ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on November 13, 2008
ICES Journal of Marine Science: Journal du Conseil 2009 66(1):82-89; doi:10.1093/icesjms/fsn181
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This article appears in the following ICES Journal of Marine Science issue: European Symposium on Marine Protected Areas as a Tool for Fisheries Management and Ecosystem Conservation [View the issue table of contents]
Selecting MPAs to conserve groundfish biodiversity: the consequences of failing to account for catchability in survey trawls
1 FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, Scotland, UK
2 Wageningen IMARES, Haringkade 1, 1976 CP IJmuiden, The Netherlands
Correspondence to H. M. Fraser: tel: +44 1224 295439; fax: +44 1224 295511; e-mail: h.fraser{at}marlab.ac.uk.
Fraser, H. M., Greenstreet, S. P. R., and Piet, G. J. 2009. Selecting MPAs to conserve groundfish biodiversity: the consequences of failing to account for catchability in survey trawls. – ICES Journal of Marine Science, 66: 82–89.Fishing has affected North Sea groundfish species diversity. Defining Marine Protected Areas (MPAs) to address this will rely on groundfish surveys. Species-specific catch efficiencies vary between trawl gears, and apparent species diversity distributions are influenced by the type of gear used in each survey. It may be that no single survey depicts actual diversity distributions. Two MPA scenarios designed to protect groundfish species diversity are described, the first based on unadjusted International Bottom Trawl Survey data and the second based on the same data adjusted to take account of catchability. Spatial overlap between these scenarios is low. Assuming that the adjusted data best describe the actual species diversity distribution, the level of diversity safeguarded by MPAs, based on unadjusted data, is determined. A fishing effort redistribution model is used to estimate the increase in fishing activity that is likely to occur in MPAs that take catchability into account, if closed areas based solely on the unadjusted groundfish data were implemented. Our results highlight the need to take survey-gear catchability into account when designating MPAs to address fish-species diversity issues.
Keywords: benthic mortality, ecosystem approach to fishery management, GOV trawls, groundfish species diversity, International Bottom Trawl Surveys
Received 24 October 2007; accepted 3 April 2008; advance access publication 13 November 2008.
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Introduction |
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 Top Introduction MPA scenariosEffort displacementModel analyses and resultsDiscussionReferences |
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Concerns about the wider impacts of fishing activity are reflected in political commitments, such as the Convention for the Protection of the Marine Environment of the Northeast Atlantic (OSPAR), the Convention on Biological Diversity (CBD), and the recent European Union Marine Strategy Directive (MSD). These policy changes have stimulated the development of an ecosystem approach to management (EAM; Gislason et al., 2000; Sainsbury and Sumaila, 2001; Hall and Mainprize, 2004; Cury and Christensen, 2005; Garcia and Cochrane, 2005). Globally, Marine Protected Areas (MPAs) have been commonly used to address ecological objectives (Carr and Reed, 1993; Allison et al., 1998; Houde, 2001; Roberts et al., 2001; Botsford et al., 2003; Gerber et al., 2003), and their use is advocated in many of the political drivers underpinning the EAM in the North Sea. Designing MPAs to address ecological objectives requires informed scientific advice. Limitations in the data available could result in MPAs that fail to meet their ecological objectives and still affect the fishing industry adversely.
Safeguarding biodiversity is central to an EAM, and MPAs have been suggested as a possible means to achieve this (Kaiser, 2005). Implementation is likely to rely heavily on fishery-independent data, such as the long-term, internationally coordinated, groundfish bottom-trawl surveys. Analyses of North Sea groundfish survey data, for example, have already been used to detect changes in demersal fish species diversity linked to fishing (Greenstreet and Rogers, 2006). International Bottom Trawl Survey (IBTS) data may therefore be used to locate MPAs in relation to spatial variation in groundfish species diversity and to reduce the risks of further declines.
All sampling processes are influenced by sampling efficiency (Cam et al., 2002; Colwell et al., 2004; Wintle et al., 2004), and trawl sampling of groundfish populations is no exception (Harley and Myers, 2001). Early studies demonstrated species-related variation in catchability in survey trawls (Yang, 1982; Sparholt, 1990). More recently, the influence of fish body length on catchability has been described (Fraser et al., 2007). Species- and size-related variation in catchability can affect species relative abundance in the samples obtained, and so influence the values of species diversity indices derived from such data. In situations where groundfish species composition varies across marine regions, this can result in misleading perceptions of spatial variation in groundfish species diversity (Fraser et al., 2008). In this paper, we assess the importance of taking catchability in survey trawls into account when using groundfish survey data to select MPAs that are intended to safeguard areas of high groundfish species diversity.
