From Alaska Dept. of Fish and Game, 12/12/16
SITKA SOUND HERRING FISHERY ANNOUNCEMENT
The Alaska Department of Fish and Game announced today the guideline harvest level (GHL) for the 2017 Sitka Sound sac roe herring fishery is 14,649 tons based on a 20% harvest rate of a forecast mature biomass of 73,245 tons.
The forecast indicates that the mature biomass in 2017 will consist of 6% age-3, 6% age-4, 73% age-5, 2% age-6, 6% age-7, and 7% age-8+.
This forecast uses spring commercial purse seine weights at age from last year’s fishery and the average weights were as follows: 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.
This forecast will not be updated with winter test fishery weights as has been done in previous years. This change was made to save on costs associated with processing winter test samples and staff time developing and reporting an updated forecast and GHL. The accuracy of the forecast is 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.
To forecast biomass, the department uses an age-structured assessment model with a long time series of egg abundance, and age composition data from department surveys conducted during and following 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 during the spring of 2016, compared to the recent 10-year average of 65.0 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% age-3, 79% age-4, 2% age-5, 8% age-6, 1% age-7, and 8% age-8+.
The hindcast of 2016 mature biomass (pre-fishery biomass; 84,732 tons) exceeded the 2016 forecast (74,707 tons); this is primarily due to a re-estimation of the size of the 2012 cohort (age-3 fish in 2016) now that there are two years of data with which to estimate the size of that cohort.
The 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 age-3 recruits in 2017 balance the decreases due to natural mortality.