The relationships between growth rates, fecundity and lengths and ages
at sexual maturity are explored, with examples primarily in spiny lob
sters, to illustrate the underlying principles involved. Changes in th
e dynamics of exploited populations of both fish and crustaceans often
have important implications for egg production. Two independent proce
sses are thought to be involved, genetic changes caused by the removal
of certain genotypes by size-selective fisheries, and growth changes
caused by altered stock densities and/or changed food availability. Th
e impacts of these processes on maturity length appear to be antagonis
tic, giving rise to some contradictory effects in different exploited
stocks, some stocks exhibiting increases and others showing decreases
in length at maturity.