For the purpose of this demonstration, we assume that indices of species diversity, based on groundfish survey data adjusted to account for species- and size-related variation in catchability in the survey trawl, provide the best indication of appropriate locations for "ideal" MPAs. In the absence of catchability information, MPA locations would be selected using unadjusted survey data, giving rise to poorly designed, and possibly ineffective or "flawed" MPAs. In such circumstances, fishing activity that would normally have occurred within such flawed MPAs will be redirected throughout the region remaining accessible to fisheries. Consequently, some of this effort will actually impinge on the areas that in reality have higher diversity and should have been selected as MPAs, based on catchability information and adjusted survey data. Two MPA scenarios aimed at safeguarding areas in the North Sea, where demersal fish species diversity is highest, are therefore compared: a flawed scenario where MPA rectangles are selected using unadjusted groundfish survey data and an ideal scenario where the data are adjusted to account for species- and size-related catchability in the survey trawl (Fraser et al., 2007). An effort-displacement model is then used to estimate the amount of fishing effort that would actually be displaced into the ideal MPA rectangles when MPAs are instead selected using unadjusted data.
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MPA scenarios |
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TopIntroductionMPA scenariosEffort displacementModel analyses and resultsDiscussionReferences |
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Recent analyses of North Sea Quarter 3 (Q3) IBTS data have provided maps of spatial variation in demersal fish species diversity that take account of species- and size-related variation in catchability in the Grande Ouverture Verticale (GOV) trawl survey and the sampling effort necessary to provide robust measures of species diversity (Fraser et al., 2007, 2008; Greenstreet and Piet, 2008). Cluster analysis of these adjusted species relative abundance data revealed three distinct community types. High diversity rectangles representative of each major community type were designated as potential MPAs (Figure 1). In all, 14 rectangles were selected, representing an ideal scenario, amounting to 46 595 km–2, 7.7% of the total North Sea area (Figure 1). Unadjusted IBTS data were also analysed and revealed similar community clustering across the North Sea, although in this instance, four distinct community types were recognized (Figure 2). However, spatial variation in species diversity differed markedly from the distribution obtained when using IBTS data adjusted to take account of catchability in the GOV trawl, resulting in the selection of a different set of rectangles as MPAs designed to protect areas of highest species diversity in each community type (Figure 2). This second set of 14 rectangles, based on unadjusted survey data, and which therefore take no account of species- and size-related variation in catchability in the GOV trawl, and called here the flawed scenario, covered 48 234 km2, representing 7.9% of the area of the North Sea (Figure 2). Only two rectangles, in the central North Sea, were common to both selections.
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Figure 3 confirms that when each of the two datasets were considered in isolation, rectangles selected for MPA status did indeed provide protection for areas of highest groundfish species diversity. However, if diversity maps based on data corrected to account for the influence of catchability are correct, as we assume, then in the southern and central North Sea, rectangles selected in the flawed scenario were less species diverse than rectangles selected in the ideal scenario (Figure 4). The difference was not as pronounced in the northern North Sea, where species diversity was generally lower.
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Effort displacement |
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TopIntroductionMPA scenariosEffort displacementModel analyses and resultsDiscussionReferences |
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Greenstreet et al. (2009) present an effort displacement model that estimates the increase in fishing effort in all ICES rectangles remaining open to fishing in the North Sea, following the designation of specific ICES rectangles as MPAs closed to fishing activity. The model uses international fishing effort and landings data for the period 1997–2004 in 215 North Sea ICES rectangles (Greenstreet et al., 2007). The model assumes that the catches normally taken inside MPA rectangles would be taken from rectangles remaining open to fishing if no actions were taken simultaneously to reduce total allowable catches (TACs). The model then estimates the effort that would be required to take these additional landings in each open rectangle. Here, we use this displacement model to estimate the increase in fishing effort likely in the ideal set of ICES rectangles, based on abundance data adjusted to take account of catchability in the survey trawl, when instead the selection of rectangles for MPA designation is flawed because catchability in the survey trawl is ignored.
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Model analyses and results |
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TopIntroductionMPA scenariosEffort displacementModel analyses and resultsDiscussionReferences |
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Sole, and to a lesser extent plaice and cod, were the species most affected by the closure of rectangles aimed at protecting groundfish species diversity based on unadjusted IBTS data. Beam trawl and seine nets appeared to be the gears most affected (Figure 5). To determine the amount of effort displacement into the ideal MPAs, the effort displacement model was run for two periods, 1997–2000 and 2001–2004. Sole were assumed to be the principal species targeted by beam trawls, cod the target of otter trawls directed at fish, Nephrops the target of otter trawls directed at Nephrops, and cod to be the principal species targeted by seine nets.
