Long-term impact of incidental catches by tuna longlines: the black escolar (Lepidocybium flavobrunneum) of the southwestern Atlantic Ocean

Fisheries Research 58 (2002) 203–213

Long-term impact of incidental catches by tuna longlines: the black escolar (Lepidocybium flavobrunneum) of the southwestern Atlantic Ocean Andre´s C. Milessia,b, Omar Defeoa,* a

Laboratorio Biologı´a Pesquera, CINVESTAV-IPN Unidad Me´rida, AP 73 Cordemex, Me´rida 97310, Yucata´n, Mexico b Instituto Nacional de Pesca, Constituyente 1497, 11200 Montevideo, Uruguay Received 12 January 2001; received in revised form 17 July 2001; accepted 2 August 2001

Abstract We analyze intra and inter-annual harvesting patterns of the black escolar Lepidocybium flavobrunneum (Gempylidae), an important component of the incidental catch of the Uruguayan tuna fleet (UTF) at the southwestern Atlantic Ocean (SAO), based on daily information of catch, fishing effort and individual weight of all the specimens caught during 16 years (1981–1996). The analysis also compares the activities of two fishing fleets that operated in two different periods of fishery development: category A, which comprises Japanese vessels that operated between 1981 and 1991, and category B, consisting of American and Spanish vessels that operated between 1992 and 1996. The relative representation of the total incidental catch significantly increased during the 16-year period of activity of the UTF, reaching 43% of the total catch in 1995. A recurrent seasonal pattern in fishing effort, catch and catch per unit of effort (CPUE) was observed, peaking in austral winter and spring. The daily number of hooks and total catch were significantly higher for category A, which exerted 2.7 times higher number of hooks and obtained catches 3.33 times higher than category B. Differing with the above trends, mean daily CPUE of a category B vessel was 16% higher than category A, which can be attributed to the increase of fishing power. The mean individual weight decreased almost 40% in 15 years, i.e., from 23.2 kg in 1982 to 14.1 kg in 1996, suggesting overexploitation risks of this incidental species. The effect of increasing fishing power and the effectiveness of management measures for large pelagics at the SAO are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Black escolar; Lepidocybium flavobrunneum; Incidental catch; Tuna fisheries; Southwestern Atlantic; Management; Harvesting patterns

1. Introduction Three main groups of species can be distinguished from fishing activities: target, incidental and discarded. The last two components are basically nontarget species incidentally captured by the fishing gear, even though some individuals of the target * Corresponding author. Fax: þ52-99-812334. E-mail address: [email protected] (O. Defeo).

species are discarded or released alive because of their size, condition, etc. Moreover, some incidental species have market value, generally lower than that of the target species, and thus they are commercialized (Hall, 1996). The majority of these species are rarely caught, but others constitute an important percentage of the total catch, which is recurrent over space and time (Alverson et al., 1994; Hall, 1996, 1998). Incidental catch constitutes an important source of uncertainty in both assessment and management of

0165-7836/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 3 6 5 - 4

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fish stocks. It is difficult to quantify because of the scarcity of statistical information about catch, effort and population features as individual weights and sizes. Given its secondary nature from an economic point of view, these species are left aside when planning management regulations. Nevertheless, as in the case of target species, fishing pressure affects the distribution, abundance and population structure of incidental species (Harris and Poiner, 1991; Alverson et al., 1994; Philippart, 1998) and thus overexploitation risks are high, especially under high fishing effort levels on co-occurring target species (Heessen and Daan, 1996). Pelagic longlines are passive-attractive fishing gears used to capture highly migratory species like tunas (Hoey, 1995; Bjordal and Løkkeborg, 1996). In spite of their relatively high selectivity, they are not excluded from bycatch and discard (Au, 1984; Boggs, 1992). In the southwestern Atlantic Ocean (SAO), longline fisheries are particularly relevant. Several international fishing fleets obtain excellent yields because of high concentration of tunas that massively migrate to this zone for feeding. The Uruguayan tuna fleet (UTF) operates with pelagic longlines within the 200 miles of jurisdiction in Uruguayan waters and also in adjacent international waters (Fig. 1). It began its activity in 1981 with freezing vessels and Japanese pelagic longlines made of cotton multifilament. In 1991, smaller vessels with fewer people on board replaced them. They process fresh marketed products and use ‘‘Florida’’ and ‘‘Spanish’’ longlines made of nylon monofilament. These vessels resulted in a

