Food for Thought Studying the behaviour of fishes in the sea at Loch Torridon, Scotland
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ICES Journal of Marine Science (2020), doi:10.1093/icesjms/fsaa118
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Food for Thought
Studying the behaviour of fishes in the sea
at Loch Torridon, Scotland
1
Anthony Hawkins * and Colin Chapman2
1
Loughine Ltd, Kincraig, Blairs, Aberdeen AB12 5YT, UK
2
Scott Garden, Kingsbarns, St. Andrews KY16 8TL, UK
*Corresponding author: tel: þ122 486 8984; e-mail: a.hawkins@btconnect.com.
Hawkins, A. and Chapman, C. Studying the behaviour of fishes in the sea at Loch Torridon, Scotland. – ICES Journal of Marine Science,
doi:10.1093/icesjms/fsaa118
Received 12 May 2020; revised 13 June 2020; accepted 15 June 2020
In the early 1960s, the Marine Laboratory Aberdeen began to examine the behaviour of fish in relation to mobile fishing gears. We were asked
to investigate the role of sound in fish behaviour. We decided that our experimental work had to be done in the sea, as under “free-field” con-
ditions the acoustic stimuli could be accurately presented and monitored. We located a suitable site at Loch Torridon and set up a field sta-
tion there. We carried out unique experiments on the hearing of fishes, their behavioural responses to different sound stimuli, and the sounds
made by the fishes themselves. Work was also carried out on the reflection of sounds by fishes, the noise made by fishing vessels and other
sources, and the movements and foraging activity rhythms of Atlantic cod. The cod generally showed limited movements within defined
home ranges. A large number of scientific papers were written, many of them in collaboration with scientists from other institutes, and other
countries. This paper considers the lessons learned from our work, and especially the advantages of observing fish behaviour and carrying out
experiments on fishes in the sea. We learned that the sound in the sea was very important to fishes, both the natural sounds, some of which
they produce themselves, and sounds made by humans, which could have adverse effects upon them. We hope that this review will encour-
age a new generation of scientists to carry out field work, similar to ours, in other areas. Since our work, there has been a large increase in an-
thropogenic noise, particularly from offshore energy sources, but very little work has been done to help regulate and mitigate their effects
upon fishes.
Keywords: effects of ambient noise, fish behaviour, fish movements, hearing, responses to sounds, sound production
Introduction fishing gears and worked with John Blaxter on the physiology and
In the early 1960s, the Marine Laboratory Aberdeen (MLA), in behaviour of fishes (e.g. Blaxter and Parrish, 1965).
the north east of Scotland, part of the Department of Agriculture Colin Chapman had studied fish physiology as a part-time stu-
and Fisheries for Scotland, embarked upon fish behaviour studies. dent at the Fisheries Laboratory in Lowestoft, England, before
There was particular concern by fisheries managers over the perfor- joining the MLA fish behaviour team in 1962. Anthony Hawkins
mance of fishing gears, and in particular how to reduce the numbers joined in 1966. He had started studying sound production by
of smaller fish being caught. The UK government had diverted some fishes at Bristol University, but moved to the MLA because it had
money from defence spending to the MLA, to help the fishing indus- excellent aquarium facilities. In 1967, David MacLennan, a physi-
try (which had lost distant water fishing grounds off Iceland and cist, joined the MLA to investigate fishing gear performance and
elsewhere). The emphasis was on recruiting new gear technologists. fish detection by sonar.
Basil Parrish (later the Director of the MLA, and eventually the The MLA work carried out on fishing gears has recently been
General Secretary of ICES) began research on the functioning of reviewed by MacLennan (2017). An English aircraft
C International Council for the Exploration of the Sea 2020. All rights reserved.
V
For permissions, please email: journals.permissions@oup.com
2 A. Hawkins and C. Chapman
manufacturing company (Saunders Roe) had designed a new pure tone sounds (Chapman, 1964; Welsby et al., 1964). It was
trawl that was 50% wider and had twice the headline height of evident, however, that studies of captive fish in tanks were very
the traditional Scottish “Granton” trawl. The two gears were limited, and it was realized that the behaviour of wild fishes
compared in 1963, but results showed little difference in their needed to be studied in the sea. The sonar system that was used
catch rates. It was realized that trawl performance did not just in- had enabled the movements of fish to be examined in detail, and
volve sieving a volume of water. It became apparent that fishes it was later used to study wild fish movements in the sea.
