© 2004 International Council for the Exploration of the Sea
The consequences of different scenarios in the management of the gillnet and purse-seine fisheries targeting Pomatomus saltatrix and Cynoscion guatucupa in southern Brazil: a bio-economic approach
CEFAS Lowestoft Laboratory Pakefield Road, Lowestoft, Suffolk NR33 OHT, England, UK
*Correspondence to F. Lucena: Present address: UFPA, Departamento de Oceanografia, Centro de Geociências, Campus do Guamá, C.P. 8617, Belém, PA, Brazil, Cep: 66073-110; tel: +55 91 31831983; fax: +55 91 31831608. e-mail: flucena{at}ufpa.br.
The bluefish Pomatomus saltatrix and the striped weakfish Cynoscion guatucupa are two of the most important commercially exploited species in southern Brazil. These species overlap spatially and temporally in southern Brazil and are exploited by a multi-purpose fleet. We analyse the consequences of different management options in terms of the bio-economy of the gillnet and purse-seine fisheries by considering P. saltatrix and C. guatucupa as the main targets. Development of the bio-economic model was based upon multi-fleet, single species models for P. saltatrix and C. guatucupa. The results are compared at the medium-term forecast level from stochastic simulations, and the technical interactions between the fleets that exploit the bluefish and striped weakfish are also addressed. The purse-seine fishery has a strong impact on the gillnet fishery. A management plan, which allocates moderate levels of effort for all co-existing fleets and interacting species, seems efficient, but effort should be controlled for P. saltatrix (for both fleets) and, for the gillnet fleet, relatively more trips should be allocated for C. guatucupa. For P. saltatrix, the best management relies upon the scenarios harvesting the oldest individuals.
Keywords: bio-economic, multi-purpose fleet, southern Brazil, technical interactions
Received 18 February 2003; accepted 26 November 2004.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The objective of fisheries management in most countries is to ensure the continuing existence of the resource, whilst improving the economic and financial status of the fishing operators (Pascoe, 2000). Although fisheries management is mainly dominated by biological considerations, during the last decade, new trends in fisheries science focused on integrating various intrinsic relationships within and between the different components of the fishery, i.e. the resources and the fishers (Ulrich et al., 2002).
The interrelation between fish stocks may rely on biological (such as predator–prey and competition relationships) and technical interactions. The latter may include (i) harvesting by different fleets at different stages in the life cycle of the fish stock; (ii) simultaneous or nearly simultaneous harvesting by different fleets; or (iii) sequential harvesting by the same fleet but at separate times (Charles and Reed, 1985). The technical interaction arising from co-existing fleets exploiting the same resource may involve conflicts between users of the fish resource, and management must consider that fisheries of the same stock are linked through their exploitation (Charles and Reed, 1985). Technical interactions have traditionally been promoted separately from biological interactions among species. Recently, these interactions have been incorporated into bio-economic models (Pascoe, 2000; Ulrich et al., 2001, 2002).
The bluefish Pomatomus saltatrix (L.) and the striped weakfish Cynoscion guatucupa (Cuvier) overlap spatially and temporally in the area. Also, the same boats exploit both species sequentially or simultaneously, but target one species or the other on a given trip. For gillnets, the allocation of effort towards one or other species depends on a combination of factors such as species availability and market price (Lucena, 2000). Moreover, each of the two species is a bycatch within the targeted gillnet fishery of the other, which make them technically dependent (Lucena et al., 2000).
