HELGOI_~NDER MEERESUNTERSUCHUNGEN
Helgolhnder M e e r e s u n t e r s . 49, 507-518 (1995)
Physical and physiological aspects of gear efficiency in
North Sea brown shrimp fisheries
R. B e r g h a h n 1, K. W i e s e 2 & K. L f i d e m a n n 1
t Institut ffir Hydrobiologie und Fischereiwissenschaft der Universit~t Hamburg,
Elbelabor; Grol3e Elbstrai3e 258, 2276F Hamburg, Germany
2 Zoologisches Institut und Zoologisches Museum der Universit~t Hamburg;
Martin-Luther-KJng-Platz 3, 20146 Hamburg, Germany
ABSTRACT: In search of m e a n s to reduce the b y - c a t c h of juvenile flatfish in the shrimp fishery,
vibrations a n d c h a n g e s in current velocity c a u s e d by shrimp trawls were investigated in the field a n d
in the laboratory. Buried as well as e m e r g e d shrimps (Crangon crangon) exhibit tailflips 5-10 cm
before being touched by the rollers of a shrimp g e a r approaching t h e m at a speed of 0.5 m - sec -1, as
was revealed by slow motion video recordings in aquaria u n d e r artificial light. Hence, the signal
effective in triggering escape m u s t be a t t e n u a t e d strongly with increasing distance. S e d i m e n t
vibration, commonly a s s u m e d to be an important signal in triggering escape of shrimps, w a s found to
decrease by a factor 100 9 m -1. Signals from the rollers of a commercial shrimp g e a r in operation
(towing s p e e d 1 m 9 sec 1) were directly recorded with an accelerometer. Their f r e q u e n c y r a n g e d
from 50 to 500 Hz a n d r e a c h e d an acceleration of 40 m 9sec -2 on soft bottom or up to 100 m - sec -2 on
hard substrate. Accelerometers, which h a d b e e n buried right at the surface of a tidal s a n d fiat during
low tide, produced only one sharp signal of 100 Hz with an acceleration of 24 m 9 sec -2, w h e n a
shrimp gear swept t h e m on the s u b m e r g e d tidal flats. However, in aquaria short sinusoidal signals
,< 5 m - s e c - 2 : 2 0 to 300 HzJ m a d e buried s h r i m p s a n d flatfish IPleuronectes platessa. Solea solea.
l~icrostomus kittl hide rather t h a n flee. T h e vibrations recorded directly at the rollers a n d the
underlying jolting m o v e m e n t s of the rollers induce corresponding pulses in the water s u r r o u n d i n g
the rollers in a Layer of approximately 10-15 cm. Similar water d i s p l a c e m e n t of high acceleration
was experimentally p r o d u c e d by a spring loaded t r a n s p a r e n t lucite piston (7 cm in diameter) fitted to
an accelerometer. Accelerating this piston I12-116 m - sec -2. 50-200 Hz rangel from 5 cm above
towards the shrimp p r o d u c e d escape r e s p o n s e s in up to 94 % of the tests. Arthropods are k n o w n to
perceive m e d i u m d i s p l a c e m e n t rather t h a n pressure. Hence. strong and rapidly rising water
currents c a u s e d by the rollers rather t h a n s e d i m e n t vibration are a s s u m e d to mainly trigger the
escape reaction, which m a k e s Crangon accessible to the gear.
INTRODUCTION
B r o w n s h r i m p (Crangon crangon L.) f i s h e r i e s i n t h e c o a s t a l w a t e r s of t h e N o r t h S e a
c o n t r i b u t e to d e a t h s a n d i n j u r i e s of j u v e n i l e f i s h ( T i e w s , 1983; B e r g h a h n e t al. 1992;
L f i d e m a n n , 1993). T h e p r e s e n t d e s i g n of t h e g e a r h a s b e e n d e v e l o p e d b y t h e f i s h e r m e n
d u r i n g t h e l a s t d e c a d e s a c c o r d i n g to t h e i r d a y - t o - d a y e x p e r i e n c e w i t h r e g a r d to s h r i m p
c a t c h e s . F i s h e r m e n b e l i e v e t h a t t h e r o l l e r s of t h e g r o u n d r o p e u s e d b y t h e s h r i m p t r a w l e r s
cause sediment vibrations which make buried shrimps emerge from the sediment.
