Unravelling the many mysteries of Tope in the Northeast Atlantic
An angler with a female tope tagged as part of Inland Fisheries Ireland’s Marine Sportfish Tagging Programme
The Background
The tope (Galeorhinus galeus), also known as the school or soupfin shark in other regions, is one of the most popular target species for sea anglers around Ireland’s coast. This medium sized shark represents one of the largest species which can be captured on typical non-specialist fishing gear, attaining lengths of almost 2m and weights of over 80lb. While tope numbers (and tope fishing) suffered major decline in the Northeast Atlantic during the latter part of the 20th century, with a 76% reduction since the 1940s (Walker et al., 2020), recent decades have seen an apparent resurgence in tope numbers in some regions and a uptick in the popularity of tope angling.
On a broader scale, however, the story of this species is far more concerning. Tope have a worldwide distribution encompassing the temperate waters of the Atlantic, Pacific and Indian oceans, including a circumglobal range in the Southern Hemisphere. Like many shark species, tope are in global decline, with an 80% reduction in their stocks since the 1940s in the Southern Hemisphere (Walker et al., 2020). Commercial exploitation is widespread within the Southern Hemisphere, where tope often represent a key commercial species. This contrasts with the Northeast Atlantic, where tope are rarely commercially targeted, being typically caught inadvertently as by-catch.
Given the high level of commercial fishing pressure and continued decline in most populations, tope are currently listed a ‘critically endangered’ globally by the International Union for Conservation of Nature (IUCN), the highest threat level which can be assigned to non-extinct species. In contrast, tope are currently ‘only’ classified as ‘vulnerable’ within Europe and the Mediterranean, with these populations being the least depleted globally. Given current population trends, it is possible that, within our lifetimes, the Northeast Atlantic may hold the last surviving wild tope.
Given the importance of this region to the global survival of this species, it is concerning that there is currently a major lack of knowledge for these regions, including regarding population size(s), as well as movement of individuals and genetic exchange between regions. On a basic level, it is still unclear exactly when and where these animals mate and give birth, or whether the sharks found in the Mediterranean and Northeast Atlantic represent separate populations. These knowledge gaps therefore present a major challenge for the management and conservation of this species.
Tope tagging
Inland Fisheries Ireland and its predecessor have managed a nationwide tagging programme (the Marine Sportfish Tagging Programme) for several shark and ray species since the 1970s. Under this programme, anglers and charter boat skippers tag sharks with uniquely numbered tags, recording the date and location of capture, alongside a range of basic biological information, such as length and sex. This enables identification of individual sharks following any subsequent recaptures, enabling scientists to estimate growth rates giving use clues about individuals’ movement patterns. Tope represent one of the most frequently tagged species, with almost 10,000 fish tagged and over 500 recaptures.
Earlier this year the author of this article, along with other Irish-based scientists, published a new paper piecing together insights from the last 5 decades of tope tagging around Ireland.
Our Findings
Tope were mostly captured within 7 major clusters around the Irish coast, covering the Northern, Northwestern, Western and Southwestern part of Ireland’s Atlantic coast, as well as the Irish Sea, Northern part of the Celtic Sea and the North Channel, between Northern Ireland and Scotland. However, the majority of sharks were captured from the Irish Sea, or along the Southwestern or Northern areas of Ireland's Atlantic coast, with these collectively representing almost 90% of total catches. Annual catches varied greatly between years, with some having almost 500 sharks tagged, while the lowest year had only 3, with the contributions of each area also varying greatly between years. Outside Irish waters, almost 400 sharks were tagged in the English Channel, the North Sea and the waters off the West coast of Scotland.
Of the sharks which were recaptured, most had spent at least 3 years at liberty, with one shark at liberty for over 25 years demonstrating the longevity of this species, while another shark was recaptured on the same day as it was tagged. Although almost half of these recaptures were from the Irish coast, sharks were also recaptured from as far as the Arctic Circle and Danish Straits in the North, to the Azores and coast of Morocco in the South, and as far East as the coast of Sicily, in the Mediterranean. Taken together this suggests, although cannot be taken as direct confirmation, that tope in the Northeast Atlantic and Mediterranean all belong to a single population.
Examining the trends in catches shows not only an increase in the number of sharks tagged since the early 2000s but also an increase in the average size of both male and female sharks. This was most pronounced in the Irish Sea, where both male and female average lengths increase by over 30 cm across a similar period, from 117cm to 148 cm (2000-2014) in males and 120cm to 151cm (2000-2019) in females. While we can only speculate at the cause of this pattern, and must caution that self reported data cannot be considered 100% reliable, it may be due to a reduction in mortality rates in recent years, with more sharks therefore surviving long enough to reach larger sizes.
