Science Advisory Report 2014/051

Precautionary reference points consistent with the Fishery Decision-Making Framework for Arctic Char (Salvelinus alpinus) in Cambridge Bay, Nunavut

Summary

• The biomass-based catch-per-unit-effort (CPUE) for Arctic Char in Cambridge Bay waters is standardized by a single-mesh-sized (140 mm) gillnet in August and used as an indicator of changing stock status.
• During 1972-2006, the catch and fishing effort data were intermittently collected, yielding a total of 12 CPUE estimates. Pair-wise correlations between the CPUE and large-scale climate change variables indicated that wintertime Arctic oscillation index (AOI) with a five-year lag was the best explanatory variable. This relationship can better predict CPUE for years missing in the time series.
• Hierarchical Bayesian state-space (HBSS) models were employed to reconstruct historical trends and harvest removal series of population biomass. The model assessment was evaluated by using deviance information criterion (DIC) and multimodel inference (MMI). The best model scenarios were uniform probability distribution function for K and r (UKR: 7% DIC weight) and lognormal probability distribution function for K and r along with time-varying catchability (LNKRWQ: 93% DIC weight).
• Essentially, maximum surplus production (MSP) and biomass at MSP (BMSP) were estimated by HBSS model at 93 and 517 t, respectively. The harvest report rate (HRR), which mainly accounted for subsistence uses, was estimated to be 34%, which amounted to 32 t on the basis of MSP.
• With the data currently available it was not possible to estimate the Limit Reference Points for individual stocks (i.e., waterbodies). This does not imply that there should be a change to the current management units or structure of the collection of fishery statistics. However, information should be collected, if possible, that would facilitate definition of individual stock Limit Reference Points.
• For the combined fishery the Limit Reference Point is located at a standing biomass of 207 metric tonnes (t), representing the lowest biomass recorded for the stock (0.0539 t/gillnet). The Upper Stock Point is at a standing biomass of 414 t (0.1078 t/gillnet). The Target Reference Point is located at a stock status of 0.1348 t/gillnet, which is equivalent to a standing biomass of 517 t; the maximum removal rate is 0.1805. These interim reference points should be re-examined and revised as new information is obtained
• The lack of sufficient information regarding mixed-stock CPUE, the stock-recruitment relationship, age structure, discrete stock discrimination, current levels of subsistence harvest, vulnerability of Arctic Char to fishing effort, and localized variations in productivity and environmental factors is responsible for observation uncertainties in the risk assessment. Future research to address these knowledge gaps is required.

This Science Advisory Report is from the June 11, 2014 Assessment of the 4R herring stocks in 2013. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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