significant increase in the magnitude of incidental catch, and commercialized species already discarded by freezing vessels (Mora, 1996; Domingo et al., 1996). The black escolar Lepidocybium flavobrunneum (Gempylidae) is a cosmopolitan species distributed between 408S and 408N (Nakamura, 1981; Nishikawa and Warashina, 1988; Nakamura and Parı´n, 1993) and constitutes an important component of the incidental catch of the UTF at the SAO, which has swordfish and bigeye tuna as primary targets. The species is landed and has a commercial value between 800 and 2600 US$/ton (INAPE, 1996). Nevertheless, we lack information about biological and socioeconomic aspects of the species. This is of utmost importance if we consider that the black escolar and other components of the incidental catch could be subject to sequential overfishing and stock collapse even though they constitute secondary targets. In this work, we explore the following questions: (1) Has the relative importance of the total incidental catch of the UTF varied in the long-term? (2) Is it possible to identify long-term patterns of fishery performance variables related to the black escolar? (3) Is it possible to use long-term changes in the mean weight of the black escolar as an indicator of the status of the stock? (4) Has the fishing power of the UTF varied in the long-term? To this end, we carried out a long-term study of the fishing activities and population structure of L. flavobrunneum, based on daily information of catch, fishing effort

Fig. 1. Southwestern Atlantic Ocean showing Uruguayan and international waters where the UTF operates.

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and individual weight of all the specimens caught during 16 years (1981–1996).

generalized linear model (GLM), as applied to fisheries (Hilborn and Walters, 1992), was used in the following model: CPUEij ¼ CPUE11 ai bj eij

2. Materials and methods From 1981 to 1996, daily information from fishing vessels of the UTF logbooks were compiled on catch and effort. All the vessels of the UTF make one set per day, with an approximated set time of 10 h. Thus, set time was assumed constant for all vessels throughout the study period (Rı´os et al., 1986). Since the longline fishery is multispecies, it was impossible to discriminate between the effort effectively applied to each resource. To circumvent this, our database contains information only for trips in which the escolar was fished, i.e., we discarded data of those trips where the escolar was absent in the logbook. The catch per unit of effort (CPUE) was then estimated for each vessel as the ratio between the daily escolar catch and the corresponding total number of hooks. Mean values of CPUE, catch, fishing effort and individual weights (see below) were estimated per fishing set and vessel. Seasonal and annual estimates were calculated from individual observations. Data were tested for normality and homoscedasticity and compared by ANOVA procedures (Zar, 1996). The analysis was also stratified by vessel type (Table 1): category A comprises Japanese vessels that operated between 1981 and 1991, whereas category B comprised Florida and Spanish vessels that operated between 1992 and 1996. A multiple standardization of CPUE was made, using: (a) the two vessel categories and (b) the four seasons of the year as explanatory variables. The Table 1 Main features of UTF vessel categories analyzed between 1981 and 1996

Number of vessels Mean length (m) Longline Main line material Mean TRB Product type Period of activity Mean crew members