are active animals, responding to sound, visual and other stimuli, While working at Cairnryan, we discovered that the haddock
and that these responses have to be taken into account when de- Melanogrammus aeglefinus made underwater sounds (Hawkins
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signing or monitoring fishing gears. Consequently, the MLA and Chapman, 1966). The sounds made during haddock spawn-
started a major research programme on fish behaviour. Colin and ing were then studied in the MLA aquarium. Our paper was the
Anthony were especially interested in the significance to fishes of lead article in Nature, with a photograph of spawning haddock
the underwater sound from fishing gears and the effects of other on the front cover of the printed journal (Hawkins et al., 1967).
anthropogenic (human-made) sources of sound. We then detected haddock sounds in the sea at Loch Ainort, Isle
At that time, there was little information about the noise levels of Skye, on the west coast of Scotland. We caught some of the fish
generated by fishing vessels and their trawls. In 1964, however, we at the loch and supplied them to some physicists for acoustic tar-
were able to fill that gap. In a cooperative exercise with the get strength measurements (McCartney and Stubbs, 1971).
Admiralty and the White Fish Authority, we monitored the Further work on behaviour and sound production by the had-
sounds produced by the research vessel “Explorer” while it was dock then took place in Broad Bay, Isle of Lewis, in the Outer
travelling and towing an “Aberdeen” trawl. The noise trials were Hebrides, in 1965, during a cruise by the Scottish research vessel
conducted on an Admiralty acoustic range on the west coast of “Mara.” Bill Hemmings and John Hislop from the MLA exam-
Scotland, using calibrated hydrophones placed close to the seabed ined the survival of haddock after trawl capture by placing them
(Chapman and Hawkins, 1969). The vessel noise dominated until in cages on the seabed. We placed a hydrophone in one of the
the trawl moved very close to the hydrophone, when the trawl cages to monitor haddock sounds. Unfortunately, the weather de-
noise dominated. Ship noise was higher when towing a trawl, as a teriorated and the Mara dragged the hydrophone cable, causing
result of the heavier load placed on the propulsion system. the cages to collapse. It was clear that attempting this type of re-
Rattling noise was generated by the trawl, with metallic sounds search from on board a ship was fraught with difficulty, and we
generated by shackles and chains, and lower frequency sounds decided that further monitoring of fish behaviour in the sea re-
generated by the ground ropes and trawl otter boards. It became quired a marine research facility, situated close to the shore.
evident that fishes would be able to detect the sounds generated
by ships and trawls and might well respond to them.
The initial MLA experiments on fish behaviour were carried Setting up our field station
out in a concrete tank on the shore at Cairnryan on the south In 1966, the MLA allowed us to establish a field station on the
west coast of Scotland. The tank was large enough to be able to west coast of Scotland. Colin travelled along the coast, looking
tow components of trawls, and it was possible to monitor the for an area with limited fishing activity, which had relatively deep
responses of fish using a high-resolution sector-scanning sonar water close to the shore. He decided that Upper Loch Torridon
developed at the University of Birmingham by Welsby and Dunn (Figure 1) was an ideal location. The site chosen was on the Aird
(1963). Shoals of Atlantic herring (Clupea harengus) showed clear Mhor Peninsula on the south side of Upper Loch Torridon
avoidance responses to the playback of trawl noise and also to (Figure 2). We built a small laboratory there on the shore,
Figure 1. The location of Upper Loch Torridon on the west coast of Scotland.
Studying the behaviour of fishes 3
comprising two garden sheds, a diesel generator, a seawater Fish hearing studies
pump, and header tank (Figure 2), used in connection with an Fish do not have external ears, but it was thought that the otolith
acoustic range in the sea. Underwater scuba diving took place in organs, within the head, constituted inner ears (reviewed by
the loch, especially by Colin and other members of the MLA div- Parker, 1903). It was established many years ago that fishes were
ing group, to set up research facilities on the seabed. Diving took able to detect sounds. However, hearing experiments carried out
place from a small boat with an outboard motor. The diving in aquarium tanks showed very variable results, even for the same
revealed the presence of lots of scallops and Norway lobsters, species. Parvulescu (1964) pointed out the pitfalls of carrying out
which provided us with additional research opportunities. Our acoustic experiments in small tanks and specifying the sounds
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initial research work included studies of the hearing abilities of solely in terms of sound pressure. Per Enger, from Norway, car-
fishes, their responses to anthropogenic noise, and their produc- ried out especially interesting experiments on fish hearing in the
tion of sounds. sea, and demonstrated that particle motion was an important pa-
Loch Torridon is a fjord on the west coast of the Northwest rameter in determining auditory thresholds for some species
Highlands, surrounded by mountains. The loch was created by (Enger and Andersen, 1967).