R e c e n t l y , i n c r e a s i n g a t t e n t i o n is b e i n g c o n c e n t r a t e d o n t h e r e d u c t i o n of b y - c a t c h m
t h e s h r i m p f i s h e r y ( B e r g h a h n , 1993). A l o n g t h e c o a s t , s h r i m p i n q is c a r r i e d o u t i n w a t e r s of
cD Biotogische Anstalt Helgoland, H a m b u r g
508
R. Berghahn, K. Wiese & K. Li i d em an n
h i g h turbidity. C a t c h e s are positively correlated with turbidity and a d v e r s e l y affected by
light. This points to visual detectability of the g e a r which m ay highly i n f l u en ce the catch
r e g a r d l e s s of w h e t h e r the animals are b u r i e d or e m e r g e d . Consequently, g e a r efficiency
of shrimp trawls for C. crangon, as w e l l as for u n w a n t e d by-catch such as j u v e n i l e fish,
cannot realistically be d e t e r m i n e d by m e a n s of v i d eo recordings only. Information on the
sensory capacity a n d the physiological control of e s c a p e in both shrimps an d potential bycatch organisms is essential for i m p r o v i n g the selectivity of the fishing method. Escape
response in Procambarus has b e e n a focus in n e u r o p h y s i o l o g i c a l research for a long time
(Wine & Krasne, 1982). However, the results from the laboratory, as well as from other
crustacean species, cannot directly be transferred to Crangon or to fish species. In order
to close this g a p of k n o w l e d g e , the following e x p e r i m e n t s w e r e carried out.
MATERIAL AND M E T H O D S
Animals w e r e k e p t in a e r a t e d 100-1 tanks at a m b i e n t t e m p e r a t u r e s for a few days.
Only specimens, w h i c h had survived the catch virtually unhurt w e r e s e l e c t e d for the
experiments. Artificial s e a w a t e r (INSTANT O C E A N ~) was used. The bottom of the tanks
was c o v e r e d with a 2-cm layer of sand. C h o p p e d m e a t of mussel or fish was p r o v i d e d in
excess every s e c o n d day.
Experiment
1: S h r i m p b e h a v i o u r i n f r o n t of a p p r o a c h i n g
gear
40 shrimps (5.0 to 6.5 cm in length) w e r e p l a c e d on the sand bottom of an a q u a r i u m
(2.5 x 0.5 m; S ~ 25, T - 15 ~ light: O s r a m L36 30 W, Warmton, at a distance of 1.0 m)
an d g i v en 15 min to adapt. T h e n a single roller of a commercial shrimp g e a r (rubber disk,
20 cm in diameter, w id th 10 cm, w e i g h t 6.6 kg, 2.5 kg in water) was pulled with a nylon
string from one side of the basin to the other at a s p e e d of 0.5 m 9 sec - I (approximately
1 knot). The e s c a p e reaction of the animals was r e c o r d e d on video tape.
E x p e r i m e n t 2: S h r i m p r e a c t i o n to w a t e r p u l s e s
Shrimps w e r e placed individually in 10 circular cages in separate 10-1 a q u a r i a {18.5
• 28.5 cm. water d e p t h i1 cm S = 30. T -- 15~ light: Osram L36 30 W, W a r m t o n at a
distance of 1.0 m: Fig. 1). A circular t r a n s p a r e n t disk (7 cm diameterl m o u n t e d to the end
of a t ran s p aren t rod w a s thrust from a distance of 10 cm in a fast and short (3-5 cm) pulse
towards e a c h specimen. Trials w e r e also c on d u ct ed with a n o n - t r a n s p a r e n t disk. The
acceleration of the disk was m e a s u r e d with an a c c e l e r o m e t e r (Bruel & Kjaer type 8319)
a t t a c h e d to the device. The r e s p o n s e of the shrimps was noted. After a series of 10
s p e c i m e n s h ad b e e n treated, t h e y w e r e r e p l a c e d by 10 fresh ones from the stock tank,
which w e r e g i v e n 1 min to adapt.
