Can Atlantic cod (Gadus morhua) recover? Exploring trophic explanations for the non-recovery of the cod stock on the eastern Scotian Shelf, Canada

Abstract

The Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf collapsed in 1993. Over a decade later, in spite of a fisheries moratorium on cod fishing, this stock is at an all-time low. In parallel with the collapse of the cod stock, the abundance of large cod prey, including forage fish, shrimp, and snow crab, has greatly increased. The key question, which we explore using trophic mass-balance models, is what processes are preventing cod from recovering on the eastern Scotian Shelf? Cod were split into large and small cod. Modelling results indicate high predation pressure on small cod. In addition, small cod compete with the abundant forage fish for decreasing prey, and are in below-average condition. Large cod incur high, but unidentified, mortality that we suggest is derived from the poor condition of small cod carried through to adulthood. As a consequence of the removal of cod by fishing and an ensusing trophic cascade, eastern Scotian Shelf cod are trapped in a vicious circle: their abundance is being kept low by predation, causing an abundance so low that cod cannot compete for prey with their exceptionally abundant competitors. Furthermore, these competitors may also prey on younger stages of cod.

Decline and Recovery of Atlantic Cod (Gadus morhua) Stocks throughout the North Atlantic

“…If the predator becomes less abundant, its prey becomes more abundant, through released predation, and then keeps the predator under control at a low level by preying on or competing with its early life-history stages. Such a mechanism has been hypothesized for cod and its planktivorous fish prey in the Baltic Sea (Köster and Möllmann 2000) and on the eastern Scotian Shelf (Bundy and Fanning 2005).…”

“…In contrast to the positive association often seen elsewhere, the condition of cod and other groundfish species on the eastern Scotian Shelf declined significantly (Choi et al 2004) at the same time as the biomass of pelagics increased. Bundy and Fanning (2005) hypothesized that the poor condition observed in small cod is due to competition with increased populations of planktivorous fish, and proposed that cod that are in poor condition when small may remain that way when they become larger.…”

Prehistoric versus modern Baltic Sea cod fisheries: selectivity across the millennia

“…Northwestern stocks ('northern cod') are so low that recovery of some may be in doubt (Bundy & Fanning 2005). Eastern Atlantic stocks are also in severe decline in the North, Skagerrak and Baltic Seas (ICES 2007), with such low spawning stock biomasses, poor recruitment and lack of signs of recovery that commercial extinction of some stocks appears to be a real possibility ( Jonzén et al 2001).…”

Apex predators and trophic cascades in large marine ecosystems: Learning from serendipity

“…For example, after Atlantic cod stocks collapsed, both Canada and the United States closed large areas to fishing in the early 1990s. Fisheries managers expected full recovery within a decade but after nearly two decades cod stocks remain at historically low levels of abundance (15). Such protracted lags in population or ecosystem recovery often result from reinforcing ecological feedbacks (14).…”

“…One theory gaining support in both the eastern and the western North Atlantic is predator-prey reversal between cod and forage fish. Forage fish examples include herring (15) or caplin (16) in the Atlantic and sprat in the Baltic (17). Large cod feed predominantly on forage fish but when cod are fished to low abundance, forage fish feed on cod eggs and/or larvae.…”