glacial action and is about 15 miles (25 km) long with several Our hearing experiments were carried out in mid-water at
parts: Upper Loch Torridon is on the landward side, joined by Loch Torridon, at about 100 m from the shore. An acoustic range
a relatively narrow entrance to Loch Shieldaig and Outer Loch was set up at a depth of 21 m. A tower was placed on the seabed,
Torridon, on the seaward section of the loch. It proved easy to constructed from rigid PVC tubing perforated with holes to ad-
persuade scientists from other institutes to take part in projects mit water and release air. Underwater sound projectors (Dyna-
with us there and even to use our field station for their own Empire Inc., types J9 and J11), capable of generating frequencies
work. The research carried out expanded considerably, to include down to about 30 Hz, were moored at different distances and in
work on the acoustic tracking of fishes and crustaceans, observa- different angular positions relative to the top of the tower, where
tions on fishing gear operations, the calibration of sonar systems, the small fish cage was positioned (Figure 3). Calibrated hydro-
and the biology of Norway lobsters and other shellfish. Work phones were placed beneath the fish cage to measure the sound
continued there until 1993, when the site was finally closed down. pressure. In the free sound field, it was possible to estimate the
There had been an increase in fishing activities within the loch, particle motion levels by measuring the sound pressure. This was
especially for shellfish, and the loch was also being used for one of the key reasons for working in the sea. The fish were
salmon and mussel farming. caught on hand-lines and held in onshore tanks, and cages within
the loch, for use in the experiments. They were caught at shallow
depths (
4 A. Hawkins and C. Chapman
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Figure 4. Hearing thresholds obtained at Loch Torridon for the cod,
mainly sensitive to sound pressure, and the salmon and dab,
sensitive only to particle motion. The natural level of ambient sea
noise can vary, affecting the ability of cod to detect sounds,
especially at low frequencies.
Olav Sand, a student of Per Enger, came to Torridon from
Norway in 1971. With Colin, Olav investigated the hearing abili-
ties of two species of flatfish, the plaice, Pleuronectes platessa, and
the dab, Limanda limanda, both of them lacking a gas-filled swim
bladder. The swim bladder acts as a transformer between sound
Figure 3. The acoustic range within the loch consisting of several pressure and particle motion in gadoid species. The sound stimuli
sound projectors, located at different distances and angular in the flatfish experiments was varied in two ways (Chapman and
positions, relative to the top of a tower, where a small fish cage Sand, 1974). First, sound projectors were placed at different dis-
was placed by divers and connected to the shore by cables. tances from the fish to vary the ratio between sound pressure and
particle motion. Second, the effect of sound radiation from a gas-
same species in laboratory tanks. The acoustic conditions in Loch filled balloon on the auditory sensitivity was examined by placing
Torridon were much quieter than in most laboratory tanks. Only it close to the head of the dab. Thresholds for the plaice and dab
one small fishing vessel passed by the site occasionally, and the were not as low as they were for gadoid species and their fre-
ambient noise levels were quite low. However, some natural var- quency range did not extend as high. There were, however, very
iations in the ambient noise level occurred as the wind and wave clear differences between sound pressure thresholds obtained at
conditions changed, resulting in changes to the auditory thresh- different distances, showing that the otolith organs, in the absence
olds for the cod. It was evident that the detection of pure tone of a swim bladder, are sensitive to particle motion rather than
signals was being masked by the ambient sea noise. These results sound pressure. However, the presence of a gas-filled balloon
underlined the importance of performing hearing experiments close to the head of the dab resulted in much lower thresholds
under quiet conditions, and showed the wisdom of working and a more extended frequency range. This experiment provided
in the sea, under the soundscape conditions that fish normally strong evidence of the role of gas-filled bodies, including the
experience. swim bladder, in augmenting hearing in fishes. It verified the im-
Sounds were presented to the cod from sources at different dis- portance of carrying out hearing experiments in the sea.
