FI SH S IZE FREQ UEN C Y
F4
What
What Is It?
BENCHMARK
Fish size refers to the estimated length of a fish.
No decrease of average fish size.
No
Fishes are often grouped by size into the following
size categories: : 0-5 cm, 6-10 cm, 11-20 cm, 21-30
cm, 31-40 cm and > 40 cm). Fish lengths can also be
TA R G E T
converted to biomass by using available conversion
To
To be developed.
factors14.
Why
Why Do We Measure It?
RED FL AG
Characterizing fish populations in terms of size can
To be developed.
help us to understand changes in fish population
dynamics and to identify human-induced influences
such as overfishing and habitat destruction.
(AGRRA) size categories: 0-5 cm, 6-10 cm, 11-20 cm,
The abundance of large herbivores such as
21-30 cm, 31-40 cm, and > 40 cm)3. The method of
parrotfishes and surgeonfishes is a good indicator of
data collection is the same as for Fish Diversity (S2),
potential herbivory. (Herbivory serves to reduce algal
but with a focus on fish length or biomass rather than
overgrowth and is discussed in more detail under F11
diversity of species. Key fish families and species in
- Herbivorous Fish Abundance.)
the western Atlantic are listed in Appendix 3, under
Indicator S2.
Large groupers and snappers are used as indicators
of the status of important commercial species and
Fish density (individuals per unit area) can be
the effectiveness of fishing regulations. They are often
calculated from the count data. Density and size are
the first species to show decreases in average size as
in turn used to calculate biomass.
fishing pressures increase.
Overfishing has resulted in a reduction of fish sizes,
especially for species targeted during spawning
aggregation events (e.g., Nassau and black groupers,
mutton and cubera snappers). Removing large-
sized individuals decreases spawning potential
exponentially and has the indirect effect of reducing
larval recruitment.
The exponential increase in fish egg production
associated with increasing size is a well-established
biological association that also applies to most
invertebrates (e.g., lobster and conch)15. For example,
a 40 cm grouper produces about one million eggs
but a 100 cm grouper produces about 15 million
eggs16. Thus, size does matter -- bigger fish produce
more eggs, and more eggs produce more fish. This is
one of the primary benefits of fully protected marine
reserves, the last refuges for most "Big Mamma" fish
that are commercially exploited.
How
How Do We Measure It?
Divers count and measure key species encountered
along ten transects (2 m wide by 30 m long) at each
site. The fish are typically classified according to these
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