ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on April 16, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(4):652-660; doi:10.1093/icesjms/fsm030
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Sustainable management of mixed demersal fisheries in the North Sea through fleet-based management—a proposal from a biological perspective
1 Federal Research Centre for Fisheries, Institute for Sea Fisheries, Palmaille 9, D-22767 Hamburg, Germany
2 Federal Research Centre for Fisheries, Institute for Gear Technology, Palmaille 9, D-22767 Hamburg, Germany
3 European Commission, DG Joint Research Centre (JRC), IPSC Institute for the Protection and Security of the Citizen, AGRIFISH Unit, TP 266, 21020 Ispra, Italy
Correspondence to H-J. Rätz: tel.: +49 40 38905 169; fax: +49 40 38905 263; e-mail: hans-joachim.raetz{at}ish.bfa-fisch.de
Rätz, H-J., Bethke, E., Dörner, H., Beare, D., and Gröger, J. 2007. Sustainable management of mixed demersal fisheries in the North Sea through fleet-based management—a proposal from a biological perspective. – ICES Journal of Marine Science, 64: 652–660.Cod, haddock, whiting, saithe, plaice, sole, and Norway lobster are main target species for the mixed demersal fisheries of the North Sea, Skagerrak, and eastern Channel. Management by total allowable catch has not been able to constrain exploitation of individual species, so the potential for a fleet-specific effort management system to reach the management objectives established for the stocks is simulated. Relative fleet-specific effort factors are estimated based on the sum of partial fishing mortalities of the species caught, weighted by the stocks at risk of reduced reproductive capacity. The strategy promotes the use of selective gear and levies non-selective gear. The factors were applied in medium-term simulations of the annual decision process in accordance with existing and proposed multi-annual management plans (including for cod recovery). Strict effort reduction would be required for fleets targeting cod, plaice, or sole (specifically large- and medium-mesh trawler fleets, beam trawlers, and gillnetters) for
5 years, reducing the exploitation rates on all stocks substantially. Cod and plaice are predicted to recover by 2010 and cod catches to exceed recent levels continually, with the more selective longliners and some other gear types profiting most. Management objectives for cod dominate annual effort adjustments, resulting in substantial underexploitation of other stocks. However, even a 10% bias caused by non-compliance would largely halt cod recovery and the restoration of other stocks.
Keywords: mixed demersal fisheries, multi-annual fleet effort management, North Sea
Received 30 June 2006; accepted 30 January 2007; advance access publication 16 April 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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For several decades, landings from the demersal fisheries in the North Sea, the Skagerrak, and eastern Channel (here referred to for convenience as part of the North Sea) have been declining as a consequence of the deterioration in target stocks (ICES, 2005a). The seven main demersal species are cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), saithe (Pollachius virens), plaice (Pleuronectes platessa), sole (Solea solea), and Norway lobster (Nephrops norvegicus). Figure 1 illustrates the average geographical distribution patterns of the international landings of these species during the period 2000–2004. Haddock and saithe are caught mainly in the northern North Sea, and cod and whiting in the northern and southern North Sea, the Skagerrak, and eastern Channel. The flatfish plaice and sole are caught mainly in the southern North Sea and eastern Channel, and Norway lobster overlapping with gadoids in the northern North Sea and the Skagerrak.
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European fleets exploit these stocks with non-selective gears in "mixed fisheries". Mixed fisheries often cause management conflict between ecological (Greenstreet and Hall, 1996; Jennings et al., 1999; Pope et al., 2000) and economic objectives. They are also characterized by high levels of discarding (Pastoors et al., 2000; Cotter et al., 2002). Single-species quota allocations may add to the discard problem because fishing may continue when the quota for one or more species is exhausted. This is perhaps one reason that total allowable catch (TAC) regulations, based on single-stock considerations, have failed to curtail the exploitation of demersal species to sustainable levels, and that the cod and plaice stocks are now failing to show signs of recovery, despite the conservation measures enacted recently.
