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Trout Anatomy

Trout Anatomy Basics

Students are likely to gain more from their exploration of the world of the Brook Trout if they have a basic understanding of the anatomy and physiology of fish.

The obvious difference between fish and land-based animals is in the way they “breathe”. Fish obtain oxygen from the water using their gills. As water passes across the gills, oxygen molecules dissolved in the water diffuse across the gill membrane and enter the bloodstream. At the same time carbon dioxide and ammonium, metabolic waste products, diffuse across the gill membrane from the bloodstream into the water and are carried away.

Fins are an important feature of fish anatomy. They come in different shapes and sizes and are attached in different positions. They perform a variety of functions. The tail or caudal fin propels the fish through the water. The Brook Trout has the “square-shaped” tail fin characteristic of a strong swimmer who needs to be able to swim through swift water currents. The dorsal fin and ventral or anal fin provide stability, allowing the fish to maintain an upright position in the water. The pectoral and pelvic fin are used much like the rudder on a boat, allowing the fish to steer or change direction. The adipose fin is a vestigal fin of unknown function.

Like most animals, fish have skin. Many fish, including the Brook Trout, have an additional outer covering of scales embedded in the skin. Scales are arranged much like the shingles on a roof. Scales protect the fish. The scales of the Brook Trout are very small. Fish also have a protective film of mucous called a slime coat.

Students often wonder how well a fish sees, smells, and hears. In fact, many of these senses are highly developed in fish. Brook Trout have large eyes and use them to hunt for their prey and avoid predators. Likewise, Brook Trout have an acute sense of smell and can use a series of nasal receptors to sense the particular odors of the streams in which they live. A series of cells arranged in a linear pattern along both sides of the fish called the lateral line allows fish to sense vibrations and disturbances in the water – including those caused both by predators and prey. The lateral line is somewhat analogous to both the sense of hearing and a motion detector.

Another unique feature of many fish is the air bladder. This “bag of air” changes the buoyancy of fish, enabling them to maintain a certain position in the water column. Air can be added or removed from the bladder by the fish to move up or down in the water. The air bladder reduces the energy a fish must expend to maintain a certain position in the water.

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