Feasibility study of Imaging Sonar at Lillooet River to assess Coho Salmon escapement to the Lillooet Conservation Unit

Inconsistent, and sometimes absent, annual funding and a lack of resources to develop a Coho centric assessment program have resulted in DFO being unable to generate an estimate of Coho salmon escapement to the Lower Fraser River Management Unit (LFRMU) from observed data. The absence of reliable escapement estimates for Coho in the LFRMU and its three component Conservation Units (Lillooet, Boundary Bay and Lower Fraser) represents a critical information gap for Southern Boundary Coho salmon Management.

Funding from the PSC in 2017 enabled the development of a simulation-estimation procedure which found that an index-style approach is the best method for aggregate escapement estimation for the LFRMU. However, due to the poor data and its limited time series, appropriate factors for in-filling cannot be estimated, thus a focused, long-term and dedicated strategy is required.

From our work in 2017, we found that escapement to the Lillooet is more variable than either the Lower Fraser CU or the Boundary Bay CU, and is thus more difficult to estimate properly through traditional stream survey assessments. The high underlying variability in abundance, coupled with the fact that Coho returning to the Lillooet CU must transit through the Lower Lillooet River, provide the unique potential for a census-style assessment for the entire CU. A census method would negate the need to determine in-filling factors, would provide a consistent and repeatable escapement estimate, and would not require additional effort to account for the CUs natural variability. Imaging sonar would be the most applicable census method and as such we propose to assess the feasibility of deploying imaging sonar on the Lower Lillooet River for the purpose of estimating Coho salmon escapement to the Lillooet CU.