Spawning time variation in Icelandic summer-spawning herring (Clupea harengus)

Different fish stocks of the same species are defined by spatial and temporal differences in spawning, though spawning time can vary within and among stocks. Here we address spawning time variation in Icelandic summer-spawning (ISS) Atlantic herring (Clupea harengus). We do so by examining influencing factors that include variation in stock size structure, spawning experience (recruit VS. repeat spawners), spawning stock biomass (SSB), sea temperature, and combinations thereof. Our results, based on temporal variation in ovary weight (OW), relative ovary weight (RG), and maturation stage, across a time series of nearly 50 years and eight length classes of ISS herring, show that spawning occurs over a relatively invariant 30-day period defined by day-of-the-year. Logistic regression analyses and temporal development in OW and RG show that maximum spawning invariably occurs on day-of-the-year 200 +/- 15 days, and seasonal variation in mesenteric fat content is physiologically consistent with the timing. We show that the smaller recruit spawners tend to spawn similar to 17 days later, on average, than the larger repeat spawners. Spawning occurs similar to 7 days later when SSB is greater than the long-term average, and spawning may be delayed by as much as 10 days during periods of colder than average ocean temperature.