E x p e r i m e n t 3: S e n s i t i v i t y of s h r i m p a n d f l a t f i s h to s e d i m e n t v i b r a t i o n
Sensitivity to vibration of the sea bottom was studied using the following setup
(Fig. 2): a t ran s p a r e n t plastic b a g filled with 0.5 k g sand and 0.5 k g s e a w a t e r from the
a q u a r i u m was put onto the m e m b r a n e of a passive l o u d s p e a k e r a t t a c h e d to an electro-
Gear efficiency in North Sea b r o w n shrimp fisheries
509
~ROD
9zJ "-RELEASE
I
~.~-SPRING
:ii
c
~
DISC
JARIUM
ND
Fig. 1. Experimental set-up for testing reaction of shrimp to water pulses
d y n a m i c vibrator (Bruel & Kjaer Mini-Shaker type 4810) at its centre/cf. Heinisch & Wiese
[1987]). The shaker performed sinusoidal oscillations at clearly defined frequencies
provided by a function generator (Impo FD 13.07). Onset of the stimulus, pulse width
pulse duration a n d amplitude could be precisely adjusted employing a n additional
device. Accelerations actually occurring in the sand were recorded by m e a n s of an
accelerometer (Bruel & Kja~r type 4338) directly m o u n t e d on the shaker. The signal was
amplified by m e a n s of a microphone power supply (Bruel & Kjaer type 2804) and
displayed for m e a s u r e m e n t on an oscilloscope (Kikusui COS 5020). Since the plastic b a g
was s u s p e n d e d from an adjustable boom, pure sinusoidal oscillations could easily be
achieved at any given frequency.
S p e c i m e n were tested by transferring them individually into the bag. A d a p t a t i o n
time was 15 min. Usually, animals s u b m e r g e d into the sand d u r i n g this period. At each
f r e q u e n c y a n d each amplitude tested, stimuli were presented in 3 s e q u e n t i a l trials at a
pulse duration of 600 ms. Interstimulus then was 10 sec. U n d e r artificial light conditions
fOsram L36 W/30, Warmton. distance 2 ml, a clearly defined response could be recorded
510
R. B e r g h a h n , K. W i e s e & K. L i i d e m a n n
- - ~ - PLASTIC BAG
-----------WATER
/---SAND
PASSIVE
LOUDSPEAKER
SHAKER
,_ FUNCTION
~
MICROPHONE F-POWER
I Z
SUPPLY
/
~
RATOR
AMPLIFIER 1
L ~ J SWITCH
ZOSCILLOSCOPE
Fig. 2. Experimental set-up for testing sensitivity of shrimp and flatfish m s e d i m e n t vibration
for e a c h s p e c i e s . A s t i m u l u s w a s c o n s i d e r e d s u b t h r e s h o l d if t h e a n i m a l f a i l e d to r e s p o n d
i m m e d i a t e l y i n at l e a s t 2 o u t of 3 s e q u e n t i a l trials. E x p e r i m e n t s w e r e r u n w i t h a n
i n t e r s t i m u l u s of 3 m i n for a m a x i m u m d u r a t i o n of 1 h p e r a n i m a l a n d d a y . I n s o m e c a s e s ,
e x p e r i m e n t s h a d to b e c o n c l u d e d e a r l i e r d u e to h a b i t u a t i o n . T h e e x p e r i m e n t s w e r e
c o n d u c t e d w i t h 8 p l a i c e (Pleuronectes platessa L.), 6 s h r i m p s , 2 soles (Solea solea L.) a n d
a l e m o n sole (Microstomus kitt W a l b . ) at 20 ~ F o u r s p e c i m e n s of p l a i c e w e r e also t e s t e d
a t 16~
Experiment
4: G e a r
and
sediment
vibration
in the field
T w o a c c e l e r o m e t e r s (see ~ x p e n m e n t 2) w e r e t i g h t l y c o n n e c t e d w i t h a s t e e l p l a t e
(37 x 3 • 0.4 c m ; 400 g) a n d b u r i e d a t a d i s t a n c e of 1 m o n a t i d a l s a n d flat d u r i n g l o w
tide. A c c e l e r a t i o n s w e r e p r o d u c e d b y a n a u t o m a t i c h a m m e r mill w h i c h w a s p u t i n t o u s e
o n t h e s u b m e r g e d t i d a l flats r i g h t b e s i d e t h e first a c c e l e r o m e t e r . T h e i r d a m p i n g w a s
m e a s u r e d a t a d i s t a n c e of 1 m b y c o m p a r i s o n w i t h t h e s e c o n d a c c e l e r o m e t e r . T h e n t h e
G e a r efficiency in North S ea b r o w n shrimp fisheries
511
area was r e p e a t e d l y swept with a commercial shrimp trawl. The a c c e l e r o m e t e r signals
w e r e amplified with a "Line Drive Supply, Br~el & Kjeer 2813", recorded with a "Racal
Thermionic Ltd. 4D" tape recorder which was b a s e d on an a n c h o r e d boat b esi d es the
track, and analysed later on a "Real-Time A u d i o s p e c t r u m Analyser, H e w l e t t Packard
8064A" in the laboratory. Moreover, an a c c e l e r o m e t e r was attached to the axis of one of
the rollers in operation and the signals r e c o r d e d on the shrimp vessel (Fig. 3).
