The effects of survey design and circulation pattern on the perceived abundance of herring larvae: a case study for Norwegian spring spawning herring ( Clupea harengus )

Abstract

A larval survey is used in the annual assessment as an index of the spawning stock size of Norwegian spring spawning herring ( Clupea harengus ). To test how inter‐annual fluctuations in circulation pattern, survey design and execution of the survey affected the larval abundance estimate we conducted simulated surveys using a model framework with idealized assumptions to model larval drift and sampled larvae using several realistic survey scenarios. The results suggest that inter‐annual variations in circulation pattern alone can have a profound effect on the perception of larvae abundance and that the direction of the survey (north to south versus south to north) can have a significant effect on the estimated abundance, particularly if hatching occurs over a short period of time. Additionally, disruptions to a continuous survey schedule also have an effect and, as such, sampling strategies in case of disruption to the survey are proposed.

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