Abstract
The Atlantic cod (Gadus morhua) stocks in the Newfoundland-Labrador Shelves (NL) and Barents Sea (BS) ecosystems have shown divergent trajectories over the last 40 years. Both stocks experienced either an important decline (BS) or a collapse (NL) in the mid-1980s and early 1990s, respectively. After these population reductions, the BS stock quickly rebounded and it is currently at record high levels, while the NL stock, despite showing some improvement since the mid-2000s, remains at low levels. Fishing and environmental conditions are known to be important drivers of cod dynamics in both ecosystems, especially the availability of high energy prey like capelin (Mallotus villosus), however, the question of how different or similar these two stocks truly are remains. Could, for example, the NL cod stock rebuild if presented to conditions like the ones experienced by BS cod? To explore such questions, we developed a simple biomass dynamic model for cod using a bioenergetic-allometric approach. This model includes fisheries catches and capelin availability as external drivers and was implemented for both ecosystems. Despite the contrasting trends, the model produced very good fits, and showed some remarkably similar estimated parameters in both systems. We explored these similarities by (a) performing the thought experiment of transferring cod stocks between ecosystems by switching estimated key parameters between models and comparing the output, and (b) implementing an integrated model architecture which allowed fitting common parameters for both stocks to evaluate the similarity of key vital rates. Our results indicate that cod trajectories in NL and BS can be reliably described using simple bioenergetic-allometric arguments, fishery catches, and capelin availability. Model parameters that encapsulate intrinsic vital rates were not significantly different between stocks. This indicates that NL and BS cod stocks are biologically similar, and that the differences in their trajectories are driven by the ecosystem context in which these stocks are embedded, and suggests that the NL stock would be expected to rebuild if enough capelin were available. This also indicates that capelin status and trend should be an important consideration for effective management of these cod stocks.
Highlights
The Newfoundland and Labrador Shelves (NL) and the Barents Sea (BS) are considered typical high-latitude shelf ecosystems (Figure 1), implying strong seasonality in physics and biological production, high productivity, and low biological diversity (e.g., Dayton et al, 1994; Wassmann et al, 2006; Gaichas et al, 2014)
The results indicate good model fits for the NL and BS cod stocks (Figure 3)
Most residuals are contained within the one standard deviation envelope; the only ones departing beyond that are the negative residuals for NL cod in 1992–1994, which correspond to the aftermath of the stock collapse; even these residuals are contained within two standard deviations
Summary
The Newfoundland and Labrador Shelves (NL) and the Barents Sea (BS) are considered typical high-latitude shelf ecosystems (Figure 1), implying strong seasonality in physics (e.g., sea ice, temperature) and biological production, high productivity, and low biological diversity (e.g., Dayton et al, 1994; Wassmann et al, 2006; Gaichas et al, 2014). The oceanography of the NL ecosystem is characterized by the unidirectional north to south flow of the cold Labrador current, while the BS oceanography is characterized by a more counter-clockwise flow within its basin, defined by the warm continuation of the Atlantic current, the Norwegian current, and inflows from the Arctic (Figure 1) Another important difference is that, while both ecosystems are showing warming signals in recent decades, NL has shown a more cyclic pattern in ocean conditions, with a very cold period in the late 1980s and early 1990s, clear warming trends from the mid2000s to the mid-2010s, and a return to average conditions since (e.g., Colbourne, 2004; Drinkwater et al, 2013; Lind et al, 2018; Cyr et al, 2020). This change has led to a reduction in the domain of Arctic waters in BS, triggering an expansion of boreal fish communities and a reduction in Arctic ones (Fossheim et al, 2015)
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