Improving Fishery-Independent Acoustic and Larval Indices for Use in the Upcoming Benchmark Atlantic Herring Assessment

Three factors have been identified as potential biases for abundance indices derived from the acoustic and larval surveys of Atlantic herring. These are: 1) changes in larval growth rate, which could affect the larval index calculation; 2) changes in the location of spawning, which
could influence the proportion of the adult stock detected by the acoustic survey, and 3) changes in the timing of spawning, which could also influence the proportion of the adult stock available to the acoustic survey. Here we propose to examine these potential biases to provide quantitative information for the next assessment. If there is evidence for biases, we will develop yearly standardizations (sy) for the acoustic index and include environmentally explicit functions in the calculation of the larval index. Our specific objectives are to:
1) develop a temperature and preydependent
growth equation for larval herring, which will be used to include a year varying
growth function in the larval index and to develop a time varying age-length key for estimating the age of all larvae captured in the NEFSC larval surveys;
2) estimate the spatial distribution of spawning using numerical circulation modeling to backtrack each larva collected to a hatch location and to test whether spawning distributions have changed through time; and
3) calculate a yearly standardization, sy, using the annual spawning time and distribution results coupled with the spatial and temporal sampling effort of annual acoustic survey. Our work will lead to an improved larval index through the inclusion of a temperature-prey specific growth function and an assessment of changes in spawning season, and will lead to an improved acoustic index through an examination of potential biases and the estimation of an annual standardization index (sy). The calculation of the standardization index will be operationalized in the NEFSC Oceanography Brach (responsible for the EcoMon surveys) and will be made available with the acoustic survey index for future assessments.