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MONITORING OF AQUATIC ENVIRONMENT ^{ocv fran /wlr::o}

^/a1.t

BENIN NATIONAL TEAI"I

Surrrmary

of

report

The 1984

rePort is a

^summ ary

of the results

obtained

in the

Southern Extension zone

in Benin.

No conunent

wiLl

be made on

the effects of

the

insecticides

on

the various

taxonomic

groups.

The

report will deai solely with the

seasonal

fluctualions

observed

in

those groups.

Results and observations

1.

Hydrology

The 1983 annual

report

ment,ioned

the deplorably

_low

rainfall.

rhis year rainfall

has been spread somewhat more evenly

in time.

^However

recordings

for the station of

Beterou show

that the

mean maximum

for

19g4

is

below

that of 1983. rt

should, however, be noted

that

because

rainfall

was

norBally distributed in time, _{the water}

_{1evel was} maintained, as compared

r.rith last year. At the

Kouffo

station,

i9g4 was

a

year

of high flood.

2.

Invertebrates Surber-sampling

Beterou

The

results

obtained

are

shovm

in

Table

1.

Comparison

of the results

recorded shows

that

Trichoptera markedly predominate.

The

relative

importance

of the other

groups which come

next in line after the Trichoptera,

_such

_as the Ephem".opt"i., the

Chironomidae and the Simuliidae, _changed irom month

to

month.

-

^The mean

densities

per m2

varied greatly

from

53

763.51

in July,

up

to

99 527182

in

August and down

to 63

^gg6.94

in

^November 19g4.

-

^The mean

density

per m2 rose from

29

437

in

^November 19g3

to

63 ^gg6.94

in

November

1984. This result is

^probably due to,

differences in the

hydrolo-

gic;iI conditions during the

two years under study.

Atchakpa

A comparative study

of the resuLts in

Tabie

rrr

shows

a net

predominance

of the

Triehoptera groups

for the

same

periods in

1983 and

19g4.

Numbers,

frequency _{and mean}

density per

m2

varied

considerably between 19g3 and 19g4.

For

example,

in the

case

of

Trichoptera

Numbers Z frequency

84 .67 78. 50

(1983) _270.8 (1984) _1e3.6

.t; | -2-

Densities

From 14 2LL,9

in

^{November 1983}

the figure fell to

10 958.90

in

^November 1984.

Drltts

B6t6rou ^(Ou6m6)

Generally

the ratio H:

^{has been}

quite high, higher in fact

than

in

1983.

Atchakpa ^(Ou6m6)

The

results

obtained here

for the different factors are

about

the

same as

at the

B6t6rou

station.

Lanta

^(Kouffo)

Hydrological conditions this

^{year made}

it possible to take

samples from

the

Lanta

station.

^A request had been made

to

change

this station.

General"Ly

the ratio ffi'

^{has been}

^very

^high.

Conclusion

In

entomology,

the

work was

carried out

according

to the

planned programme

for 1984.

The Lanta

station

on

the

Kouffo was maintained although changing

it

has been envisaged because

of the hydrological

conditions.

3. I"Ehy.l-ggl

ft

was

possible to carry out only

two

missiorsout of four for

two reasons:

(a)

The mission ^planned

for

February-March was

not

undertaken because

of the ichthyologists' training

course and

the

1984

hydrobiologistsr

meeting.

(b) Mr.

Tauzes,

the

teamrs

ichthyologist

was evacuated

for health

reasons

following

an

accident,

as mentioned

in the

1983

report.

Note:

N.D. =

night-time drift

D.D. = day-time

drift