EBM Opinion: EBM Is About More than Just Managing Fisheries

The Skimmer on Marine Ecosystems and Management

By John F. Caddy

[Editor's note: Caddy is formerly of FAO and now an independent consultant based in Rome. E-mail: jfcaddy [at] yahoo.co.uk]

I see EBFM (Ecosystem-Based Fisheries Management) as a subset of EBM (Ecosystem-Based Management). Any kind of fisheries management that separates itself from changes in the ecosystem, the habitat, and the environment, is in danger of neglecting dangerous changes. We expressed that opinion over twenty years ago (see Caddy and Sharp 1986*), and I see no reason to change my opinion now.

When describing what now seems to be my minority view, I have to query the majority view, namely that ecosystem issues must be addressed through trophic analysis. Over-promoting the axiom that ecosystems are maintained by the flow of carbon through a food web, however true, casts a shadow over other important issues in marine ecology. Wherever EBM is discussed you will see trophic models offering pre-digested solutions to your ecosystem modelling needs. I appreciate the utility of these and other models used in EBFM, but have a bone to pick with scientists who use any model without considering its assumptions, or advocates of ecosystem conservation who adopt any explanation that proves their point of view.

When we discuss that mysterious procedure referred to as EBFM - or any other science-based endeavour for that matter - progress will be made only if we avoid paradigm fixation, and compare different hypotheses for ecosystem change. In the 1970s-1990s we were perhaps over-fixated on size frequency analysis, but it seems illogical to abandon size-based methods now in favor of a classification by trophic levels. Size, scale, and spatial dimensions are critical to understanding trophic interactions at different life history stages, but are also needed to understand size-related interactions of fauna to their marine habitat. And habitats often show fractal characteristics of size that add complexity to understanding ecosystems.

Fishing is not the only cause of ecosystem change

From an EBM perspective, in most coastal seas anthropogenic effects start within estuarine systems and wetlands: these are highly vulnerable ecosystems and are under threat. Progressive nutrient enrichment affects demersal food chains through seasonal and then permanent hypoxia, and results in "unnatural" and often toxic algal blooms. Hence I cannot accept the dominance of the "Fishing Down The Food Web" (FDTFW) hypothesis in all cases. It singles out the impact of the fishing industry on the upper trophic levels as the predominant cause of ecosystem change, and absolves the rest of humanity from responsibility for its "footprint" on coastal seas. Urban humans also cause bottom-up effects: we are responsible for pollutant runoff for example, as well as the water shortages our needs can cause. We also create the market demand that the fishing industry tries to satisfy.

Effectively, semi-enclosed and coastal seas act like large estuaries, and human actions within catchment areas can seriously impact adjacent marine ecosystems by reducing freshwater runoff and increasing nutrient and sediment outflows. Should we exclude these effects from the models we use? How can we avoid incorporating habitat and environmental change into our working hypotheses? While all ecosystem stresses can cause consequences similar to those described in the FDTFW hypothesis, the assumptions of a predominantly top-down effect, and of fixed trophic level components within a static food web, are not always compatible with reality.

An incident at the ICES/SCOR Symposium on the Ecosystem Effects of Fishing in 1999 emphasizes how poorly my point of view has been received. I had been asked to talk about fishing effects in semi-enclosed seas. This posed a problem, since in addition to effects of trawling such as those on seagrass beds in the Mediterranean and other semi-enclosed seas (e.g., Caddy 1993**), ecosystem components were disappearing in areas with high turbidity and eutrophication. In the Black and Baltic Seas, resulting problems of hypoxia make bottom trawling a high risk activity. Other anthropogenic influences that I classified as "Marine Catchment Basin effects" have shown dramatic impacts on Black Sea fisheries, and bottom-up factors appeared to be responsible for an apparent increase in productivity in other Mediterranean areas. Therefore in my talk I chose to compare two sources of ecosystem stress: fishing and eutrophication. My presentation was interrupted by one organizer who came on stage to claim that this conference was on the effects of fishing and not on environmental change!

Although trophic models provide useful background information, ecosystem change is primarily what we have to document at this time, and we need to identify the causes of it, which may not be simply a consequence of trophic imbalances.

I do not discount the overcapacity of fishing fleets as a major factor damaging marine ecosystems, or that predator decline may affect lower trophic levels - this has been evident for decades. What I have warned against is choosing a single explanation for the deterioration of marine ecosystems, not to mention the dangers of adopting a canned software approach without considering the evidence and assumptions of the model (which is, of course, the responsibility of the user).

There are several other potential causes for stock declines and ecosystem change, such as the degradation of habitats by fishing and eutrophication, declines in spawning stock size, the loss of spawning or nursery habitats, cover and connectivity, the conversion of sources of recruitment within a metapopulation into sinks, changes in environmental temperatures, or the introduction of harmful exotics. Unless you can identify the single (or more usually, the several) key problems that apply locally, working toward a solution through EBFM or EBM will be difficult.

For additional reading

* An Ecological Framework for Marine Fishery Investigations, by J.F. Caddy and G.D. Sharp. 1986, United Nations Food and Agriculture Organization. www.fao.org/docrep/003/T0019E/T0019E00.htm

** "Towards a comparative evaluation of human impacts on fishery ecosystems of enclosed and semi-enclosed seas", by J.F. Caddy. 1993, Reviews in Fisheries Science 1(1): p 57-95.

Marine Habitat and Cover, by J.F. Caddy. 2007, UNESCO. http://publishing.unesco.org/details.aspx?Code_Livre=4538

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