Food web models and data for studying fisheries and environmental impacts on Eastern Pacific ecosystems
There are various ways ecosystem "control", and two of these are 'top-down control' and 'bottom-up control', usually set as alternatives. This dichotomy has various incarnations; in the Pacific Northwest it is referred to as the 'Thompson-Burkenroad debate', with the former associated with top-down control (here: of halibut biomass, by fishing), and the latter bottom-up control (with environmental variability responsible for changes in the recruitment, and eventually, the biomass of halibut). When applied to ecosystems, more often than not, the 'bottom-up' part of this dichotomy has more evidence in its favour, particularly in the Pacific Northwest, where 'regime shifts' tend to be invoked almost exclusively to explain ecosystem changes. The main reason for this asymmetry, however, is that it is easier to measure temperature and its variability, or chlorophyll and its variability, than to construct and fit ecosystem models and test how much they explain of the variability at hand. However, it has now become possible to straightforwardly construct models of ecosystems, and to fit them with time-series data, and thus to test top-down control hypotheses, i.e., to separate out top-down from bottom-up effects. These tests, which required ecosystem models such as documented in this report, have not shown regime shifts to be unimportant. Rather, they have shown, at least for the North Pacific, that bottom-up and top-down processes occur simultaneously, and that both must be taken in account when modelling these ecosystems. Thus, this document is part of what will take us beyond the dichotomy, toward the complex hypotheses that these complex ecosystems deserve.
Director, Fisheries Centre, UBC
Fisheries, the environment, or what? An introduction
The North Pacific is a hot-bed for understanding how marine populations are impacted by humans as well as by environmental conditions. The "Thompson-Burkenroad debate" has been ongoing since the late-1940s: what drives the marked fluctuations in Pacific halibut that has been observed over the past century? Dr William Thompson, who started up the work of the International Pacific Halibut Commission, IPHC, argued that the changes in halibut abundance could be fully explained by changes in fishing pressure, i.e. that they were the result of successful management on the part of IPHC, while his adversary, Dr Martin Burkenroad questioned if the populations trends could be accounted for by fishing pressure on its own, or if wasn't rather a question of environmental factors impacting halibut recruitment. While Thompson and Burkenroad actually never debated the relative role of fisheries and the environment - indeed it may well be that they would actually agree that one factor in itself would not suffice to give us the full explanation their debate has lived on, and both sides still have proponents arguing for one over the other. Examining the Pacific halibut trends now, nearly 60 years after the debate started, still yields inconclusive answers only. We cannot name the culprit.
The debate has widened since Thompson and Burkenroad's days, and we regularly hear about regime shifts in connection with the North Pacific. A notable debate in this context has emerged, seeking explanations for why the Steller sea lions have declined to become threatened in major parts of their North Pacific distribution area, while increasing in others. A multitude of explanations have been brought forward, and considerable research has been aimed at understanding the importance of nutritional conditions, of predators and of prey, of competition with commercial fisheries, of parasites and diseases, of the Pacific Decadal Oscillation Index, and of the potential impact of incidental culls, to mention some. As for the halibut, no conclusive explanation has emerged.
Asking then, if the non-emergence of a single clear explanation may be due to the Steller sea lion being impacted by a combination of factors the North Pacific Universities Marine Mammal Research Consortium and the North Pacific Marine Science Foundation initiated a project "Ecosystem analysis of Steller sea lion dynamics and their prey" through NOAA funding. The project, which was the brain child of Andrew Trites (Director of the Marine Mammal Research Unit, Fisheries Centre), employs ecosystem modelling of North Pacific ecosystems (Southeast Alaska, the Central Gulf of Alaska and the Western Aleutian Islands) in an attempt to evaluate (quantify!) the relative role the various factors may have played in determining population trends. The methodologies applied for the modelling along with some of the preliminary findings from the study are described in this report. Notably, the models indicate that no single factor by itself can explain the population trends of Steller sea lion, several have to be invoked.
In parallel to the work centred on Steller sea lion, the UBC 'Sea Around Us' project (www.seaaroundus.org) through funding from the Pew Charitable Trusts initiated a series of workshops aimed at evaluating the relative role of fisheries and environmental factors for North Pacific ecosystems. Bringing together researchers from the Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo; the NOAA Alaska Fisheries Science Center, Seattle; the University of Washington, School of Fisheries, Seattle; and the University of British Columbia, Fisheries Centre, Vancouver, to analyse a series of ecosystems ranging from the Bering Sea to the Northern California Current, and coordinate the methodologies. We present descriptions of some studies in this report, while most of the findings are published separately. The present report also includes a reconstruction of North Pacific whale catches for the 20th century, which served to estimate the whale population at different periods in Southeast Alaska and the Western Aleutians. Finally, in the interest of preparing future work related to evaluating nutritional aspects of North Pacific ecosystems we have included a compilation of the energy content of invertebrates, fish and mammals in the Gulf of Alaska. The present report is freely available at the website of the Fisheries Centre of the University of British Columbia. ( www.fisheries.ubc.ca/publications/reports/fcrr.php ).