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Nature 477, 86 - 89 (7362)
Published online: 2011-09-01; | doi:10.1038/nature10285

Article: Transient dynamics of an altered large marine ecosystem

  1. #29513
    Date:
    2011-11-03 12:33:57 PM
    Brian Rothschild said:

    Alternative interpretation of trophic cascade

    Brian J. Rothschild
    School for Marine Science and Technology
    University of Massachusetts Dartmouth
    706 South Rodney French Boulevard
    New Bedford, MA 02744-1221, USA
    brothschild@umassd.edu
    tel: 508-910-6382

    ?Transient dynamics of an altered marine ecosystem? portrays the spectacular collapse of ?large benthic fish? (LBF) in the early 1990s in the waters of the Scotian Shelf as a ?trophic cascade?1. Frank et al. define the Scotian-Shelf trophic cascade as a fishing-driven, top-down, restructuring of an ecosystem where top predators of pelagic fish (PF) are removed by fishing; as a consequence, the PF populations explode; the LBF and the PF reverse trophic roles (?predator-prey reversal??the PF prey on young stages of LBF); and increased predation by PF suppresses the ability of the LBF to increase to their pre-collapse abundance. The PF also feed on zooplankton. Eventually, feeding by PF on zooplankton reduces the abundance of zooplankton; this causes the PF to decline and the LBF begin to increase in abundance. Frank et al. imply that in the course of these trophodynamic changes, physical forcing was minimal.

    However, the trophic-cascade interpretation forwarded by Frank et al. is challenged by reexamining their data and considering literature they have omitted. In particular, the northwest Atlantic shelf was pervaded in the early 1990s with a major freshening^2,3,4^, and it can be argued that this freshening, along with other environmental factors, induced the observed dynamics, rather than a top-down trophic cascade driven by fishing and operating in an unchanging environment. To support this point of view, we consider 1) overfishing, 2) predator-prey reversal, 3) the recovery, and 4) body-weight anomaly.

    Overfishing ? Three recent papers^5,6,7^ conclude that the primary cause of the collapse of the LBF was an environmental perturbation, not fishing. The LBF during the period of their decline, and in the presence of very low fishing mortality, have exhibited decreases in growth rates, decreases in condition factor, change in maturity, and a several-fold increase in natural mortality rate. These changes must be attributable to environmental forcing rather than fishing. If the changes owe to environmental forcing, then the decline of the LBF and the increase in PF would both be attributable to an environmental cause, in contradiction to the claim that fishing initiated the trophic cascade.

    Predator-prey Reversal ? A key point is that predator-prey reversal would only affect the recruitment of the LBF, not the observed LBF decreases in growth rate, condition factor, change in maturity, or increases in natural mortality of the LBF. In addition, cod recruitment declined from 1985 to 1995, but the PF population did not explode until 1994. Support for a predator-prey reversal by Frank et al. is based upon Fauchald (2010)8. Fauchald?s work is based upon relating cod recruitment to cod spawning stock biomass (SSB), herring SSB, and Calanus in the North Sea. The North Sea has a long history of studying this problem without definitive results^9^. Long time series from the North Sea as they might apply to the Scotian Shelf require critical examination owing to confounding significant warming and increased warm-water plankton^10^.

    The Recovery of LBF ? The ?recovery? of LBF occurred in c. 2005. Frank et al claim that the recovery is generated by a decline in PF, which began in 1994, almost ten years earlier. They claim that the decline in PF results from a reduction in the biomass of large zooplankton. However, the abundance of large zooplankton and the abundance of PF are not correlated over the full extent of the time series.

    Body Weight Anomaly ? An alternative interpretation is that the increasingly negative anomaly is being caused, not by decreased growth, but by increased predation on the pelagic fish (where increased predation reduces average age, and consequently average size). The increased predation could arise from the recovery of the cod and increased predation by predators not thoroughly considered, such as dogfish and seals.

    Assertions of trophic cascades have an important impact on our understanding of ecosystem functioning and fisheries management. Accordingly, it is important to give broad consideration to alternative explanations of observed phenomena. In particular, reducing fishing mortality will not cause stocks to increase if their decline is induced by environmental factors, and every decline in a fish stock is not the result of ?overfishing?.

    1. Frank, K. T., Petrie, B., Fisher, J. A. D., & Leggett, W. C. Transient dynamics of an altered large marine ecosystem. Nature 477, 86-89 (2011).
    2. Belkin, I. M. Propagation of the ?Great Salinity Anomaly? of the 1990s around the northern North Atlantic. Geophysical Research Letters 31, L08306. doi:10.1029/2003GL019334 (2004).
    3. Colbourne, E. B. Decadal changes in the ocean climate in Newfoundland and Labrador waters from the 1950s to the 1990s. J. Northw. Atl. Fish. Sci. 34, 41-59 (2004).
    4. Mountain D. G. & Kane J. Major changes in the Georges Bank ecosystem, 1980s to the 1990s. Mar Ecol Prog Ser 398, 81-91 (2010).
    5. Rothschild, B. J. Coherence of Atlantic cod stock dynamics in the northwest Atlantic Ocean. Trans. Am. Fish. Soc. 136, 858-874 (2007).
    6. Hilborn, R. & Litzinger, E. Causes of decline and potential for recovery of Atlantic cod populations. The Open Fish Science Journal 2, 32-38 (2009)
    7. Halliday R. G. & Pinhorn, A. T. The roles of fishing and environmental change in the decline of Northwest Atlantic groundfish populations in the early 1990s. Fisheries Research 97, 163-182 (2009).
    8. Fauchald, P. Predator-prey reversal: A possible mechanism for ecosyst em hysteresis in the North Sea? Ecology 91(8), 2191-2197 (2010).
    9. Cushing, D. H. The decline of the herring stocks and the gadoid outburst. J. Cons. int. Explor. Mer 39(1), 70-81 (1980).
    10. Beaugrand, G. The North Sea regime shift: evidence, causes, mechanisms and consequences. Progress in Oceanography 60, 245-262 (2004).

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