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Category A

Category B

12 47.8 Japanese Cotton multifilament 310.3 Frozen 1981–1991 21

8 26.7 Florida and Spanish Nylon monofilament 183.8 Fresh 1992–1996 13

where i and j are vessel type and seasons of the year, respectively, CPUE11 the catch per unit of effort obtained by category A vessels in season 1 (summer), considered as standard; ai the efficiency of category B vessels when compared with category A; bj represents the relative fishing power of autumn to spring when compared with summer; and eij the error term. Parameters were estimated by maximum likelihood and the significance of main effects was tested via the Wald statistic test. A logarithmic link function was used to relate the expected catch rates to the categorical predictors. A gamma distribution was used since the frequency distribution of the CPUE was skewed to the left (Gon˜ i et al., 1999). The population structure of the catch was reconstructed by each fishing vessel to its absolute total annual landings, and the corresponding mean individual weight w was obtained by the following equation: P wkm Ckm w¼ P Ckm where wkm is the mean daily individual weight for vessel k and set m and Ckm the total daily catch for vessel k obtained in the set m. Between-year comparisons were explored to examine temporal differences in population structure, reflected in the weight-frequency distributions (WFDs). The size structure of black escolar was determined by distinguishing normally distributed components in the WFDs, using a class interval of 5 kg. Afterwards, modal values were estimated by maximum likelihood through the routine NORMSEP (Gayanilo et al., 1996).

3. Results 3.1. Long-term trends Target and incidental catches showed similar long-term patterns, peaking respectively in 1984 (2521 t) and 1985 (925 t), decreasing thereafter till reaching a minimum in 1991. From 1992 onwards they increased again, following the increase of fishing

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The number of vessels reached a maximum of 10 in 1984, decreasing thereafter till reaching a minimum of one vessel in 1991; from 1992, it increased again up to six vessels in 1996 at the end of the study period (Fig. 3). The effective fishing effort significantly differed among years (ANOVA: F15;384 ¼ 38:59, p ! 0:01, Fig. 3), showing a clear decreasing trend from 1984 ð166 598  38 904 hooksÞ to 1993 ð4136  2144 hooksÞ, increasing slowly afterwards. Post-hoc LSD test identified four independent year groups: (a) 1985, 1986 and 1987; (b) 1984, with the maximum effort values; (c) 1983, 1988–1990 with intermediate values; and (d) the 1991–1996 period, in which category B operated. 3.2. Seasonal fluctuations

Fig. 2. (a) Annual values of the target and incidental catches (t) obtained by the UTF between 1981 and 1996; (b) regression estimates (95% CI) between annual incidental catch (%) and time of fishery development (t ¼ 1: 1981; t ¼ 16: 1996).

effort (Fig. 2a). The relative representation of the total incidental catch significantly increased during the 16year period of activity of the UTF (r ¼ 0:64, p ! 0:01), reaching a maximum in 1995, i.e., 43% of the total catch (Fig. 2b).

The long-term analysis showed a recurrent intraannual pattern in fishing effort (Fig. 4a), catch (Fig. 4b) and the corresponding CPUE (Fig. 4c). In 9 (effort and CPUE) or 10 (catch) of the 16 years, they peaked in austral winter (from July to September), the remaining years peaking in spring. These fishery performance variables showed an overall increasing trend from autumn to spring, thereafter decreasing until reaching a minimum in summer, when they always dropped down to the lowest levels. As a result, the three variables significantly differed among seasons (ANOVA, p ! 0:01).

Fig. 3. Long-term fluctuations in nominal (*: No. of vessels) and effective (*: No. of hooks  1 S.E.) fishing effort exerted by the UTF between 1981 and 1996. The vertical line separates the operation periods of categories A (1981–1991) and B (1992–1996).

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Fig. 4. Seasonal fluctuations of: (a) fishing effort; (b) catch; (c) CPUE of the black escolar between 1981 and 1996.