Fig. 3. Accelerometer attached to a roller of the groundrope of a commercial shrimp gear. Arrow
indicates the part of the gear for the dose up. 1 = rubber disc; 2 = steel mountings; 3 = line to
ground rope; 4 = ground rope and net; 5 = steel plate with accelerometer. Bar = 10 cm
RESULTS
Experiment
1: S h r i m p b e h a v i o u r i n f r o n t of a p p r o a c h i n g g e a r
Buried as well as e m e r g e d shrimps exhibit tailflips well before b e i n g t o u c h e d by the
rollers of an a p p r o a c h i n g shrimp gear, as was r e v e a l e d by slow motion video recordings
u n d er artificial light in aquaria. T h e direction of the a p p r o a c h was not important. Th e
s p e c i m e n s a l r e a d y e m e r g e d 5-10 cm before the rollers. Habituation was c o n s i d e r a b l e at
5 rain intervals.
E x p e r i m e n t 2: S h r i m p r e a c t i o n to w a t e r p u l s e s
T h e t r a n s p a r e n t disk p r o v e d to be less effective at i n d u c i n g escape reactions than t h e
visibIe version (Table 1). Tailflips could be i n d u c e d in both e m e r g e d and b u r i e d s p e c i mens. Buried small s p e c im e n s of 4 cm could be stimulated more easily than s p e c i m e n s of
about 6 cm. Small s p e c i m e n s w h i c h had b e c o m e b u r i e d and w e r e treated b y this m e t h o d
lost their sand c o v e r by t h e a c c o m p a n y i n g spray of sand and exhibited e s c a p e responses.
Habituation wa s considerable.
R. B e r g h a h n , K. W i e s e & K. L i i d e m a n n
512
Table 1. Response of shrimps to water pulses
Tension
of spring
Max. acceleration of disc
[m 9 sec-21
Angle of
pulse [~
n
N u m b e r of
tail-flipping
specimens
% tail-flipping
specimens
116
72
42
32
38
12
t16
90
90
90
90
90
90
45
20
11
19
17
34
42
19
14
10
14
15
32
38
12
70
91
74
88
94
90
63
Disc transparent, no sediment:
Free fall (damped)
32
Tension
2.5
68
(damped)
90
90
30
49
11
34
37
69
90
30
12
40
Disc visible, no sediment:
4
3
2
1
Free fall
Free fall (damped)
4
Disc transparent, specimens buried in sand:
3
72
Experiment
3: S e n s i t i v i t y
of shrimp
and flatfish to sediment
vibration
S h r i m p s r e s p o n d e d to s e d i m e n t v i b r a t i o n w i t h m o v e m e n t s of t h e i r l a r g e s e c o n d
a n t e n n a e . Plaice r e s p o n d e d w i t h o n e e x t r a m o v e m e n t of t h e o p e r c u l u m . In c o n t r a s t , o n e
s p e c i m e n of sole s t o p p e d its gill v e n t i l a t i o n for a s e c o n d , w h e r e a s t h e s e c o n d s p e c i m e n ' s
r e a c t i o n w a s s i m i l a r to t h a t of p l a i c e . T h e l e m o n sole t e s t e d d i s p l a y e d a q u i c k t u r n of its
eyes.
