Research Document - 2005/066

An Assessment of White Sturgeon Stock Status and Trends in the Lower Fraser River

By Walters, C., J. Korman, S. McAdam

Abstract

We used a variety of information sources to estimate natural and fishing mortality rates and to assess recent trends and current abundance of white sturgeon in the lower Fraser River from Yale to the Strait of Georgia. These sources included:

1. estimates of current population structure and recruitment-mortality rates over the last few decades, from age composition sampling in 1995-99 and size composition sampling since 1999;
    
2. estimates of capture and recapture rates of over 18100 sturgeon tagged with PIT tags since 1999, which provide information on growth, mortality rate, total abundance, and movement;
    
3. estimates of historical harvests since 1880, which can be used to back-calculate how large the stock must have been before fishing and how much it must have declined in order to be near the current size indicated by the PIT tag data.
    
4. indices of recent abundance trend from the gill net test fishery at Albion.

The age and size composition data suggest a stock that has relatively healthy numbers of older fish and is either stable or increasing over time. The PIT tag data indicate that natural mortality plus emigration rates may be as low as 7%/yr or as high as 14%/yr. Higher rate estimates are obtained when the data are analyzed with simple aggregate abundance models while lower rates are indicated by models which recognize that data come from a complex spatial sampling regime with variable marking and capture rates among sections of the lower river. Reconstructions of the stock history from catch data indicate that the stock is probably now at about 10% of its natural annual egg production, and may have been impacted most severely over its history by gill net fishing in the 1960-90 period rather than the pre-1920 period. Catches in the Albion test fishery suggest a very rapid increase in abundance of small fish (ages 1-7) since 2000, and this apparent rapid increase is also evident in the size structure of fish captured for PIT tagging. Although the stock is most likely increasing in abundance, it is also most likely to be well below its most productive level for harvest management, and its recovery to that level could be severely delayed by allowing any retention fisheries or through increases in by-catch mortality associated with catch and release angling or commercial and first nations gillnet fisheries.

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