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