Pomatomus saltatrix and C. guatucupa are two of the most important commercially exploited species in southern Brazil, with annual landings over the last decade ranging from 585 to 5400 t and from 2800 to 12 400 t, respectively (Lucena, 2000). Pomatomus saltatrix is exploited by drift gillnets with a stretched mesh size of 90 mm and by purse-seines (Lucena and Reis, 1998). The species is caught mainly from June to September (96% of annual catches), in the subtropical coastal waters of Rio Grande do Sul. C. guatucupa is caught throughout the year by trawlers, and during the autumn and winter, by bottom gillnet with a stretched mesh size of 90 mm (Vieira, 1990; Boffo, 2000). The percentage of P. saltatrix and C. guatucupa landings (by weight) within the overall gillnet annual fleet catch are approximately 11% and 28%, respectively. The greatest proportion of catches comes from the targeted Micropogonias furnieri fishery (36% of catches by weight) carried out from October to January (Lucena, 2000). For the purse-seine fishery, P. saltatrix is 64% (by weight) of the annual catch (Lucena, 2000).
In southern Brazil, a number of species have been identified as either over- or intensively fished which has resulted in fleet reductions (Reis, 1992; Haimovici, 1998; Boffo, 2000). For P. saltatrix, there is a reported decrease in average fish length during the years 1992–1998, an increase of 87% in the length of netting deployed and 166% in the setting period (Lucena and Reis, 1998; Lucena, 2000). Also, the spawning-stock biomass (SSB) decreased from 23 to around 2.5 thousand tonnes and the exploitation patterns increased (Lucena et al., 2002). For C. guatucupa, the SSB has decreased sharply and exploitation rates have been increasing through time (Lucena, 2000). The number of boats has been decreasing from 1991 to 1998 in most southern Brazilian fleets: 73% for purse-seines, 50% for trawlers, and 11% for gillnets (Lucena and Reis, 1998; Lucena, 2000).
In this study, we analyse the consequences of different management options based on the bio-economics of the gillnet and purse-seine fisheries where P. saltatrix and C. guatucupa are the main targets. We use numerical multi-fleet, single-species simulation to compare the performance of a discrete set of management scenarios defined by economic, social, and biological objectives.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Data
Annual number of trips of the purse-seine fleet, in which catch depends on effort, is adopted as the unit of effort for the present study. The annual number of trips for the study period, 1992–1998, varied from 14 to 90, with the length of trip varying from one to five days. The number of boats operating within the fishery varied between 14 and 43 for the period 1992–1998. The purse-seine fleet in 1998 comprised of 14 boats.
The total trips per year targeting P. saltatrix or C. guatucupa was regarded as the unit of effort for the gillnet fleet. During the period of 1992–1998, the annual number of gillnet trips targeting P. saltatrix varied from 69 to 342, and the number of gillnet trips targeting C. guatucupa varied from 219 to 550. The number of boats operating within the fishery remained relatively constant and in 1998, the gillnet fleet comprised 14 boats. For both species, trips are limited by the boat storage capacity and are typically no longer than three days with between one and three sets per trip.
For trawlers, the number of trips was also adopted as the unit of effort. The annual number of trips for the period 1993–1998 varied from 398 to 792; with the length of trip varying from 8 to 19 days (fish are kept on ice in the hold). The number of boats operating within the fishery varied from 80 to 126 from 1992 to 1998. Pair trawlers consisted 96% of southern Brazilian trawl fleet and effort.
The biological component
Development of the biological component of the model was based upon multi-fleet, single species models for P. saltatrix and C. guatucupa, described by Lucena (2000) and Lucena et al. (2002). For P. saltatrix and C. guatucupa, the exploitation is gear specific and selectivity coefficients are derived by considering the effect of the different gears on the stock structure. The catch-at-age model requires data on numbers-at-age and fishing effort by gear and by year and an assumption about natural mortality. To consider a seasonal model (as for P. saltatrix) it is also necessary to have knowledge of the fishery distribution throughout the year. A catch-at-age model was implemented in the program AD Model Builder (Fournier, 1996).
The biological model was applied for the southern Brazilian stock of P. saltatrix for the period 1992–1998. Let i be the index year, j the index age class, and g the index gear. Denote catchability by gear (qg) and the selectivity coefficients by age and gear (Sg(j)). Assume that the instantaneous fishing mortality rate, Fg(ij), is given by:
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| (1) |
g(i) are deviations from the expected relationship between the observed fishing effort and the resulting fishing mortality in year i. This term is modelled as a random error term.