T h e l o w e s t t h r e s h o l d of p e r c e p t i o n for v i b r a t i o n of s a n d or w a t e r w a s 4 0 c m 9 s e c -2
{ 2 0 - 2 0 0 Hzl in s h r i m p s , a n d s l i g h t l y l o w e r in flatfish. S e n s i t i v i t y to s e d i m e n t v i b r a t i o n
w a s a b o u t t h e s a m e m s h r i m p a n d f l a t f i s h It w a s g r e a t e s t i n sole (10 c m - s e c -2 a t 20 to
170 Hz). A t l o w f r e q u e n c i e s t h e d i f f e r e n c e f r o m t h e o t h e r s p e c i e s w a s t e n t i m e s g r e a t e r .
T h r e s h o l d s of r e s p o n s e in p l a i c e w e r e s i m i l a r to t h o s e in l e m o n sole a n d w e r e l o w e s t
b e t w e e n 100 a n d 200 H z (Fig. 4). A t f r e q u e n c i e s b e t w e e n 20 a n d 100 H z , t h r e s h o l d s
d e c r e a s e d in w a r m e r w a t e r . S h r i m p s w e r e a little m o r e s e n s i t i v e a t l o w f r e q u e n c i e s
c o m p a r e d to trials f r o m 140 H z u p w a r d s (Fig. 4). W h e n p u l s e w i d t h w a s e n l a r g e d f r o m
600 m s to 3 0 0 0 ms. t h e t h r e s h o l d i n s h r i m p w a s l o w e r . At f r e q u e n c i e s h i g h e r t h a n 300 Hz,
s e n s i t i v i t y d e c r e a s e d d r a s t i c a l l y i n b o t h s h r i m p a n d flatfish. A t all f r e q u e n c i e s t e s t e d ,
e v e n v e r y s t r o n g a c c e l e r a t i o n s (e.g. 200 m 9 s e c -2 at 200 Hz) did n o t i n d u c e b u r i e d
s p e c i m e n s , w h e t h e r s h r i m p s or flatfish, to e m e r g e f r o m t h e s a n d .
Experiment
4: G e a r , w a t e r
and sediment
vibration
in the field
T h e s i g n a l p r o d u c e d b y t h e h a m m e r mill w a s d a m p e d to 1 / 1 0 0 a t a d i s t a n c e of 1 m .
S i g n a l s f r o m t h e rollers of a c o m m e r c i a l s h r i m p g e a r i n o p e r a t i o n , w h i c h w e r e d i r e c t l y
r e c o r d e d w i t h a n a c c e l e r o m e t e r , r a n g e d f r o m 50 to 5 0 0 H z (Fig. 51 a n d r e a c h e d 4 0 m 9
s e c -2 o n soft b o t t o m a n d u p to 100 m 9 s e c - 2 o n h a r d s u b s t r a t e . S w e e p i n g t h e a c c e l e r o m e -
Gear efficiency in North Sea b r o w n shrimp fisheries
crangon
plaice
10
10
~
513
8
E
6
0
~,~
5
0
0
3
40
70
100
140
170 200
250
300
400
0
frequency [Hz]
9 min
9 max
40
70
100
140
170 200
250
300
400
frequency [Hz]
x mean
9 min
'= m a x
x mean
Fig. 4. Sensitivity of shrimp and plaice to sediment vibration. Water temperature 20 ~
ters with the rollers of a commercial shrimp g e a r resulted in a short p r o n o u n c e d signal of
24 m . sec -2 (Fig. 6). First signals of 6 m 9 sec -2 w e r e r e c o r d e d at a distance of 15 cm from
the a p p r o a c h i n g rollers.
DISCUSSION
As for the rate of the b e h a v i o u r a l reaction of Crangon to isolated stimuli of s e d i m e n t
vibration, vibration p r o d u c e d by the a p p r o a c h i n g rollers is unlikely to trigger tailflips in
buried animals. Tailflips, however, are essential to get shrimps netted.
