Oscillating reproductive strategies of herring in the western Atlantic in response to changing environmental conditions

Herring are broadly distributed in the western Atlantic, but reproductive activity is restricted to the central portion, from Cape Cod to northern Newfoundland. Spawning extends from early spring to late autumn. Two spawning types are recognized: spring spawners and autumn spawners. At the southern extreme, spawning occurs almost exclusively in autumn, whereas in the north, spring spawners prevail. Mid-range, both strategies are common, with spawning occurring throughout the entire season. We postulate that the temporal intra-stock dominance of spawning type is an adaptive reproductive strategy to changing environmental conditions. A conceptual model is presented to describe how reproductive success can oscillate between spawning strategies. In most years, environmental conditions appear to favour one type or the other, although strong year classes can co-occur when conditions are right. The recent change in environmental conditions, characterized by a general warming trend, favours autumn spawners and is consistent with our model. At the distributional extremes, stocks are restricted to a single spawning strategy, and broad-scale environmental changes can introduce large interannual variability and limit the reproductive and recovery potential of a stock. The dependence of a stock on a single spawning season may result in longer times to recover to target levels.