Example: threatened euryhaline elasmobranchs

Threatened euryhaline elasmobranchs — what the draft found, why risk is high, what we propose next

Purpose

Frame a constructive, Cooke‑style re‑elicitation around the TAP’s own findings for threatened euryhaline elasmobranchs (northern river shark, speartooth shark, largetooth sawfish), and convert those findings into plan‑relevant safeguards and metrics.

What the draft has already established

  • Asset is in scope and high‑value; species are EPBC‑listed and culturally significant (Table 3.2).
  • Risk ratings: high at 10 years under the high‑volume scenario; extreme at 50 years under the high‑volume scenario; medium under the medium and low scenarios (confidence mostly low) (Tables 6.1–6.2).
  • Ecological rationale: dependence on freshwater pulses and brackish salinity gradients for movement, recruitment and prey; higher stranding risk with rapid recession; potential increase in overlap with predators if flows are reduced (Appendix A).
  • Modelling constraints the TAP faced: whole‑of‑catchment extraction allocation, a single reporting point (Dirty Lagoon), no salinity modelling, event snapshots rather than flow‑regime statistics; hence the precautionary scoring and low confidence (Section 5.3; Section 6.1.1).

Why this group is exposed (in plain terms)

  • States: small, slow‑reproducing populations; any sustained recruitment shortfall matters.
  • Rates: movement, recruitment and survival modulated by rate‑of‑rise/fall, duration and magnitude of wet‑season flows. Rapid draw‑down raises stranding risk.
  • Traits: reliance on brackish niches (≈0.5–13.6 ppt for early life stages cited for co‑occurring euryhaline fauna), strong cueing to flow sequences; sensitive to shifts in salinity wedge position.

Cooke round‑1 re‑elicitation (4–6 metrics; TAP‑owned; policy‑relevant)

Target metrics that the plan could monitor and manage. Six potential metrics are provided below as an example.

Each expert gives:

  1. a best estimate
  2. 5th/95th percentiles, and
  3. seed‑question[^seed] answers for calibration (seed questions are not outlined here).

For the following six metrics (for example):

  1. Probability of late‑wet season stranding of juvenile sawfish/sharks on the falling limb under a given take rule (per event and per season).
  2. Median recession rate threshold (cm/day) beyond which stranding risk sharply increases.
  3. Number of days per wet season with brackish habitat available (e.g., 0.5–13 ppt) in tidal freshwaters used by juveniles. Proxy now; couple to salinity modelling later.
  4. Frequency of functional floodplain connectivity events (access to nursery/prey areas) per decade under candidate take rules.
  5. Recruitment index (directional change in juvenile abundance/CPUE class) under sequences of wetter vs drier years with and without extraction.
  6. Overlap index with predators/competitors (expert‑scored change in encounter risk as freshwater cues weaken).
Note

Seed questions (for calibration weights): are questions that the experts do not know the answers to, but we can answer now or within a week. They are used to calibrate the experts’ estimates for future weighting.

How we connect to the plan (safeguards to test with the experts)

  • Event‑based pump‑start and shut‑down: raise thresholds and impose ramp‑down to cap rate‑of‑fall during juvenile egress windows.
  • Seasonal guardrails: no take during nominated recession weeks; contingent “off switches” tied to brackish‑days metric.
  • Spatial realism: concentrate assessment at likely take locations (near Dirty Lagoon), not whole‑catchment smoothing.
  • Flow‑regime framing: assess rules across long sequences (1900–present) to reflect prior drier regimes as context; ask if 2030 extractions still leave flows above 1980 baselines for key events.

What we ask of the TAP now

  • Endorse the short metric set above for Round‑1 Cooke pooling.
  • Nominate seed questions and weights basis; accept that we will publish both.
  • Validate a briefing on NT regulatory settings and how this risk work feeds the plan’s safeguards and the future EIS context.
  • Note modelling gaps (salinity, tides, spatial take) and agree the minimum upgrades needed to support the metrics, while proceeding with Round‑1 using proxies where necessary.

Why this is a safe step

It builds on the draft’s judgments, explains the “extreme” call, and converts it into a few auditable numbers that a Minister, community and proponent can understand. The Cooke weights carry through to later rounds as data improve.