Here, we investigate the potential for fleet-specific effort management in the North Sea mixed demersal fisheries with the aim of maximizing catches within constraints set by stock conservation, as defined in multi-annual management or recovery plans (CEC, 2004, 2005a) and their associated interim measures (CEC, 2002, 2003, 2005b, 2006), and by the precautionary approach (FAO, 1995). Effort management and its decisive procedures are simulated in medium-term predictions over ten years.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Stock parameters and management objectives
Of the seven main target species, cod, plaice, and sole stocks are the most severely reduced (see Table 1 for the essential stock parameters used in medium-term predictions and management benchmarks). Since 2000, the spawning-stock biomass (SSB) of cod has remained below the limit reference biomass (Blim = 70 000 t; ICES, 2005a) that should be avoided because it implies an immediate risk of reduced reproductive capacity. Recently, estimates of SSB have ranged around 40 000 t. Annual exploitation rates, expressed as mean rates of fishing mortality (F) on age groups 2–4, increased steadily until 1980, to exceed the limit reference point for F (Flim = 0.86). Subsequently, exploitation remained too high and is held responsible for the current poor status of the stock. Mean stock and catch weight-at-age, maturity ogives, and natural mortality represent long-term averages used in the most recent assessment (ICES, 2006a). Initial population estimates (of numbers at 1 January 2005) were generated from the short-term prediction outputs for 2005 (ICES, 2005b) based on status quo exploitation patterns because conflicting survey indices and uncertainties in catch statistics prevented an updated assessment. However, F-at-age in 2005 was reduced by 40% to reflect the reduced mortalities deduced from survey indices (ICES, 2006a). As the stock remained depressed and F remained high, the model results are robust against the starting values for 2005.
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The plaice stock has also been depleted by exploitation rates that were too high. Discard rates (in numbers) exceed 80% (Pastoors et al., 2000). Since the early 1990s, SSB has fallen by
50%, but remained above the limit reference level (Blim = 160 000 t). Since the mid-1950s, F has increased steadily, varying around Flim (0.74) over the past 10 years (ICES, 2006a). The historical dynamics of recruitment are largely unknown because of extremely variable discard rates. The limit and target reference points listed in Table 1 allow for the recently proposed management plan (CEC, 2005a). In contrast to cod and plaice, haddock and saithe stocks recently matched what is thought to be their full reproductive capacity. However, discard rates of haddock have been high, and some saithe quota has not been utilized as a consequence of poor prices. Differences between the SSB estimated by short-term prediction (ICES, 2006a) and the medium-term estimates presented here, result from the latter being based on the much higher weight-at-age derived from long-term averages.
The sole stock has sustained intensive exploitation for decades, but also appears to be overfished in terms of maximum sustainable yield (MSY; ICES, 2005a). The target F is taken from the proposed management plan and is consistent with its geographic interaction with plaice.
Unfortunately, the data available for whiting prohibit a reliable assessment of stock status. The six functional stock units distinguished for Norway lobster display constant or increasing abundance, while total landings of around 20 000 t have been maintained. However, an equally rigorous assessment as for demersal fish is not possible, so whiting and Norway lobster have been excluded from the analysis.
Fleet-specific effects
The definition of fisheries and fleets requires precise knowledge of their specific effect on exploited stocks through landings and discarding. Since 2001, sampling of landings and discards by fleets and gear type has been supported and regulated by a data collection regulation (CEC, 2001). Catch data from various European fleets for 2004 have been collated by the Scientific, Technical and Economic Committee for Fisheries (STECF, 2005). Table 2 lists the quantitative effects of six gear categories on the five demersal stocks for which sufficient data were available, expressed as relative partial fishing mortalities (Rijnsdorp et al., 2006) based on the catch-at-age proportions taken (excluding unallocated catches). This information is regarded as representing technical interaction in the mixed fisheries. The small-mesh (16–31 mm) demersal trawl is entirely disregarded in the analyses because it is used mainly by industrial fisheries directed at sandeel (Ammodytes spp.) and Norway pout (Trisopterus esmarki), and has minor effects on the stocks considered here.
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Cod biomass appears most affected by large-mesh demersal trawls (
100 mm), whereas medium-mesh demersal trawls (70–99 mm), beam trawls (80 mm), and static gears have less, but still measurable, effect (ICES, 2006a). Haddock and saithe are almost exclusively taken by large-mesh demersal trawls, though small shares are also taken by medium-mesh trawls. Plaice catches are widely spread over the gear categories, but most are taken by beam trawls. Sole are caught mainly by beam trawls and to a small extent by static gear (e.g. trammel-nets and tangle nets).