The natural mortality rate M is assumed to be 0.25 for all age classes. This value was obtained from averaged estimates derived from the empirical formulae of Pauly (1980) and Alagaraja (1984), and has been used for the US Atlantic P. saltatrix (Buckel et al., 1999). For P. saltatrix, to calculate the total mortality (Z) attributable to exploitation and natural causes, the seasonality concept is incorporated. Let f be the index fishery season and o the index off season, and Fl(ij) and Fp(ij) denote the fishery mortality by year and age for gillnet and purse-seine exploitation, respectively. The fishery (F(ij) = Fl(ij) + Fp(ij)) for southern Brazilian P. saltatrix occurs from June to September and total mortality is obtained accordingly (Zf(ij) = F(ij) + (M/3)). During the rest of the year, total mortality is restricted to the natural (non-fishery) causes (Zo(ij) = M x 2/3). The beginning of the fishing year will be considered as 1 June. Total mortality by year and age class is given by:
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Given estimates of the initial population in June of year 1, N 1,1, N 1,2..., N1,j, and knowledge of recruitment in each year, the seasonal exponential decay equation yields:
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The SSB at the beginning of the year is calculated through the equation:
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| (6) |
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Parameter estimation is based on maximum likelihood with catches assumed to follow a Normal distribution, with likelihood functions given by:
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| (9) |
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| (10) |
The biological model was applied for the southern Brazilian stock of C. guatucupa for the period 1993–1998. The model for C. guatucupa is similar to that used for P. saltatrix, but as C. guatucupa is exploited throughout the year (by trawlers; gillnets only operate from May to August), the analysis becomes simpler. For the southern Brazilian C. guatucupa, estimates derived from the empirical formulae of Pauly (1980) and Alagaraja (1984) vary from 0.13 to 0.38, and M=0.20 was assumed as reasonable in view of the life history of the species.
As no seasonality is incorporated, Equation (2) of total mortality (Z) simplifies to give:
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The exponential decay equation is simply:
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The SSB at the beginning of the year on 1 January is calculated as in Equation (5) and the weights-at-age for C. guatucupa are derived from the equations Lt = 501 (1 – e–0.25 (t–0.19)) (Vieira and Haimovici, 1993) and Wt = 3 x 10–5 x Lt2.88 (Lucena, 2000). The proportion of mature fish by age is to that given by Vieira and Haimovici (1997). Mature individuals are found from age 1.
The predicted catch by gear (t, l), year i, and age j for C. guatucupa is given as in Equations (6) and (7) for P. saltatrix, but where catches for the purse-seine are substituted with trawler catches. Parameter estimation is based on maximum likelihood, given by maximizing the sum of the independent log-likelihoods log L1 + log L2 + log L3 as for Equations (8–10) but also catches for the purse-seines are substituted with trawler catches.
The economic component
The economic model used comprises an economic analysis of part of the year (June to September – indexed as Jun–Sept) when P. saltatrix and C. guatucupa are the dominant catches. Data on costs and prices were derived from catch records obtained from coastal landing sites during the entire years of 1995 and 1996. These include information on the price of target and bycatch species, and costs for the gillnet and purse-seine fleet collected directly from fleet managers. A total of 238 trips for the gillnet fleet (23% of the total number of trips for the years 1995 and 1996) from 22 different boats and 10 trips for the purse-seine fleet (16% of total number of trips for the years 1995 and 1996) from two different boats were obtained. All prices and costs are converted on a yearly basis from the Brazilian Real (R$) to the American dollar ($) in order to facilitate comparisons among years.