3.3. Analysis by vessel category The daily number of hooks deployed by each vessel of the UTF significantly differed among categories (ANOVA: F1;3530 ¼ 13251:22, p ! 0:001) between 1981 and 1996 (Fig. 5a). Category A used 1809  46 hooks=day, a value 2.7 times higher than those used by category B ð658  55 hooks=dayÞ. The mean daily catch of black escolar was also significantly higher for category A (ANOVA: F1;3530 ¼ 365:90, p ! 0:001, Fig. 5b), which caught 140  10 kg=day, a value 3.33 times higher than the daily catch of a category B vessel ð42  5 kg=dayÞ. Differing

with the above trends, mean daily CPUE per vessel did not differ between categories (Kruskal–Wallis test: H1;3532 ¼ 0:91, p ¼ 0:34, Fig. 5c). Moreover, mean (S.E.) CPUE value of a category A vessel was 0:069  0:005 kg=hook, while those from category B was 0:081  0:016 kg=hook, i.e., 16.1% higher than category A. 3.4. Multiple standardization of CPUE The GLM corroborated the trends observed above, showing a clear effect of time (Table 2). The base catch rate, defined for category A vessels that fished on

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Fig. 5. Mean (1 S.E.) daily values of: (a) effort; (b) catch; (c) CPUE for the 20 fishing vessels of the UTF that operated between 1981 and 1996. Category A: (*) vessels 1–12; category B (*): vessels 13–20. The vertical line separates the operation periods of both categories.

fishing ground A during summer, was estimated to be 0.04 kg/hook. The expected CPUE for category B vessels was 17% higher than the CPUE achieved by category A. Winter and spring were by far the most Table 2 Coefficients and associated statistics from a GLM fitted to CPUE data, incorporating a logarithmic link function and a gamma distributiona Parameter

Estimate

S.E.

Wald statistic

P

CPUE11 a b2 b3 b4

3.27 0.15 0.10 1.04 0.60

0.17 0.16 0.21 0.21 0.22

366.95 0.86 0.24 25.00 7.64

0.0000 0.3532 0.6219 0.0000 0.0057

a CPUE11: catch per unit of effort of category A vessels during summer; a: efficiency of category B vessels relative to category A; b2, b3 and b4: catch efficiency in autumn, winter and spring relative to summer. Significant results are highlighted.

productive seasons, with expected catch rates 2.84 and 1.81 times higher than in summer. Autumn had intermediate values, with an estimated catch rate 1.11 times higher than in summer. 3.5. Population structure Fig. 6 shows the annual WFDs of black escolar between 1981 and 1996. The distributions were unimodal in all cases, and the position of the mode, estimated by maximum likelihood, varied from 24.0 kg in 1982 to 15.8 kg in 1996. The mean individual weight, which roughly coincided with the mode, decreased almost by 40% in 15 years, i.e., from 23.2 kg in 1982 to 14.1 kg in 1996. The bivariate negative correlation between mean individual weight and time of fishery development was highly significant (r 2 ¼ 0:68, p < 0:0001), showing a marked long-term decrease of individual sizes (Fig. 7).

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Fig. 6. Black escolar. Annual frequency distributions of individual weights (kg) obtained by the UTF for the period 1981–1996. Note the different scales in the y-axis.

Fig. 7. Bivariate correlation and 95% CI of the regression between time of fishery development and annual mean individual weight of black escolar (t ¼ 1: 1981; t ¼ 16: 1996): categories A (*) and B (*).

