The IUCN/SSC Shark Specialist Group
Shark News 5: October 1995
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What Future for Manta Rays?
Giuseppe Notarbartolo-di-Sciara, Tethys Research Institute
Rays belonging to the family Mobulidae, the devil rays, are considered
by many, scientists and lay-persons alike, among the most fascinating
and mysterious of marine animals. Yet, because of their present
vulnerability to man's activities, we may no longer have the opportunity
of finding out the details of their biology, ecology and behaviour.
Entire populations, if not species, may disappear without anybody
even noticing it. Back in the early 1980s, I approached a community of
fishermen near the southern tip of Baja California to collect data on
mobulids for my doctoral thesis. According to the existing literature,
there should have been two species of Mobula in that area, however the
fishermen insisted that there were four. Of course they were right, and
one of those species turned out to be new to science. Unfortunately, I
also found out that my friends were mostly catching immature rays.
Seventy-two per cent of bentfin devilray M. thurstoni - the most frequent
species in their catch - had not had the chance of reproducing. I can
hardly think of a better example for an unsustainable fishery.
A fisherman fillets a spinetail devilray Mobula japanica in Baja Califonia.
Photo: G. Notarbartolo-di-Sciara
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I had stumbled on such conclusions by pure serendipity. However,
how many situations such as this one exist, or have existed, scattered
around the world's tropics? How many mobulid populations have been
studied, what data are being or have been collected and published
concerning the presence of mobulid rays in any of the world's
fisheries' catch or by-catch? I regularly monitor Current Contents,
and obtained a clean zero during the past ten years. Yet, we know
from anecdotal sources that mantas and mobulas are often involved
in fishing activities, be it in organised direct catches, by opportunistic
harpooning, or in the by-catch of large-scale industrial fishing, such as
pelagic driftnets or tuna purse seines.
Mobulids are extremely vulnerable animals. First of all, their
reproductive rate is among the lowest of all Elasmobranchs, with a
single huge pup being produced by each female presumably every 2 to
3 years, or longer. Secondly, although there are no data on population
sizes, one can presume that these large-bodied rays are rare and live in
very low densities. Finally, mantas and mobulas are very easy to
harpoon or to entangle in a gillnet, and for most fishermen living in
precarious conditions these rays provide a very tempting source of extra
proteins for their table.
One can think of several reasons why all possible action
should be undertaken to prevent the disappearance of mobulid
populations and species from the world's oceans. Obviously,
to begin with, there is the catch-all but deservedly sacrosanct
concept of preserving biodiversity. Secondly, mobulids are
evolutionarily extraordinary batoids in many ways, having
left the bottom for the surface, having attained the largest
body size, having adopted a filter-feeding habit, and having
developed the largest elasmobranch brain. It would be nice
to be able to make some sense of all this before eating them
out of existence.
In addition, mobulids (and particularly manta rays) are
becoming a major attraction for many diving locations, and
can thus be considered assets of economic importance for the
tourism industry. Finally, I think mantas and mobulas have an
important symbolic value, because they could be excellent
indicators for the plight of the forgotten species - those which
are disappearing without anyone knowing it. As such symbols,
they could serve to greatly increase public awareness of the
need for preserving marine biodiversity. In the Mediterranean
Sea, for example, a war has been raging for almost a decade between
fishermen and environmental groups over the use of driftnets. This was
mainly due to the environmentalists' awareness of the tremendously
large cetacean by-catch in this fishery. I feel personally very sympathetic
with the plight of cetaceans in the Mediterranean, and spend a great
deal of my energies in this field, realising that dolphins and whales are
taking a very severe beating from human activities in this region. But at
least we all know about this. By contrast, how many hundreds, or how
many thousands of the giant devil ray Mobula mobular were obliterated
from the Mediterranean by the very same driftnets? We will never know.
Most environmentalists are not even aware of the existence of mobulid
rays in the Mediterranean.
Mantas should have now the spotlights turned on them, to
remind us of all those marine species, large and small, that are
disappearing daily under our unseeing eyes.
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