© 2005 International Council for the Exploration of the Sea
Developing an operational reference framework for fisheries management on the basis of a two-dimensional index of ecosystem impact
a WWF Mediterranean Programme Canuda 37, 08002 Barcelona, Spain
b Institute of Marine Sciences (ICM-CSIC) Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain
*Correspondence to S. Tudela: tel: +34 93 3056252; fax: +34 93 2788030. e-mail: studela{at}atw-wwf.org.
A composite quantitative index for the percentage of primary production required to sustain fisheries and the average trophic level of catch (%PPR–TLc) was employed to develop ecosystem-based reference functions suitable for fisheries management. Established ecosystem models, characterized by pairs of %PPR–TLc, were classified as either sustainably exploited or ecosystem overfished, on the basis of the results of factorial correspondence analysis applied to selected ecological indices, and on information from various sources. Canonical discriminant analysis of these pairs was applied to establish the discriminant function to separate the two exploitation classes. Next, reference functions related to different probabilities of ecosystem overfishing were developed to obtain an operational framework for ecosystem-based fisheries management. Values of ecosystem-based maximum sustainable catches associated with different probabilities of belonging to a sustainable situation were calculated. Overall, results show that most current fishing scenarios entail high risks of ecosystem overfishing.
Keywords: ecosystem approach to fisheries, ecosystem index, ecosystem overfishing, fisheries management, primary production, reference function, trophic level of catch
Received 1 April 2004; accepted 3 November 2004.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The primary production required to sustain fisheries (PPR; typically expressed as t km–2 y–1) has been conceived as an ecological footprint that highlights the role of fishing, in channelling marine trophic flows towards human use (Pauly and Christensen, 1995), and is routinely deduced by Ecopath modelling (Christensen and Walters, 2004). The trophic level (TL) identifies the position of organisms in the food chain (Lindeman, 1942; Odum and Heald, 1975). By convention, primary producers and detritus have TL = 1; values for other groups are determined using mass-balance models, gut content analysis, or isotope data (Briand, 2000). The average trophic level of the catch (TLc) reflects the strategy of a fishery in terms of foodweb components selected, and is calculated as the weighted average of TL of harvested species (Christensen and Walters, 2004).
Tudela (2003) proposed the use of %PPR (relative PPR, calculated by dividing by the total primary production available) in combination with TLc as a quantitative ecosystem index to capture the effect of fisheries. The reason given was that, for a given %PPR, a fishery with a higher TLc would be intrinsically less disruptive than a fishery with a lower one, and for a given TLc, a lower %PPR would be less disruptive than one with a higher %PPR. Therefore, the sensitivity of a system to the effects of fishing depends on both indicators, and it cannot be deduced from either metric independently. Accordingly, Tudela (2003) defined a theoretical framework to relate the %PPR–TLc parameter space to concepts of sustainable exploitation and ecosystem overfishing (EO; Figure 1). The challenge then is to establish the appropriate limit, as well as target reference functions (Caddy and Mahon, 1995) based on available ecological information.