Fleet-based management
Based on the data, fleet-specific effects on the five demersal fish stocks in the North Sea (i.e. our original seven, but without whiting and Norway lobster) can be quantified for 2004. We believe that such knowledge can be used to safeguard sustainable catches (as well as meeting the goals of multi-annual recovery plans) of all species caught in mixed fisheries. Important issues are the number of fleets involved and the number of stocks, their exploitation rates, and their SSBs in relation to sustainable levels (stock status).
The fleet-specific effect on different stocks can be derived from the partial fishing mortality (Fpar) on each, which is defined as the proportional contribution of a fleet to the total catch times F (averaged over an appropriate range of age groups; Rijnsdorp et al., 2006). We express the relative partial F (Fparrel) by fleet as
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| (1) |
Fleet-based effort management in a mixed fishery may be visualized as a scheme that allows some maximum effort for each individual fleet, which is determined by individual effort factors on each species. The fleet- and species-specific reciprocal value of the relative partial F (rFp(f,s)) appears to be a suitable starting point for calculating an effort factor for all species combined: if rFp(f,s) is high, the effect of a given fleet on a given stock is low, and vice versa. The sum of rFp(f,s) over all fleets by species equals 1, and the sum over all species by fleet represents the numerically assessed effect of that fleet on the species mix relative to other fleets (the sum of these values over all fleets equals the number of fleets). In this way, a relative fleet-specific effort factor can be derived that constrains the least selective (or least targeted) fleet in the mixed fishery through a handicap. In practice, we deal with species that need different levels of effort constraints because of their stock status. Therefore, it would seem appropriate to put a weighting factor on rFp(f,s) that depends on the assessed SSB relative to the value considered sustainable. Because we use forward projections, the precautionary reference point (Bpa) rather than Blim is appropriate here, and the relative fleet-specific effort factor (Efac(f)) would become
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| (2) |
Because extremely low bycatches of any species could have a major effect on Efac(f), we put a lesser constraint of 0.01 on the value that Fparrel(f,s) can take, although low bycatches ought to be neglected in considerations of the catch composition of the main target species.
For making medium-term predictions of stock-specific parameters (SSB, catch, and F) and fleet-specific parameters (relative effort changes and catches) for the period 2005–2015, Efac(f) values were proportionally increased until either Fpa or Bpa was reached. The model simulates annual management decisions on effort adjustments by fleet, the constraining decision being recorded each year. The forecast scenarios were also constrained by a target F, a proxy for FMSY. Reaching this target was to be realized through annual reductions of a maximum of 10%, as stipulated in the multi-annual management plans for stocks shared with Norway and currently under review for plaice and sole. For cod, the reference SSB was calculated according to the recovery plan (CEC, 2004) to ensure an annual increase of SSB by 30% during the years 2005–2015. However, the agreed maximum annual variation in the TACs for cod, saithe, plaice, and sole (±15%) had to be disregarded, because that constraint was quite often exceeded in the simulations, implying conflicting decision rules when the maximum variation was exceeded for more than one species at the same time.
The forecasts assume a Ricker (1975) stock-recruitment function for all stocks. Recruitment was generated allowing for random error and with a minimum value set at the 5% quantile of the distribution of recruitment observed (Table 1). Random errors (±10%) were generated on the stock- and fleet-specific catch weights-at-age, maturity, and natural mortality; stock size in numbers and partial mortality-at-age were varied by ± 20%. To simulate effects of non-compliance with the regulations, stock numbers were subjected to a negative bias (overestimation) and partial Fs to positive biases (underestimation) of 10% and 20%, respectively. Simulations without any bias (i), and with a 10% (ii), or 20% bias (iii) were run iteratively 100 times.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Figure 2a illustrates the productivity potential of the five stocks under the effort-management regime outlined, given the current management objectives (excluding the ± 15% constraint on annual variation in catches). All stocks grew steadily except haddock, which decreased temporarily until 2007 as a consequence of the strong 1999 year class being fished out. However, with a 10% bias attributable to non-compliance, the cod stock did not recover and the SSB of other species hardly improved relative to the present. With a 20% bias, the cod stock disappeared and estimates of SSB for all stocks dropped substantially, and projected catches largely followed the same pattern (Figure 2b). Specifically and without a non-compliance bias, annual catches of cod grew steadily, including during the recovery phase, until 2010. The technical interactions of the main fleets in terms of catch composition were responsible for marked initial reductions in the catches of other stocks, whereas the large reduction in haddock catches was driven by a declining abundance of the 1999 year class. Under the simulation, cod catches rose to 250 000 by 2015, and catches of haddock, saithe, and plaice stabilized at around 40 000, 70 0000, and 90 000 t, respectively. The interaction between plaice and sole in the beam trawl fleet constrained the catches of sole below 10 000 t. Even a 10% bias attributable to non-compliance would markedly reduce catches of all species relative to current levels.
