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Batoids in a coastal trawl fishery of Argentina
Fisheries Research 77 (2006) 326–332
Batoids in a coastal trawl fishery of Argentina L.L. Tamini, G.E. Chiaramonte ∗ , J.E. Perez, H.L. Cappozzo Divisi´on Ictiolog´ıa and Estaci´on Hidrobiol´ogica de Puerto Quequ´en, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Angel Gallardo 470, C1405DJR Buenos Aires, Argentina Received 23 February 2005; received in revised form 16 August 2005; accepted 20 August 2005
Abstract Elasmobranchs have become an important element of the bycatch in the fisheries worldwide and an important resource for most of Argentine trawl fisheries. This work is the first analysis of the diversity of batoid fishes in the bycatch of the Argentine coastal fisheries. Between July 1998 and June 2000, an onboard observer program was carried out and 89 tows were monitored. A total of 1354 batoid specimens were obtained from the bycatch and 11 different species were identified: Atlantoraja castelnaui, A. cyclophora, Dasyatis pastinaca, Dipturus chilensis, Discopyge tschudii, Myliobatis goodei, Psammobatis bergi, P. extenta, Rioraja agassizi, Sympterygia bonapartii and Zapteryx brevirostris. The electric ray D. tschudii and the sand skate P. extenta were the most abundant species and were always discarded. The commercial capture of batoids was estimated at 33.6 kg h−1 and showed no clear pattern of temporal variation. © 2005 Elsevier B.V. All rights reserved. Keywords: Elasmobranchs; Commercial catch; Bycatch; Discard; Fishing effects; South West Atlantic
1. Introduction Bycatch has been defined by Hall (1996) as “is that portion of the capture that is discarded at sea dead (or injured to an extent that death is the most likely outcome) because it has little or no economic value or because its retention is prohibited by law”. Sharks, skates and rays (Elasmobranchii) typically exhibit rapid declines in catch rates (‘boom and bust’ yields), with fisheries collapsing soon after initiation of heavy exploitation (Holden, 1973, 1977; Anderson, 1990; Bonfil, 1994). Although there is some evidence that some elasmobranch species could be exploited sustainably at low fishing pressure, elasmobranchs taken as bycatch in fisheries targeting other species could be extirpated long before appropriate management policies could be implemented (Walker, 1998). Skates and rays are considered to be particularly vulnerable to over-exploitation by fisheries as a result of their K-selected life-history strategy (Stevens et al., 2000). The ∗ Corresponding author. Tel.: +54 11 4982 9410x219; fax: +54 11 4982 5243. E-mail address: [email protected] (G.E. Chiaramonte).
0165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2005.08.013
biology of most skate species (low fecundity and high age and size at maturity) makes most of them susceptible to increased mortality, for example by fisheries (Holden, 1973, 1974, 1977). Examining fisheries catch trends often assessed the status of rays and skates but these data are not generally recorded on a species-specific level (Dulvy et al., 2000) and in most fisheries, the discard of rays and skates is not recorded. Some cases of local extinctions of skates have been recently recorded (e.g. the disappearance of the common skate Dipturus batis by the trawling fishery operating in the Irish Sea (Brander, 1981) and the close-to-extinction status of the barndoor skate Raja laevis in northwest Atlantic (Casey and Myers, 1998)). The negative impact of fisheries highlights the need to assess not just the conservation status of target species but also the status of the bycatch, particularly rays and skates discarded in trawl fisheries of coastal areas. Overall, high-seas fisheries have more complete elasmobranch bycatch data than coastal fisheries, probably because coastal fisheries are much more diverse and complex (Bonfil, 1998). In the multispecies bottom trawl coastal fishery of Puerto Quequ´en, Argentina (38◦ 37 S, 58◦ 50 W; Fig. 1), Tamini (unpublished data) found an important bycatch
L.L. Tamini et al. / Fisheries Research 77 (2006) 326–332
327
Fig. 1. Puerto Quequ´en area in the South West Atlantic. Shaded squares are fishing grounds of bottom trawlers with onboard observers. Wi98 = winter 1998; Sp98 = spring 1998; Su99 = summer 1999; Au99 = autumn 1999; Wi99 = winter 1999; Sp99 = spring 1999; Su00 = summer 2000; Au00 = autumn 2000.
biomass which fluctuated seasonally between 44.5 and 67.5% of the total capture. In this fishery commercial batoid fishes were the largest and most abundant species within the elasmobranch group. The non-commercial batoid species and undersized specimens of commercial species were incorporated to the bycatch, from which 44.8% were elasmobranchs. In the two years period of this study the national fisheries authorities reported 1757 t of skates landed at Puerto Quequ´en (5.3% of the total landed skates in Argentina). In order to assess the status and the fishing impact on batoid fishes in the coast of south Buenos Aires province, this paper describes the catches taken by bottom trawl vessels, calculates the seasonal yield of commercial batoids and examines the relative abundance of the main discarded species by season during a period of two years and analyses the data in a wider context of South West Atlantic fisheries.
