats, when it eats, how successful it was, and sometimes, what parasites and non-food items it’s accidentally consumed. Fisheries biologists call this diet analysis, and it’s one of the most revealing ways to understand the ecology of a water body.

Why Diet Analysis Matters for Management

If a lake’s trout population is declining and you find that the trout are full of non-native crayfish instead of the native insects and minnows they evolved to eat, you now have a management hypothesis: the non-native crayfish may have disrupted the food web. Diet analysis has revealed invasive species impacts, documented cannibalism in stressed populations, confirmed the effectiveness of hatchery stocking (hatchery fish eating wild food sources = survival), and identified lead sinker ingestion in loons and ducks.

Getting Your Three Fish

The requirement is explicit: you don’t need to catch the fish yourself. Options include:

Fish cleaning stations: If you’re near a lake or river with public fishing access, ask anglers at a cleaning station if you can examine the stomachs of fish they’re already cleaning. Most are happy to help and find it interesting. Bring a labeled container for the stomach contents.

State hatchery: Contact your state hatchery and ask if a biologist would supervise you examining stomachs from fish they’re working with. Many hatcheries regularly collect diet data.

Bait shop: Ask if they know of a local fishing guide or tournament angler who might be willing to let you examine a catch.

Your own fishing: If you fish, you can examine the stomachs of fish you’re already cleaning for the table. The fish are not being harmed specifically for this requirement.

Fisheries biologist cooperation: Your merit badge counselor may be able to connect you with a state biologist who is conducting a fish survey and would include diet analysis.

Conducting the Analysis

Equipment Needed

• Dissection scissors or small sharp scissors
• Blunt-tipped forceps (tweezers)
• A white dissection pan, white plate, or white paper — to make contents visible
• Hand lens (10x) for identifying small items
• Field guides or printed reference photos for invertebrate identification
• Labeled containers for each fish’s stomach contents
• Data recording sheet

Procedure

1. Identify and record the fish: species, estimated length, where and when it was caught (if known)
2. Remove the stomach: the stomach is a J-shaped pouch between the esophagus (coming down from the mouth) and the intestine. Cut it free and place it in a labeled container.
3. Cut the stomach open along its length and spread the contents in the white pan
4. Record all visible items: What do you see? Use these categories:
   • Insects/invertebrates: Mayflies, caddisflies, midges, dragonfly nymphs, beetles, crayfish, amphipods — note whether whole or fragments
   • Fish: Scales, bones, whole small fish — note species if identifiable
   • Plant material: Algae, aquatic plants, seeds
   • Miscellaneous: Fish eggs, mud (incidental ingestion), parasites, human debris (plastic, lead sinkers)
5. Estimate relative volumes: What fraction of the stomach was insects? Fish? Plants? An “empty with two caddisfly cases” is very different from “packed with minnows.”
6. Note digestion state: Freshly eaten items are intact; partially digested items are harder to identify; highly digested items appear as mush

What Your Findings Tell You

For each fish, interpret your findings:

• A stomach packed with one prey type (all mayfly nymphs, for instance) indicates a concentrated food event — a hatch, or a location where that prey is abundant
• A mix of prey types indicates opportunistic foraging
• An empty stomach could mean the fish was stressed and regurgitated before it died, or that feeding activity was low
• Parasites (nematodes, tapeworm segments, spiny-headed worms) are common findings and worth noting — heavy parasite loads can indicate stressed or crowded populations
• Lead sinkers or soft plastic fishing lures are documented findings in bass and pike stomachs — evidence of accidental ingestion of human debris
• A stomach full of crayfish in a trout, when crayfish are a non-native species in that water, is evidence relevant to invasion ecology

Diet analysis connects individual fish to the entire food web of the water body. The next option takes that ecosystem view even further.