Research Document - 2016/065

Principal predators and consumption of juvenile and adult Atlantic Herring (Clupea harengus) in the southern Gulf of St. Lawrence

By Hugues P. Benoît and Jean-François Rail

Abstract

Herring (Clupea harengus) is a key prey species in the diets of numerous fishes, marine mammals, seabirds, and large pelagic predators in many North Atlantic ecosystems, including the southern Gulf of St. Lawrence (sGSL; NW Atlantic, Canada). Like many small pelagic forage fish species, predation can be a significant source of mortality in Herring, rivaling or exceeding fishery removals. Changes in the abundance of Herring and their predators can therefore lead to important non-stationarity in Herring productivity, which, if unaccounted for, can bias the perception of stock status and its response to given levels of fishing. In this report we review the available information on interannual and seasonal trends in the abundance, distribution and diet of the major predators of Herring in the sGSL. This information was assembled in support of an assessment framework review for sGSL fall-spawning Herring that took place on April 13-15, 2015. Sufficient information was available to estimate the annual consumption of sGSL Herring by Northern Gannets (Morus bassanus), Cormorants (Phalacrocorax sp.), Grey Seals (Halichoerus grypus), Bluefin Tuna (Thunnus thynnus), White Hake (Urophycis tenuis) and Atlantic Cod (Gadus morhua). The reliability of those estimates varied between predators and was best for the groundfish predators. Consumption by cetacean predators could only be estimated for the mid-1990s, where it represented 37% of total estimated consumption by predators, indicating that consumption estimates for other years could be significantly biased low. For 2013, consumption by all predators other than cetaceans was almost equal to landings in the fishery, confirming the relative importance of natural mortality for this forage fish. Consumption of Herring by Cod, a historically important predator, peaked in the mid-1980s and has been declining since then. The declines since 1990 in consumption by Cod and White Hake have been offset by estimated increases in consumption by Grey Seals and Northern Gannets. The amount of Herring consumed annually by Bluefin Tuna is somewhat uncertain, but is likely to have increased to a relatively important level since 2000. Additional abundance surveys for certain key predators (e.g., cetaceans, Bluefin Tuna) and ongoing monitoring of predator diets will improve consumption estimation, ideally eventually to a point where this information can be more directly incorporated in the assessment of sGSL Herring. For now, the available information is most useful for identifying key predators, changes over time in their relative importance and an indication of the overall scale of predation mortality for sGSL Herring.

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