It is expected that prices might vary within a fishing season. In the southern Brazilian gillnet and purse-seine fisheries, price does not vary with catch volume and it has been assumed that prices, for both P. saltatrix and C. guatucupa, were constant with respect to quantity landed and months of the fishing season. However, prices by species do vary with age (but not with gear), as larger fish are more valuable than smaller ones. The bycatch for both P. saltatrix and C. guatucupa for gillnets are also considered as potential revenue but, for each species incidentally caught, an average price was assigned for all age classes. The bycatch of the targeted bluefish and striped weakfish gillnet fleet was obtained from catch records from coastal landing sites collected directly from fleet managers for the period 1994–1997. These comprised the specific catch of 1050 trips operated by 25 gillnet boats. For the P. saltatrix target fishery, an average of 17% (by weight) of the total catch is bycatch, and C. guatucupa is a common incidentally caught species. The bycatch of the targeted C. guatucupa is mainly Argentine croaker Umbrina canosai, codling Urophycis brasiliensis, P. saltatrix, elasmobranchs and white croaker M. furnieri (21% of the total catch; Lucena et al., 2000). The value of the catch in the purse-seine fleet is merely that of the targeted fishery, and the purse-seine bycatch is considered irrelevant (Lucena, 2000).
Let PPs, PCg index the prices derived from the targeted P. saltatrix and C. guatucupa, respectively. Prices vary from age 1 to Jl and Jp (for the gillnet and the purse-seine fisheries, respectively). Let Cbycatch be the index incidental catch, and Pbycatch be the index price. Subscripts Ps and Cg refer to P. saltatrix and C. guatucupa, respectively. The number of species in the bycatch varies from 1 to S (SPs and SCg index the maximum number of species in the bycatch of the gillnet fishery targeted for P. saltatrix and C. guatucupa, respectively). Revenue, or gross sales, correspond to the total annual value of the catch for the entire fleet. The revenue (Rl(i)) in the gillnet fishery is given by:
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Revenue, or gross sales, for the purse-seine fleet is given by:
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For both fleets, the analysis of variable costs is considered for the whole fleet rather than for individual boats. The gillnet fleet is considered as relatively uniform in size, characteristics, and behaviour (Lucena, 1997), and variable costs are similar for all trips that operate with this gear. For the purse-seine fleet (targeting P. saltatrix), variable costs can vary between trips, as fishing grounds are located in waters between 8 and 90 m deep. Hence, costs such as fuel and oil would increase if the fleet harvests the oldest fish in the deeper water. However, the relative increase of the total variable costs for the purse-seine attributable to the fleet operating in deeper water could not be quantified and, owing to the lack of precise data, it will be considered as an average value.
The variable cost (VC) is the operating cost of the boat by trip (effort, E). Effort is considered the total number of trips by year for the fishing season. For the gillnet, it is the sum of the variable cost attributable to the P. saltatrix's targeted fishery and C. guatucupa's targeted fishery. Variable costs for the gillnet and purse-seine fleet are given by:
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Crew costs for the entire fleet are based upon the share system between the owner and the crew. For the gillnet fleet, the crew share is 50% of the revenue minus the variable costs, while for the purse-seine fleet crew share is 40% of the revenue minus the variable costs. Fixed costs (FC) are reported as annual costs and do not vary according to the level of fishing activity (Hanesson, 1993). They depend upon the characteristics of the fleet and may vary between boats of the same fleet (Coglan and Pascoe, 1999). Fixed costs are considered to include an annual payment for insurance and an allowance for economic depreciation of the boat. For the calculation of the fleets' fixed costs, the fishing season is only during one-third of the fishing year (for both the gillnet and purse-seine fleet). For both gears, it is assumed that fixed costs do not vary between boats and an averaged value is adopted.
