The purpose of this chapter is to help students understand the factors influencing the survival of organisms in an environment, focusing on the interdependent relationships in an ecosystem. It calls attention to the limitations of our resources and to the human impact on the environment. This activity will help students develop the skills needed for scientific inquiry such as interpreting data to make inferences about fluctuations in fish populations and analyzing the effects of human use and habitat changes on fish populations. This activity, with teacher input, can also be used to help students understand the nature of scientific inquiry and the nature of scientific explanations as they consider the reliability and validity of data to support their claims and as they sort through the data to identify specific relationships between factors that exist in a complex system of interactions. Students can learn to use causal patterns as they reason about patterns in relationships and forms of cause and effect with teacher guidance.