1. PURSE-SEINE CATCH ESTIMATES DETERMINED FROM OBSERVER GRAB SAMPLES
CORRECTED FOR SIZE SELECTIVITY

2. MFCL PURSE-SEINE CATCH DATA — STANDARD PRACTICE
LOGSHEETS
SKJ ↑ YFT+BET ↓
ANNUAL CATCH ESTIMATES
Total Catch of
SKJ + YFT + BET
GRAB SAMPLES
UNCORRECTED
SKJ ↓ YFT ↑ BET ?
S_BEST
UNADJUSTED
1x1 x Month
SKJ ↑ YFT+BET ↓
GRAB SAMPLES
UNCORRECTED
YFT and BET
in YFT+BET
S_BEST
ADJUSTED
1x1 x Month
SKJ ↑ YFT ↓ BET ↓
MFCL INPUT DATA
MFCL Area x Quarter
SKJ ↑ YFT ↓ BET ↓

3. GRAB SAMPLES CAN BE CORRECTED FOR SIZE SELECTIVITY BIAS
UNCORRECTED
SKJ ↓ YFT ↑ BET ?
PAIRED SPILL AND GRAB SAMPLES
Size Selectivity Bias
GRAB SAMPLES
CORRECTED
SKJ  YFT  BET 

4. MFCL PURSE-SEINE CATCH DATA — ALTERNATE PRACTICE
LOGSHEETS
SKJ ↑ YFT+BET ↓
LOGSHEETS
SKJ ↑ YFT ↓
ANNUAL CATCH ESTIMATES
Total Catch of
SKJ + YFT + BET
ANNUAL CATCH ESTIMATES
Total Catch of
SKJ + YFT + BET
GRAB SAMPLES
UNCORRECTED
SKJ ↓ YFT ↑ BET ?
GRAB SAMPLES
UNCORRECTED
SKJ ↓ YFT ↑ BET ?
PAIRED SPILL AND GRAB SAMPLES
Size Selectivity Bias
PAIRED SPILL AND GRAB SAMPLES
Size Selectivity Bias
S_BEST
UNADJUSTED
1x1 x Month
SKJ ↑ YFT+BET ↓
S_BEST
UNADJUSTED
1x1 x Month
SKJ ↑ YFT ↓
GRAB SAMPLES
UNCORRECTED
YFT and BET
in YFT+BET
GRAB SAMPLES
UNCORRECTED
YFT and BET
in YFT+BET
S_BEST
ADJUSTED
1x1 x Month
SKJ ↑ YFT ↓ BET ↓
S_BEST
ADJUSTED
1x1 x Month
SKJ ↑ YFT ↓ BET ↓
GRAB SAMPLES
CORRECTED
SKJ  YFT  BET 
GRAB SAMPLES
CORRECTED
SKJ  YFT  BET 
MFCL INPUT DATA
MFCL Area x Quarter
SKJ ↑ YFT ↓ BET ↓
MFCL INPUT DATA
MFCL Area x Quarter
SKJ ↑ YFT ↓ BET ↓
MFCL INPUT DATA
ADJUSTED
MFCL Area x Quarter
SKJ  YFT  BET 
MFCL INPUT DATA
ADJUSTED
MFCL Area x Quarter
SKJ  YFT  BET 

5. Status of Paired Grab and Spill Samples on Purse Seiners
From 2008 to mid 2010
Data for 17 trips were used in analysis presented at SC6 (2010)
Data for 4 trips were unusable
Still missing data for 4 trips in Solomons in 2009
Mid 2010 to now
Seven trips completed: US x 2, KR x 2, TW x 3
Data from 3 are unusable: KR x 2, TW x 1
Data from 4 should be usable.
Next?
EU = Spain : No response to proposal following SC6
Japan : Currently under negotiation
Others : More KR? TW? New FSM, Marshalls, Vanuatu? China, Philippines?
Data Collection Officer will be recruited to implement paired samples

6. Vessel Nationality / Arrangement
Progress with the design for collecting paired samples
proposed in SC5-ST-WP6
Vessel Nationality / Arrangement
Target # Trips
# Trips End of 2010
FSM Arrangement 8
15 (all in PNG)
China 2
Japan 6
Korea
2 bad
New Zealand
4 bad
Philippines
Solomon Islands
4 missing
Chinese Taipei
1 bad + 2 good
United States of America 4
Vanuatu
EU and EPO-based fleets
TOTAL 50
bad = 32

7. Status of Analyses of Paired Grab and Spill Samples
2010
Grab sample selectivity bias was estimated from paired samples collected during 17 trips.
Historical grab samples were corrected for selectivity bias.
MFCL catch data were generated with species compositions determined from corrected grab samples.
2011
Estimates of selectivity bias will be updated, pending the availability of data.
MFCL catch data will be updated.
MFCL length data will be generated with grab samples corrected for selectivity bias.

8. Grab Sample Size Selectivity
The length frequencies determined from the spill samples (red) and the grab samples (green) show that the grab samplers miss the smaller fish.

9. The paired spill and grab samples can be used to quantify the selectivity bias in grab samples. See ST-WP-2, equation (1).

10. Data covering very large fish are insufficient to reliably estimate the selectivity bias, but the use of cubic splines suggests that it is similar to very small fish .

11. In 2010 the estimates of selectivity bias were used to correct the species compositions determined from grab samples collected from 1995 to 2009, which were then be used to adjust the purse-seine catch data.
MFCL input catch data, , unadjusted (left) and adjusted with grab samples corrected for selectivity bias (right):

12. Discussion
The estimation of selectivity bias assumes that the species compositions determined from spill samples are unbiased. Comparison with cannery receipts and port sampling of landing categories for trips for which both spill samples and port samples were collected may give us more confidence in this assumption.
During the 17 trips analysed, grab samples were collected by 14 observers. Additional paired samples with different grab samplers are required to obtain estimates of selectivity bias that are representive of the population of observers.

13. Discussion (continued)
Additional paired samples are required to obtain reliable estimates of selectivity bias for fish less than 35 cm and greater than 54 cm; paired samples of large fish in unassociated schools would be particularly useful.
The analyses have assumed that selectivity bias depends only on the size of fish; other variables — such as species, school association and the observer — should also be examined as the data permit; again, paired samples from unassociated schools would be particularly useful.