Salinity pulses and a freshwater crayfish survey
A river reach below an irrigation district is sampled for the threatened freshwater crayfish Cherax tenuimanus. Two sites are compared. Site A lies 1.2 km downstream of a return-flow drain; Site B lies 9 km upstream in a forested tributary. Over 14 days, loggers record Site A conductivity rising from 480 to 2100 µS/cm after a storm, while Site B stays between 410 and 520 µS/cm. Capture effort is identical: 40 trap-nights per site. Site A yields 6 crayfish, all juveniles, in the first 5 days before the peak; Site B yields 19 crayfish across all size classes throughout. Water temperature and dissolved oxygen are similar at both sites. A student concludes that salinity has eliminated the Site A population and recommends the reach be listed as a population sink.
1. Evaluate whether the data support the student's conclusion that salinity has eliminated the Site A population.
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The data are consistent with a salinity effect but do not establish elimination. Three alternative explanations must be excluded first.
- Temporal confounding. All six Site A captures occurred before the conductivity peak. The later zero catch could reflect reduced activity or trap avoidance during the pulse rather than mortality, so absence of captures is not evidence of absence of animals.
- Size-structure signal. Only juveniles were caught at Site A. This could mean adults are more sensitive, or simply that large individuals avoided traps under stress. Without mark-recapture or direct mortality counts, mortality is inferred, not measured.
- Single-pulse design. One storm event over 14 days cannot show whether the population recovers. Elimination requires evidence across at least one full generation or repeated pulses.
A defensible conclusion is that the storm pulse is associated with a collapse in catch and a juvenile-only size structure at Site A, which is a plausible sublethal or lethal impact. Listing as a sink requires demographic data showing recruitment failure exceeds immigration, which this survey does not provide.
2. Design a two-season follow-up study that could distinguish mortality from trap avoidance, stating the variables and the decision rule you would use.
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Design. Repeat sampling at both sites in the wet and dry seasons, with 40 trap-nights per site per visit and at least three visits per season. Add two independent methods that do not depend on bait attraction: baited vs unbaited traps, and a fixed quadrat search or eDNA water sampling at each site.
Variables.
- Independent: site (A vs B), season, and conductivity measured continuously at each trap position.
- Dependent: catch per unit effort, size-class distribution, eDNA detection probability, and where possible survival of caged sentinel crayfish exposed at each site.
Controls. Identical trap type, soak time, depth and temperature logging at both sites; sentinel cages placed at matched depths.
Decision rule. If eDNA and sentinel survival at Site A remain high during a pulse while trap catch falls, trap avoidance or reduced activity is supported. If eDNA detection falls to near zero and sentinel mortality is high at conductivity above about 1500 µS/cm but recovers by the next season, a transient lethal pulse is supported. Persistent absence across both seasons plus no recruitment would be needed before calling the reach a sink.