Introduction

Fish are the most abundant, widespread and diverse group of vertebrates, comprising about 22 000 species with a dazzling variety of form, size and habits. To exploit this profusion of potential food, humans have devised a wide array of gears to capture fish. Ecologists rely heavily on modified forms of these methods to census fish populations, since fish are often difficult to observe in their natural habitat.

Sampling fish requires a high level of resources (e.g. time, labour, cost of equipment), and this increases with the size of the habitat (e.g. a pond versus the sea). Many commercial-scale techniques (e.g. deep-sea seines and trawls) are beyond the scope of ecological sampling, but they can be scaled down to suit smaller habitats. To census fish in the largest aquatic systems, we recommend using data from commercial catches, where available, or visiting markets where fish are landed. The use of local knowledge and technology, particularly where resources are limited, is always recommended.

Methods for capturing fish fall into two categories: passive methods, which rely on the fish swimming into a net or a trap; and active methods, in which fish are pursued. The fact that fish are cold-blooded influences the choice of method and timing of sampling. For example, in temperate zones, active methods may be more successful in winter when fish are less mobile, whereas passive techniques may work best in summer when fish are more active. The choice of method will also be guided by gear selectivity.