2. Materials and methods Data were collected seasonally by on board observers at Puerto Quequ´en bottom trawlers during 22 trips (89 tows;
∼0.64% of the fleet tows) between July 1998 and June 2000 and samples of capture were obtained in only 73 tows. The variation in the distribution of the observed fishing grounds by season is shown in Fig. 1. Random sampling was not attempted. We decided to work with all available hauls, since observers had no option to choose in which commercial vessel or trip to work on. Bottom trawlers ranged from 12.25 to 17.25 m long with engines of 120–240 HP. Nets with footropes of length 25 m were used, while codend mesh size was 38 mm (stretched) and trawling speeds were between two to three knots. Position, time, duration and depth were recorded for each trawl. One sample crate (∼5% of the capture of each tow) was taken from the middle of the right deck when the codend was opened. After the crew opened the footrope of the net and once the catch was released on the deck, the identities and numbers of the commercial batoids species from the crate (∼50 kg; 44 cm × 65 cm × 21 cm) were recorded and then, left in the commercial crates. Commercial and bycatch specimens were determined using the crew’s criteria. Each specimen was identified to species level following Menni et al. (1984) and reassessed using Cousseau et al. (2000), sexed and the disc width (DW) was measured to the nearest mm.
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For a comparison with other studies the DW–total length (TL) relationship of the five most common species (Discopyge tschudii, Psammobatis extenta, Psammobatis bergi, Atlantoraja cyclophora and Sympterygia bonapartii) was calculated by a regression analysis or obtained from the literature. For these five species DW–TL relationships, maximum DW (DWmax ), maximum TL (TLmax ), DW of 50% of maturity (DW50 ) and TL of 50% of maturity (TL50 ) were estimated from our and published data. Total landed weight of commercial batoid fishes was calculated using an estimated weight of crates landed for the last six of the eight analysed seasons. Thus, yield by seasons (kg of batoids per hour of trawl) was estimated. Fishery statistics were obtained from published reports by the national fisheries authorities (Direcci´on Nacional de Pesca, SAGyP). The seasonal alternating abundance between the two most common species (D. tschudii and P. extenta) was analysed by a Spearman rank correlation coefficient (rs ) corrected for ties (Zar, 1996) using the number of specimens catch per hour.
was 2.457 t, and of these, 1.658 t were discarded while 0.799 t were landed. The sampling batoid capture was 0.746 t, and of these, 0.529 t were discarded while 0.217 t were landed. A total of 1354 specimens of batoid fishes were identified during the study (average 18 batoid fishes per sample). These batoid fishes were classified into five families and 11 species (Table 1). The DW–TL relationship, DWmax , TLmax , DW50 and TL50 are shown for the five most frequent species in the bycatch (Table 2). The lowest yield of commercial skates was found during 1999-summer season (9.5 kg h−1 ) and highest in 2000summer season (62.1 kg h−1 ) with an average of 33.6 kg h−1 . Yield seemed to increase for the second half of the study but the sampled period was not extended enough to draw further conclusions (Fig. 2). In five of six seasons when we were able to calculate the commercial–bycatch relationship, the batoid fishes discards exceeded 50% of the total batoid fishes caught and reached the 48% of the capture in one season (Fig. 3). However, during summer 2000 when the discards of batoid fishes were 21%, we found out that five of the seven samples of that season came from a fishing ground different than the usual (Fig. 1). This also agreed with the highest fishing yield during the studied period (Fig. 2). Seasonally, of the sampling batoid capture for the last six seasons, commercial batoids were 1.8–19.6% (in weight, Fig. 3), while the batoids bycatch comprised 5.0–48.0% (in weight, Fig. 3). Winter 1999 showed that a great proportion of the capture (48% was batoids bycatch). Summer 2000 was the only season when commercial landings of batoids were higher than batoid bycatch (Fig. 3).
3. Results The vessels operated in depths of ca. 35–56 m and trawling lasted for 1.5–3.0 h, both during the day and night. The main target species were the volute Zidona dufresnei (Neogastropoda: Volutidae), flatfishes (Osteicthyes: Paralichthyidae) and the smoothound Mustelus schmitti (Chondrichthyes: Triakidae); in addition, other 20 fish species of less economic value were commonly landed. The sampling total capture
Table 1 Status, number, percentage number and weight in relationship to the total sampling number and weight, and percentage of occurrence of batoid species in the catch from 73 commercial bottom trawls off Puerto Quequ´en, July 1998–June 2000 Taxon
Status
Common name
Common name in Argentina
Number
% Number
Torpedinidae D. tschudii
B
Electric ray
Torpedo
730
53.9
53.9
70.0
70.0
64.4
B B B/C B/C B/C B/C
Skate Skate Skate Skate Skate Skate Skate
Rayita de orlas Raya reticulada Raya de c´ırculos Raya marmorada Raya pintada Raya lisa
410 90 50 38 6 4 6
30.3 6.6 3.7 2.8 0.4 0.3 0.4
84.2 90.8 94.5 97.3 97.8 98.1 98.5
5.3 8.4 4.0 6.9 0.4 0.1 0.6
75.3 83.7 87.7 94.6 95.0 95.1 95.7
76.7 43.8 34.2 21.9 2.7 2.7 6.8
Rajidae D. chilensis
B/C
Skate
Raya hocicuda
2
0.1
98.6
0.3
96.0
2.7
Rhinobatidae Zapteryx brevirostris
B
Small guitarfish
Guitarra chica
16
1.2
99.8
2.7
98.7
17.8
Myliobatidae Myliobatis goodie
B
Eagle ray
Chucho
1
0.1
99.9
0.6
99.3
1.4
Dasyatidae Dasyatis pastinaca
B
Eagle ray
Chucho
1
0.1
100.0
0.7
100.0
1.4
Arhynchobatidae P. extenta P. bergi A. cyclophora S. bonapartii Atlantoraja castelnaui R. agassizi Arhynchobatidae Unid.
