Factors in the control of a sea fishery

[First paragraph(s)...]
The fundamental equation for the balance between incomings and outgoings of fish stocks in a fishery was first expressed by E. S. Russell in 1930: "stock this year equals stock last year plus growth during year plus recruitment of young fish during year mm.us natural mortality during year minus fish caught during year." The last, the fish caught, is the only one of these variables known to us from the start. The others must be determined by sampling.
The principal samphng tool in the hands of the biologist is the same gear used by the commercial fisherman-notably the trawl and other types of net. This gives the basic data required-the size frequency diagram-provided always that we are right in assuming that the sample is a represent- ative one. This is often far from certain. Many other techniques may be used for checking and inter-relating the estimates but the commercial fishing gear still always supplies the fundamentals.