Stimulus response to s e d i m e n t vibration o b s e r v e d in shrimp w e r e the s a m e as those
r e c o r d e d by Heinisch & Wiese (1987). T h e y consisted of reflex m o v e m e n t s of a n t e n n u l e s
an d an t en n ae. Since an increase of the stimulus resulted in more p r o n o u n c e d m o v e m e n t s
of the second a n t e n n a e up to burying, the reaction can be i n t er p r et ed as hiding
behaviour. Th e clear response to s e d i m e n t acceleration at low frequencies (Fig. 4) is in
a g r e e m e n t with the findings of Heinisch & W iese (1987) plotted in terms of d i s p l a c e m e n t
instead of acceleration. In Procambarus, sensitivity is also p r o n o u n c e d l y b e l o w 100 Hz
(Breithaupt & Tautz, 1990). Consequently, similar m e c h a n i s m s of stimulus detection can
be assumed. A n t e n n a l flagellae and sensory hairs are k n o w n to be h y d r o d y n a m i c
receptor systems in m a n y crustaceans (Bleckmann et al.. 1991). The latter are mainly
located on the carapace, the telson, the p a r a e o p o d s, an d on the first and s e c o n d antenna.
In plaice, vibratory stimuli caused one extra m o v e m e n t of the operculum. In case of
an increase of the stimulus, burying b e h a v i o u r could often be observed. For that reason,
hiding intention can be assumed, as well. Th e same holds true for sole. This result
corresponds with the hiding b e h a v i o u r of sole in the field, w h e n it is a p p r o a c h e d by a
b e a m trawl (Berghahn, 1984). Lemon sole, however, s e e m s to b e c o m e alert w h e n
514
R. B e r g h a h n ,
K, W i e s e & K, L i i d e m a n n
A
8
C
Fig. 5. A c c e l e r a t i o n s r e c o r d e d b y a m e t e r a t t a c h e d to a roller of t h e roller g e a r in s h r i m p i n g
o p e r a t i o n (Fig. 3). W e i g h t of t h e roller 6.6 k g (2.5 k g in water). A = t o w i n g s p e e d i n c r e a s e d from 0 to
2 knots, b a r = 3 sec; B = soft bottom, m a x . 40 m 9 s e c -2, c o n s t a n t t o w i n g s p e e d of 2,1 knots, b a r =
0.05 sec; C -- h a r d bottom, m a x . 100 m - sec-21 c o n s t a n t t o w i n g s p e e d of 2.1 knots, b a r = 0.05 sec; 5 0
to 500 Hz c o m p o n e n t s
G e a r e f f i c i e n c y in N o r t h S e a b r o w n s h r i m p fisheries
515
Fig. 6. Accelerations recorded by a meter which was buried on a tidal sand flat during low tide and
swept with a commercial shrimp gear during high tide. Peak 24 m 9 sec-2; bar = 0.1 sec
s t i m u l a t e d . This is in a c c o r d a n c e w i t h the g e n e r a l o b s e r v a t i o n from t a n k e x p e r i m e n t s ,
that l e m o n sole tries to a v o i d active g e a r b y s w i m m i n g o v e r it, w h i c h is in c o n t r a s t to
p l a i c e or f l o u n d e r (Platichthys flesus L.). T h e t h r e s h o l d s for t h e sensitivity to s e d i m e n t
v i b r a t i o n r e p o r t e d h e r e m a y b e e v e n l o w e r in b o t h shrimp a n d flatfish, if o t h e r t h a n
b e h a v i o u r a l r e s p o n s e s are c o n s i d e r e d . N e v e r t h e l e s s , t h e s e t h r e s h o l d s are two o r d e r s of
m a g n i t u d e b e l o w t h e p e a k signals r e c o r d e d w i t h an a c c e l e r o m e t e r in situ ( E x p e r i m e n t 4).
M o r e o v e r , no e s c a p e reactions, b u t only h i d i n g b e h a v i o u r c o u l d b e i n d u c e d by m e a n s of
s e d i m e n t v i b r a t i o n s in shrimp or flatfish, e v e n if a c c e l e r a t i o n s w e r e i n c r e a s e d by o n e
o r d e r of m a g n i t u d e .
