Does the Porpoise-PAL really reduce harbor porpoise by-catch?
Yes, by-catch reduction is around 77 %. The Thünen Institute for Baltic Sea Fisheries (Rostock) carried out 750 experiments between 2014 and 2016 in the professional western Baltic Sea fishery. Fishermen set around 4,500 km of nets. Twenty-three harbour porpoises were by-caught in total: only 5 in PAL nets, but 18 in nets without the alerting devices.
Does the Porpoise-PAL affect the catch of target species?
Not at all: The experiments conducted by the Thünen Institute in professional fisheries showed no differences in the catch of target species between control and PAL nets.
How do fishers get along with the PAL?
Very well: Since April 2017 a large-scale pilot study is underway, supervised by the Baltic Infocenter in Eckernförde. The fishermen find no impact on the yield of their target species and there are no issues with the PAL on board. The tested attachment technique ensures smooth fishing operations without net entanglements.
Are there indications that the Porpoise-PAL attracts seals?
No: During the experiments in professional fisheries, no damaged fish were found in the nets. No seals were reported as by-catches.
Can grey and harbour seals hear the Porpose-PAL at all?
Possibly: Porpoise-PAL emits at around 133 kHz. Within about 20 meters there are weak resonance vibrations down to 50 kHz, which reach into the hearing range of harbour and grey seals. However, these signals are much weaker than in conventional pingers. A response of harbour seals to the PAL was not observed in the field.
Are there indications that the Porpoise-PAL attracts or repells seabirds?
Contrary to the yellow devices of several pinger manufacturers, PAL is completely black. The scant data collected to date show no correlation between PAL and seabird bycatch.
Can seabirds hear the Porpoise-PAL?
Probably not. A recent study by the University of Southern Denmark shows that seabirds such as cormorants can only hear under water in the 1-4 kHz range.
What specifications does the PAL have?
The PAL is lighter than water and floats with 60 grams of lift. It has an impact-resistant, cold-resistant and UV-resistant plastic housing and has been tested to a depth of 320 m. The battery is foam padded and replaceable. The housing is screwed together and a greased O-ring is used as a seal. Every PAL is programmable and can be updated to the latest software or acoustic signal version.
How do you attach a PAL to the net?
With the help of sheathed flexible lines, so that the PAL is not damaged by on-board machinery. The lines are tightly attached to the nets floatline to ensure that no mesh become entangled. This attachment method has been extensively tested by various fishermen and ensures smooth operation on board.
How many PAL will I need?
As a rule of thumb: Net length in meters / 200 plus one. So in a set consisting of 1,000 m length you'll need 6 PAL. All experiments of the Thünen Institute in professional fisheries refer to a distance between PAL of 200 m. All PAL must be aligned in the same direction (please note the arrow on the device) and the last device on the set points in the opposite direction, towards the net.
On what gillnet types should I use the Porpoise-PAL?
Actually on all of them. As a rule of thumb, the higher the net stands, the greater the risk that porpoises will get caught in it. The experiments of the Thünen Institute were carried out on bottom-set cod gillnets. Currently, fishermen use PAL in cod and turbot gillnets.
What time of the year should the Porpoise-PAL be used?
Throughout the year: The Thünen Institute's experiments in the professional gillnet fishery in the Western Baltic showed that porpoise bycatch occurs year round.
How long does the PAL battery last?
PALs have a battery life of approximately 4 years on the shelf, over 1 year during continuous operation (temperature dependent). We estimate up to 2 years normal operation. The battery is replaceable by the manufacturer.
What happens if a PAL doesn't work an more?
Please do check all PALs weekly to bi-weekly. The devices are rugged and durable, yet it cannot be ruled out that the battery is dead or the housing damaged. Defective PAL cannot protect the net area concerned. If bycatch occurs, it is very important to check the neighbouring PAL and to collect the dead animal for analysis, in order to clarify the circumstances.
How can the functioning of the Porpoise-PAL be checked?
