Research Document - 2009/073

Modeling herring population dynamics Herring Catch-at-Age Model version 2

By L.B. Christensen, V. Haist and J. Schweigert

Abstract

The herring catch-at-age model (HCAM) is an age-structured stock assessment model developed specifically for Pacific herring which is assumed to be a multi-stock population that has experienced periods of significant fishery impact and a recent decline in size-at-age (Figure 56 in Appendix II). The first implementation of the HCAM model, HCAMv1, used in the 2006 and 2007 stock assessments has some lingering issues concerning estimates of the proportion of mature fish and natural mortality rates and annual fluctuations in these estimates as well as potential problems with retrospective bias. To address these issues we develop a second implementation of the HCAM model referred to as HCAMv2. In this implementation we simplify the model and change some structural assumptions, resulting in a model that we feel more accurately represents the stock dynamics of the five geographic stocks of Pacific herring. HCAMv2 is a more numerically stable model and retrospective bias, which is a major concern when predicting future stock sizes, is addressed so that such impacts are minimized. We compare the 2007 stock assessment results produced using HCAMv1 and HCAMv2 respectively. HCAMv2 results are in general more conservative than estimates from HCAMv1. The paper also touches on estimates of unfished biomass assumed in current assessments relative to the estimates produced in both HCAMv1 and HCAMv2.

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