Feeding, somatic condition and growth of Baltic herring juveniles in different salinities
Marjut Rajasilta*, Päivi Laine* and Jorma Paranko**
*) Archipelago Research Institute, University of Turku
**) Department of Biomedicine, University of Turku
FIN 20014 Turku, Finland
Introduction
In the Baltic, herring reproduces in varying salinity conditions depending on the location of each spawning area.
Larval nursery grounds are situated in or near the spawning areas, where also juveniles spend the first months
of their life. Adult herring tolerate a wide salinity range (Holliday and Blaxter 1961) and live even in lakes (Neb
1972; Høgnestad 1994), but to reproduce successfully herring requires saline water. Griffin et al. (1998) showed
experimentally that eggs of the Baltic herring are fertilised in a wide salinity range (4-32 psu), but the best result
was obtained in 8, which corresponds to the surface water salinity conditions in southern Baltic Sea. The
influence of salinity on the embryonic development of Baltic herring has been studied by Ojaveer (1981) and
Klinkhardt (1984), whereas for juvenile herring, the effects of salinity are not known.
3
Atlantic
Ocean
7
Experimental
Ba
lti
c
15
8
Herring juveniles were caught in July in the Archipelago Sea (AS), and reared until autumn in net
pens in the sea. Fish were fed daily with dry pellets (standard meal for juvenile rainbow trout), and
they were also able to feed on natural zooplankton. In October, ca 500-600 individuals were moved
to a large tank in a laboratory hall and kept in constant conditions (ambient water temperature and
salinity, daily feeding) until the start of the experiment. In December, when the fish were
approximately 5 months old, ca 200 individuals were carefully taken and placed into four
2
experimental tanks, 40-50 fish per each. Square tanks with rounded edges (area 1 m ; height 0.5
m; water volume 300 liters) were used for the experiment. The lowest salinity level was 5.7
(ambient), and higher salinities (8, 12 and 15 psu) were obtained by mixing a suitable amount of
sea salt (food quality) to sea water. Temperature of the laboratory hall and the experimental tanks
was kept at 6 °C, and constant photoperiod was used in the experiment (12 hrs light : 12 hrs dark).
Fish were fed ad libitum daily with dry pellets. In the daily rearing routines, fish behaviour was
recorded by eye, tank water was partly changed (ca 10 % of the total volume), and salinity and
oxygen were controlled. Dead individuals were removed and counted. Growth rate was determined
at the end of the experiment from fish scales, using a method described by Goolish and Adelman
(1983). At the end of the experiment 10 fish/tank were anesthetised with MS-222, a sample of
scales (several tens) was removed immediately, and the fish were killed. Total length (in mm) and
weight (in g) was determined, and the amount of visceral fat (fat index) was estimated visually
using a relative scale of 0-3 (0 = no fat; 1-2 = increasing amount of fat around the intestine; 3 = rich
3
deposits of fat around the intestine). Condition factor (CF) was calculated as CF=100*wt/le .
60°
Se
25
North Sea
AS
a
We studied the influence of water salinity on feeding, condition and growth of Baltic herring juveniles in
laboratory conditions using four salinity levels, the lowest being the ambient salinity in the nursery areas of the
Archipelago Sea.
5
20°
Measurements of the growth rate
Scales were removed from the upper caudal part (above the lateral line). This is the body area where
otolith and scale ages are equal in the Baltic herring (Eklund 1999). The scales were put into
Eppendorf tubes containing physiological teleost saline buffered with HEPES (pH 7.5), freshly added
14
unlabeled glycine (3.2 mg/ml) and 2 µl C-glycine with specific activity of 430 mCi/mmol (Amersham).
The tubes (3 radiolabelled, 1 control) were incubated in a water bath in 25 °C for 120 minutes, after
which the scales were washed thoroughly with saline buffer. From each tube, 10 scales were taken for
the measurements of the growth rate using two different methods: (G1) scales were taken in random
order and (G2) scales of uniform size were selected, as suggested by Busacker and Adelman (1987).
The area of the scales was measured individually under a dissecting microscope.
The scales were rinsed with saline and transferred to clean tubes, digested in a tissue solubilizer,
homogenised and incubated at 55 °C overnight. After adding 1 ml scintillation medium (OptiPhase
Supermix, Wallac, Finland), radioactivity in the scales was recorded with the Microbeta Plus Liquid
Scintillation Counter (Wallac, Finland). Counting time was 2 minutes. Scale growth was expressed as
2
a number of counts per scales area; counts/ mm .