In c o n t r a s t to s e d i m e n t vibration, w a t e r p u l s e s c a n easily i n d u c e tailflips in b o t h
b u r i e d as w e l l as e m e r g e d shrimp. This s t i m u l u s s e e m s to h a v e h i g h priority in eliciting
tailflips a n d in t h e c a t c h i n g p r o c e d u r e , r e s p e c t i v e l y . If so. the s t i m u l u s a n d t h e a l a r m
d i s t a n c e of a n a p p r o a c h i n g g e a r c a n b e e x p e c t e d to i n c r e a s e w i t h t o w i n g s p e e d
S t i m u l a t i o n of t h e t r i g g e r n e u r o n s of e s c a p e , the m e d i a l a n d the lateral g i a n t n e u r o n of
the c r u s t a c e a n v e n t r a l cord, is u n d e r c o m p l e x control of d i v e r s e i n p u t s a n d inhibitions
(Glantz & V i a n c o u r , 1983). T h e i n p u t s are m a i n l y r e c e p t o r s on a n t e n n u l e s a n d a n t e n n a e
(Wine & Krasne, 1982). S t i m u l a t i o n c a n b e g r e a t e r in e m e r g e d t h a n in b u r i e d s p e c i m e n s .
since t h e e s c a p e r e a c t i o n is i n h i b i t e d in c o v e r e d crayfish (Krasne et al., 1990).
Rollers w h i c h are c o m m o n l y e m p l o y e d in c o m m e r c i a l s h r i m p i n g are 10 c m w i d e and
20 c m in d i a m e t e r . C o n s i d e r i n g the slight r o c k i n g a n d jolting of the rollers w h e n t o w e d
across t h e s e d i m e n t , t h e rollers c a n b e r e g a r d e d as d i p o l e s o u r c e s of m e d i u m d i s p l a c e m e n t (Wiese et al., 1980). F r o m t h e a c c e l e r a t i o n m e a s u r e d at the axle of the roller m
fishing o p e r a t i o n s at a t o w i n g s p e e d of 1 m sec -1. it is p o s s i b l e to c a l c u l a t e t h e u n d e r l y i n g a m p h t u d e of roller jolting, a s s u m i n g a d o m i n a n t f r e q u e n c y of t h e s p e c t r u m of
v i b r a t i o n s o b s e r v e d . If t h e p e a k v a l u e of a c c e l e r a t i o n , b. e q u a l s t h e a m p l i t u d e ot
oscillation m u l t i p l i e d b y co2 (co = 2z~ 9 f), the c o r r e s p o n d i n g oscillation a m p l i t u d e for 100 m
9 s e c -2 a c c e l e r a t i o n at 100 Hz is 250 ~m. A t t e n u a t i o n of this m o v e m e n t of t h e roller in the
w a t e r c a n b e d e s c r i b e d b y the e q u a t i o n d = A 9R ~ 9r -3 (Van B e r g e i j k , 1967). T h e d i s t a n c e
r is m e a s u r e d f r o m t h e c e n t r e of t h e roller; R is t h e r a d i u s of t h e roller: d is t h e w a t e r
d i s p l a c e m e n t m e a s u r e d at a d i s t a n c e r from t h e axle of t h e s p h e r e . If t h e original
516
R. Berghahn, K. Wiese & K. Ltidemann
oscillation of the roller is 250 ~m in amplitude, a point 5 cm in front of the a p p r o a c h i n g
roller will be affected by a vibration with a n amplitude of d = 250 - 1000 c m . 3375 cm -1 =
72 ~m. This v a l u e is more than 2 orders of m a g n i t u d e above the sensitivity thresholds for
m e d i u m d i s p l a c e m e n t in crustaceans (Procambarus 0.5 ~m: Tautz & S a n d e m a n , 1980;
Euphausia superba 0.5 ~m: Wiese & Marshall, 1990; Crangon 0.5 ~m: Heinisch & Wiese,
1987; sensitivity to flow in Crangon a n t e n n u l e s 0.5 m m . s e c - l : Liihr & Wiese, 1990) a n d is
p r o b a b l y apt to excite the lateral giant n e u r o n to the level of spiking. Eliciting escape by a
spring loaded piston at a distance of 5 cm could b e achieved with accelerations down to
12 m 9 sec -2. In addition, a head is also p r o d u c e d by the s m a l l - m e s h e d n e t (mesh size
11-12 mm; knot to knot).
According to fishing experience, Crangon catches increase with w a t e r turbidity or
with d e c r e a s i n g light intensity. In the W a d d e n Sea, high catches can also be m a d e during
low tide, which Dahm (1975) attributed, on the basis of laboratory experiments, to the fact
that Crangon is not b u r i e d during stow water, b u t is m u c h more active in search of food.
