1. Improving the accuracy of aerial surveys for dugongs: implications for management of Indigenous hunting in Torres Strait Helene Marsh, Ken Pollock, Ivan Lawler and Matthew Alldredge

2. Basis for estimating sustainable anthropogenic removals Potential biological removal (PBR) = maximum number of animals not including natural mortalities that may be removed sustainably from a marine mammal population PBR = n min * 0.5 r max * recovery factor

3. Parameters of PBR n min 20th percentile of log-normal distribution based on absolute population estimate < 4 years old r max maximum rate of increase RF between 0.1 and 1 depending on status of population 0.1 endangered species or stocks 0.5 depleted or threatened stocks or stocks of unknown status Challenge: estimating the absolute size of a dugong population

4. The probability of detecting a group of dugongs is made up of a probability that the area is sampled plus an availability process and a detection process P [animal detected] = P[area sampled] * P[animal available] * P [ animal detected given it is available]

5. Chief source of variation is water turbidity which affects probability that a dugong is available: heterogeneous at fine temporal and spatial scales

6. Estimation P [animal available]= p a Must be done external to survey with additional data Dugong models used to estimate depth at which dugongs visible in various turbidities and sea states. Dive profiles of 15 individually monitored dugongs recorded. Combination allows probability of dugongs being available for detection under various conditions of depth, turbidity and sea state to be estimated.

7. Dugong models fitted with timed depth recorders were raised from the bottom until they become visible from a helicopter at aerial survey height in water of varying turbidities and sea states

8. Zone of non-availability Dive profiles measured for 15 wild dugongs fitted with timed depth recorders ~ 40,000 dives from 15 dugongs

9. AVAILABILITY PROBABILITIES FOR VARIOUS STRATA OF SURVEY DEPTHS, TURBIDITIES AND SEA STATES Water qualityDepth range Visibility of sea floor Maxim depth models visible P a (se) ClearShallowClearly visibleAll1 Variable Visible but unclear 2.44 m0.652 (0.0452) Clear>5mNot visible4.32 m0.462 (0.057) TurbidVariableNot visible1.23 m0.474 (0.0525) Optimal survey sea state Also developed for marginal survey sea state

10. ASSUMPTIONS FOR AVAILABILITY PROCESS ESTIMATION The depth at which dugong models became visible measured without error. The depth at which dugong models became visible was the same as for real dugongs. Depth profiles of individually monitored dugong are representative of the dugongs studied in the aerial survey Flight speed fast enough that the dugongs are only available for an “instant”.

11. MODELING PERCEPTION PROCESS Estimate of P[animal detected given animal available] = p d Done internal to the aerial survey using two independent observers and a mark recapture model X 11 - no. detected by both observers X 10 - no. detected by mid observer only X 01 - no. detected by rear observer only n 1 - no. detected by mid observer n 2 - no. detected by rear observer

12. Seating arrangement in the aircraft

13. ASSUMPTIONS FOR DETECTABILITY PROCESS ESTIMATION Counts within the strip of 200 metres are measured accurately. There are no matching errors between the two observers so that the assignment to X 11, X 10, X 01 are accurate Equal detection probabilities for all groups for each observer.

14. MODELING PERCEPTION PROCESS Two Independent Observer Method Generalizations using Program MARK Fits generalized Lincoln-Petersen models which allow for detection probability conditional on availability to vary by seat (mid or rear), side(port or starboard), and location of the survey. Uses AIC technique to pick the simplest adequate model Determines if detection probability conditional on availability is dependent on individual group covariates such as size of group, sea state, glare, distance class etc.

15. Example of Results using Program MARK Probability of a small group of available dugongs being detected by one Observer = 0.72 (s.e. 0.0159) Probability of a small group of available dugongs being detected by at least one Observer = 0.92 (s.e. 0.0159)

16. Calculation of dugong population of Torres Strait P [animal detected] = P[area sampled] * P[animal available] * P [ animal detected given it is available] N = 14106 + s.e. 2134 in December 2001

17. Crude estimates of current catch ~ 1000 dugong p.a. Recovery Factor r max =0.01 r max =0.02 r max =0.03 r max =0.04 r max =0.05 0.5316192123154 162122184246308 N = 14106 + s.e. 2134 N min = 12297 PBR for Torres Strait

18. Conclusion The 2001 aerial survey estimate of the absolute abundance of dugongs in the Torres Strait region indicates that the dugong harvest is far too high to be sustainable Indigenous leaders agree with this assessment and a pushing for urgent management action