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## What is MULTIFAN-CL

# MULTIFAN-CL MULTIFAN-CL is a computer program that implements a statistical, length-based, age-structured model for use in fisheries stock assessment. The model is a convergence of two previous approaches. The original MULTIFAN model (Fournier et al. 1990) provided a method of analysing time series of length-frequency data using statistical theory to provide estimates of von Bertalanffy growth parameters and the proportions-at-age in the length-frequency data. The model and associated software were developed as an analytical tool for fisheries in which large-scale age sampling of catches was infeasible or not cost effective, but where length-frequency sampling data were available. MULTIFAN provided a statistically-based, robust method of length-frequency analysis that was an alternative to several ad hoc methods being promoted in the 1980s.

However, MULTIFAN fell short of being a stock assessment method as the end point of the analysis was usually estimates of catch-at-age (although later versions included the estimation of total mortality and yield per recruit).

The second model (actually the first, in terms of chronology) was that introduced by Fournier and Archibald (1982). The FA model was a statistical, age-structured model in which estimates of recruitment, population-at-age, fishing mortality, natural mortality and other estimates useful for stock assessment could be obtained from total catch and effort data and catch-at-age samples. In principle, the estimates of catch-at-age obtained from the MULTIFAN model could be used as input data to the FA model and a complete stock assessment analysis conducted.

Such a sequential approach to length-based stock assessment modeling had several serious limitations. First, it was extremely unweildy. Second, it was difficult to represent and preserve the error structure of the actual observed data in such a sequential analysis. This made estimation of confidence intervals for the parameters of interest and choice of an appropriate model structure for the analysis problematic. It was clear that an integrated approach was required, one that modeled the age-structured dynamics of the stock, but which recognized explicitly that the information on catch-at-age originated with length-frequency samples.

The early versions of MULTIFAN-CL, which were developed for an analysis of South Pacific albacore (Fournier et al 1998), provided the first attempt at developing a statistical, length-based, age-structured model for use in stock assessment. Subsequent versions of the software have added new features, the most important of which have been the inclusion of spatial structure, fish movement and tagging data in the model (Hampton and Fournier 2001).

MULTIFAN-CL is now used routinely for tuna stock assessments by the Oceanic Fisheries Programme (OFP) of the Secretariat of the Pacific Community (SPC) in the western and central Pacific Ocean (WCPO). Beginning in 2001, the software gained additional users, with stock assessment applications to North Pacific blue shark, Pacific blue marlin, Pacific bluefin tuna, North Pacific swordfish and Northwest Hawaiian lobster underway or planned. The more widespread use of the software prompted the development of a user's guide and software availability via this web page. Further development of the software, and of the user's guide, will be an ongoing, collaborative effort.

**References**

- Fournier, D., and Archibald, C.P. (1982). A general theory for analyzing catch at age data. Canadian Journal of Fisheries and Aquatic Sciences} 39, 1195-1207.

- Fournier, D.A., Sibert, J.R., Majkowski, J., and Hampton, J. (1990). MULTIFAN: a likelihood-based method for estimating growth parameters and age composition from multiple length frequency data sets illustrated using data for southern bluefin tuna (Thunnus maccoyi). Canadian Journal of Fisheries and Aquatic Sciences 47, 301-317.

- Fournier, D.A., Hampton, J., and Sibert, J.R. (1998).MULTIFAN-CL: a length-based, age-structured model for fisheries stock assessment, with application to South Pacific albacore, Thunnus alalunga.(pdf - 287k) Canadian Journal of Fisheries and Aquatic Sciences 55, 2105-2116.

- Hampton, J., and D.A. Fournier. (2001). A spatially disaggregated, length-based, age-structured population model of yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean.(pdf - 4502k) Marine and Freshwater Research 52, 937-963.