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4. Discussion 4.1. Inter-annual patterns The long-term analysis showed a significant increase of the relative representation of the total incidental catch through time, reaching a maximum of 43% in 1995, i.e., one year before the end of the analyzed time series. This fact can be considered as a potential indicator of overexploitation of target species (Hall, 1996). A hypothesis to explain the relative increase in incidental catches of the UTF in the long term could be the occurrence of cascade effects and/or sequential depletion of primary targets. As these primary species are being overexploited, the fishery and the market shift targets onto formerly low-value species once captured incidentally, increasing their unit price and generating a new categorization of the targets (Crowder and Murawski, 1996; Caddy, 1999). Thus, the general trend worldwide is the overexploitation of species less valued in the market, i.e., secondary targets or components of the incidental catch (Hall, 1996; Daves and Nammack, 1998). 4.2. Intra-annual patterns Fishery performance variables of the UTF showed clear and highly recurrent seasonal oscillations over 16 years, with maximum values in winter and spring. In the case of catch and fishing effort, this trend was accompanied by an overall decline since 1984. In contrast, CPUE did not differ between years and showed maximum values in the springs of 1992 and 1993. The primary targets of the UTF, swordfish and bigeye tuna followed the same intra-annual pattern as black escolar (Rı´os et al., 1986; Mora, 1987, 1996). Other incidental species, such as mako (Isurus oxyrinchus) and porbeagle (Lamna nasus) sharks, as well as other sharks with subtropical distribution, showed the same seasonal patterns (Mora, 1987, 1996; Menezes de Mello et al., 1993; Domingo et al., 1996). The marked recurrence of these long-term patterns both for target and incidental species could be attributed to a similar trophic and reproductive behavior. Concerning the former, the black escolar and cooccurring large pelagic species are considered as top predators (Collette and Nauen, 1983; Compagno, 1984; Zavala-Camı´n, 1986; Vaske and Castello, 1991;

Massutı´ et al., 1998) and the SAO constitutes a major feeding area for large pelagics. The subtropical convergence, produced by the mix of the Malvinas/ Falklands (subantarctic cold waters) and Brazil (subtropical warm water) currents, has a major incidence in the study area during winter and spring (SHN, 1981; Podesta´ , 1987; Menezes de Mello et al., 1993; Severov and Korobochka, 1998). This convergence generates a marked thermal front and promotes high levels of primary and secondary production, including crustaceans, mollusks and fishes of high commercial value (Nion et al., 1986; Podesta´ , 1987; Niggemeyer et al., 1990) such as large pelagics (Rı´os et al., 1986; Menezes de Mello et al., 1993; Mora, 1996). Several prey species of the large pelagics are also found, such as squids (Illex spp.) and anchovy (Engraulis spp.) (Zavala-Camı´n, 1986; Podesta´ , 1987; Vaske and Castello, 1991). On the other side, the low catches in summer and autumn could be explained by reproductive migrations to lower latitudes, as demonstrated for other sympatric species (tunas) of the black escolar (Collette and Nauen, 1983). The same trophic– reproductive seasonal pattern has been described for other large pelagic fisheries around the world (Parin, 1986; Sharp and Dizon, 1978; Stretta, 1991; Hurley, 1998; Lu et al., 1998; McKinnell and Seki, 1998). 4.3. The effect of catching power The impressive increase in fishing power of industrial vessels that occurred over the last two decades determined progressive and yet unmeasured changes in catchability, which is also subject to variations in fishing intensity and stock biomass (Sanders and Morgan, 1976; Greenstreet et al., 1999). These interactions have resulted in a poor ability to calibrate fishing effort, and hence, in an important uncertainty component when estimating this variable in the long term (Caddy, 1996, 1999; Caddy and Defeo, 1996). The switch from category A to B by the UTF illustrates this improvement of fishing power. Our longterm analysis suggests that category B, even with a smaller number of hooks, obtained catch rates that did not differ significantly from those of category A. The increase in fishing power of category B could be associated with improvements in vessel design and fishing gear. These vessels are smaller, have sophisticated satellite navigation and fish finding systems,

