A harpoon, a satellite, and a computer – tools of the trade for swordfish researchers

Swordfish are notoriously difficult creatures to track and study. They are a highly migratory species and scientists simply didn't know how North Atlantic populations interrelated – if they related at all. Yet Canadian biologists needed to track their movements to understand how best to manage the species.

Dr. John Neilson, his colleagues at Fisheries and Oceans Canada's St. Andrews Biological Station and the Nova Scotia Swordfish Harpoon Association tackled the problem with an innovative blend of age-old hunting skills and space age technology. Their target: the swordfish of the Northwest Atlantic – specifically those off Georges Bank.

The scientists were eager for answers because the International Commission for the Conservation of Atlantic Tunas (ICCAT) considers the North Atlantic swordfish (Xiphias gladius L.) as a single stock that mixes and breeds together. Single stocks are managed as a unit – to set fishing quotas, for example. But the genetic analyses used to identify fish stocks are not always definitive. What if the designation were inaccurate? What if separate groups of fish did not breed with each other or otherwise interact? Combining them as a single stock could lead to over-fishing of one group or another.

The DFO scientists knew that swordfish move into Canadian waters each summer after wintering from January through March in subtropical and tropical areas. Swordfish spawn in three seasons and utilize both the Caribbean Sea and five regions in the western North Atlantic. They are fished off the Scotian Shelf, Georges Bank, and also off the southern edge of the Grand Bank of Newfoundland. But this was not enough information to draw conclusions about stocks.

For a clearer picture of the North Atlantic population, the scientists decided to use pop-up satellite archival tags (PSATs) to track the swordfish's movements. The pop-up tags are tiny waterproof computers that can endure dives up to 1800 metres. They are programmed to record the depth at which the fish is swimming, the temperature of the water, and the length of daylight. Data are recorded until the tag is released and pops up to the surface days to months later. The release mechanism is pretty clever. Researchers decide how long they want the fish to be tagged. Once the time has elapsed, the PSAT actively corrodes the pin which tethers the pop-up tag to the animal. The tag pops to the surface, where it relays its information to the satellite. The information is then downloaded to a computer for scientists to analyze.

John Neilson knew that pop-up tags were a promising technology for studying highly-migratory species like swordfish. But the common method of tagging fish for study can be risky for the animal. The fish is normally caught on a line, hauled aboard a vessel and then tagged and released. Some don't survive the process.

Swordfish, however, have a distinctive feeding behavior that had advantages in this case. They dive deeply to feed during the night (up to 350 m or so), and then bask on the surface in the daytime. Basking likely allows swordfish to recover from the rigors of a deep, cold dive, but as they bask, they are vulnerable to the local harpoon fishery.

For the researchers, however, the basking behavior and existence of a harpoon fishery provided a golden opportunity. A basking fish could be harpoon-tagged. An experienced harpoon fisherman in a specialized vessel could tag the lolling fish without removing them from the water – thereby increasing the fish's chance of surviving.

The plan worked well, although interpreting the tracking data was a challenge. PSATs record changing light levels, which scientists use to determine the fish's location. For example, as the fish moves latitudinally (from the poles to the equator), the length of the days will change. Similarly, changes in the timing of sunrise and sunset as the fish moves east to west or vice versa, allow scientists to determine its longitude.

The swordfishes' deep and rapid diving behavior made calculations more difficult. Because the PSATs depend on recorded light levels to chart the fishes' movements, anything that alters light levels affects the data. The diving behavior could confuse calculations of day length or the sunrise-sunset schedule. Also, proximity to the spring or fall equinox reduces differences in day length, making it harder to determine north-south movements.

While tag manufacturers have provided software that allows users to edit light records considered unreliable, the process makes reconstructing migrations a bit subjective. To compensate for these light-related issues, the researchers considered sea surface temperature data. Changes in water temperature can indicate that a fish has been moving from southern to northern waters or vice-versa. Although the mix of ocean currents makes temperature-based tracking rather complex, temperature data helped complete the picture of fish movements, to create a more precise track of migration for the selected fish.

Of the 25 fish tagged through 2007, 22 PSATs “reported,” though not all remained attached to the fish as long as intended. Nonetheless, 10 remained attached for at least 75 days. The results represent some of the longest deployments reported to date for pop-up satellite tags applied to marine fish (up to 411 days, a world record!). They have yielded valuable insight into the annual migration of mature swordfish off North America.

The collaboration of the Nova Scotia Swordfish Harpoon Association and Captain Larry Sears was vital to the research, says John Neilson. “Without his expertise with the harpoon, this would not have been possible”.

The study found “very specific migration patterns” and “very specific homing behavior,” says John Neilson. In fact, one of the fish released in 2006 was recaptured the next year only five kilometers from its point of release. This limited movement surprised the researchers, since they are well aware of a variety of spawning locations and discrete feeding areas off southern Newfoundland. Yet the tagged swordfish from Georges Bank showed no movements across the North Atlantic, remaining in the Northwest Atlantic. It appears that North Atlantic swordfish have a complex population structure and several migratory pathways.

The study concluded that the seasonal movements of the tagged swordfish are inconsistent with the ICCAT designation of all North Atlantic swordfish as a single stock unit. More study would be useful, since research was limited to fish tagged off Georges Bank. Swordfish aggregations elsewhere in the western North Atlantic might mix with those in the eastern North Atlantic population. Also the animals tagged were predominately large, mature fish. The situation could be different for other life stages. For example, long distance movement and migrations of Pacific halibut (Hippoglossus stenolepis) are thought to occur at relatively young ages.

Unlike land animals, marine creatures like swordfish easily disappear beneath the waves. This makes them hard to follow and difficult to study. Satellite pop-up tags, creative methods and sound scientific analysis are important tools for researchers studying these captivating but elusive creatures.