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Following these strategies, landings of all species were sufficient to compensate for the shortfalls normally taken from within the MPA rectangles. In fact, bycatches of haddock, whiting, and saithe exceeded the landings normally taken from within the MPAs, indicating the need for some discarding to avoid exceeding TACs, but this was not excessive. Without better detailed knowledge of current discard practices, it is not possible to judge the practical significance of this. For otter trawls directed specifically at fish, increased effort in rectangles outside the MPAs exceeded the effort normally expended within the MPA rectangles, leading to an overall increase in fishing effort across the whole North Sea (Table 1). The same was also true for beam trawls and seine nets; for the former, the overall increase in effort was marginal in both periods, whereas for the latter, the increase was relatively large between 1997 and 2000, but marginal between 2001 and 2004 (Table 1). Over the whole North Sea, otter trawls directed at Nephrops decreased marginally in the earlier period and increased fractionally in the later period (Table 1). Table 1 also indicates the changes in fishing effort in just the rectangles remaining open to fishing following this particular MPA scenario. Changes in fishing activity in each ideal MPA, based on adjusted groundfish survey data, are illustrated in Figure 6. In five of the six rectangle groups, significant increases in fishing effort occurred, with the type of fishing involved varying between the different MPAs. In the sixth MPA, substantial reductions in fishing activity took place, but this was only because two of the ICES rectangles involved were also included in the flawed MPA designation, based on the unadjusted groundfish survey data.
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Discussion |
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TopIntroductionMPA scenariosEffort displacementModel analyses and resultsDiscussionReferences |
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Species diversity frequently increases following the introduction of MPAs (Halpern, 2003; Lubchenco et al., 2003; Friedlander et al., 2007; McClanahan et al., 2007). Benefits are often most notable among larger fish (Barrett et al., 2007), re-establishing lost predatory interactions and consequent cascade effects down marine food chains (Guidetti, 2006). Conversely, reopening marine reserves to fishing can cause species diversity to decline (Russ and Alcala, 1989). Given the political concern over marine biodiversity and a clear intention to incorporate MPAs as part of an EAM, MPAs may well be employed to address fish biodiversity issues in the North Sea. Groundfish survey data will be invaluable to scientists providing advice to support this type of management. The use of such data to address fish biodiversity issues already has a long history (Greenstreet and Hall, 1996; Greenstreet et al., 1999; Piet and Jennings, 2005; Greenstreet and Rogers, 2006). However, these previous analyses have all focused on temporal changes in diversity. Advising on appropriate MPA strategies to conserve or restore fish species diversity will involve spatial analysis of these data and, as we have demonstrated, this is where problems start to emerge.
Groundfish surveys were initiated with a specific task in mind: to provide independent indices of abundance to fine-tune the single-species stock assessments that are the mainstay of traditional fishery management (ICES, 2007). It is readily acknowledged that catchability affects groundfish survey abundance estimates (Harley and Myers, 2001; Fraser et al., 2007), but because each assessment considers a single species only, indices of relative change in abundance are all that are required from the groundfish survey data. Assuming that catchability (at length) in survey trawls remains constant for each species, then catchability simply introduces a constant bias in each species’ abundance estimate, which does not detract from the survey’s ability to portray relative trends in individual species abundance. As such, groundfish surveys are fit for purpose in supporting traditional fishery management. The EAM, however, was always intended to be holistic in its perspective (Pikitch et al., 2004), and using MPAs to conserve or restore biodiversity provides a prime example of the multispecies and spatial nature of the type of issue that this alternative approach to marine resource management will be expected to address.
When using particular datasets for purposes other than those for which they were originally intended and designed, or in applying novel or unplanned analyses to such data, all the caveats associated with the data need to be properly taken into consideration. The critical importance of catchability in relation to spatial analysis of groundfish survey data to address fish species diversity issues is just starting to be realized. The underlying reason for this is that the species composition of the demersal fish community varies markedly across the North Sea (Daan et al., 1990; Greenstreet and Piet, 2008). Therefore, two different groundfish surveys, one using a beam trawl, which is designed primarily to catch flatfish, which dominate the southern North Sea groundfish assemblage, and the second using an otter trawl, which preferentially catches the round-bodied fish that dominate the northern North Sea community, produced markedly different maps of species diversity (Fraser et al., 2008). Species- and size-related catchability in the GOV otter trawl has been evaluated for species sampled in the Q3 IBTS between 1998 and 2004 (Fraser et al., 2007). Application of these catchability coefficients to Q3 IBTS data had a profound effect on the resultant maps of groundfish species diversity (Fraser et al., 2008).