tances, following the method introduced by Enger and Andersen Together with Alistair Johnstone from the MLA, Anthony
(1967). Thresholds at frequencies between 60 and 160 Hz were studied the hearing abilities of Atlantic salmon, Salmo salar
largely independent of sound source distance. At frequencies be- (Hawkins and Johnstone, 1978). In the sea, salmon responded
low 60 Hz, the thresholds were lower when the source was very only to low-frequency pure tones (below 380 Hz). The salmon
close to the cod, where particle motion amplitudes were higher for was relatively insensitive to sounds compared to the cod
a given sound pressure. It was concluded that the auditory system (Figure 4). Masking of the thresholds did not take place under
of the cod was effectively sensitive to sound pressure, but at fre- natural conditions of sea noise but could be imposed by creating
quencies below 60 Hz, the ear can respond directly to particle mo- higher levels of artificial noise. Again, use was made of the near
tion when the sound source is close to the fish. field effect to expose the salmon to different ratios of sound
Studying the behaviour of fishes 5
pressure and particle motion. As with the dab and the plaice, it Directional hearing
was confirmed that the salmon was sensitive to particle motion Location of the source from which a sound is coming is likely to
rather than sound pressure. Although the salmon has a swim be important to fishes. It may enable them to seek out prey, avoid
bladder, this species is a physostome, with an open connection predators, find mates, and detect important spatial sound cues.
between the swim bladder and the oesophagus. Physostomes can Early sound localization experiments gave negative results, and it
rapidly empty the swim bladder, which they normally do when was thought unlikely that fishes utilized the same direction-
frightened. Hence, it is uncertain if the salmon had gas in the finding mechanisms as terrestrial vertebrates (reviewed by
swim bladder during these experiments. Hawkins and Popper, 2018). However, it was evident from our
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observations on wild cod in Loch Torridon that fishes could
The acoustic characteristics of the swim bladder swim towards or away from some underwater sound sources.
Olav and Anthony later measured the sound fields re-radiated by This led to us wondering whether fish could discriminate between
the swim bladders of living cod. This required the fish to be sounds from different directions and distances. Colin showed
moved at up and down to different depths and it was easier to do that the masking effect of noise on the detection of a pure tone by
these experiments in a flooded quarry near Oban, on the west the cod was reduced when the masking noise was transmitted
coast of Scotland. A technique for doing this work had previously from a different direction (Chapman, 1973). Colin and Alistair
been applied in experiments carried out beneath a ship at Loch did further experiments with cod using four sound projectors,
Torridon (McCartney and Stubbs, 1971). In the Oban ex- allowing a wider range of angular separation between the signal
periments, the cod was placed inside a large, ring-shaped, piezo- and noise sources (Chapman and Johnstone, 1974). Experiments
electric sound transducer. The fish was lowered as sounds were were then carried out where cod and haddock were conditioned
presented and changes in the resonance frequency at different to a short period of switching of a pulsed tone from one projector
depths were monitored (Sand and Hawkins, 1973). It was con- to another at different angles of azimuth. The fish readily
cluded that the extra auditory gain provided by a swim bladder is responded to the switching when the projectors were separated
mainly in a frequency range below resonance, and that the swim by 20 or more. We later demonstrated that cod are also able to
bladder oscillations are heavily damped. From theoretical consid- discriminate between sound sources in the median vertical plane
erations, it was also apparent that the swim bladder provides no (Hawkins and Sand, 1977). We concluded that the otolith organs
auditory gain below a certain frequency, which depends on both are involved in directional hearing, through the detection of par-
the swim bladder volume and depth. Subsequently, Olav Sand re- ticle motion.
