The species composition and size composition of the catch taken by purse seiners in the Western and Central Pacific Ocean can be determined from samples collected by observers. However, grab samples — wherein the observer grabs five fish per brail as the fish are transferred from the net to the vessel — are known to be subject to size selectivity bias. Spill sampling — wherein fish are spilt from the brail into a bin — was developed so that the observer does not select the individual fish.

    Sampling tuna onboard a purse seiner after spilling fish from the brail into a bin
 

Paired spill and grab samples have been collected on purse seiners since 2008 so that the size selectivity bias of grab samples can be estimated. Analyses of size selectivity bias, and other factors affecting the estimation of the species composition of the purse-seine catch, are found in the papers linked below.

Lawson, T.A. 2008. Factors affecting the use of species composition data collected by observers and port samplers from purse seiners in the Western and Central Pacific Ocean. Working Paper SC4–ST–WP3. Fourth Regular Session of the Scientific Committee of the Western and Central Pacific Fisheries Commission, 11–22 August 2008, Port Moresby, Papua New Guinea. Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New Caledonia

Lawson, T.A. 2009. Selectivity bias in grab samples and other factors affecting the analysis of species composition data collected by observers on purse seiners in the Western and Central Pacific Ocean. Working Paper SC5–ST–WP3. Fifth Regular Session of the Scientific Committee of the Western and Central Pacific Fisheries Commission, 10–21 August 2009, Port Vila, Vanuatu. Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New Caledonia

Lawson, T.A. 2010. Update on the estimation of selectivity bias based on paired spill and grab samples collected by observers on purse seiners in the Western and Central Pacific Ocean. Working Paper SC6–ST–WP2. Sixth Regular Session of the Scientific Committee of the Western and Central Pacific Fisheries Commission, 10–19 August 2010, Nuku’alofa, Tonga. Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New Caledonia