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ADVANTAGES OF PERCH (Percafluviatilis) SEASONAL MIGRATIONS TO BRACKISH WATERS.
Linas Lozys, Laboratory of Marine Ecology, Institute
2600, Lithuania, lozys(dawekoi.lt
Introduction. Perch (Percafluviatilis) are known as
seasonally migrating fish species in southern part of
the Baltic Sea. Perch inhabit brackish coastal waters
of the southern Baltic in summer time, while they
usually winter in the freshwater lagoons. The
objective of the study was to explain experimentally
perch spring migration behaviour while they migrate
from the freshwater Curonian Lagoon to the brackish
Baltic Sea, Lithuania, and to prove a cause of their
backward migration to the Lagoon in the autumn.
Methods. Field study. Material for the perch in situ
growth study was collected during 1998 - 2000, in
September when growth in length' and accumulation
of energy resources reached the yearly maximum.
The growth study focused on perch individuals of
1997-year class, since this generation dominated in
both water bodies: individuals of the other age groups
were scarce. All perch were weighed (± 1.0 g) and
measured (total length L, ± 0.5 cm). Condition factor
(CF) and fatness coefficient (Kf) were calculated as
follows (Bagenal & Tesch, 1978; Bukelskis &
Kublickas, 1988): CF = W/L3 X 100, where W, body
weight in grams, L, total length in cm and KF = w /
W x 100, where w, weight of fat on the intestines, in
grams, W, body weight, in grams. Since differences
in the growth rate of perch for the same age group is
considerable, the parameters were calculated for the
same length, but different age perch individuals. Both
sexes were combined in calculations. Fat content (CF)
in muscle tissues was expressed as a percentage of
wet weight. Perch age was estimated from growth
zones of opercular bones. Only females were aged.
For statistical analysis t-tests was used.
Experimental study of growth. To evaluate the
effect of salinity on perch growth, a survey of perch
YOY growth under different salinity conditions was
conducted. The first experiment was performed at
"high temperature" range (mean water temperature
19.2) and the second - at "low temperature" range
(mean water temperature 12.4). Available laboratory
facilities restricted the experiment to YOY providing
sufficient density of fish for statistical analysis. Perch
YOY were collected by seine netting at Vente cape, a
coastal area in the eastern part of the Curonian
Lagoon (55°21' N, 21°12' E) in August 2001 and
2002. After 5 days acclimatization in fresh water, fish
were randomly divided into 15 experimental groups.
Each group comprised 10 fish, which were reared in
identical 100-1 glass aquaria under continuous
aeration. The influence of salinity on the perch
growth was studied under certain three experimental
salinity conditions: five aquaria contained water at 5
ppt, five at 2 ppt and five contained fresh water. The
duration of the experiment in "high temperature"
range was three weeks, while the duration of the
experiment in "low temperature" range was six
weeks. Water temperature was ambient in all aquaria
and during the experimental period, fluctuated from
24.9 to 16.00C in "high temperature" range, while in
of Ecology, Vilnius University, Akademijos st. 2, Vilnius
"low temperature" range - from 14.1 to 11.70C. All
fish were provided with the same quantity of live
Chironomus spp. larvae, weighed with accuracy of
0.01 g. To ensure similar food consumption between
replicates, each new portion of food was introduced
immediately following consumption of the previous
ration. Total weight (±0.01 g, after excess water
removed) for each replicate was measured prior to
the beginning of the experiment and after each week.
To assess differences in growth under different
treatments, the weight increments of perch YOY
groups of each replicate were analysed using Tukey
honest significant difference test.
Results. Perch start to migrate from the Curonian
Lagoon after wintering and spawning. Field
observations demonstrated better growth in length
and condition (Fig. 2, 3) in the sea despite lower
water temperature (Fig 1).
24
22
20
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- 16
0
&- 14
4   12
o  10
E8
0 6
4
2
w       IV      V       V
Vi            VII             KX
Month
Figure 1. Water temperatures in the Curonian Lagoon
(-) and the Baltic Sea (A) during March -
September 1998-2000 (vertical bars represent 1 SD).
E
U
0
-a
I-
1998
1999
2000
1+          2+           3+
Figure 2. The total length of perch of 1997 year class
in the Curonian Lagoon (i) and the Baltic Sea (A)
during September of 1998-2000 (vertical bars
represent 1 SD) (Lozys 2001).
The results of the experiment in "high
temperature" range demonstrated the significant
affect of salinity on YOY perch growth at both 5 and
2 ppt salinity conditions (Fig. 4) (Lo'ys, in press).
Differences in growth between 2 and 5 ppt salinity
replicates were insignificant. Salinity affect was
insignificant at the "low temperature" range (Fig. 5).
77
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