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Visual Sampling of Fishes

Visual Sampling of Fishes. Introduction to Sampling Visual Sampling Methods Evaluation of Effectiveness. GOAL: Describe Complex Biological Communities. PROBLEM: Complete census is impossible!!. APPROACH: ‘sub-sample’ several units. Progressive levels of sampling effort.

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Visual Sampling of Fishes

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  1. Visual Sampling of Fishes • Introduction to Sampling • Visual Sampling Methods • Evaluation of Effectiveness

  2. GOAL: Describe Complex Biological Communities PROBLEM: Complete census is impossible!! APPROACH: ‘sub-sample’ several units

  3. Progressive levels of sampling effort. Time (= cost) increases with each level Qualitative - estimates species present, no abundance information Semi-quantitative - estimates species present andrelative abundance Quantitative - estimates species present and densities

  4. Random vs Fixed Stations Random sampling:addresses some parametric test assumptions Fixed-station sampling:greater power when funding limited

  5. Spatial and Temporal Considerations Many projects sample at established: time of day, lunar cycle, tidal cycle... Sampling day and night, or among seasons Distributional studies sample randomly through space

  6. 1. Qualitative Description • Presence/absence only – no abundance estimates • Example: Timed Swim • establish specified time • haphazard swim • record species observed

  7. Species-site Checklist REA Fish Survey Depth/habitat distributions Island checklist of species New records, range extensions

  8. 2. Semi-Quantitative DescriptionRapid Visual Transect (RVT) Relative abundances estimated Species-time relationships Commoner species sighted before rarer species

  9. Rapid Visual Transect • Establish time survey duration • (e.g., 0.5 hrs, 1.0 hrs, etc.). • Divide survey period into sub-intervals • (e.g., 5 or 10 mins). 3. Surveyor swims freely within sampling habitat.

  10. Each species is recorded once • In the time interval first seen.

  11. 6. Species assigned scores based on time seen: TimeIntervalScore 00:00-04:59 1 5 05:00-09:59 2 4 10:00-14:59 3 3 15:00-19:59 4 2 20:00-24:59 5 1 7. Scores from multiple surveys are averaged. • Rank mean scores • (higher score = common; lower score = rare).

  12. 3. Quantitative Description 1. Densities are estimated 2. Sampler records: • a. species observed • b. number of individuals of each species • c. size/position of each individual (optional)

  13. Quantitative Descriptions Methods taught at QUEST: Strip Transect (SST) Stationary Point Count (Circular Plot - CPLOT)

  14. Strip Transect (SST) Deploy transect line Record species and number (and size) in assigned segment

  15. In QUEST: four 25 m transects (paired) sampled per team 4 m 2 m 4 m 25 m 25 m 2 m 50 m transect line

  16. Fish Belt-Transect - NOAA Program Method • Fish identified to “species” & counted • Swim-out: record fish >20 cm in 4m wide swath • Swim-back: record fish <20 cm in 2m wide swath Transect 2 25 m Diver 2 2m • Common species listed • Enter number and • size of each Transect 1 25 m Diver 1

  17. Stationary Point Count Circular Plot – ‘C-PLOT’ Popular in Caribbean Plots usually chosen randomly or haphazardly In QUEST: Transect line is deployed Team waits for fishes to settle Then proceeds to pre-determined ‘point’

  18. Record all fishes within acylinder set radius height above bottom (at QUEST r = 5 m, h = 1 m) r Cylinders cannot overlap Each cylinder treated as a replicate

  19. Fish Stationary Point Count (SPC) • 4, 15-m diameter “cylinders”, 12-15m deep • 1st , 5 min. – create a list of species observed; • NO counts or sizes recorded. • After 5 min. – divers count & size individuals 7.5m radius circle 7.5m radius circle 30m transect Diver 2 Diver 1

  20. END Multiple methods are used in some Programs

  21. Evaluation of Visual Surveying Effectiveness

  22. A. Principal Advantages 1. least disruptive and biased method for surveying fishes (compared to ???) 2. fairly inexpensive

  23. Principal disadvantage: • some species chronically under-estimated: 1. large, mobile species (jacks, sharks).

  24. Principal disadvantage: • some species chronically under-estimated: • schooling fishes with patchy distributions • (small jacks, mullet, some goatfishes).

  25. Principal disadvantage: • some species chronically under-estimated: • cryptic species • (cardinalfishes, eels, gobies, blennies).

  26. Principal disadvantage: • some species chronically under-estimated: 4. rare fishes

  27. 2. Swimming speed affects accuracy • at fast speeds, uncommon species • overlooked. b. at slow speeds, mobile species overestimated. c. rare species overlooked at all speeds.

  28. Happy Hunting!

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