The need to investigate and understand the nature of changes in abundance of economically important marine living resources has been widely recognized. They may be naturally occurring fluctuations, cycles or undirectional processes, or changes in response to artificial influences such as pollution or commercial harvesting. As direct investigation of status and population structure of some of these resources can often prove difficult, or incompatible with continuing exploitation, attention has also focussed on the identification and study of more convenient species which may be used as indices of environmental change.