Sondre Aanes

Precision in estimates of density and biomass of Norwegian spring-spawning herring based on acoustic surveys

Abstract The abundance and biomass of the Norwegian spring-spawning herring (NSSH) stock are assessed annually using a virtual population analysis (VPA) applied to catch-at-age data from the fishery and fishery-independent abundance indices derived from research vessel surveys for calibration (‘tuning’). The most important of these surveys is the International Ecosystem Survey in the Nordic Seas (IESNS) and this is highly influential for the outcome of the assessment. Until now, the abundance indices from the IESNS have been reported without any measure of uncertainty. In this study, the sampling errors associated with the density estimates of NSSH from the IESNS for the years 2009–2012 are estimated using design-based survey sampling theory. The annual survey estimates of number and biomass of herring per square nautical mile (nm2) were relatively precise for all age groups combined, with relative standard error (RSE) ranging from 8% to 15%, while for each individual age group (3–12) the RSE was less than 30% for most years. For age groups 1 and 2, and all age groups older than 12 years, the density estimates are highly imprecise, with RSE ranging from 30% to 100%. The precision in estimated density provided here indicates that the time series from the survey can be used to study trends in overall abundance and biomass of the stock. It is recommended that statistical assessment models that can account for sampling errors in input data from survey indices by age group and catch-at-age be tested in the assessments of the NSSH stock.

Age determination of Northeast Arctic cod otoliths through 50 years of history

Abstract Norwegian and Russian Marine Research Institutes have investigated the possibility that biases in age determination from otoliths may have contributed to the long-term trends observed in the Northeast Arctic cod stock productivity: changes in growth, maturation rate, fecundity, etc. To determine the effects of changes in age reading protocols between contemporary and historical practices, randomly chosen material from each decade for the period 1940–1980s was re-read by experts. The quality of old otoliths stored in paper bags for more than 50 years was also assessed. Although some year-specific differences in age determination were seen between historical and contemporary readers, there was no significant effect on length at age for the historical time period. A small systematic bias in the number spawning zones detection was observed, demonstrating that the age at first maturation in the historic material as determined by the contemporary readers is younger than that determined by historical readers. The difference was the largest in the first sampled years constituting being approximately –0.6 years in 1947 and 1957. Then it decreased with time and was found to be within the range of –0.28–0 years in the 1970–1980s. The study shows that cod otoliths could be used for age and growth studies even after long storage.