Every PAL can be checked directly on board with the naked ear or using a bat detector. The signal is generated in air for another 20 minutes and is faintly audible on board. Under water, PAL can be detected with suitable hydrophones up to a distance of 170 m.
How often does the Porpoise-PAL transmit its signal?
Porpoise-PAL transmit continuously in cycles while submerged. To prevent a habituation effect, the device randomly generates 1-3 warning signals consecutively during each cycle, followed by random pause intervals of between 8 and 30 seconds. So every cycle is different.
Is the PALactive on deck?
No: PAL has a water switch and will only become active within a minute of submerging it in the water. After retrieval, the devices transmit for another 20 minutes to allow a functionality check.
Will the PAL effect diminish with time?
Probably not: During the three-year study conducted by the Thünen Institute there was no evidence for a decrease. This is also unlikely, as porpoises will probably not get used to the warning signals of their own kind.
In what areas did the Thünen-Institute conduct the study?
In the western Baltic Sea: The professional fishermen set their nets in their traditional fishing areas around the Isle of Fehmarn, in the area of Eckernförde and Damp, as well as in the Öresund between Denmark and Sweden, from Copenhagen north to the Kattegatt.
How was the by-catch study of the Thünen-Institute conducted?
Professional, commercial fishermen conducted their normal fishing activities with around half of their gillnets equipped with PAL. Simultaneously, they set the other half of their nets without PAL as controls, at least 500 m away. The nets were identical in terms of setting time, duration, length and condition. The fishermen recorded yields and bycatches in special protocols. In addition, fishery observers and video monitoring were used.
How do porpoises react to the signal of the Porpoise-PAL ?
In preliminary investigations, various signals were tested. The best signal for alerting the animals imitates their aggressive communication sounds. Their response to a PAL transmitting this signal was observed from a cliff and recorded under water using a click detector. When Porpoise-PAL was active, harbour porpoises increased their minimum distance to the sound source by on average 19 m. At the same time they increased their echolocation by on average 10%. These effects probably lead to an earlier perception of the nets as well as a reduction of collision probability.
Will harbour porpoises approach nets equipped with Porpoise-PAL?
Yes. Autonomous acoustic detectors recorded porpoises on both control and PAL-equipped nets.
What is the difference between a Porpoise-PAL and a conventional pinger?
The Porpoise-PAL produces artificial communication sounds of harbour porpoises at a high ultrasonic frequency of 133 kHz. Experiments with the Porpoise-PAL show that the animals hardly avoid it, but increase their echolocation by 10%. Pingers are available in different versions: as audible 10 kHz pingers, ultrasonic broadband pingers with signals between 20 and 160 kHz and whale pingers with a low 3 kHz. Studies show that harbour porpoises are deterred by pingers. They also reduce their echolocation, presumably to avoid attracting the attention of potential enemies. As a result, they may become entangled in nets near defective or missing pingers.
Will I have no porpoise bycatch at all when using Porpoise-PAL?
The porpoise PAL reduces bycatch by 77 %. This means that small numbers of by-catches will probably still occur. The state of Schleswig-Holstein is promoting studies on whether this may particularly concern sick or hearing impaired harbour porpoises.
- Bonn Convention on the Conservation of Migratory Species of Wild Animals (CMS)
COP12 “inflight magazine”, TRACKS, p. 41. Download PDF of article here.
- International Council for the Exploration of the Seas. Working Group on Bycatch
ICES Reports on Bycatch of Protected Species (WGBYC). Download PDF of 2017 Report here.
- Acoustic protection for marine mammals: new alerting device PAL
Proceedings 43. Annual Acoustics Conference DAGA, 2017, Kiel. Download PDF of article here.
- Porpoise alerting device (PAL)
Proceedings of the 4th PMCE Conference 2015. Download PDF of article here.
- New mitigation methods and evolving acoutic exposure guidelines.
Download PDF of ECS 2015 Workshop proceedings
- Synthetic communication signals influence wild harbour porpoise behaviour