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and new refrigeration systems for rapid fish processing. Fishing gear improvements include chemical light to attract fish, use of synthetic fibers (polyamide monofilament) in the main line and curved hooks. Taking into account that category B vessels have also fewer crew, lower variable costs and thus higher economic benefits than category A could be expected. These improvements did not reduce the impact on black escolar (this paper) and other incidentally species (Marı´n et al., 1998). Besides, the relative representation of the incidental catch has drifted upwards, and the individual mean weight of the black escolar systematically decreased, constituting dramatic evidence of impact on the large pelagic fishery in the SAO. 4.4. Population structure: long-term patterns Individual weight of the black escolar significantly decreased in the long-term. This is the first time that a significant long-term pattern of decreasing weight is documented for a pelagic species incidentally captured in the SAO. As both vessel categories operated in the same fishing area with the same hook selectivity, this consistent long-term pattern could be invoked as an indicator of overexploitation of the black escolar in the SAO. This finding coincides with those of Menezes de Mello et al. (1993) for several target tuna species in Brazilian waters. However, these authors did not observe this impact on the population structure of incidental species. Our work gives strong support to the sequential depletion hypothesis, which means an overexploitation of target species first and incidental ones later on (Hall, 1996; Orensanz et al., 1998). Coinciding with the conclusions of Orensanz et al. (1998) in their comprehensive work in the Greater Gulf of Alaska, the pattern of collapse would tend not to be haphazard but proceeds serially, starting with the most valuable resources. Other hypotheses should not be discarded, e.g., differences in selectivity patterns between hooks and changes in the areas of operation of the fleets. This last explanation is not feasible in the present case, as the fishing fleets operated in the same grounds during the last 20 years (Milessi, unpublished). The marked decrease in individual sizes suggests that high fishing pressure and increasing fishing power could also affect incidental species. This situation is particularly relevant in long-lived species with

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low growth rates and low reproductive potential (Dayton et al., 1995; Heessen and Daan, 1996; Ross and Hokenson, 1997; Sinclair and Murawski, 1997; Philippart, 1998; Rochet, 1998; Greenstreet et al., 1999). Domingo et al. (1996) also observed a tendency of the UTF to switch from target to incidental species in the short-term, which could alter the abundance and population structure of incidental species. 4.5. Effectiveness of the management system in the SAO The accelerated trend towards overexploitation of tuna species all over the Atlantic Ocean (ICCAT, 1993; Mahon, 1996) and particularly in the SAO determined the implementation of regulation measures. In Uruguay, this included minimum sizes for primary and secondary targets (Mora, 1996) and restriction of the number of vessels (MGAP, 1997). However, as demonstrated in this work, these regulations were unsuccessful: both aggregated (relative representation of the incidental catch) and specific (individual mean weight) variables are still indicating an increasing impact on the UTF. The high spatial cooccurrence of target and incidental species in the SAO, make them very sensitive to overexploitation. Management failure probably relies on the inadequate single-species management scheme, where in fact the UTF affects a multispecific resource, harvested incidental or intentionally by a fleet which had suffered drastic variations in fishing power through time. Quota control has led to a neglect of the technological issues of fishing power of the UTF, and suggests the need to correct for variations in fleet composition and characteristics to be introduced in fishery models and management regulations. Improvements in fishing power supplied the reduction of nominal effort, and continued affecting target and incidental species, thus generating an overexploitation scheme ‘‘legalized’’ by an institutional framework only restricted to impose operational measures based on single species controls of catch, effort and individual sizes (Milessi and Defeo, 2000). This constitutes a serious deficiency in the control mechanisms directed to balance fishing fleet capacity and productivity of fish stocks (Caddy, 1999). The lack of international cooperation and agreement in managing highly migratory fish stocks like

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black escolar is one of the major problems to be solved in the short-term in world fishing (Caddy, 1999). Tuna fisheries management involves high enforcement or policing costs resulting from the implementation of management schemes and allocation of property rights. Policing areas are so extensive and accessible to third parties that policing effort is ineffective. Indeed, not only the UTF operates in the SAO, but also the Brazilian tuna fleet and a large number of noncompliant vessels (sensu Seijo et al., 1998) are not authorized by ICCAT. This fact, together with a limited capacity to introduce technologically sophisticated management and control systems, contributes to an increase in fishery uncertainty and to management inefficiency. The high costs of obtaining reliable information about the dynamics of the resource and the fishing fleet exponentially increase overexploitation risks. All the above topics are of utmost importance in reducing the multiple uncertainty that characterizes the increasingly threatened highly migratory stocks around the world.

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