The annual allowance for fixed costs for the P. saltatrix and C. guatucupa fishing season (for both fleets) is represented by :
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Two types of indicators of economic performance are calculated in the model: profit (PF) and rate of return of capital (RR). Profit for the entire and both fleets is given by:
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The rate of return of capital is calculated by dividing the profit by the value of capital invested in the fishery (I). The value invested in the fishery is the sum of the present value of the boat, gears, and licenses. The rate of return of the capital can be compared to an expected rate of return to determine whether or not the boat is covering the opportunistic cost. The opportunistic cost of the capital can be defined as the value of the goods that could be produced if the harvested resources used in the fishery were used for a different purpose (Clark, 1985). In this study, this different purpose is considered as the capital obtained if the money invested in the fishery were to have been invested in a bank. Rates of return greater than the opportunistic cost of capital are generally indicative of the level of resource rent in the fishery. As for the calculation of the fixed costs, the rate of return of the capital and opportunistic cost of the capital are considered for the fishing season accordingly. The rate of return of the capital is given, for both fleets, from June to September as:
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Prediction of socio-economic indicators of performance in the medium term
Advice was compared using medium-term forecasts derived from stochastic simulations to investigate the performance of a discrete set of management scenarios in terms of a number of economic, social, and biological objectives. The economic component of the model is largely driven by the level of effort and the selectivity coefficients, which are transformed into catches, revenues, and profits. Within the biological sub-models, for simulating catches and future SSB in the medium term (here considered as the year 2005), different scenarios were conducted with two constraints: (i) varying the number of trips allowed by fleet within a fishing season (effort) and (ii) varying the selectivity coefficients by gear (technical) (Table 1). For P. saltatrix, only scenarios where exploitation relies on the oldest ages (at least for one gear) or as it was reported in 1998 (ages 3 and 4) were considered suitable for the maintenance of the SSB in the medium term (Lucena et al., 2002). For C. guatucupa, only one scenario concerning the gillnet selectivity (the status quo situation) is considered, since varying the selectivity coefficients for the species towards either the youngest or the oldest ages did not significantly decrease (or increase) catches, in the medium term (Lucena, 2000). Trawler selectivity was considered for the biological model but not tested for the economic component. This choice is based on the fact that trawlers are multispecific and operated widely (up to 200 m deep). Any attempt to propose management options, which would restrict the area of operation, would have to consider also the distribution and availability of the other targeted species.
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For both species, in southern Brazil, the stock-recruitment relationship is not obvious, as time-series are too short to detect a pattern. Hence, for medium-term advice, the recruitment reported for the years of the analysis was randomly resampled (20 times with replacement) for 1999 up to 2005. Mean estimates of catches and SSB from different simulations were converted into profits under different selectivity and effort scenarios. Results are compared at the forecast year 2005. Considering each simulation; namely, the combination of effort for each fleet; the randomly resampled recruitment was kept constant for each of the selectivity scenarios. This would allow the comparison of selectivity scenarios under the same effort simulation.
For simulating catches and future SSB under different management options, the year 1998 was taken as the last estimated year of analysis. For this study, the number of boats is assumed fixed at the level of 1998. The maximum likelihood estimate for the predicted SSB in 1999 using the exploitation level of 1998 was 7900 thousand tonnes (Table 2). For the purpose of management options, which would ensure sustainability of the stock, we have assumed that a nominal level of SSB of 7900 t must be at least maintained (Lucena et al., 2002). Considered next for the economic purpose is effort control, which will achieve this target. The stock of C. guatucupa has been sharply decreasing over the past six years (from 250 thousand tonnes in 1993 to 68 thousand tonnes in 1998). The predicted SSB for 1999 based upon the 1998 exploitation rate was 50 thousand tonnes (Lucena, 2000). Considering the SSB trend for this species, and aiming to ensure the sustainability of the stock, we have assumed, for the economic analysis that effort, for the gillnet, should be decreased by 25% or at least maintained (Lucena, 2000)
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Specifically, we explore four options: (i) Allocating different levels of effort for the gillnet fleet towards C. guatucupa only (P. saltatrix's gillnet fishery is banned) with a maximum number of trips allowed for the gillnet (550 trips per year) and for the purse-seine fleet (50 trips a year); (ii) Allocating different levels of effort for the gillnet fleet towards P. saltatrix only (C. guatucupa's gillnet fishery is banned) with a maximum number of trips allowed for the gillnet targeting P. saltatrix (200 trips per year) and for the purse-seine fleet (30 trips a year); (iii) Allocating different levels of effort for the gillnet fleet towards P. saltatrix and C. guatucupa (up to 800 trips a year). The purse-seine fleet targeting P. saltatrix is not allowed to operate; and (iv) Co-existing fleets. Both the gillnet (P. saltatrix and C. guatucupa) and the purse-seine (P. saltatrix) fleets are allowed to operate at different levels of effort. In the gillnet fishery, a maximum of 750 trips a year can be allocated for both P. saltatrix and C. guatucupa. For the purse-seine fleet, a maximum of 30 trips per year is allocated towards P. saltatrix.
