State fisheries officials have announced a guideline harvest level of 14,649 tons for the 2017 Sitka Sound sac roe herring fishery, based on a 20 percent harvest rate of a forecast mature biomass of 73,245 tons.
A year ago the Alaska Department of Fish and Game put the preliminary guideline harvest at 15,674 tons, based on a 20 percent harvest rate of a forecast mature biomass of 78,372 tons, but only just under 10,000 tons was landed.
The forecast indicates that the mature biomass in 2017 will consist of 6 percent age-3, 6 percent age-4, 73 percent age-5, 2 percent age-6, 6 percent age-7 and 7 percent age 8+. The forecast uses spring commercial purse seine weights at age from last year’s fishery and the average weights were age-3, 64 grams; age-4, 95 grams; age-5, 104 grams; age-6, 132 grams; age-7, 149 grams; and age-8+, 178 grams.
State fisheries officials said this forecast would not be updated with winter test fishery weights as has been done in previous years. The change was made to save on costs associated with processing winter test samples and staff time developing and reporting on updated forecast and GHL. The accuracy of the forecast was not expected to be impacted significantly by not updating the model with the winter test fishery weights at age due to the relatively small variability seen in weight at age, they said.
To forecast biomass, ADF&G uses an age-structured assessment model with a long time series of egg abundance, and age composition data from department surveys conducted during and after the spring fishery. Herring egg abundance is estimated using aerial surveys, designed to map the length of shoreline receiving spawn, and dive surveys, which are used to estimate the density of eggs and average width of the spawn.
The department mapped 63.3 nautical miles of herring spawn in the Sitka Sound area in the spring of 2016, compared to the recent 10-year average of 65 nautical miles. The estimated post-fishery spawning biomass in 2016 was 74,676 tons, the total sac roe harvest was 9,833 tons, and an additional 223 tons were harvested in personal use and test fisheries.
Estimated age composition of spawning herring in 2016 was 2 percent age-3; 79 percent age-4; 2 percent age-5; 8 percent age-6; 1 percent age-7, and 8 percent age 8+.
State biologists said mature biomass forecast for 2017 is similar to the spawning biomass in 2016 because the increase in maturity of the 2012 and 2013 cohorts and the additional ag-3 recruits in 2017 balance the decreases due to natural mortality.
