Integrating Acoustic, Midwater and Bottom Trawl Data to Assess Sampling Strategies and Abundance Trends of Spawning Stock of Atlantic Herring (Clupea harengus) in the Georges Bank-Gulf of Maine Region

Fisheries independent surveys provide estimates of relative or absolute abundance for assessment and management of fish stocks. Atlantic herring (Clupea harengus) is a key species in the northwestern Atlantic ecosystems. Over the last 14 years, abundance trends of herring between the NEFSC acoustic-midwater survey and bottom trawl survey matched well until 2002, when the acoustic survey showed a sharp decline in herring abundance on Georges Bank with consistently low levels whereas the bottom-trawl survey maintained a relatively high level. This disparity has created much debate over catchability and availability of herring to the acoustic-midwater survey. Given the timing of the two surveys is isolated (early for acoustic survey and late for bottom trawl survey), we hypothesize that the two surveys actually sample the same stock complex and the difference in abundance trends potentially relate to growth of herring over the interval of the two surveys. We analyzed spatially-explicit biological data such as sex ratio, maturity status, age, and length structures within a fixed region, which has been sampled by both acoustic-midwater survey and bottom trawl survey from 1998-2012. Statistical tests showed that there is no significant difference in proportion patterns for sex ratio and age structure between the two surveys. However, maturity status suggests pre-spawning fish are surveyed by midwater trawl and post-spawned fish are surveyed by bottom trawl, and length structures show interesting trends that may indicate migratory or movement patterns relative to survey timing.