In this paper, we have considered the problem facing scientists asked to select 
8% of the North Sea (14 ICES statistical rectangles) for designation as MPAs to protect fish species diversity hot spots from fishing. With no knowledge of catchability in the GOV trawl, their selection would be based on a spatial analysis of the unadjusted data. Because such an analysis ignores species- and size-related catchability in the survey trawl, we consider the advice that would emerge to be flawed. Information on species- and size-related catchability in the survey trawl allows this shortcoming to be addressed. Unfortunately, this considerably alters our perception as to where demersal fish species diversity hot spots are actually located, leading to very different advice as to which ICES rectangles should be designated as MPAs. Of 14 ICES rectangles in each of the flawed and ideal selections, only two were common to both.
As a result of reduced fishing mortality, fish abundance within MPAs generally increases (Polacheck, 1990; Pipitone et al., 2000; Côté et al., 2001; Fisher and Frank, 2002; Halpern, 2003; McClanahan and Graham, 2005; Friedlander et al., 2007; McClanahan et al., 2007), particularly among the larger fish that tend to be the target of fisheries (Willis et al., 2003; Barrett et al., 2007). This can cause density-dependent emigration out of MPAs (or spillover) into adjacent areas (Alcala and Russ, 1990; McClanahan and Kaunda-Arara, 1996; Russ and Alcala, 1996; McClanahan and Mangi, 2000; Fisher and Frank, 2002; Russ et al., 2004; Abesamis and Russ, 2005). It has therefore been argued that MPAs tend to enhance fishery yields, so fishers should not suffer from their implementation (Gerber et al., 2003; Russ et al., 2004; Gaylord et al., 2005; but see Gardmark et al., 2006). Only by accessing these spillover fish will fishers be able to make up for the catches normally taken inside reserve boundaries. Almost inevitably therefore, MPAs affect areas outside their boundaries through the displacement of fishing effort (Roberts et al., 2001; Halpern et al., 2004; Murawski et al., 2005; Kellner et al., 2007). Only under circumstances where catches associated with spillover exceed catches normally taken within MPAs, thereby increasing cpue outside the reserves, will effort displacement be negligible (Sanchirico et al., 2006). This is difficult enough to achieve with MPAs designed specifically to enhance fishery yield; attaining such a situation with MPAs directed towards conservation objectives is unlikely.
Our effort-displacement simulations suggest that areas that should have been designated MPA status, based on survey data adjusted to account for catchability in the trawl, are likely to be adversely affected by displaced fishing activity, if MPA strategies based on unadjusted data are implemented instead. Use of unadjusted survey data to select MPAs intended to conserve areas of high species diversity could easily result in poor choices of location for MPAs, because the spatial pattern of diversity has not been represented accurately. In the absence of measures to limit effort displacement, fishing effort will be redistributed to areas outside the selected MPAs, including the areas that are, in reality, diversity hot spots. Failure to take account of shortcomings in the data available to scientists responsible for the provision of advice in support of an EAM could lead to MPA management that is actually detrimental to the marine ecosystem as a whole. It has been argued that using MPAs to address conservation goals is likely to be most successful when combined with concomitant measures, such as TAC reductions or effort restrictions to limit fishing activity and so reduce effort displacement (Zeller and Russ, 2004; Hiddink et al., 2006). Our results suggest that such an approach is essential where any doubt exists over the capacity of the data to support advice on MPA strategies. Simultaneous reductions in TAC should reduce the risk of adverse ecological consequences as a result of MPA management. By definition, therefore, the imposition of measures to limit effort displacement, simultaneously with the introduction of MPAs, must invariably lead to reductions in fishers’ catch rates (Hilborn et al., 2006). This suggests that, perhaps, regional scale conservation goals might be better achieved through a general reduction in fishing effort than through the use of MPAs (Kaiser, 2005; Holland and Schnier, 2006).
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Acknowledgements |
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We thank all our colleagues who participated in the MAFCONS (Managing Fisheries to Conserve Groundfish and Benthic Invertebrate Species Diversity) project for which the datasets used in this study were compiled. Discussions at several of the MAFCONS meetings led to the development of the ideas explored here. We are grateful to all staff at the various institutes involved for their efforts in extracting the original raw data. We also thank staff at ICES for access to the IBTS data. This work was supported by the Scottish Executive Environment and Rural Affairs Department under ROAMEs MF0753 and MF0168. The MAFCONS project was funded in part by the European Commission (Q5RS-2002-00856). Stuart Rogers provided valuable comments on the manuscript.
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  S. P. R. Greenstreet, H. M. Fraser, and G. J. Piet Using MPAs to address regional-scale ecological objectives in the North Sea: modelling the effects of fishing effort displacement ICES J. Mar. Sci., January 1, 2009; 66(1): 90 - 100. [Abstract] [Full Text] [PDF]
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