By Dr. Chris Harvey-Clark
This guest post is brought to you by Dr. Chris Harvey-Clark who is the University Veterinarian as well as the Director of Animal Care in the Department of Psychology at Dalhousie University. In an interview on CBC's Mainstreet program, Dr. Harvey-Clark spoke to the success of small-scale aquarium such as the ones found in St. Andrew's, New Brunswick and Petty Harbour, Newfoundland. We reached out to him following this interview at which point Dr. Harvey-Clark agreed to serve as one of our valuable advisors. He has taken the time to tell us more about one of his areas of interest in the post below.
People might be surprised to know that a big marine predator that kills fish using electrical shock is coming in increasing numbers to shallow waters around Nova Scotia!
This late summer visitor is the Atlantic torpedo ray, Tetronarce (formerly Torpedo) nobiliana, a large, enigmatic member of the skate and ray family (the Batoids) found from tropical to temperate waters on both sides of the North Atlantic inshore and in deep waters. By far the largest of 17 species of electric rays worldwide, the torpedo ray can weigh 90kg and have a body disc diameter approaching 2m in mature females.
This species uses electrogenic organs comprised of modified muscle cells in the lateral margins of the body disc to generate controlled DC current bursts in excess of 200 volts. This shocking power can snap the back of a mackerel in tetanic convulsions and is also used for discouraging predators. A friend of mine who was shocked by this species while diving lived to tell the tale and likened the sensation to putting your finger into a dryer socket.
Dr. Fred Whoriskey views the first ever satellite tagged Atlantic torpedo ray
Photo credit: Dr. Chris Harvey-Clark
The electrogenic tissues of torpedo rays have been extensively studied at the cellular and molecular level, with thousands of citations in the scientific literature. Some of the earliest work on the neurotransmitter acetylcholine and its effects on muscle tissue were first characterized in torpedo ray tissues. It is a paradox that despite extensive study at the cellular level, little is known of the ecology, movement and behaviour of T. nobiliana. In fact, decades of fishing for use in neuroscience research depleted local populations of this species in the vicinity of the Marine Biological Laboratory at Woods Hole Mass.
The fact remains that virtually nothing is known about the basic biology of T. nobiliana. The size and age structure of the Atlantic population, depth, substrate and temperature preferences, onshore/offshore movements of this species, prey preferences, longevity, reproductive parameters and life cycle are all poorly known.
The IUCN (International Union for Conservation of Nature) Red List indicates the species is data deficient, in common with the majority of sharks, skates and rays. In this respect, our knowledge of T. nobilana resembles the former state of knowledge of many large charismatic species such as sharks, tunas, sea turtles and many marine mammal species prior to the development of modern electronic tracking technology beginning two decades ago.
Like the curious case of the dog that failed to bark in the night, the fact that this species is rarely reported as bycatch despite intense commercial fishing within its known range begs the question: where do these animals go? What is their role in the seasonal summer assemblage of large pelagic and forage fish species that occurs in boreal seas around Europe and North America annually?
The habit and habitat of this species remains a mystery. Observations exist of occasional individuals in shallow water sand and mud bottom habitats from Nova Scotia to the Florida keys and from northern Scotland to West Africa, into the Mediterranean, usually from fisheries bycatch inside continental shelf depths. Fishbase and similar database sources cite depth data for this species from shallow water to 800 meters and report their presence as rare fisheries bycatch in the Mediterranean. Several references claim the rays are benthic bottom dwellers when younger and become more pelagic dwellers as they get older, but there is little evidence in the primary scientific literature to support this claim.
In the fall of 2015, Dr. Fred Whoriskey and myself tagged a female Atlantic torpedo ray with a satellite tag near Halifax, NS. The tag was programmed to pop up to the surface 95 days later, and report its position and other data to a geosynchronised satellite. I had theorized that the rays were following the shallow continental shelf migrations of forage fish like herring and mackerel - north in the summer and south in the winter. Imagine my surprise when the tag reported 95 days later from an offshore location over 900 km out in the North Atlantic, from an area where the bottom is in excess of 4000 m. This single record indicated that in at least one case, this species does in fact act as a pelagic animal, quite amazing for a ray we had found dug in to the bottom while scuba diving in 20 m of water.
This discovery has led to plans for a more extensive study of the movement and behaviour of this species. Volunteers interested in helping the torpedo ray tagging team can contact me at Dalhousie University: firstname.lastname@example.org.