The retrospective problem in sequential population analysis: An investigation using cod fishery and simulated data

Abstract

The retrospective problem is a systematic inconsistency among a series of estimates of population size, or related assessment variables, based on increasing periods of data. In some stocks, this problem is of such magnitude that sequential population analyses (SPA) are deemed inapplicable. The eastern Scotian Shelf (ESS) cod fishery, which displays the retrospective problem, and simulated data are analysed to provide insight into the causes and potential solutions to this problem. The retrospective problem is shown to be a result of the traditional analysis techniques and a non-stationarity in the data used in the population analysis. A moving window analysis is developed which allows the non-stationarities to be identified and in some cases rectified. Recommendations are also made for ad hoc investigations of the data. The analysis suggests that failure to correct the retrospective problem for a stock with data like ESS cod could lead to catch-level advice that would be twice or more the intended level.