Mechanisms for migration of anadromous herring: An ecological basis for effective conservation

Land use planners have the challenge of incorporating biologically sound guidelines into development plans to balance human development with conservation of natural resources. Valued as an important component of the natural heritage of the northeastern United States, anadromous river herring (Alosa pseudoharengus, A. aestivalis) represent a model system to look at how ecological data can help conserve biological diversity in systems impacted by humans. Juvenile river herring spend 3–7 months in freshwater then migrate to the ocean. However, factors that trigger migration, and consequently influence distribution and abundance, are not well understood. Thus, our objectives in this study were to (1) describe juvenile river herring migration patterns, both “peak” (\u3e1000 fish/wk) and “all” (\u3e30 fish/wk) migration; (2) identify potential cues for this migration; (3) examine effects of one type of ecosystem alteration, low water levels, on river herring; and (4) suggest how this information can be incorporated into an effective conservation plan. Weekly during June–November 1994, we sampled both migrating and nonmigrating river herring and seven associated abiotic and biotic variables in one continuous and one intermittent flow system on Cape Cod, Massachusetts. We then used multiple logistic regression to predict when juvenile river herring migrate. In the continuous-flow system, juvenile river herring migration primarily occurred during the midday hours from July through early November, with the peaks of migration, comprising \u3e96% of all migrating fish observed in early July and early September. The peaks of migration occurred during the new moon, when Bosmina spp. density was low, and all periods of migration occurred when water visibility was low, during decreased amounts of rainfall. In the intermittent-flow system, juvenile river herring migration was frequently inhibited due to low water levels, and herring were on average 25 mm smaller than those in the continuous flow system. Using these results, managers can more effectively monitor river herring populations by identifying factors associated with migration and isolating critical flow periods when fish movement is likely to occur. Thus, we can detect changes in herring population size due to human impacts. Ultimately, these data can be incorporated into an ecologically sound conservation plan for juvenile anadromous herring that may help ensure their survival while balancing human needs for natural resources