  The size structure of fish-communities is an emergent high-level property of
marine food webs responsive to changes in structure and function. To measure
this food web property using data arising from routine fisheries surveys, a
simple metric known as Typical Length has been proposed as more suitable than
the Large Fish Indicator, which has been highly engineered to be responsive to
fishing pressure. Typical Length avoids the inherent dependence of the Large
Fish Indicator on a parameter that requires case-by-case adjustments. Using
IBTS survey time series for five spatial subdivisions of the Greater North Sea,
we show that the Typical Length can provide information equivalent to the Large
Fish Indicator when fishing is likely the strongest driver, but differences can
also arise. In this example, Typical Length exhibits smaller random
fluctuations ("noise") than the Large Fish Indicator. Typical Length is also
more adaptable than the Large Fish Indicator and can be easily applied to
monitor pelagic fish in addition to demersal fish, and together with
information on the potential growth of the fish community, a proxy of which can
be derived from the Mean Maximum Length indicator, it is possible to partition
change in community composition from change in size structure. This suggests
that Typical Length is an improvement over the Large Fish Indicator as a food
web indicator with the potential to offer further insight when considered in
conjunction with indicators of community composition.