This interpretation, however, contrasts with the g e n e r a l finding that in areas with strong
tidal currents an e n d o g e n o u s circatidal rhythm is established in Crangon with high
activity a r o u n d high tide a n d no activity a r o u n d low tide (A1-Adhub & Naylor, 1975). The
high catches d u r i n g low tide can rather be explained by the migratory b e h a v i o u r of
Crangon. A great part of the population in the tidal c h a n n e l s exhibits tidally-phased
feeding migrations onto the s u b m e r g e d tidal flats (Berghahn, 1987). During low tide, the
population is concentrated in a m u c h smaller area, l e a d i n g to higher catches.
The differences in the shrimp catches in the course of the tidal cycle, as well as
d u r i n g different light a n d turbidity conditions, can be explained as follows: g i v e n the
average towing speed of a commercial shrimper of 2 knots (approx. 1 m 9sec-1), e v e n the
observed distance of 10 cm for escape from the rollers approaching with 1 k n o t would be
sufficient for Crangon > 4 cm in total l e n g t h to avoid the gear, if the water is clear e n o u g h
to detect the gear visually. This is b e c a u s e the b e a m is only 0.5 to 0.7 m a b o v e the bottom
a n d shrimping has to be carried out with the current, which also supports the escape
reaction of shrimps. Since the b e a m is a h e a d of the major part of the g r o u n d rope (Fig. 3),
its visual detection may lead to a n earlier escape reaction. A r o u n d high- a n d low-tide
slack water, the escape reaction of shrimps is not supported b y the current, since current
velocity in the tidal c h a n n e l s and the m a i n s h r i m p i n g areas is close to zero. This may
explain the higher catches around high a n d low tide e v e n though w a t e r turbidity
decreases d u r i n g slack water. During low tide the catches are furthermore i n c r e a s e d b y
higher shrimp densities in the subtidal due to tidal migration of shrimps from the tidal
flats. In dark or turbid water, the escape reaction is not directed away from the gear, b u t
in all directions, again yielding higher catches. U n d e r such conditions, a n additional
g r o u n d r o p e which is rigged in front of the gear a n d acts like a snow-plow will chase away
up to 60 % of fish such as gadoids, smelt a n d clupeids without diminishing shrimp catches
(Berghahn, 1993), even though the visual stimulus from the approaching n e t is the m a i n
factor for fish reaction a n d escape b e h a v i o u r (Parrish, 19691.0-group flatfish, however, in
particular the small specimens which are not sufficiently sorted out by the selective
shrimp trawl (Mohr & Rauck, 1979; Berghahn. 1993), c a n n o t b e p r e v e n t e d from b e i n g
caught with this gear modification, since they are h e r d e d in front of the g e a r a n d their
s w i m m i n g e n d u r a n c e as well as their b u r s t s p e e d is too low (Blaxter & Dickson, 1959;
Blaxter. 1969). Consequently, attempts to reduce the by-catch of small flatfish should
G e a r e f f i c i e n c y in N o r t h S e a b r o w n s h r i m p f i s h e r i e s
517
f o c u s o n w a t e r d i s p l a c e m e n t b e f o r e t h e n e t a s a m e a n s to m a k e t h e g e a r m o r e s e l e c t i v e
for s h r i m p s .
I n c o n c l u s i o n , (1) l o w e r c a t c h e s d u r i n g t h e d a y a n d / o r in w a t e r of l o w t u r b i d i t y s e e m
to r e s u l t f r o m s h r i m p e s c a p i n g t h e g e a r d u e to its v i s u a l d e t e c t i o n ; (2) a t t e m p t s to r a i s e
t h e s e l e c t i v i t y of t h e g e a r s h o u l d f o c u s o n d i s p l a c e m e n t , f l o w a n d a c c e l e r a t i o n of w a t e r
rather than sediment vibration.
Acknowledgements. T h a n k s to the f i s h e r m e n Reinhold a n d Claus Herpel for their valuable support.
This study is s u p p o r t e d by the Federal E n v i r o n m e n t a l Agency, Environmental Research Plan of the
Minister for the Environment, Nature Conservation a n d Nuclear Safety of the Federal Republic of
G e r m a n y (Grant 108 02 085/01), a n d by the state of Schleswig-Holstein. This is publication No. 86 of
the project Ecosystem Research W a d d e n Sea.
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