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Horst Bleckmann et al.
of the submarine. The main propeller of the submarine was started under water via a
solenoid switch. Due to the momentum of the submarine, a mercury-inclination switch
started one of the small propellers, thus inducing a change of swimming direction. It was
unpredictable at what time which of the two small propellers was started during the course
of a trial. For further explanations see text
The above experiments clearly show that background flow in combination with
dipole stimuli is a powerful tool to analyze lateral line function. Flow tank experiments immediately reveal why running water fish have only a small number of
superficial neuromasts (Coombs et al. 1992). In running water DC flow immediately drives superficial neuromasts into saturation and therefore renders them
useless. We believe that we will uncover a clear fonn-function relationship of the
various lateral lines only if we study the respective system under hydrodynamic
conditions which come close to the hydrodynamic conditions a fish faces in real
life.
10 Detection ofFish-Borne Wakes
The idea that hydrodynamic sensory systems like the lateral line are useful for the
detection of complex water motions like fish wakes (cf. Fig. 3) is supported by
recent experiments with Harbor seals. By means of their facial vibrissae, Harbor
seals detect minute water movements (Dehnhardt et al. 1998). Blind seals successfully hunt in the open ocean (Newby et al. I 970). Thus seals can sense, locate,
and pursue fish without vision. One potential cue which might guide seals to their
prey are the hydrodynamic trails generated by swimming fish (see above).
Recent experiments (Dehnhardt and Mauck, unpubl.) suggest that blindfolded
seals can track fish wakes. Blindfolded seals were trained to follow the wake of a
miniature submarine (Fig. 6, inset). In order to exclude acoustic cues the seal was
stationed with its head in a hoop so that it was outside the water up to its
foreflippers. While in the hoop station the seal additionally wore head phones
playing white noise. When the seal was stationed the submarine started from a
platform in an arbitrary direction and unpredictably changed its direction while
moving across the experimental pool (Fig. 6). After 5 s the motor of the submarine
turned off and the seal was allowed to start its search 2 seconds after the submarine surfaced (Fig. 6). In 78% of the trials (n = 326) the blindfolded seal
successfully tracked the wake. From its hoop station the diving seal always started
its search by swimming straight into the pool. As soon as it came across the point
where the submarine had started, it immediately changed its swimming direction
and exactly followed the hydrodynamic trail. In trials in which the seal missed the
submarine's trail, it started its search deeper or shallower than the submarine's
starting position. However, when it found the trail, the seal never had a problem to
find the submarine. Using vibrating spheres as a stimulus we would have totally
Horst Bleckmann et al.
of the submarine. The main propeller of the submarine was started under water via a
solenoid switch. Due to the momentum of the submarine, a mercury-inclination switch
started one of the small propellers, thus inducing a change of swimming direction. It was
unpredictable at what time which of the two small propellers was started during the course
of a trial. For further explanations see text
The above experiments clearly show that background flow in combination with
dipole stimuli is a powerful tool to analyze lateral line function. Flow tank experiments immediately reveal why running water fish have only a small number of
superficial neuromasts (Coombs et al. 1992). In running water DC flow immediately drives superficial neuromasts into saturation and therefore renders them
useless. We believe that we will uncover a clear fonn-function relationship of the
various lateral lines only if we study the respective system under hydrodynamic
conditions which come close to the hydrodynamic conditions a fish faces in real
life.
10 Detection ofFish-Borne Wakes
The idea that hydrodynamic sensory systems like the lateral line are useful for the
detection of complex water motions like fish wakes (cf. Fig. 3) is supported by
recent experiments with Harbor seals. By means of their facial vibrissae, Harbor
seals detect minute water movements (Dehnhardt et al. 1998). Blind seals successfully hunt in the open ocean (Newby et al. I 970). Thus seals can sense, locate,
and pursue fish without vision. One potential cue which might guide seals to their
prey are the hydrodynamic trails generated by swimming fish (see above).
Recent experiments (Dehnhardt and Mauck, unpubl.) suggest that blindfolded
seals can track fish wakes. Blindfolded seals were trained to follow the wake of a
miniature submarine (Fig. 6, inset). In order to exclude acoustic cues the seal was
stationed with its head in a hoop so that it was outside the water up to its
foreflippers. While in the hoop station the seal additionally wore head phones
playing white noise. When the seal was stationed the submarine started from a
platform in an arbitrary direction and unpredictably changed its direction while
moving across the experimental pool (Fig. 6). After 5 s the motor of the submarine
turned off and the seal was allowed to start its search 2 seconds after the submarine surfaced (Fig. 6). In 78% of the trials (n = 326) the blindfolded seal
successfully tracked the wake. From its hoop station the diving seal always started
its search by swimming straight into the pool. As soon as it came across the point
where the submarine had started, it immediately changed its swimming direction
and exactly followed the hydrodynamic trail. In trials in which the seal missed the
submarine's trail, it started its search deeper or shallower than the submarine's
starting position. However, when it found the trail, the seal never had a problem to
find the submarine. Using vibrating spheres as a stimulus we would have totally