In this post Jeremy Vaudo and Mahmood Shivji discuss their article ‘Long-term satellite tracking reveals region-specific movements of a large pelagic predator, the shortfin mako shark, in the western North Atlantic Ocean’
In your paper you used satellite telemetry to investigate movements and seasonal distributions of shortfin mako sharks (Isurus oxyrinchus) in the western North Atlantic Ocean. What were the biggest challenges for this work?
The biggest current challenge for this kind of work is the high cost of the satellite tags and the logistics of getting these tags deployed on the sharks. The shortfin mako shark typically occurs farther offshore (more than 30 km), requiring appropriately-sized and equipped fishing vessels to catch them. Furthermore, this species occurs in relatively low abundance and requires a lot of fishing effort to catch enough sharks for tagging. The process of attaching the tags to the sharks, once you catch them, isn’t too difficult, but you’re also at the mercy of the weather. We had many instances of rough seas keeping us off the water, reducing our field time.
Once the tags are deployed, the waiting game begins. Even though we know the theoretical battery life of the tag, the information we will get from each shark is unpredictable. We were able to track some sharks for over a year, while other sharks transmitted data only for a few months, or in a few cases just a few weeks. The reasons for this inconsistent reporting performance are difficult to identify. Some tags possibly stop reporting because of electronic malfunction problems (after all, the tag is immersed in salt water while attached to a deeply diving, fast swimming animal) or damage to the tag from interaction with another animal. Or the shark does not come to the surface often enough for the fin-mounted tag to make contact with the receiving satellites, or even due to death of the animal. All you can do is keep your fingers crossed that you get enough data to put together a good analysis. Fortunately, and although we did not know this at the start of the study, it turns out that mako sharks spend a lot of time at the sea-surface, and so the tags made frequent contact with the satellites and generated a lot of detections for us. This surfacing-preference behaviour of the makos allowed us to reconstruct high resolution tracks from which to draw our inferences.
What new insights did the study provide on shortfin mako sharks habitat use?
Despite being a shark captured in fisheries worldwide, there wasn’t much known about the habitat use of mako sharks. Our study focused on the western North Atlantic, where capture information obtained from fisheries showed that they occur along the northeastern coast of the USA and southern Canada during the summer and fall; but that is also when those fisheries are most active in those areas, which can skew inferences about the shark’s movements and habitat use. Fisheries-dependent, conventional identification tag and recapture information also suggested they moved into the Sargasso Sea during the winter, leading to a hypothesis of a circular migration pattern between the USA, Canada, and Sargasso Sea. There was hardly anything known about mako shark movements in the Caribbean and Gulf of Mexico.
By tracking satellite-tagged individual sharks over extended periods of time, we were able to test the hypothesized migratory pathways and identify core areas of mako shark use. The mako shark is one of only a handful of sharks that possesses a partially endothermic physiology, which would be expected to influence their movements in relation to temperature. In this context, we were also able to examine if mako sharks in subtropical vs. temperate waters differed in their movement patterns. Our results showed that the hypothesized movements of mako sharks in the western North Atlantic as derived from conventional tags were quite oversimplified. The satellite tracking revealed that during the winter, movements in the western North Atlantic were much more geographically dispersed, with many sharks also moving far out into pelagic waters during the cooler months (winter and spring), and some even making unexpectedly directed migrations down as far as Venezuela and back without much pausing along the way. In contrast, mako movements in the Gulf of Mexico were much more restricted and displayed no obvious seasonal patterns; the core usage area here was virtually unchanged year round. These differences indicate that mako shark movements are region specific. Surprisingly, there was almost no overlap between the movements of mako sharks in the western North Atlantic and Gulf of Mexico, despite the absence of physical barriers between these areas and the ability of this species to move really long distances. We also found that mako sharks are truly international rovers, crossing the management jurisdictions of at least 17 countries.
What implications do the results have for shortfin mako shark management?
The results of our study have several applications to management and conservation of shortfin makos: The identification of core habitat use areas by makos off the western North Atlantic and in the Gulf of Mexico provides a focal area for investigating interactions with fisheries in more detail, and also focusing spatial and temporal regulation of fishing effort, as necessary. The fact that the mako sharks demonstrated little spatial overlap in habitat use between the western North Atlantic and Caribbean/Gulf of Mexico and also showed region-specific movement patterns suggests mako sharks in these two areas may constitute different demographic stocks, and therefore should be managed on this basis. Additionally, the substantial cross-national boundary migrations by the sharks underscore that their management will require well-coordinated, international planning to be effective. A big surprise from this study also was the rather large proportion (7 of 32; 22%) of our satellite tagged sharks that were harvested in fisheries. Five of these sharks were harvested within the high use areas identified. The percentage of sharks harvested was twice that known from fisheries-dependent, conventional tagging studies, suggesting that fishing pressure on this species may be greater than thought. This result plus the locations of harvest suggest high overlap between areas used by the mako sharks and fisheries. This is a dangerous combination for a k-selected species. Lastly, understanding which areas are used by mako sharks over the course of the year can help inform stock assessments – the outcomes of which drive management policy – which currently have a great deal of acknowledged uncertainty because of a lack of biological information on mako sharks.
Where do you see this work go, what are future challenges and applications?
The next steps for us with this work encompass both basic ecological research as well as management-driven research questions.
We plan to look in detail at the environmental correlates associated with the movements of mako sharks, as these factors are likely to influence the region-specific movement patterns we observed. Once we figure out the important environmental drivers of movement, we can develop models to predict areas of high mako shark usage on a global scale.
The more management-directed questions we are pursuing are made possible by the ability to track individual, satellite tagged sharks in high resolution. For example, the high proportion of sharks harvested revealed by tracking them in near real-time provides an opportunity to more accurately and directly assess fishing mortality, a key parameter in stock assessments. Additionally, we plan to combine the information from our shark tracks with the increasing information becoming available from the Global Vessel Monitoring System about spatial activities of fishing vessels. This will allow us to more robustly determine overlap in space use between the sharks’ movements and spatial fisheries efforts.
The challenges of this work in the near future will remain the high cost of the satellite tags, but the information being generated has tremendous utility for improving the conservation of such apex marine predators.