Results:
Salinity affected the schooling behaviour of juveniles. At salinities 5.7 and 8, fish were scattered around, timid, moved little and sought for sheltered areas in the
corners. At higher salinities (12 and 15 psu), a dense school was formed immediately, and the fish maintained normal schooling behaviour throughout the
experiment. The daily rearing routines seemed to have no effect on their behaviour. The behaviour was only recorded by eye and no detailed data are available.
60
5.7 (n=56)
8 (n=33)
12 (n=44)
15 (n=43)
40
20
7
6
Weight (g)
CF*100
Length (mm)
100
90
5
Weight (g)
The survival rate in the test tanks
were quite equal during the first
weeks. Between weeks 3 and 4,
mortality increased rapidly in all
other salinities except for the
highest one. Mortality was most
severe in the lowest salinity, and in
the highest salinity tank (15 psu),
the survival of the herring juveniles
was as high as 51%.
80
% Survived
Length, weight, and condition factor of herring
juveniles in different salinites
Survival of herring juveniles
in different salinites
80
4
70
3
60
2
1
50
0
40
Length (mm); CF*100
100
Total length, weight and
condition varied among the
salinities, the largest
juveniles were found in 15
psu and smallest in 5.7 psu,
but differences were not
significant. The condition
factor of the fish reared in 12
psu was significantly higher
than that of fish in all other
salinities.
0
1
2
3
4
5
6
7
Experiment Week
Amount of visceral fat in
herring juveniles
Fat index
3
2
1
0
5.7
8
12
Salinity
15
8
In all test tanks fish were feeding
successfully. This is indicated by
the amount of fat in their body
cavity at the end of the experiment.
Fat index (FI) indicated significant
differences in the fat reserves of
fish in different salinities (p=0.001).
In lower salinities (5.7 and 8), the
majority of juveniles had a small
amount of fat (FI=1) on the surface
of the intestine and rich deposits
(FI=3) were found only in few
individuals. In 12 psu, all fish had
rich fat deposits in the body cavity
(mean FI = 3). Mean FI was
significantly (p<0.05) higher in 12
psu tank.
5.7
8
12
15
Scale growth (=incorporated
radioactive glycine) was
highest at salinities of 8 and
12, and in salinities of 5.7 and
15 the values were clearly
lower. Analysis of scale growth
also indicated, that the scale
sampling method suggested in
literature (Busacker and
Adelman 1987) yielded in nongrowing scales. Consequently,
in herring juveniles, scales for
growth determination should
be taken randomly, so to
include size variation in scales.
Salinity
Scale growth rate of herring juveniles
in different salinities
12
10
Counts / mm²
0
G(I)
G(II)
G(control)
8
6
4
2
0
5.7
8
12
15
Salinity
In our experiment, the accumulation of fat, somatic condition and growth rate all suggest that Baltic herring juveniles have their optimum salinity in 8-12 psu,
which is higher than the salinity in most of the nursery grounds in the Baltic Sea.
References:
-Busacker GP, Adelman IR (1987) Uptake of 14C-glycine by fish scales (in vitro) as an index of current growth rate. In: Summerfelt RC and Hall GE (eds) The age and growth of fish, Iowa State University Press, Ames, Iowa, pp 355-357.
-Eklund J (1999) Herring growth and age estimates from otolith and scales. Study report, Archipelago Research Institute, University of Turku, 4 pp.
-Goolish EM, Adelman IR (1983) Effects of fish growth rate, acclimation temperature and incubation temperature on in vitro glycine uptake by fish scales. Comp Biochem Physiol 76A:127-13.
-Griffin FJ, Pillai MC, Vines CA, Kääriä J, Hibbard-Robbins T, Yanagimach R, Cherr GN (1998) Effects of salinity on sperm motility, fertilization and development in the Pacific herring, Clupea pallasi. Biol Bull 194: 25-35.
-Høgnestad PT (1994) The Lake Rossfjord herring (Clupea harengus L.) and its environment. ICES J mar Sci 51: 281-292.
-Holliday FGT, Blaxter JHS (1961) The effects of salinity on herring after metamorphosis. Mar biol Ass UK 41:37-48.
-Klinkhardt M (1984) Zum Einfluss des Salzgehaltes auf die Befruchtungsfähigkeit des Laiches der Rügenschen Frühjahrsheringe. Fischerei-Forschung 22:73-75.
-Neb K-E (1970) Über die Heringe des Windebyer Noors. Ber Dt Wiss Komm Meeresforsch 21: 265-270.