Problems with an Environmental-Based Recruitment Index: Examples from a New Zealand Snapper Assessment (Pagrus auratus)
M.N. Maunder
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Description
This paper summarizes the different methods used to incorporate annual recruitment variation into the assessment of a New Zealand snapper stock and presents the implications of these methods. Recruitment in the Hauraki Gulf snapper stock has been related to water temperature and relationships with temperature are used to determine year-class strengths used in stock assessment models. Using a recruitment series derived from environmental factors has two major problems. (1) Environmental factors are used to predict recruitment from outside of the period that the relationship was derived and small changes in the relationship can influence the estimates of current biomass, its relative position to optimal biomass, and the age structure. (2) Assumptions about the period of historic recruitment used to estimate future recruitment influences forward projections and yield calculations. The most defensible approach is to limit the time-frame of the model to recent history and assume that recruitment in the near future is similar to recent history. The problem with limiting the time-frame of the model is that the analysis becomes less model-dependent and more data-dependent, increasing the uncertainty and providing little management guidance. Catch-at-age data have been used as an alternative method to estimate recruitment and have also indicated other problems such as incomplete recruitment due to annual variation in growth rates.
Item details
- Item number: AK-SG-98-01aj
- Year: 1998
- DOI: https://doi.org/10.4027/fsam.1998.36
