The 'direct'
measurement of growth and production is difficult in
many species, especially bivalve molluscs because (a)
a large proportion of the total production can be lost in
the form of gametes (either in a single complete release
or by gradual spawning), (b) it is impracticable to
measure weight changes in animals with shells and
with variable amounts of seawater, and (c) it is not
sufficient to measure changes in shell length because
there is no apparent tight coupling between shell
growth and tissue growth