joined Per Enger in Norway and they provided further informa- While our experiments were being carried out at Loch
tion on the auditory function of the swim bladder in the cod Torridon, Arie Schuijf and his colleagues from the University of
(Sand and Enger, 1973). Utrecht were carrying out experiments on directional hearing by
fish in a Norwegian fjord (reviewed by Schuijf, 1975). Arie and
Masking of sound detection by ambient noise his colleagues trained fish to show which of two alternative sound
It was evident from our Loch Torridon experiments that the de- projectors was active by swimming to either of two opposing cor-
tection of sounds by fishes like the cod and haddock is often ners of a cage in return for a food reward. Working with his stu-
masked by natural variations in the levels of ambient sea noise, al- dent Rob Buwalda, Arie showed that the fish could discriminate
though no masking occurs under the quietest ambient noise con- sound waves travelling towards the head from those travelling
ditions. Masking occurs for species like the plaice, dab, and towards the tail (Schuijf and Buwalda, 1975). They then came
salmon in the presence of anthropogenic noise. This suggests that to work with us at Loch Torridon. We showed that cod could dis-
the hearing abilities of fishes are closely matched to the levels of criminate between pure tones emitted alternately from two
background noise in the environment. There is a real need for aligned sound projectors at different distances from the fish
hearing and fish behaviour experiments to be carried out at (Schuijf and Hawkins, 1983). In a key paper (Schuijf, 1976), Arie
acoustic field facilities like those at Loch Torridon, ideally at a lo- had proposed that directional detection might involve two dis-
cation with minimal noise interference from shipping and other tinct processes: determination of the axis of particle motion by
human activities. vector weighing and then removal of any remaining 180 ambigu-
Some additional experiments were carried out in Loch ities by analysis of the phase relationship between sound pressure
Torridon on masking, involving the presentation of pure tone and particle motion. At Torridon, we carried out experiments be-
stimuli in the presence of different noise frequency bands neath a raft to test the validity of the phase model in three-
(Hawkins and Chapman, 1975; Hawkins and Johnstone, 1978). dimensional space (Buwalda et al., 1983). The experiments in-
We showed that fish, like humans and other mammals, use audi- volved a complex configuration of sound projectors around the
tory frequency filters to improve the detection of signals in the fish. We showed that cod can discriminate between two sources
presence of ambient noise. of low-frequency sound, positioned opposite one another in the
Ambient noise levels are now often much higher in the sea, median vertical plane.
lakes, and rivers because of human activities. Masking by anthro- It was evident that most behavioural studies of directional
pogenic noise can prevent the detection of the sounds made by hearing have to be carried out in a free sound field, however,
fish themselves and other sound signals of importance to them. physiological approaches would be possible in a laboratory.
This is likely to have detrimental effects, adversely affecting the During a short visit to the MLA by Per Enger, a discussion took
ability of fish to find prey, avoid predators, navigate, migrate, and place over conducting laboratory experiments on the mechanisms
spawn successfully. There is now a need for more research on of directional hearing in fish. We decided to investigate the direc-
aquatic soundscapes, and how they may be deteriorating as a re- tional properties of the otolith organs by vibrating a fish in differ-
sult of human activities. ent directions. Microphonic potentials were recorded from the
6 A. Hawkins and C. Chapman
inner ear of a haddock. The fish was mounted in air on a vibra- or 10 min (Chapman, 1975). The fish observed mainly comprised
tion table and artificially respirated with water through a tube. three gadoid species, the cod, the saithe Pollachius virens, and the
The table consisting of a rotatable metal slab resting upon a foam pollack, but it was also possible to extend the observations to a
rubber bed. The slab was driven back and forth by an electromag- flatfish, the dab.
netic vibrator. The amplitude of the potentials proved to be a Initially, the gadoid fishes showed consistent avoidance reac-
function of both the stimulus strength and the direction of vibra- tions to low-frequency narrow band noise stimuli, but as the
tion (Enger et al., 1973). Different groups of hair cells within the bandwidth was reduced, the avoidance was less marked and when
otolith organs showed different patterns of directional sensitivity low-frequency pure tones were transmitted, there was a reversal
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when stimulated by the particle motion stimuli. More refined ver- in response and fish became attracted to the stimuli. In general,
sions of this method were later employed for examining the direc- the attraction response increased in proportion to sound inten-
tional responses of the inner ear of fishes (e.g. Sand, 1974; sity. When the tone transmission was switched between loud-
Hawkins and Horner, 1981; Fay, 1984). It became evident that speakers, the fish always gathered at the active sound source.
the directional information conveyed by particle motion can be Later, we found that the gadoid fishes and dabs were strongly
extracted from the incident sound by comparison of the outputs attracted to all our low-frequency sounds, both pure tones and
of differently orientated groups of hair cells. narrow band noise. We were able to show that this behaviour was
related to our diving activities in the area. In diving to set up ap-
paratus in the sea, we noticed that fish seemed to be attracted to-
Behavioural responses of fish to sounds wards us, and they appeared to be feeding on the benthic
During the collaborative work carried out with McCartney and organisms disturbed when we dived close to the seabed. Counts
Stubbs (1971) on fish target strengths, we were able to observe of fish observed by sonar and TV camera were then made before
the responses of wild shoals of whiting, Merlangius merlangus, a and after a diver was positioned at a particular location. This
pelagic gadoid species, to a seismic “air gun” sound source. Our demonstrated strong fish attraction towards the diver and to the
research ship, Mara, was above the whiting shoals and the fish ob- playback of noise recordings of the “scuba” breathing apparatus.
