Evaluation of an index method (AIM) for data poor stocks

All stock assessments must ultimately answer two simple questions: (1) Will the stock increase or decrease in response to fishing? and (2) What is most appropriate catch limit next year? Both questions can be answered without knowledge of absolute abundance by relating the rate of change in a relative abundance index to the relative fishing mortality rate. The rate of population change (i.e., the replacement ratio) can be expressed as a ratio of current abundance index to a moving average of recent indices. Relative fishing mortality rate can be expressed as the ratio of catch to a moving average of survey indices. Comparisons of AIM modeling results with those from more complex assessments in the Northeast US and from simulations indicate that AIM correctly tracks population trends and generates appropriate catch limits given current relative population abundance. However, as with more complex models, simple models are not immune to model mis-specification, especially when underlying processes are not stationary. AIM is available on the NOAA Fisheries Toolbox website (http://nft.nefsc.noaa.gov).