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Keeping an accurate count

Posted by Mark Maunder
5 October 2012

Mark Maunder, chair of the 2012 ISSF Stock Assessment Workshop: Understanding Purse Seine CPUE, currently leads the Inter-American Tropical Tuna Commission’s Stock Assessment Program.

High quality data is integral to the sustainable management of tuna fisheries. When scientists have access to more comprehensive data, the more effectively fisheries, RFMOs, scientists, advocates, and other stakeholders can target their sustainability efforts.   That’s why ISSF sponsored a Stock Assessment Workshop in Rome this July that was focused on how one particular unit of measurement in purse seine fisheries – the catch per unit effort (CPUE), or simply the catch rate – is used when calculating the health of particular tuna stocks.

Scientists use CPUE data from various fisheries to track changes in the abundance of a stock. This assumes that the CPUE is proportional to abundance. Traditionally, tuna stock assessments rely heavily on CPUE data from longline fisheries, where the rate measures how many fish are caught per hooks set.  However, it is more difficult to understand how CPUE data from purse seine fisheries is related to stock abundance because these vessels employ many techniques to improve their catch rates (such as using FADs, bird radars, sonars, etc.).

Workshop participants completed a preliminary analysis of the available CPUE data and made a number of recommendations for future analyses, including new studies to better understand what makes catchability change over time (catchability is the constant of proportionality between stock abundance and CPUE) and ensuring that CPUE for purse seine vessels includes information about the use of floating objects (including fish aggregating devices, FADs) so that we could standardize the data appropriately and potentially use FADs as observatories for tuna densities. The Workshop also examined other potential alternatives to using CPUE, such as the biomass of tunas associated with FADs, and data derived from Vessel Monitoring Systems.

Generally, when evaluating the health of stocks, we examine how much purse seine vessels catch, and their rate of success when setting nets; how many times they set when fishing and where they set; and the size, weight, and composition of their catches.  We look at information about the tuna’s life cycle, including how fast they grow, how quickly they reproduce, and their natural mortality rates.   And we use information about other environmental factors that may affect how, where, and why fisheries set their nets or how fish respond.

The workshop report notes that there is a desire to have more data on the use of manmade floating objects (FADs) and ISSF has demonstrated a commitment to collecting this information. During the workshop Victor Restrepo, chair of ISSF’s Scientific Advisory Committee, outlined for us the details of an effort that is already underway to help fishers collect the informationscientists are looking for. The FADTrack electronic logbook, as it’s called, catalogues data during a fishing trip such as when the vessel uses a FAD, how often and where and how successful they are at helping to attract fish. The program is designed as an efficient way to collect data during fishing operations, with the electronic information from each fishing trip being sent to fishery management organizations’ scientific bodies. That data can be used in stock assessments and other analyses, helping scientists provide fishery managers with advice on crafting more effective management plans.

While efforts like FADTrack and scientific observer programs will fill data gaps, there continues to be difficulty in using purse seine CPUE to determine stock abundance but the participants all agreed that there has been progress “in terms of describing historical changes in technology, in data that should be collected, and on further analyses to be carried out.”

The bottom line is that the better the data is, the more effectively fisheries, RFMOs, scientists, advocates, and other stakeholders can target their sustainability efforts.


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