served by means of the ship’s echosounder. The shoals extended We concluded that the fish had learned to associate the noise gen-
from 15 fathoms (27.4 m) down to 30 fathoms (54.9 m) in water erated by divers with feeding opportunities (Chapman et al.,
that was almost 50 fathoms (91 m) deep, and they had probably 1974; Chapman, 1975).
entered Loch Torridon to spawn. The air gun was fired inter- Our work also showed that the behaviour of fish may be
mittently, generating a series of high amplitude, low-frequency strongly influenced by other underwater sounds made by
sounds. The whiting shoals showed strong downward move- humans. Although fish may swim away from mobile fishing gears,
ments, forming a more compact layer beneath 30 fathoms they can also move towards such gears, including bottom trawls,
(Figure 5). The air gun was fired several more times over a period seines, and shellfish dredges, all of which cause much disturbance
of 1 h, during which the fish habituated to the sounds and of the seabed. Buerkle (1973) confirmed that the noise produced
steadily ascended. Later on, the sounds were produced again and by trawling can influence their behaviour.
the fish descended once more (Chapman and Hawkins, 1969).
We later used the Torridon acoustic range to undertake further
experiments on the reactions of wild fishes to sound stimuli. Sound production by fishes
Observations were made using the Birmingham University high- Anthony’s supervisor at Bristol University, Dr H. P. Whiting, had
resolution sector-scanning sonar system (Welsby and Dunn, been a naval officer during the Second World War, and he had
1963), accompanied by an underwater TV camera. Counts of fish, been involved in locating submarines by listening for their
observed by the sonar and/or TV camera, were made before, dur- sounds. He had detected sounds that he believed had been made
ing, and after periods of sound transmission, generally lasting 5 by fishes. He handed over his naval hydrophone to Anthony and
asked him to listen to fishes, both in aquarium tanks and in the
sea. Initially, Anthony examined the sounds made by gurnards
(Triglidae), both in aquarium tanks and in waters off the coast of
Devon. It was evident that sound production is important to
some fishes, and he based his PhD on sound production by ma-
rine fishes.
When Anthony moved to Aberdeen, he and Colin began work-
ing together, initially focussing on the sounds made by haddock.
At Loch Torridon and elsewhere, it became clear that some other
gadoid fishes were vocal, including the Atlantic cod, pollack, ling,
tusk Brosme brosme, and the tadpole fish Raniceps raninus. Later,
Hawkins and Rasmussen (1978) were able to show that the main
differences in the calls of different gadoid fishes were based on
their temporal structure, all the calls being made up of low-
frequency pulses that were repeated at different rates and in dif-
ferent groupings. The sounds were generated by the repetitive
contraction of specialized “drumming muscles” attached to the
swim bladder. Examination of a number of other gadoid species
Figure 5. The responses of whiting shoals to sounds from showed that they also possessed drumming muscles, although
a seismic airgun. sounds had not been recorded from them. It later became evident
Studying the behaviour of fishes 7
that many species of fish are vocal, and that the sounds they pro- Other work on fishes at Loch Torridon
duce are used to communicate with one another (Hawkins and The movements of cod
Myrberg, 1983). Several research students came to work with us
Interest in the behaviour of cod within the loch prompted us to
on fish sounds: from Denmark (Knud Just Rasmussen), Portugal
develop a fish tracking system, to follow the movements of indi-
(Clara Amorim), and Italy (Licia Casaretto and Marta Picciulin).
vidual fish. An ultrasonic transmitter, developed by the Fisheries
Their work provided more detailed information on sound pro-
Laboratory at Lowestoft, was placed in the stomach of the cod or
duction by a range of fishes.
