---
title: "By-catch — AP Environmental Science Definition & Exam Guide"
description: "By-catch is the unintended capture of non-target species during fishing, a key overfishing problem in AP Enviro Unit 5 that threatens biodiversity."
canonical: "https://fiveable.me/ap-enviro/key-terms/by-catch"
type: "key-term"
subject: "AP Environmental Science"
unit: "Unit 5"
---

# By-catch — AP Environmental Science Definition & Exam Guide

## Definition

By-catch is the unintended capture of non-target species (like dolphins, sea turtles, or juvenile fish) during commercial fishing, often killing or injuring them and contributing to the biodiversity loss tied to overfishing in AP Environmental Science Topic 5.8.

## What It Is

By-catch is everything a fishing operation pulls up that it wasn't trying to catch. When a boat sets out for tuna or shrimp, its nets and lines don't discriminate. They scoop up dolphins, sea turtles, seabirds, sharks, and undersized fish too. Most of that unwanted catch gets thrown back dead or dying.

In [AP Enviro](/ap-enviro "fv-autolink"), by-catch lives under **Topic 5.8, Impacts of Overfishing**. It matters because it's a hidden cost of how we fish. Methods like longlining, trawling, and drift nets are efficient at catching the target species, but they're also indiscriminate. So even a fishery that's technically managing its target stock can still be wrecking [populations](/ap-enviro/unit-3 "fv-autolink") of other species it never wanted. That's why by-catch is treated as one of the core *problems* of overfishing, not a separate issue.

## Why It Matters

By-catch sits in **[Unit 5](/ap-enviro/unit-5 "fv-autolink"): Land and Water Use** and supports learning objective **AP Enviro 5.8.A**, "Describe causes of and problems related to overfishing." The essential knowledge says overfishing can lessen biodiversity in aquatic systems and harm people who depend on fishing. By-catch is a direct mechanism for both. When fishing gear kills non-target species, it removes organisms from the [food web](/ap-enviro/key-terms/food-web "fv-autolink") that no one was even harvesting, dragging down overall biodiversity. If you're asked to explain a *problem* of overfishing on the exam, by-catch is one of the cleanest, most specific answers you can give.

## Connections

### Overfishing and Fish Stock Collapse (Unit 5)

By-catch is one piece of the larger [overfishing](/ap-enviro/unit-5/impacts-overfishing/study-guide/3qhg3l72iRrDu0YemvRP "fv-autolink") picture. Overfishing crashes target species, but by-catch quietly drains the non-target species at the same time, so the total damage to the ecosystem is bigger than the catch numbers suggest.

### [Biodiversity Loss (Unit 2)](/ap-enviro/key-terms/biodiversity-loss)

By-catch is a real-world example of how [human activity](/ap-enviro/key-terms/human-activity "fv-autolink") reduces species diversity. When you remove sea turtles or sharks accidentally, you shrink the variety of life in that ecosystem, which connects straight back to why biodiversity matters for ecosystem stability.

### Trophic Levels and Food Webs (Unit 1)

Killing non-target species through by-catch disrupts food webs. Remove a [predator](/ap-enviro/unit-1/intro-ecosystems/study-guide/IR1y6F8EcTH2cQsPbCHQ "fv-autolink") like a shark and its prey population can explode; remove prey species and the predators starve. By-catch shows how pulling one organism out ripples through the whole trophic structure.

### Sustainable Yield and Fishing Practices (Unit 5)

Solutions to overfishing, like turtle excluder devices and selective gear, exist largely to cut by-catch. Sustainable fishing isn't just about catching fewer target fish; it's about gear that stops catching the wrong things in the first place.

## On the AP Exam

By-catch shows up as a named *problem* of overfishing. On multiple-choice questions, expect stems describing a fishing method and asking which environmental consequence follows, or asking you to identify by-catch from a description of dolphins or turtles caught in tuna nets. On free-response, you'll typically need to either define by-catch in your own words or describe it as a cause of declining biodiversity. The 2018 SAQ that used an Arctic food web is a good model for how the exam links species removal to food-web disruption, which is exactly the reasoning by-catch requires. To earn the point, name the mechanism clearly: non-target species are caught and killed unintentionally, which lowers biodiversity.

## By-catch vs Overfishing

Overfishing is taking too many of the target species, so a fishery harvests faster than the population can reproduce. By-catch is catching species you weren't aiming for at all. They're related (by-catch happens during fishing operations), but on the exam, overfishing depletes the fish you want, while by-catch kills the fish, turtles, and mammals you didn't.

## Key Takeaways

- By-catch is the unintended capture of non-target species during fishing, and most of it dies or is discarded.
- It lives in AP Enviro Topic 5.8 and supports learning objective AP Enviro 5.8.A on the problems of overfishing.
- By-catch reduces aquatic biodiversity by removing organisms no one intended to harvest, like dolphins, sea turtles, and sharks.
- Indiscriminate fishing gear such as longlines, trawls, and drift nets is the main cause of high by-catch.
- By-catch and overfishing are different problems: overfishing depletes the target species, by-catch kills the non-target ones.
- Solutions like turtle excluder devices and selective gear exist specifically to reduce by-catch.

## FAQs

### What is by-catch in AP Environmental Science?

By-catch is the unintended capture of non-target species during fishing operations, often killing or injuring them. It falls under Topic 5.8, Impacts of Overfishing, in Unit 5 and is treated as a major problem that lowers aquatic [biodiversity](/ap-enviro/unit-2/intro-biodiversity/study-guide/c77aT0cHPSCwPKS87s5o "fv-autolink").

### Is by-catch the same as overfishing?

No. Overfishing means taking too many of the species you're targeting, faster than it can reproduce. By-catch is accidentally catching species you weren't trying to catch at all. Both happen during fishing, but they damage the ecosystem in different ways.

### Why is by-catch bad for the environment?

By-catch removes organisms like sea turtles, dolphins, sharks, and juvenile fish that no one intended to harvest, which lowers biodiversity and disrupts food webs. Killing a predator or prey species through by-catch can throw off whole trophic levels.

### What causes by-catch?

Non-selective fishing gear is the main cause. Methods like longlining, bottom trawling, and drift nets are efficient at catching target species but can't tell the difference between a tuna and a dolphin, so they trap everything in their path.

### How can fisheries reduce by-catch?

More selective gear is the key fix, including turtle excluder devices, escape panels, and modified nets that let non-target animals get out. These are exactly the kinds of solutions the exam expects you to connect to sustainable fishing practices.

## Related Study Guides

- [5.8 Impacts of Overfishing](/ap-enviro/unit-5/impacts-overfishing/study-guide/3qhg3l72iRrDu0YemvRP)

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