Long Distance Movements of Atlantic Sharks from the NMFS Cooperative Shark Taggi

Fish tagging can have several objectives, including studies of migrations, age, rates of growth, identification of different stocks, and population dynamics (e.g., assessing the size of the population, the size and age of individuals making up the populations, whether the population is increasing, decreasing, or remaining stable). Tagging data also provide information for monitoring shark populations that can help detect changes in species composition, geographical distributions, and size compositions. These variables can then be compared to shifts in prey abundance or environmental changes such as annual rainfall, water temperature, or the effects of fishing, pollution, and other man-made influences.

In addition to the scientific interest and the practical applications of tagging information to immediate management initiatives, there are also discoveries that gain the attention of a broad segment of society simply because they are “interesting.” Questions commonly asked include: How many sharks have been tagged? How fast do they swim? What is the longest time at liberty? How long does a shark live? How far do they travel? One problem with attempting to answer questions about “sharks” is that there are an estimated 350 species in the world, and they are so different in many respects that very few answers can be applied to all sharks. Accordingly, the rest of this article is offered in answer to the question, “How far have tagged sharks traveled?”

For the sake of convenience, the sharks considered here can be categorized as: (1) highly pelagic (those that range over broad geographical areas, sometimes occupying entire ocean basins), (2) coastal pelagic (those that are generally confined to the continental shelves but have shown movements exceeding 1,000 miles), and (3) local or resident (those that apparently spend most of their lives in a limited range of a few hundred miles or less). Bull, nurse, and bonnethead sharks are examples of local species. Although we are not considering the movements of these and other local species in this article, we hasten to point out that populations of local elasmobranchs are highly important components of marine ecosystems. Moreover, they are the most vulnerable to the impact of human activities, including habitat degradation and intensive fishing.

Of the 32 shark species from which tags were recovered under the NMFS program, 10 species have demonstrated movements exceeding 1,000 miles between tag and recapture locations. The distances in miles are calculated as straight-line distances. With the exception of the oceanic whitetip shark, the maximum distance for each species is supported by additional, if somewhat shorter, long-range tag returns. For example, the averages of the five farthest distances traveled for the shark species shown in the figures are as follows: blue (3,383 mi.), sandbar (1,994 mi.), dusky (1,888 mi.), mako (1,909 mi.), tiger (1,351 mi.), bignose (1,202 mi.), night (908 mi.), blacktip (771 mi.), and bigeye thresher sharks (745 mi.).

These maximum distances between tag and recapture locations are measured as straight lines and do not reflect random movements, or the effects of current systems, temperature zones, and other environmental features that influence migratory pathways and would certainly increase these distances significantly. For example, the maximum straight-line distance recorded for a tagged blue shark is 3,740 miles. Yet multiple recaptures suggest blue sharks may make round-trip movements between North America and Europe that exceed 10,000 miles. Sandbar sharks traveling on the continental shelf between Southern New England and Yucatan, Mexico, could easily cover 3,500 miles. On the other hand, a single long-distance recapture must be interpreted with caution since it may only reflect the stray movement of an individual shark outside the normal range of that species. For example, the oceanic whitetip shark is a highly pelagic species that is so rare in Hudson Canyon off New York that the fisherman who caught one provided a photograph of the shark being tagged to confirm the identification.

When interpreting tag-recapture data, consideration is also given to the possibility that the returns are more indicative of areas of intensive fishing, and the absence of recaptures in another area does not automatically rule out the possibility of sharks occurring there. The blue shark was considered rare in the Caribbean because it was known to be a temperate species that preferred cooler waters. However, expanding longline fisheries for tunas and swordfish in warmer regions of the Atlantic showed blue sharks to be common in the deeper offshore zones and around islands where upwelling brings cooler, deeper waters closer to the surface. Another consideration is that tagged sharks are sometimes recaptured in the same area where they were released several years earlier. Where a particular shark traveled during those years can be deduced only from other information. In addition, sharks generally segregate by size and sex, and the different segments of the population can have different migration patterns.

For these and other reasons, the NMFS long term tagging program can be likened to thousands of people working on an extremely complicated puzzle. The work is fascinating, challenging, and rewarding but at times frustrating because it requires a great deal of patience, and the picture remains far from complete. The discovery of a new piece of scientific information from tagging, particularly if it involves a record of some kind, deserves special attention because it represents a focal point in the advancement of knowledge that is a credit to everyone past and present who has participated in the research.

In addition to contributing to the basic biological knowledge of sharks, this information demonstrating movements across international boundaries is important for shark management initiatives. “Shark Management” in different parts of the world ranges from efforts to reduce shark populations where they are considered dangerous or represent costly nuisances to other fisheries, to maintaining populations at sustainable harvest levels for food and other uses. In Australia, conservation measures were enacted to protect shark populations in danger of serious depletion. During the past two decades, sharks have become more important in U.S. recreational fisheries. Over the past 10 years, the recreational catch of Atlantic sharks has averaged about 4,000 metric tons. In recent years, sharks have also become important in U.S. commercial fisheries. Between 1987 and 1988, U.S. commercial landings of Atlantic sharks increased nearly threefold, from about 2,000 to 6,000 metric tons (Leach et al., 1989). These expanding sport and commercial fisheries have prompted sufficient concern for overfishing the stocks that a Fisheries Management Plan for Atlantic Sharks has been prepared by the Secretary of Commerce and is scheduled to go into effect in 1991. The fact that fishermen from many countries have returned NMFS tags from sharks released in U.S. waters can be used to argue for international management of highly pelagic species. Other Atlantic shark species might best be managed through cooperative action between Northern and Central American countries. Finally, the responsibility for some local shark populations may rest with a few, or even single states.

Whatever management directions are taken in the future, the success of those actions will depend on a better understanding of the biology of each shark species, including additional knowledgeof their reproductive biology, food requirements, life spans, rates of growth, and migratory patterns. Fortunately, this knowledge continues to advance, and the pieces of the puzzle keep falling into place, bringing with them a clearer perspective of man’s responsibility for sharks as living marine resources.

ADDITIONAL READING:

Casey, J.G. 1985. Transatlantic Migrations of the blue shark; a case history of cooperative shark tagging. pp. 253-268. In R.H. Stroud (Ed.) World Angling Resources and Challenges. Proceedings of the First World Angling Conference, Cap d’Agde, France, September 12 to 18, 1984. Int. Game Fish Assoc., Ft. Lauderdale, FL.

Leach, P., D. Hays, P. Hooker, P. Kurkul, J. Casey, J. Castro. 1989. Draft Secretarial Shark Fishery Management Plan for the Atlantic Ocean. NMFS NOAA Review Document, 116 pp.