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Driven by the F-reduction and SSB constraints in the cod recovery plan (no bias scenario), F on all other stocks simulated reduced quickly from the starting values in 2005 (Figure 3) to below the species-specific targets and at a faster rate than intended in the management plans (>10% per year; Table 1). The target F for cod (0.4) continued to be the major constraint on all demersal fisheries, even after cod had recovered, resulting in underutilization of the other four stocks owing to their technical interactions with cod.
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The effort-management system simulated caused changes in the distribution of fleet-specific relative effort (Figure 4) and the associated catches (Figure 5). As intended, the fleets most affected were those characterized by relatively large catches of the most overexploited species. Large-mesh demersal trawls (
100 mm) contributed most to the required effort reductions until the cod recovered in 2010. A similarly large effort reduction applied to beam trawls (80 mm), which affected both cod and plaice. Effort of medium-mesh demersal trawls (70–99 mm) and static gears had to be reduced initially by about 30%, because their impact on cod and plaice is quite low. The more selective fleets using longlines and other gear benefitted initially, and their effort initially increased. After the projected cod stock recovery by 2010, there were gradually fewer fleet-specific effort changes.
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The trajectories of the projected fleet-specific catches (Figure 5) indicated immediate, substantial declines for the fleets of beam trawls (
80 mm) and demersal trawls (100 mm). In contrast, catches of static gears and medium-meshed demersal trawls (70–99 mm) appeared to be more stable until 2010 and, subsequently, to increase. Compared with early years in the simulations, the catches of fleets deploying longline or other gears almost exclusively for cod increased substantially as a consequence of both their increase in effort and cod recovery. |
Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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TAC regulations based on single-stock considerations alone appear insufficient for the effective management of mixed fisheries, because they act as an incentive for discarding and misreporting of landings that are in excess of the allocated quota (Cotter et al., 2002; Daan et al., 2005). To adjust TAC regulations to the potential catches of fleets engaged in mixed fisheries, either a dictate for landing all catches or a fleet-based effort management regime appears necessary. Closed areas are often applied to protect endangered species or habitats but, applied in a fisheries management context, would still require appropriate regulations on effort (Horwood et al., 1998; Dinmore et al., 2003).
Fleet-based management might be an effective and malleable tool for controlling F at sustainable levels, in the case of mixed fisheries through fleet-specific effort regulations, to avoid high rates of discarding and unallocated landings (Shepherd, 2001). However, effort management requires predefined, transparent management goals, and control rules. Any attempt to realize effort management immediately leads to the question, what are "good" fishing practices? A variety of adequate fleet-specific parameters and numerical procedures are conceivable, including different weightings of catch compositions. Available model approaches have recently been reviewed by ICES (2006b). Vinther et al. (2004) applied recommended rates of exploitation for the stocks under consideration in combination with stock-specific weighting factors to provide effort management advice based only on short-term management goals.
Our method considers the sum of the fleet-specific effects on jointly caught species, including ecological quality goals regarding recent stock status in relation to reference values (UN, 1995; Piet and Rice, 2004). Such a strategy promotes fleets deploying highly selective gears for undepleted stocks. This is balanced by an effort reduction in fleets deploying non-selective gears taking large proportions of their catches from overexploited stocks. Note, though, that the method proposed is based entirely on arbitrary choices of parameters and operations, which are available and considered meaningful for fleet-based effort management. Although the ultimate goal is to attain maximum catches within the constraints set by various stock conservation objectives, the agreed maximum annual variation in TACs could not be implemented, because these would imply conflicting decision rules, when they are exceeded simultaneously for more than one species.
