Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 439

METHOD 21.1.
Sampling Fish Populations
A wide variety of techniques are available for sampling fish populations. De-
termining the method to use depends on the physical constraints of the habi-
tat being sampled, the species of interest, the information being sought, and
the relative effectiveness of the method. The capture methods can be cate-
gorized as passive capture techniques, active netting, and electrofishing. All
three are used in freshwaters and require determination of catch per unit ef-
fort. If all things are equal, twice the effort should lead to capture of twice
as many fish. Of course, as population density decreases, additional effort to
capture individuals will lead to a lower amount of catch per unit effort.
Passive capture techniques work by entanglement, entrapment, or an-
gling with set lines (Hubert, 1996). Gill nets are commonly used to entangle
fish in meshes of specific size. The larger the mesh size, the greater the se-
lectivity for larger fish. Trammel nets also entangle fish. They are fine-mesh
nets with larger mesh next to the net. When the fish pushes the smaller mesh
net through the larger mesh, it forms a pocket in the smaller mesh net and
is caught. Entrapment gear allows the fish to enter, but not leave, and may
include bait to lure the fish. One type of entrapment gear has cylindrical
hoop nets with a series of funnels in which the fish are able to move into the
cylinder but not out. Fyke and other types of trap nets have additional pan-
els of netting at the entrance to a cylindrical trap that guide fishes into the
trap. Pot gears also use entrapment. These are rigid traps with funnels or
one-way entrances. Minnow traps, crayfish pots, and similar equipment are
included in this group. Weirs are barriers built across a stream to divert fish
into a trap; these work well on migratory (usually reproductive) fishes.
Active gears require moving nets through the water to capture organ-
isms (Hayes et al., 1996). Nets can be towed (trawled) at the surface,
annual marks can be observed as well (Fig. 21.10). Scales and fin rays are
commonly used to determine age because they can be removed without
killing the fish. Otoliths (calcified structures from the inner ear) yield more
reliable aging results but require sacrifice and dissection of the fish.
Once age distribution of a population is determined, recruitment and
mortality for each size class can be calculated. Reproduction and mortality
can vary tremendously (up to 400-fold) among year classes (Pitcher and
Hart, 1982), and explaining this variation is an important part of fish pop-
ulation management and ecology. Life tables such as those used by demog-
raphers and population ecologists allow mortality and recruitment rates to
be calculated for each age class in a fish population. Age-specific life tables,
for which a cohort is followed for years, are the most useful. These tables
are constructed by calculating the proportion of fish surviving each time in-
terval, generally a year. The recruitment and mortality at each size class can
be correlated with environmental factors and used to calculate the influ-
ences of natural factors and exploitation on fish populations.