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Our objective here is to define a quantitative boundary for EO in this framework, based upon available information on %PPR–TLc pairs and evidence of disruptive exploitation for different marine ecosystem types. In addition, the framework is used to evaluate ecosystems for which direct evidence of EO status was lacking, and to calculate the ecosystem-based maximum sustainable catches (EMSC) related to the chosen reference functions.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Pairs of %PPR and TLc were compiled on the basis of published Ecopath models (Christensen and Walters, 2004) for different areas and periods (www.seaaroundus.org). Models for upwelling regions were excluded, because of difficulties in distinguishing between effects of fishing and those caused by environmental factors. Because %PPR values for primary producers only and for primary producers and detritus combined are highly correlated and yield conceptually similar results, we restrict ourselves here to analyses for primary producers only. Each Ecopath model was characterized by a %PPR–TLc pair, and classified if possible either as a sustainably fished, non-disrupted ecosystem, or as an overfished, structurally and functionally degraded ecosystem. Ecosystems were given an overexploited status when cumulative fishing impacts manifested at least one overexploitation symptom, based on the EO definition proposed by Murawski (2000). The classification was based on specific information obtained from published and unpublished sources, on personal communication with experts on the ecosystem concerned, and on previous analysis of Ecopath-derived indices of ecosystem development, sensu Odum (Odum, 1969; Christensen, 1995b; Christensen and Pauly, 1998; Table 1). Factorial correspondence analysis (FCA), based on Pearson correlation coefficients and the WARD hierarchical method as aggregation algorithm (statistical package SPAD, version 5.2), was applied to a selection of Ecopath-derived indices (Table 1) from a data set comprising 27 models for different periods and six ecosystems. Table 2 lists the 49 ecosystem models included in the analysis that could be classified in terms of sustainability/overexploitation criteria, and those that could not be classified.
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Because of heteroscedasticity in the data, %PPR–TLc pairs were standardized by log-transformation. The discriminant functions associated with the classified ecosystems were then obtained through canonical discriminant analysis (CDA; using SPSS, version 11) to check classification efficiency. Limit reference functions (following the nomenclature of Caddy and Mahon, 1995) were established on the basis of 50%, 70%, and 90% probabilities that %PPR–TLc pairs belonged to a sustainably fished situation. Graphically, these functions can be used to identify the parameter space corresponding to different probabilities of EO. The discriminant functions were then applied to categorize previously unclassified models (Table 2b), and the classification obtained was evaluated against available information on exploitation effects.
Following Pauly and Christensen (1995), catches can be expressed in terms of %PPR by
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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FCA results for historical series of exploited ecosystems confirm that, among different %PPR–TLc pairs from a given ecosystem, the suite of ecological indices chosen was useful in tracking structural and functional changes within the ecosystem driven by changes in exploitation scenarios (Table 1), so were broadly consistent with the theory that resilience decreases if fishing pressure increases.
Figure 2 presents paired estimates of %PPR–TLc for classified cases (Table 2a). Sustainably fished ecosystems are limited by values of TLc ranging from 3.0 to 4.1, and of %PPR ranging from 0.3 to 14.7, with few exceptions (Newfoundland 1900; Eastern Bering Sea 1950s; northern British Columbia 1900). Overfished ecosystems have a wider TLc range of 2.2–3.9 and %PPR of 2.8–89.5, except in the case of Newfoundland (1985–1987), which has a high TLc and a low %PPR.
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CDA applied to the %PPR–TLc pairs from previously classified models correctly reclassify 88% of the cases (Figure 3). Figure 2 also shows limit reference functions associated with probabilities of the EO status, using the discriminant function from CDA. Sustainably fished ecosystems involve a TLc > 3.0 and a low to moderate %PPR, and the range of sustainable levels for %PPR increases with TLc. The likelihood of EO status clearly increases as TLc decreases, and models located within this area are characterized by fisheries exploiting small fish and invertebrates (Table 2a).
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Pairs of %PPR–TLc for unclassified models (Table 2b) were plotted within the general framework defined (Figure 4). Faroes (1997) and Norwegian and Barents Sea (1997) models were located above the (50% probability) threshold curve for EO. The rest of the temperate models fitted within the range for sustainable exploitation. All models from tropical shelves and seas and coastal areas and coral reefs were located above the threshold curve, indicating that present exploitation implies a high risk of EO status. In general, the results fit the fragmentary information available from these ecosystems (Table 2b).
