Growth of spring- and autumn-spawned larvae of Atlantic herring Clupea harengus: a long-term experiment mimicking seasonal light conditions

ABSTRACT: Atlantic herring Clupea harengus populations differ in their spawning time, and spring- and autumn-spawning populations are genetically distinct. Offspring of these populations encounter seasonal variations in productivity. We conducted a fertilization experiment using spring-spawning Atlantic herring. Offspring were reared for 3 yr with seasonal varying light cycles starting either in spring or autumn, using 2 fixed temperature levels and food provided in excess. Such long-term experiments from hatching to maturation in small pelagic fish are very rare. We hypothesized that longer daylengths early in life would provide an overall growth advantage resulting in larger size after 1 yr (same amount of light) compared to those experiencing prolonged daylight later in life due to higher size-dependent growth rates at smaller sizes. Larvae with initial spring conditions initially grew faster. However, contrary to our expectations, offspring with initial autumn conditions had caught up to similar size after 1 yr. Herring at higher temperatures grew faster, even when correcting for the amount of degree-days. After the first year, individuals hatched in spring showed higher growth at the higher temperature while herring hatched under autumn light conditions consistently had higher growth rates at lower temperatures. The somatic condition of herring followed the daylength, with best conditions during summer and poorest during winter. This was the first long-term experiment conducted on herring with varying light conditions from hatching to maturation. Our novel results indicate that herring display considerable growth plasticity, reflecting the wide range of environmental conditions and life histories sustaining herring populations.