Biological field and laboratory methods for measuring the quality of ...

Deepwater seInIng usually requires a boat.
One end of one of the hauling lines is anchored
on shore and the boat pays out the line until it
reaches the end. The boat then changes direction
and lays out the net parallel to the beach. When
all of the net is in the water, the boat brings the
end of the second hauling line ashore. The net is
then beached rapidly.
The straight seines (without bags), such as the
common-sense minnow seines, can usually be
handled quite easily by two people. The method
of paying out the seine and bringing it in is
similar to the haul seine, except the straight
seine is generally used in shallow water where
one member of the party can wade offshore
with lines.
Bag and straight seines vary considerably in
dimensions and mesh size. The length varies
from 3 to 70 meters, and mesh size and net
width vary with the size of the fish and the
depth of the water to be sampled.
Nylon seines are recommended because of the
ease of maintenance. Cotton seines should be
treated with a fungicide to prevent decay.
Seining is not effective in deep water because
the fish can escape over the floats and under the
lead line. Nor is it effective in areas that have
snags and sunken debris. Although the results
are expressed as number of fish captured per
unit area seined, quantitative seining is very
difficult. The method is more useful in determining
the variety rather than the number of
fish inhabiting the water.
2.2.2 Trawls
Trawls are specialized submarine seines used
in large, open-water areas of reservoirs, lakes,
large rivers, estuaries, and in the oceans. They
may be of considerable size and are towed by
boats at speeds sufficient to overtake and enclose
the fish. Three basic types are: (1) the
beam trawl used to capture bottom fish (Figure
2), (2) the otter trawl used to capture nearbottom
and bottom fish (Figure 3), and (3) the
mid-water trawl used to collect schooling fish at
various depths.
The beam trawls have a rigid opening and are
difficult to operate from a small boat. Otter
trawls have vanes or "otter boards," which are
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FISH SAMPLING
attached to the forward end of each wing and
are used to keep the mouth of the net open
while it is being towed. The otter boards are
approximately rectangular and usually made of
wood, with steel strapping. The lower edge is
shod with a steel runner to protect the wood
when the otter slides along the bottom. The
leading edge of the otter is rounded near the
bottom to aid in riding over obstructions.
The towing bridle or warp is attached to the
board by four heavy chains or short heavy metal
rods. The two forward rods are shorter so that,
when towed, the board sheers to the outside and
down. Thus, the two otters sheer in opposite
directions and keep the mouth of the trawl open
and on the bottom. Floats or corks along the
headrope keep the net from sagging, and the
weights on the lead-line keep the net on the
bottom. The entrapped fish are funneled back
into the bag of the trawl (cod end).
A popular small trawl consists' of a 16- to
20-foot (5- to 6- m) headrope, semiballoon
modified shrimp (otter) trawl with 3/4-inch (1.9
cm) bar mesh in the wings and cod end. A 1/4inch
(0.6 cm) bar mesh liner may be installed in
the cod end if smaller fish are desired. This small
trawl uses otter boards, the dimensions of which,
in inches, are approximately 24 to 30 (61 to
76 cm) X 12 to 18 (30 to 46 cm) X 3/4 to 1-1/4
inches (0.9 to 3.2 cm), and the trawl can be
operated out of a medium-sized boat.
The midwater trawl resembles an otter trawl
with modified boards and vanes for controlling
the trawling depth. Such trawls are cumbersome
for freshwater and inshore areas.
Trawling data are usually expressed in weight
of catch per unit of time.
The use of trawls requires experienced personnel.
Boats deploying large trawls must be
equipped with power winches and large motors.
Also, trawls can not be used effectively if the
bottom is irregular or harbors snags or other
debris. Trawls are best used to gain information
on a particular species of fish rather than to estimate
the overall fish population. See Rounsefell
and Everhart (1953), Massman, Ladd and
McCutcheon (1952) and Trent (1967) for
further information on trawls.