surgically implanted within the abdomen. The position of the fish
Our observations and sound analyses on captive fish at the
was then determined by comparing the time of arrival of the ul-
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MLA provided information for locating particular fishes in the
trasonic pulses at an array of hydrophones placed close to the sea-
sea. Listening was carried out at different locations in Balsfjord,
bed and spaced several hundred metres apart. Cod were tracked
Norway. Long sequences of repeated knocks were heard at one
continuously for up to 11 days. The tracking system was also used
particular location in the fjord, similar to the display sounds
with invertebrates (Chapman et al., 1975), and it was used by
recorded during reproductive behaviour by haddock in our
others to study the performance of fishing trawls. It was also used
aquarium (Casaretto et al., 2014). At night, the sounds merged
to examine the behaviour of cod in Loch Beag, at the western end
into a continuous low-frequency rumble, confirming that many
of Loch Hourn, to the south of Loch Torridon (Hawkins et al.,
vocal haddock were present. Spawning haddock were then found
1974). Such tracking systems are really useful for examining the
at the same unique location in Balsfjord in several successive
behaviour of fishes and invertebrates in the sea and could be used
years. Our work confirmed that listening for fish sounds provides
more widely. This method of acoustic position fixing is described
a reliable, non-invasive way of locating aggregations of spawning
in detail by Hawkins et al. (1980) and MacLennan and Hawkins
fish in the sea, allowing close definition of their spawning
(1977).
grounds. We have recently suggested that it is important to map
The tracking of juvenile cod showed that they lived near the
the spawning grounds of vocal fishes, and especially cod and had-
seabed in and around the edge of Loch Torridon, at depths be-
dock, to ensure that they are not deleteriously affected by offshore
tween 10 and 20 m, moving within restricted home ranges
human activities (Hawkins and Picciulin, 2019).
(Hawkins et al., 1980), where they searched for food. The major-
ity of Torridon cod were more active during the day than the
night, although a few were nocturnal. In Loch Beag, the cod
Other acoustic work at Loch Torridon ranged widely throughout the loch initially, following their re-
On two occasions while we were working at Loch Torridon, we lease, but later, as in Loch Torridon, they showed movements of
were asked to conduct measurements in relation to two quite dif- only limited extent within home ranges. During the night the cod
ferent sound sources: one was the sonic boom of a “Concorde” in Loch Beag moved over a wider area (Figure 6). Indeed, rela-
aircraft and the other was the sound from a new purse-seine fish- tively few positions could be plotted during the day because the
ing vessel, the “Semla,” having difficulties in catching fish! cod often occupied the same position on successive samplings.
The supersonic aircraft Concorde, flown by British Airways The areas occupied by different tracked cod were sometimes adja-
and Air France, flew at twice the speed of sound, and because of cent to one another, but they did not overlap to any great extent.
the sonic booms, it generated many countries would not allow
flights over their land. The routes were generally restricted to
ocean crossings, although fishermen raised objections to this be-
cause of possible effects upon fishes. In 1970, we were asked to
measure the underwater sound levels from the sonic booms, and
to consider whether they would affect fish behaviour. Concorde’s
test flights passed over Loch Torridon and we were able to mea-
sure the underwater sounds from the sonic booms. The sounds
reaching fishes were composed of two double pulses, one couple
passing through the water, and the other generated by substrate
transmission. Dramatic slowing of the heart rates of cod revealed
that the sounds were being detected, and that they could have ad-
verse effects upon the behaviour and physiological condition of
fishes. Anthony was invited to talk to the airlines about the effects
of supersonic aircraft upon fishes, and informed them that such
aircraft could have detrimental effects upon fishes. Following the
crash of an Air France flight, the Concorde was later abandoned.
In 1967, the Christian Salvesen Shipping Company launched a
new purse-seine fishing vessel the Semla (Registration LH454).
Early fishing trials with the vessel on herring shoals were unsuc-
cessful and it was thought that noise from the vessel was scaring
the fish. Semla came to Loch Torridon and a number of underwa-
ter noise measurements were made as the vessel operated its Figure 6. Locations of an individual cod within in Loch Beag,
purse-seine. It was found that tight manoeuvring of the vessel monitored at 15-min intervals over two successive days. The
generated rather high noise levels, and it was concluded that it positions were followed located at the following times: (a and c)
would be necessary to steer the vessel carefully, to avoid sudden from dusk to dawn (b and d) from dawn to dusk. The cod moved
changes in the engine noise as the ship approached a fish shoal. within a home range, covering a wider area at night.8 A. Hawkins and C. Chapman
The individual cod were occupying separate territories. The tim- establish research facilities, like those that we made available at
ing of their movements, and the areas covered, may perhaps be Loch Torridon, at new locations.
related to the vulnerability of particular prey.
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