Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 440

through the water column, or across the bottom. Alternatively, purse seines
can be used to encircle the fish and confine them into successively smaller
space as the net is drawn closed. Other active gears include nets that are
thrown or pulled up rapidly from the bottom, spears, dipnets, dynamite,
and rotenone (poison).
Electrofishing is a common and effective way to capture fish. Either AC
or DC current is used to alter the behavior of fish or to stun or kill them
(Reynolds, 1996). The most common strategy is to stun fish and then catch
them with a net. To stun fish, the current and its pulsing frequency are con-
trolled at a level that causes paralysis but little injury and death. Water
conductivity is an important factor in determining effectiveness of the tech-
nique. Low-conductivity waters are poor conductors of electricity and de-
crease the effectiveness of the method; very high-conductivity waters allow
the charge to dissipate too rapidly.
Fish can be counted directly when removal is not necessary. Scuba div-
ing or snorkeling are commonly employed. Habitats that are very turbid,
with excessive water velocity, or unsafe for human contact are obviously
not suitable for such techniques.
Several methods are used to determine total population size, depend-
ing on habitat and method of capture. If sampling is highly effective, such
as in a small pond with a clean bottom, the fish are collected until all are
captured. It is rarely possible to capture all the fish in a habitat, and sev-
eral other methods are used to compensate. A depletion method repeatedly
samples the same area, and the decrease in catch per unit effort is used to
estimate the original population. Alternatively, mark and recapture tech-
niques are used. In this method, a sample of fish is taken and marked. Fish
are later recaptured, and the proportion of marked fish is used to estimate
the population size. There are several marking techniques, including fin
clipping, external and internal tags, and marks (Guy et al., 1996).
Generally, a negative correlation exists between recruitment and
growth. If a large number of fish occur in a size class, then significant, in-
traspecific competition can occur. This leads to poor fish yields in the size
class. If food is not limiting (rarely the case in natural situations but pos-
sible in aquaculture), recruitment and growth may both be high.
REGULATING EXPLOITATION OF FISH STOCKS
Any fishery containing desirable fish and with unlimited human access
will be overexploited without management. The amount of fish that are
taken per unit time is the yield of a fishery. The ability to predict potential
fish yields is central to attempts to manage exploited fish stocks. For many
years, it was thought that the maximum sustainable yield (MSY) could be
calculated and used to estimate the maximum production of any fish pop-
ulation in a water body. The MSY was calculated by William Ricker