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Table 3 lists a compilation of estimates of ecosystem-based maximum sustainable catches (EMSC) for tropical and temperate shelves and seas and coastal areas and coral reefs, in comparison with current annual catches. Reported catches are far in excess of EMSC50 and EMSC70 estimates, by factors of 1.7 and 2.9, respectively, in temperate shelves and seas, 2 and 3.4 in tropical shelves and seas, and 2.7 and 4.6 in coastal areas and coral reefs.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Some limitations of the data set need to be considered when it comes to interpreting the ecosystem-based framework developed for fisheries management, and to applying it to new pairs of %PPR–TLc characterizing exploited ecosystems. First, more information was available for temperate areas than for other marine ecosystem types, and modelled ecosystems included in the analysis referring to reconstructed past situations are mainly from temperate regions. The models for historical situations are fundamental to this approach (Jackson et al., 2001), even if their precision tends to be lower than for more recent models (Pitcher, 2001). Furthermore, catches are subject to high uncertainty, owing to difficulties in estimating discards, bycatches, and illegal, unreported, or unregulated catches. However, the proposed ecosystem-based framework for fisheries management and the related reference functions can be modified easily whenever more accurate data become available.
The initial hypothesis on the behaviour of %PPR and TLc with respect to ecosystem overfishing (Tudela, 2003) has been confirmed, and can now be described mathematically, allowing limit reference functions to be established. Any new pair of %PPR–TLc for any exploited ecosystem can now be directly assessed in terms of its deviation from a situation desirable to preclude EO status. Overall, the framework appears to represent a clear step forward in the difficult process of translating concepts of "ecosystem overfishing" into operational definitions that can be used in fisheries management (Larkin, 1996), and it provides a holistic ecosystem-based evaluation of a harvesting strategy, taking into account marine foodweb structure and functionality.
Discriminant analysis correctly reclassified a large number of models. Exceptions refer to reconstructed Newfoundland (1900), eastern Bering Sea (1950s), and northern British Columbia (1900) systems, which were characterized by great fishing pressure on marine mammals and a high %PPR. Newfoundland (1985–1987) was classified as an overfished ecosystem (Heymans, 2003), but it is located within the range of sustainable exploitation. This may be due to a transitional state of the ecosystem, caused by a supposed regime shift (Steele, 1998). This stresses the importance of detailed and wide-ranging historical information when interpreting ecosystem state.
Results highlight the fact that ecosystem overfishing is reached sooner following an increase in %PPR operating on low-TL species. Clearly, disrupting energy flows lower in the trophic web has the farthest-reaching effects on the whole ecosystem. This is important in the context of evidence about the rapid worldwide depletion of predator fish, and the consequent decrease of the TLc (Christensen et al., 2003; Myers and Worm, 2003).
According to our results, %PPR values of 
20% are only compatible with sustainable exploitation if fisheries are harvesting high in the foodweb (TLc > 3.5). The sustainable ranges for TLc and %PPR values identified contrast with estimates reported by Pauly and Christensen (1995). This raises serious concern about the sustainability of marine ecosystems under current exploitation regimes, mainly characterized by fisheries focusing on low-TL species and a high harvesting intensity. If reconstructed situations from the past are eliminated, Figure 2 provides a good graphic overview of the overall impact of current fishing practices on marine ecosystems: clearly, overexploited systems now dominate.
Estimated EMSC values below current catch levels agree with previous assessments of exploited ecosystems dominated by EO situations, and point to current catches being well beyond sustainable levels. Moreover, EMSC estimates could be overestimated because they are theoretical maxima, considering a mean TLc. If the fishery is operating on a broad spectrum of TLs, sustainable maxima will be lower than those proposed, owing to the exponential relationship relating %PPR and TLc.
Catches exceeding EMSC might be maintained within chronically degraded ecosystems, although the long-term viability of such a situation is doubtful, and large-scale resource collapses may continue (Jackson et al., 2001). This conclusion reinforces the need to apply a more generous precautionary approach to fisheries management.
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Acknowledgements |
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We thank Villy Christensen for providing the database and useful advice, various scientists for supplying essential information about, and personal experience with, the ecosystems modelled, and Maria Lourdes Palomares and Niels Daan for constructive comments on the draft manuscript.
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