Orientation and Swimming Behavior of Saithe ( Pollachius virens ) Larvae Increases the Chance of Recruitment to Nursery Areas

Abstract

Saithe ( Pollachius virens ) is a semipelagic North Atlantic species supporting an important fishery. In the North Sea, saithe spawn east of the Shetland Islands, with larvae recruiting to coastal areas. Understanding the interaction between abiotic and biotic factors influencing larval dispersal is crucial for assessing their population structure. However, the swimming and orientation behaviors of saithe larvae, and their effects on dispersal, remain unknown. To address this, we quantified the in situ orientation and swimming behavior of 132 saithe larvae (48‐ to 62‐day posthatch) in the Norwegian North Sea using Drifting In Situ Chambers (DISCs). Ninety‐three percent of the larvae had exhibited oriented swimming but towards different directions. These behaviors were incorporated into an Individual‐Based Model (IBM) simulating larval dispersal from North Sea spawning grounds during the spring and summer of 2023 and 2024. Three numerical experiments were conducted: (1) passive larvae (IBM), (2) vertically migrating larvae (IBMV), and (3) larvae with observed horizontal swimming and orientation behavior (IBMVH). In 2023, IBMVH resulted in a 15% increase in cumulative recruitment compared to IBM and 57% compared to IBMV. In 2024, the increases were 11% and 39%, respectively. Recruitment was consistently lowest in IBMV. Including observed swimming and orientation behavior resulted in higher recruitment to the North Sea saithe stock management unit compared to other models. These results demonstrate the important role of larval swimming and orientation in the recruitment dynamics of saithe populations.

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