This is a troubled story, but I tell it anyway for the promise it holds. For some years my colleague, Prof. Godi Fischer, had together with his graduate students been developing a fishtag that included a complete RAFOS signal receiving system on a silicon chip. This development led to several PhD theses in recent years. Along the way various tests showed this development to hold promise. In 2017 another colleague, Prof. Jeremy Collie asked if it might be possible to use the fishtag to track lobster – and we saw no reason why not. What was even better, the tag was just about ready for its first serious application in the field, so why not on lobster? We had three sound sources available to provide the acoustic navigation.
Jeremy and a colleague, Michael Long, had funding to tag a large number of lobsters. So, in addition to their own tag work, they wanted to see if the tag could be used to study lobster movements on the bottom. Jeremy found that Gorilla glue was quite effective at attaching the tag to the back of the lobster, the carapace. The procedure was to catch a lobster, make sure it was healthy, attach the tag, check that the lobster was ok with the tag, and then release it. Some weeks or months later the lobster might get caught in a trap. The lobsterman would have been informed of the project and seeing the tag would set the lobster aside for Jeremy and Michael.
Godi and I agreed on a transmission schedule whereby I would program three RAFOS sound sources to transmit a 32 second signal, what we call a ‘pong’, every two hours. Even though it had been a long time since I had worked with the sources, I succeeded in verifying the transmission schedules. The sources were loaded onto a Galilee lobster boat, and Michael worked out a plan with the lobsterman to deploy them on moorings in protected (free from fishing) areas. Alas, just before the boat set out from Galilee I foolishly made a change in how the sound source power was to ramp up. The reason was I got concerned that the sources might damage themselves if they reached to full power before they were in the water. I shouldn’t have done that for I made a change without having the time to test that it was ok. The deployment of the sources went well (and three months later Michael did a great job recovering them.)
Of the 32 tags that Godi supplied and Jeremy attached to lobsters, 8 were returned by various lobstermen, an impressive 25% return. But when Godi examined the data stored in the tag memory, not a single signal arrival had been detected and recorded. What went wrong? I have to assume I made a mistake when resetting the ramp up schedule. I had had difficulty getting them set up to begin with (and I now know why) so I must assume they either didn’t transmit (possible) or that the clock or schedule was messed up (very unlikely). While Michael tried to verify that the sources were operating after deployment, listening conditions were not optimal, so it wasn’t clear whether absence of signal was an issue or not. That was my second mistake, I decided not to worry. Although less likely, it is possible that listening conditions are poor right on the seafloor. But surely at least some signals should have been detected. This needs to be resolved prior to any future lobster tagging.
But not all was lost, the tags recorded temperature every 40 minutes so along with where the lobsters were deployed and caught, we can piece together some interesting details about their movement. Two lobsters migrated more than 80 miles south from their release SE of Block Island out down the continental slope to 11 and 6°C temperatures or roughly 250 m and 500 m depths, respectively. As they wander south into deeper water, their temperatures decrease gradually until a certain time when suddenly they start to oscillate with a roughly but not regular 12-hour period, first at the deeper site and a couple of days later at the shallower site. It’s curious that the onset is so sudden. I have to guess that internal tide propagation slows along the shoaling slope to the point where the ambient velocity field dominates its subsequent movement. Perhaps a change in the eddy field, instead of ‘resisting’ the tide, enables it to enter the slope where the lobsters are. So not all was lost, these records just make you want to know more about what’s going on!
This website has some nice information about the lobster studies:
https://web.uri.edu/wp-content/uploads/sites/548/Ventless-Trap-Survey-2018.pdf