Single nucleotide polymorphism genotypes for the Australian blackspot shark and the milk shark in Northern Australian waters

Charles Darwin University and the Northern Territory (NT) Department of
Industry, Tourism and Trade (DITT) Fisheries Division used genetic data to
investigate the population structure of two small tropical shark species
(Milk Shark [Rhizoprionodon acutus] and Australian Blackspot Shark
[Carcharhinus coatesi]), which are caught as bycatch from commercial
fisheries in the NT.   The aim of this study was to gain
information on the genetic stock structure to inform the future management
of these two species in the NT. This project was conducted in parallel
with a PhD project investigating the biology and ecology of both species
for applications to fisheries management. There is motivation by the NT
Government to develop these two shark species into a commercial product.
This project used genetic analysis to understand the patterns of
connectivity of populations of these two shark species in NT waters and
adjacent regions, including Northern Western Australia and Papua New
Guinea.   Background   These two shark species that are
captured as bycatch in the NT Demersal Trawl fishery have the potential to
be developed into a byproduct to add value to that fishery. A sustainable
commercial harvest of these two species could greatly reduce the waste
from fisheries, where they are currently abundant and caught in relatively
large numbers. We address current knowledge gaps in biological information
about populations of R. acutus and C. coatesi to inform the potential
development of a byproduct fishery for these two species in the NT.
  Aims   Our research aimed to:   · 
       identify the genetic population
structure for R. acutus and C. coatesi in NT waters   · 
       develop capacity for genetic
research and monitoring of shark species in the NT   · 
       provide baseline information on
genetic structure to inform potential genetic monitoring of these species,
including initial estimates of effective population size.  
Methods   We used single-nucleotide polymorphism genetic analyses
to measure genetic structure among R. acutus and C. coatesi samples
obtained from commercial trawl fishing in NT waters between May 2018 and
November 2019. Our aim was to determine whether the two species each occur
as a single population in NT waters or as a set of discrete populations
that may warrant separate monitoring and management. We also analysed
samples of these species from Western Australia and Papua New Guinea to
provide broader context for the degree of genetic differentiation among
the samples from different regions in the NT.   Our secondary aim
was to provide a baseline for deciding whether genetic estimates of
effective population size could be used to monitor trends in abundance of
these species, and whether samples from across the NT could be combined
for the genetic estimation of effective population size for this purpose.
  Results   Genetic data from R. acutus and C. coatesi
strongly suggest that each species exists as a single, highly connected
population in the NT. Genetic differentiation among the sampling locations
for each species was low, and genetic clustering analyses provided strong
support for a single population of each species in the region. Sharks of
both species captured within a single location (within 50 km of one
another) were more genetically related than those further apart; however,
this does not constitute evidence for multiple, spatially discrete
populations of either species in NT waters. Preliminary applications of
effective population size estimators were used, but further work is needed
to determine if these can be used to indicate trends in abundance.
  Implications for relevant stakeholders   The immediate
implications of our research are for fisheries scientists and managers
from NT DITT. Our results indicate that these two shark species can be
monitored and managed in the NT under the assumption that each species
occurs as a single population in this region. Further information relevant
to shorter-term movements of individuals may refine management strategies
for the two species.   Our research has potential implications
for commercial fishers, particularly from the Demersal Trawl Fishery and
Australia Bay Seafoods company. Currently, those implications are
indirect, as the information from our research will flow through to the
industry by contributing to the information required to develop a
byproduct fishery for the two species, mitigating bycatch and increasing
economic return.   Recommendations   Future research
could develop genetic methods, such as effective population size or
close-kin mark-recapture, for population monitoring. Comparing the genetic
data against other data that indicate individual movement patterns on
shorter timescales would help develop a holistic understanding of shark
movement and population connectivity to inform sustainable harvest
strategies.