Complex Linkages Between Herring Abundance and Body Size and Predator Populations in the Gulf of Maine

Small pelagic fish such as herring and sardines are a critical ecological link between changes in oceanographic conditions, including plankton abundance, and populations of larger predators, such as commercial fish, marine mammals, and seabirds.  Under an ecosystem-based approach, managing commercial fisheries for small pelagic fish requires balancing commercial interests with ecosystem needs and the potential impact of harvesting on predator populations. An example from the Gulf of Maine suggests that managing for high biomass of Atlantic herring is not universally beneficial to all herring predators.  During the 1990s, herring biomass in the Gulf of Maine was at historically high levels, and herring prey abundance (large copepods) was consistently below average.  The low prey levels led to reduced herring growth rates and biased the size spectrum towards smaller herring.  While the high biomass of herring likely led to an increase in humpback whale sightings in the Gulf of Maine, the decline in the abundance of large herring (>260 mm) led to a reduction in bluefin tuna weight.  The reduction in mean prey size was associated with a shift in tuna distribution from US to Canada waters.  These results suggest that the size-spectrum of small pelagic fish is an important indicator of ecosystem status.  An improved understanding of how this spectrum responds to shifts in environmental conditions, changes in predator populations, and fishing pressure is necessary for effective ecosystem-based management.