An appropriate definition of fleet is a critical issue and probably requires more than a simple classification based on a quantification of their effects on the stocks (i.e. mainly ecological considerations). Fisheries-related economic parameters and arguments have been disregarded here largely because of a general lack of relevant information.
A disadvantage of the method is the assumed precision of stock assessments. Also, only stocks that have been assessed analytically by age, and for which precautionary reference points for SSB and F have been defined, are included. A variety of taxa taken in these gears (all invertebrate and vertebrate bycatch species, but also target species such as whiting and Norway lobster) do not fulfil these requirements and, therefore, were not considered.
The application presented here is intended to demonstrate the operational behaviour and medium-term consequences of fleet-based effort management. Major underlying assumptions are that the productivity of the stocks in terms of natural mortality, growth, and recruitment does not change relative to observations during the past 40 years. Recent analyses of the impact of global warming suggest major changes in stock productivity (Beaugrand, 2004; Cook and Heath, 2005; Drinkwater, 2005) and distribution patterns (Perry et al., 2005), though Kell et al. (2005) emphasize the robustness of stock productivity under appropriate exploitation strategies, taking into account effects induced by climate changes.
The data used here were compiled from various databases developed for providing routine management advice in the ICES area. However, grave concerns have been expressed repeatedly about the uncertainties in these databases, which would undermine their application for management purposes (Stratoudakis et al., 1999). For instance, the fleet data have been compiled for just a single year, and it seems doubtful that these could reasonably be extrapolated over a 20-year period. Slightly different extensions of the management areas for the five demersal stocks considered in the analyses have been ignored. However, for a comprehensive effort-management strategy, such discrepancies should be resolved.
The simulation results suggest that, assuming full compliance with regulations, the cod stock could recover by 2010. Until then, however, the fisheries would have to suffer from marked reductions in catch and effort. Thereafter, stocks are predicted to continue to increase to levels comparable with those observed in the mid-1960s. However, exploitation rates would have to remain below the sustainable target levels, after initial drastic reductions, except for cod. The technical interactions of all fleets catching cod cause underutilization of other demersal stocks by 30–50%. This undesirable situation might be averted through the development of more selective fishing strategies, by considering technical as well as geographical and seasonal measures (Madsen et al., 1999). Despite technical interactions between different roundfish fisheries, it is possible to take almost clean catches of a single species on specific fishing grounds (Weber, 1999). Further, the initiative to use sorting grids in Norway lobster fisheries to avoid fish bycatches should provide scope for better utilization of that valuable resource.
Global management targets in terms of MSYs to be attained by 2015 were formulated in the Johannesburg declaration (UN, 2002). Based on the simulations here, realization of these targets does not appear to be unrealistic, if appropriate measures were implemented and respected. However, even a bias as low as 10%, introduced to simulate non-compliance with the regulations, delays the cod stock recovery considerably and will not improve medium-term yields anywhere close to the maximum sustainable level (Horwood et al., 2006).
In the North Sea mixed fishery, the fleets operating demersal trawls (100 mm) and beam trawls (80 mm) would have to be cut more than those using medium-size-meshed demersal trawls (70–99 mm) and static gears. In contrast, but consistent with the assessment of good fishing practice, longline effort and the use of other gears might be increased because of the balancing element in the relative effort factor proposed. The model results appear similar to the fleet-specific effort limitations enacted since 2003 (CEC, 2002, 2003, 2005b, 2006). After cod recovery, annual effort adjustments for all the fleets defined here become gradually smaller, but they remain controlled by constraints set by the target F for cod rather than any other management or recovery plan. Such dominance of cod-related targets in the management of demersal stocks is to be expected because the historical abundance of cod, and its wide distribution, are responsible for the strong technical interactions among these fisheries.
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Acknowledgements |
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We are indebted to Chris Darby, Ewen Bell, Coby Needle, Steven Holmes, Rick Officer, Franck Coppin, Per Sparre, Stuart Reeves, Willy Vanhee, Martin Pastoors, Rob Grift, and Joakim Hjelm for their valuable contributions and comments during hard ICES and STECF meetings. The manuscript benefited from the many comments by Niels Daan and Sarah Kraak. The work itself and many of the related events and evaluations were financially supported by the Commission of the European Community, DG Fish.
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References |
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