Level of technical interaction between gears
Measuring the technical interactions among gears is a way to determine which is the most important fishing activity for the species (according to Ulrich et al., 2001). We assess the technical interactions in terms of profits for P. saltatrix only. The sensitivity coefficient (scPF), represents the decrease in profits, when the other gear increases its effort by 1% (fg = 1, fother gear = 1.01).
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Results |
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The economy of the gillnet and purse-seine fleet
For the period 1993–1998, from June to September, losses of up to $3100 per boat per year (especially for 1998) were reported for the purse-seine fleet. Gains ($5300) were only recorded for the year 1995. The fishing industry claims the low profitability for the decrease in the number of boats by 63% from 1991 to 1998. For the gillnet fleet, boats have also been leaving the fishery, but at a moderate level of 11% from 1991 to 1998. No losses were reported for the gillnet fleet for the period 1993–1998, but from 1996, there is a reported decreasing trend in profit compared with previous years. As for the purse-seine fleet, there was a reported maximum profit per boat in the year 1995 (approximately $7000 per boat per year), and minimum profit for the last years of the analysis 1997–1998 (approximately $926 per boat per year).
Level of technical interactions among gears
The levels of effort (and selectivity) exerted by one gear strongly influence the economic performance of the other gear. The purse-seine fishery has a strong impact on the gillnet fishery. A change in effort for this gear will have a large effect on profit for the gillnet fleet (Figure 1), especially for Scenarios 1 (gillnets towards the oldest and purse-seine selectivity as in 1998 – ages 2 and 3) and 8 (gillnets towards the oldest and purse-seine towards the youngest). In contrast, the gillnet fishery has little impact on the profit of the purse-seine fleet.
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Prediction of socio-economic indicators of performance in the medium term
Banning the gillnet fishery from targeting P. saltatrix and allocating the gillnet effort towards C. guatucupa would not give rise, for the gillnet fleet, to a positive profit in any situation in the medium term (2005) (Table 3). Losses for the whole gillnet fleet would vary from approximately $27 000 (Simulation 2) to $67 000 (Simulation 1), whereas high profits for purse-seine fishers for all scenarios and effort levels are reported for 2005 (Table 3). For the purse-seine fleet, the rate of return of capital is well above the opportunistic cost of the capital for all scenarios and effort simulations (Table 4). However, if maximum selectivity is maintained at the ages fished in 1998 (ages 2–4), there are lower profits than with those scenarios targeting the oldest ages. For all scenarios, the situation of banning the gillnet fishery from targeting P. saltatrix would lead to a minimum value of SSB for the species of 8500 t (Table 5).
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With the stock of P. saltatrix over- or intensively exploited (Lucena et al., 2002), the gillnet fleet has, in recent years, allocated most of its effort towards C. guatucupa, even if this species is less valuable. Thus, closing the fishery towards C. guatucupa and allocating all the gillnet effort toward P. saltatrix gives rise to low profits (Scenario 1) or losses (Scenarios 3 and 9) for the gillnet fleet (Figure 2). At high levels of effort (200 trips a year for the gillnets), especially for Scenarios 1 and 6, profits are positive and the rate of return of the capital covers the opportunistic cost of the capital (Table 4). Profits for the whole gillnet fleet are well below that obtained in 1997–1998. Purse-seine profits are not affected by these scenarios. For all simulations and effort scenarios, SSB for P. saltatrix is over 7900 tonnes (Table 5).
