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

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 ↓

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 

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 

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

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 25 + 7 bad = 32

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.

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.

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

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 .

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, 1995-2009, unadjusted (left) and adjusted with grab samples corrected for selectivity bias (right):

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.

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.