Choose the most suitable one-word substitution for the following expression.
A person who scientifically studies the behaviour and biology of fish.

The correct answer is Ichthyologist.

Let's break down the meaning of each word to understand why "Ichthyologist" is the most suitable one-word substitution:

1. Ichthyologist: A specialist or scientist who studies the biology, behavior, and classification of fish. The prefix "ichthyo-" comes from the Greek word for fish, and "-logist" refers to a person who studies a particular subject.

2. Ecologist: A scientist who studies how organisms interact with each other and their environment.

3. Biologist: A scientist who studies living organisms and life processes in general. While fish are living organisms, this term is far too broad.

4. Herpetologist: A scientist who specializes in the study of reptiles and amphibians (such as snakes, lizards, frogs, and turtles), not fish.

Therefore, a person who scientifically studies the behaviour and biology of fish is an Ichthyologist.