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Closing the purse-seine fishery targeting P. saltatrix gives rise to high levels of profit for all scenarios and effort levels for the gillnet fleet (Figure 3). In all cases, by the end of the management plan (2005), the fishing industry (and fishermen) would have the greatest benefit if the fishery was directed towards the oldest ages for P. saltatrix (Scenarios 1, 6, and 8). For all simulations of effort and scenarios, the rate of return of the capital is well above the opportunistic cost of the capital (0.036%) (Table 4) and profits, for the whole fleet, may be up to $434 000 (Simulation 6, Scenarios 1,6, and 8; Figure 3). Compared to the last years of the analysis (1997–1998), profits would be higher (at various level of effort) for scenarios directed towards the oldest ages. For the purse-seine fishery, the banning would give rise to an absolute loss of $296 000 for the whole fleet (RR = –0.094). Eliminating the purse-seine fishery heads to high levels of SSB for P. saltatrix (Table 5).
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Co-existing fisheries gives rises to acceptable profits and rate of return of the capital higher than the opportunistic cost of the capital for all scenarios and simulations tested for both fleets, expect for Scenarios 3 and 9, where most of the simulations imply a rate of capital return and profits as low as $98 000 and $97 000 for the gillnet and purse-seine fleets, respectively (Figure 4 and Table 4). Adopting Scenario 6 (both gears towards the oldest ages for P. saltatrix) as an option gives rise to higher profits at the same level of effort for the medium term (2005), when compared with other scenarios and the years 1997–1998. This is also reported for the gillnet fleet.
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Discussion |
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The addition of economic concepts to the biological approach for population management provides a powerful tool for understanding the forces that drive the fishery activity and simulating the effects of management propositions (Castro et al., 2001; Ulrich et al., 2002). According to Padilla and Charles (1994), "the value of bioeconomic modelling can be judged both by its generation of useful theoretical insights into the operation of fisheries systems, and by its application to real-world fisheries".
The decreasing status or depletion of some commercial fish stocks in southern Brazil is affecting the financial status of the fishers who are selling their boats to fishers in more productive areas of the country (e.g. the state of Santa Catarina in the north of Rio Grande do Sul (Boffo, 2000)). This is leading to increased levels of unemployment in the local communities, as fishers, boat owners, and managers are not finding other employment. Frequently, regulation will only be adopted when fish stocks and the fishing industry become so depressed that the need for action is truly compelling (Rettig, 1987). The southern Brazilian fish stock is basically unmanaged at present, i.e. there is no effort control and the current legislation (minimum landing size and closed season) is not fully respected.
Because biological and economic aspects are complementary (Pascoe, 1997), the priority in the management options is to rebuild the parental stock of P. saltatrix and to keep the level of SSB for C. guatucupa (Lucena, 2000; Lucena et al., 2002). Although these objectives are complementary, the biology (SSB) and the economy (profit) cannot be maximized at the same level of effort. Maximizing employment may conflict with biological and economic objectives (Mardle and Pascoe, 1999). In southern Brazil, it is not possible to maximize employment as it would imply a large number of boats operating. To prevent more boats from leaving the fishery, a prudent and efficient plan is required for southern Brazil. Effort control and the banning of fishing towards the juveniles of P. saltatrix are needed, especially if the management plan allows co-existing fleets to operate and interacting species to be exploited. Re-allocation of effort is essential.
