Eastern Baltic cod larvae in a salinity gradient: choice of salinity and the role of neutral buoyancy

Fish rely on multiple sensory systems to navigate in the aquatic environment, yet these mechanisms are often scarcely known during their early life history stages. Salinity is an important environmental parameter affecting the performance of fish larvae, and recent studies have shown that some larval fish use it as an environmental cue to position, orientate and navigate. Further, fish larvae are often visual feeders, and light has been suggested to be a crucial environmental cue for visual orientation and feeding. In this study, we investigate whether first-feeding larvae of eastern Baltic cod ( Gadus morhua callarias ) are driven by light and salinity. We examine this in the context of their natural vertical distribution and movement; from deeper, darker and more saline (13-17 psu) water layers towards the more light-exposed and less saline (7 psu) surface water. Further, we investigate the relationship between this movement and the neutral buoyancy measured both in active and passive larvae. We found that the larvae remain in a saline environment close to the salinity which supports their neutral buoyancy. They show negative phototaxis, suggesting an active avoidance of the lighter and less saline upper water layers. These results improve our understanding of the ecophysiology and behaviour of eastern Baltic cod larvae during the first-feeding stage and serve as an important input for restocking efforts of eastern Baltic cod and potential future production of hatchery rearing of eastern Baltic cod.