Change-in-Ratio and Index-Removal Population Estimation of Dungeness Crab in Southeastern Alaska

Change-in-Ratio and Index-Removal Population Estimation of Dungeness Crab in Southeastern Alaska

G.H. Bishop, C.E. Siddon, and J.M. Rumble

Change-in-Ratio and Index-Removal Population Estimation of Dungeness Crab in Southeastern AlaskaThis is part of Biology and Management of Exploited Crab Populations under Climate Change
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Description

The fishery for Dungeness crab, Cancer magister, in southeastern Alaska is passively managed with regulations governing size, sex, and season. In response to concerns regarding the timing of the commercial season and high exploitation rates, a pot survey program was conducted to investigate the utility of abundance-based management tools. Pre (June) and post-season (August/September) surveys were conducted in seven survey areas: Stikine Flats, Duncan Canal, Port Camden, Berners Bay, Peril Strait, Tenakee Inlet, and St. James Bay from 2000 through 2002. Data were modeled using change-in-ratio and index-removal methods to estimate legal population size, catchability, and exploitation rates.

Change-in-ratio population size estimation yielded exploitation rates averaging 93% and 99% respectively, for Stikine Flats and Duncan Canal open escape ring pots, and 83% each for Peril Strait and Tenakee Inlet closed escape ring pots. In Port Camden, St. James Bay, and Berners Bay, low, variable, and even negative exploitation rates were estimated, probably as a result of inseason recruitment molts, which violate the assumption of a closed population. The index-removal method produced exploitation rate estimates that were biased low due to catchability increasing between the two survey periods.

The variable success of the two methods applied here demonstrates a high level of spatial and temporal variability in Dungeness crab life history timing, and makes their assessment very difficult. We suggest that increased variability in life history timing could be a general effect of climate change and result in associated decreases in the precision of stock assessments.

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