Considering the socio-economic and the biological aspect of the fishery, it is not a reasonable option to reduce fishing capacity. Unless fishers are able to find suitable alternative employment elsewhere, the reduction in fishing capacity may be a serious social problem (Clark, 1985). For this reason, the banning of the purse-seine fishery targeting P. saltatrix is biologically the best solution for the threatened species, but only if fishers would be able to find other employment. At low levels of effort towards P. saltatrix, the gillnet fleet would have to allocate more effort towards the target species from October to February, M. furnieri (Lucena, 2000), and hence guarantee that the boats in the gillnet fleet would not have to leave the fishery. However, the stock of M. furnieri is showing clear signs of decline in abundance and there has been a trend of increasing effort to maintain catch levels (and hence profits) historically (Reis, 1992; Haimovici, 1998). A management plan, which allocates moderate levels of effort for all co-existing fleets and interacting species, seems practical and efficient. Effort should be controlled and reduced for P. saltatrix (for both fleets) and for the gillnet fleet, relatively more trips should be allocated for C. guatucupa at least until the parental stock of P. saltatrix is rebuilt.
In Tables 6 and 7, we select some of the best alternatives for the management of the stocks of P. saltatrix and C. guatucupa. For those options selected, the SSB of P. saltatrix is in a rebuilding phase and the SSB of C. guatucupa is maintained at moderate levels. Moreover, the purse-seine and gillnet profits (for the fishing season June–September) are higher than, or maintained at, those for the period before the management plan proposition, and the rate of return of capital covers the opportunistic cost. The maximization of all objectives (biological, social, and economic) is not an easy goal. However, we attempted to select the harvest strategies which achieve at least acceptable target levels for each objective rather than maximizing any single objective (Pascoe et al., 1997), thus equilibrating social, economic, and biological aims. The stock status, exploitation rates, number of boats, and costs involved in the fishery of both P. saltatrix and C. guatucupa must be monitored over the coming years, and the management plan adapted accordingly.
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This proposed management plan might be implemented in southern Brazil. Maximum numbers of trips allowed for the gillnet and purse-seine fleet may be divided by the number of licences allowed to operate with these gears. Moreover, considering that the behaviour of P. saltatrix varies with size, with small fish being found near the coast in shallow water and the larger fish being found offshore in deeper water (Lucena et al., 2002), controlling the minimum depth of operation for the gillnet and purse-seine fleets targeting P. saltatrix might allow regulation of selectivity. This would avoid the capture of juveniles, which is not feasible for the sustainability of the stock in the medium term.
Fish resources are an important source of food and their sustained use depends on the biology of the stock (Hanesson, 1993). However, economic analysis of fisheries increases our awareness that resources are capable of yielding high returns if utilized effectively (Arnasson, 1993). The understanding of fleet dynamics is essential for an unbiased assessment of the fish stocks involved. Some gains for the fishing industry, fishers, and species conservation, can be obtained by influencing the optimum distribution of effort for each of the species. This was the case with the re-allocation of effort for the gillnet fleet, from June to September, between P. saltatrix and C. guatucupa as the main suggestion for the management of these fish stocks in southern Brazil.
It should be stressed that the estimates obtained in this study may not represent their actual values. However, by using a scenario-based approach, clear management recommendations can emerge despite the potential complexity of the fishery and the poor level of knowledge of the stock. We recognize that, in this study, we dealt with part of the southern Brazilian fishery system. Additional analysis of the gillnet and the purse-seine fleets could be expanded and other targeted species from other fishing seasons included. A more complete, annual economic analysis of these fleets could then be undertaken.
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Acknowledgements |
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The authors are grateful to Gladimir Barenho for technical field assistance in Brazil and Sean Pascoe and Matt Dunn for comments on earlier drafts of the manuscript. This study was partially financed by CAPES ("Comissão de Aperfeiçoamento de Pessoal de Ensino Superior"), Brazil, through a grant to the first author and by the Ministry of Agriculture, Fisheries and Food, UK, with funding support (contracts MF0310 and MF0316) provided to the second author (CMO'B).
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