Trends in annual sex ratios (a) and (b) male and female mean (average) total lengths of tope for three areas around the Irish coast. Note: only male total lengths are included for the Southwest and Northern Atlantic coasts due to the low number of females caught.
Growth rates for tope in the Northeast Atlantic were also among the highest for this species worldwide, providing some explanation as to why this region seems to produce so many of the world’s largest tope. Like many other fish species, growth rates were not consistent throughout life, with most sharks reaching 1 metres in length before their 10th birthday, and 1.6 metres (and over the Irish specimen size) before turning 25, but potentially taking 60 years or more to attain Irish record size. Interestingly, like many shark species, male tope appear to grow faster than females - at least in length terms - in their early years, but with their growth rates then tailing off more sharply as they mature, meaning most of the largest specimens are female.
The Gender Gap
One of the most striking patterns apparent from this tagging work was the clear differences in the distribution of male and female sharks. Male sharks formed the majority of catches along the Atlantic coast, with this becoming more pronounced further North. In the most Northerly group almost 95% of sharks caught were male. In contrast, catches from the 3 east coast regions had approximately equal numbers of males and females. At a broader scale, females were also recaptured much further south than males on average, with catches from the Azores, Mediterranean and the western coasts of Spain, Portugal and France, as well as the English Channel all dominated by Females. Interestingly these regions also had the largest proportions of females which were large enough to likely be mature. Aside from the 3 Irish east coast regions catches from most areas to the North were then dominated by males, showing a clear North/South divide. Curiously, this separation between male and female tope appears to become more pronounced with age, with juveniles of both sexes overlapping much more than their larger adult counterparts. The Irish east coast appears to represent one of the main areas where mature male and female tope can be found at the same time of year, raising questions about whether this area is used during mating.
Image showing the proportions of male and female tope for various regions within the Northeast Atlantic and Mediterranean. Part (a) & (d) show the percentage of tope catches from each region which are made up of females. Parts (b) & (e) show percentage of females which exceed length at which 50% of sharks are mature (i.e. those exceeding this length are more likely to be sexually mature than immature). Parts (c) & (f) show the equivalent information for male sharks. Sample sizes for each marine area and subsample are denoted by hashed lines. Note: patterns in areas where the sample size (N) is less than 10, denoted by diagonal lines, should be considered cautiously.
Furthermore, while male sharks displayed no clear seasonal migration pattern, some females appear to follow a similar North-South migration pattern to other animals, moving northward as sea temperatures rise during the summer months, before returning to more deeper, more southerly waters during winter. Although it should also be noted that numerous female sharks also appear to buck this trend, being found in the far North even in the depths of winter, seemingly out of spite for the scientists attempting to characterise their behaviour. Some larger female sharks also appear to stay in more southerly waters year-round, while a number of these southerly-caught sharks were also found to be pregnant. This raises the possibility that female sharks might remain in more southerly waters during pregnancy. This behaviour has been doucmented in other shark species. It is thought that the warmer water temperatures (and thus body temperatures of the sharks themselves - remember, most sharks do not regulate their body temperature in the same way we do) may increase the growth and development rate of their embryos by boosting their metabolism.
Interestingly the majority of very small juveniles, or young-of-the-year, were found from the Irish Sea and surrounding regions, including several sharks which likely represented newborns. In other parts of the world female tope appear to migrate from offshore waters to give birth in shallower coastal areas, with these observations raising the possibility that some female tope may similarly migrate from Southerly regions to Irish coastal areas to give birth. Thus the Irish East coast may represent not only a key mating area for this species but also a juvenile nursery.
Next Steps…
While the information collected by anglers over the last 5 decades has given us an invaluable insight into the movements and biology of this fascinating, and oftentimes bewildering, species. As is often the case as scientists, we are left with more questions than answers. These mostly centre on the breeding behaviour of this species, including confirming exactly when and where these animals mate, where females reside during pregnancy, and where they give birth. These questions are particularly important for the protection and preservation of tope in the Northeast Atlantic, and must be answered through additional research. Ongoing research using satellite tags, which enable the movements of individual sharks to be tracked, may help to better define the annual movements of this species, enabling more links to be made between these and reproduction.
An adult female tope, just tagged with a pop-up satellite tag by the author, and ready for release. These tags record details of the movements, depth use, activity levels and surrounding water temperatures of these animals over a 1 year period, before popping off the animal and transmitting this information to us via satellites.
However, all this is not to say that the work is done from an Irish tope tagging perspective. Angler tagging reports remain one of our most important ways of monitoring the abundance, distribution and seasonal occurrence patterns of tope and other elasmobranch species. It is likely that such information will become increasingly important as sea temperatures continue to rise, acting as an invaluable monitor for changes in our marine ecosystems.
If you want to know more about this work you can read the full article here
