Evaluating the suitability of close-kin mark-recapture as a demographic modelling tool for a critically endangered elasmobranch population

Estimating the demographic parameters of contemporary populations is
essential to the success of elasmobranch conservation programmes, and to
understanding their recent evolutionary history. For benthic elasmobranchs
such as skates, traditional fisheries-independent approaches are often
unsuitable as the data may be subject to various sources of bias, whilst
low recapture rates can render mark-recapture programmes ineffectual.
Close-kin mark-recapture (CKMR), a novel demographic modelling approach
based on the genetic identification of close relatives within a sample,
represents a promising alternative approach as it does not require
physical recaptures. We evaluated the suitability of CKMR as a demographic
modelling tool for the critically endangered blue skate (Dipturus batis)
in the Celtic Sea using samples collected during fisheries-dependent
trammel-net surveys that ran from 2011 to 2017. We identified three
full-sibling and 16 half-sibling pairs among 662 skates, which were
genotyped across 6,291 genome-wide single nucleotide polymorphisms (SNPs),
15 of which were cross-cohort half-sibling pairs that were included in a
CKMR model. Despite limitations owing to a lack of validated life-history
trait parameters for the species, we produced the first estimates of adult
breeding abundance, population growth rate, and annual adult survival rate
for D. batis in the Celtic Sea. The results were compared to estimates of
genetic diversity, effective population size (Ne), and catch per unit
effort (CPUE) estimates from the trammel-net survey. Although each method
was characterised by wide uncertainty bounds, together they suggested a
stable population size across the time-series. Recommendations for the
implementation of CKMR as a conservation tool for data-limited
elasmobranchs are discussed. In addition, the spatio-temporal distribution
of the 19 sibling pairs revealed a pattern of site-fidelity in D. batis,
and supported field observations suggesting an area of critical habitat
that could qualify for protection might occur near the Isles of Scilly.