The trade in manta ray gill plates has considerably increased over the last two decades. The resulting increases
in ray mortality, in addition to mortality caused by by-catch, has caused many ray populations to decrease in size. The aim
of this study was to ascertain how yearling and juvenile growth and survival, and adult survival and reproduction affect reef
manta ray (Manta alfredi) population change, to increase our understanding of manta ray demography and thereby improve conservation
research and measures for these fish.
We developed a population projection model for reef manta rays,
and used published life history data on yearling and juvenile growth and adult reproduction to parameterise the model. Because
little is known about reef manta ray yearling and juvenile survival, we conducted our analyses using a range of plausible
survival rate values for yearlings, juveniles and adults.
The model accurately captured observed variation
in population growth rate, lifetime reproductive success and cohort generation time in different reef manta ray populations.
Our demographic analyses revealed a range of population consequences in response to variation in demographic rates. For example,
an increase in yearling or adult survival rates always elicited greater responses in population growth rate, lifetime reproductive
success and cohort generation time than the same increase in juvenile survival rate. The population growth rate increased
linearly, but lifetime reproductive success and cohort generation time increased at an accelerating rate with increasing yearling
or adult survival rates. Hence, even a small increase in survival rate could increase lifetime reproductive success by one
pup, and cohort generation time by several years. Elasticity analyses revealed that, depending on survival rate values of
all life stages, the population growth rate is either most sensitive to changes in the rate with which juveniles survive but
stay juveniles (i.e., do not mature into adults) or to changes in adult survival rate. However, when assessing these results
against estimates on population growth and adult survival rates for populations off the coasts of Mozambique and Japan, we
found that the population growth rate is predicted to be always most sensitive to changes in the adult survival rate.
is important to gain an in-depth understanding of reef manta ray life histories, particularly of yearling and adult survival
rates, as these can influence reef manta ray population dynamics in a variety of ways. For declining populations in particular,
it is crucial to know which life stage should be targeted for their conservation. For one such declining population off the
coast of Mozambique, adult annual survival rate has the greatest effect on population growth, and by increasing adult survival
by protecting adult aggregation sites, this population’s decline could be halted or even reversed.