Long Distance Movements of Atlantic Sharks from the NMFS Cooperative Shark Tagging Program*
by J.G. Casey and N.E. Kohler
In 1962, the U.S. Government initiated a shark tagging program in the Atlantic
Ocean with the volunteer assistance of sport and commercial fishermen. From 1962
through 1966 the study, directed by J.G. Casey, was centered at the Sandy Hook
(NJ) Marine Laboratory within the U.S. Fish and Wildlife Service of the
Department of the Interior. Since 1966, the program has been conducted from
NOAA's National Marine Fisheries Service Laboratory in Narragansett, RI. A
detailed account of the program's early history and summary of results for the
period 1962 through 1982 are provided in an earlier publication (Casey, 1985).
Currently, the Cooperative Shark Tagging Program involves about 4,000
recreational anglers, commercial fishermen, scientists, and Foreign Fisheries
Observers distributed along the Atlantic and Gulf coasts from Maine to Texas and
from the Canary Islands, England, Mexico, Spain, Portugal, Poland, Italy, and
the Bahamas. Over the years, the number of sharks tagged annually has varied
from 100 in the early years to an average of about 5,000 per year in the past
five years. In recent years, sportsmen have tagged about 50% of the sharks,
followed by NMFS and other biologists (22%), U.S. Foreign Fisheries Observers
aboard Japanese tuna boats (20%), and commercial fishermen (8%). Up to the
present, over 87,000 fish representing 46 species of sharks and 20 species of
other fishes have been tagged. Of these, over 3,200 fish from 32 species of
sharks have been recaptured (a recapture rate of approximately 4%). Recaptures
have been returned by fishermen from 24 countries and island territories. This
shark tagging program is the largest conducted anywhere in the world. It owes
its success to the thousands of fishermen who have unselfishly assisted in the
tagging effort and in providing valuable data from log books, tournament
records, and observations of sharks at sea.
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.