Rapid assessment of adult abundance and demographic connectivity from juvenile kin pairs in a critically endangered species

Abstract

The viability of spatially structured populations depends on the abundance and connectivity between subpopulations of breeding adults. Yet, for many species, both are extremely difficult to assess. The speartooth shark is a critically endangered elasmobranch inhabiting tropical rivers with only three adults ever recorded in Australia. Close-kin mark-recapture models, informed by sibling pairs among 226 juveniles, were developed to estimate adult abundance and connectivity in two Australian river systems. Sixty-eight sibling pairs were found, and adult abundance was estimated at 892 for the Adelaide River and 1128 for the Alligator Rivers. We found strong evidence for female philopatry, with most females returning to the same river to pup. Adelaide River males appear largely philopatric, whereas Alligator Rivers males are highly connected to the Adelaide River. From only 4 years of sampling, our results demonstrate that juvenile-only kin pairs can inform simultaneous estimates of abundance and connectivity in a rare and threatened species.

Fig. 1.. Concept diagram of hypothetical population connectivity scenarios.

Reproductive dispersal and spatial distribution of half-sibling pairs (HSPs) between two hypothetical rivers, denoted r₁ and r₂. Linked blue and orange dots represent HSPs that share and do not share the same haplotype, respectively. Scenarios (A) and (B) denote the extreme cases of complete philopatry and complete panmixia, respectively. In these cases, we expect to see no HSPs between rivers (philopatry) or uniformly likely (panmixia). In male-based dispersal (C), all HSPs between rivers are paternal and they only share the same haplotype by chance. In female-based dispersal (D), all HSPs between rivers are maternal and always share the same haplotype. In nature, sex-specific rates of dispersal would include potential intermediate scenarios between the cases shown here. The reproductive connectivity may or may not mean that the parent was physically present within a spatial location; mating may occur in a third unknown location, and then, mothers or young may physically move to the river.

Fig. 2.. G. glyphis distribution and sampling locations.

(A) Juvenile G. glyphis. (B) Study area in the Northern Territory, Australia, with the Adelaide River and the Alligator rivers, composed of the west, south, and east Alligator rivers. (C) Regional map with the approximate location of the documented populations of G. glyphis in Australia and Papua New Guinea. The population in the Bizant River, Queensland, is likely extinct as no sightings of the species have been recorded since the 1990s.

Fig. 3.. Spatial linkages of HSPs between river systems.

(A) The vertical axis is the cohort of the sampled juvenile, indicating that the parent bred in that river system in that year. Orange indicates that the two half-sibling juveniles share the same haplotype, and blue lines depict an HSP that do not, and hence whether the movement of reproductive allocation was likely via the maternal or paternal parent. The data (see Table 1) on cross-river HSPs (N = 6) and whether they did (N = 4) or did not share (N = 2) haplotypes indicate that males and possibly some females move from the Alligator rivers to the Adelaide River with only one male reproductive transition in the opposite direction. While the sample size creates uncertainty, the close-kin model (see Table 2 and the Supplementary Materials) estimates of probability of reproductive connectivity accounting for the observed mtDNA haplotype distribution indicate male linked connectivity being far more likely than female. (B) Numbers of samples collected in each river system through time. Note that these are assigned to an earlier cohort year based on estimated age given size.