B: bycatch. C: commercial.
Cumulative number
wt.%
Cumulative weight
% Occurrence
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Table 2 Relationship between disc width (DW) and total length (TL); maximum sizes (DWmax and TLmax ) and sizes at 50% of maturity (DW50 and TL50 ) of the five more common batoids species in the discards from bottom trawlers off Puerto Quequ´en Species
DW (mm)–TL (mm)
DWmax –TLmax (mm)
DW50 –TL50 (mm)
References
D. tschudii
Sex
DW = 0.44552 TL + 16.46582 DW = 0.65093 TL − 22.87371
257–540 231–390
139–2751 185–3201
Garc´ıa (1984) 1. TL100 (is the TL at which all specimens are mature)
P. extenta
DW = 0.5882 TL − 2.47162 DW = 0.5767 TL − 0.85752
182–3133 180–3133
144–2493 150–2623
2. This work 3. Braccini and Chiaramonte (2002)
P. bergi
DW = 0.6123 TL − 15.806 DW = 0.592 TL − 8.5194
326–558 350–605
255–442 263–458
San Mart´ın et al. (2005)
A. cyclophora
DW = 0.7811 TL + 5.96264 DW = 0.7714 TL + 7.65434
539–6825 544–6955
– –
4. This work 5. Cousseau et al. (2000)
S. bonapartii
DW = 0.6911 TL − 5.9626 DW = 0.6711 TL − 0.5228
552–808 500–746
434–636 436–651
Modified from Mabraga˜na et al. (2002)
The DW–TL relationships estimated for P. extenta and A. cyclophora were significant at P < 0.05.
Fig. 2. Mean yield (batoids caught per hour of trawl) in 85 commercial tows for eight successive seasons. Bars: standards deviation. Numbers: sample sizes. Wi98 = winter 1998, Sp98 = spring 1998, Su99 = summer 1999, Au99 = autumn 1999, Wi99 = winter 1999, Sp99 = spring 1999, Su00 = summer 2000, Au00 = autumn 2000.
Six out of 11 batoid species were always discarded and five species—A. cyclophora, S. bonapartii, A. castelnaui, Rioraja agassizi and Dipturus chilensis—were discarded or kept as commercial species depending on their size (Table 1).
Fig. 3. Captured batoids (wt.%) in six sampled seasons. Commercial batoids (black), discarded batoids (gray) and remained capture (white).
Two species were dominant in the capture, the torpedo D. tschudii and the small sand skate P. extenta. D. tschudii and P. extenta were present in 47 and 56 trawls, respectively, accounting for 84.2% in number of the discarded batoids (Table 1). From the analysis of the seasonal distribution abundance of the five more abundant species in the area we can see that D. tschudii and P. extenta showed strong changes in their occurrence in the bycatch (Fig. 4). The former was not present in the summer samples and was found in a high proportion in winter 1998–1999 and spring 1998, whilst P. extenta was present during all sampled seasons with a peak in summers. For these two species a seasonal alternating negative correlation (rs = −0.396, P < 0.001) was established. P. bergi was absent both years only in winter, A. cyclophora was more abundant in autumn. Although, no pattern was detected for S. bonapartii, it seems a little more abundant in winter and spring (Fig. 4). The DW–TL relationships estimated for P. extenta and A. cyclophora were significant at P < 0.05 (Table 2). In order to allow further comparative studies, the latter relationship (plus DWmax and TLmax , DW50 and TL50 ) was resumed for the more common batoids species in the bycatch from bottom trawlers off Puerto Quequ´en (Table 2). Fig. 5 shows the size frequency distribution by sex of those five species. The bulk of D. tschudii specimens sampled (80.8% of the sample) were mature (Fig. 5a) and showed a bimodal size frequency distribution corresponding with sex. P. extenta (Fig. 5b) did not show sexual differences in their size frequency distribution and only half of the sample (49.5%) was mature animals. P. bergi (Fig. 5c) had a bimodal distribution by sex and the number of mature specimens in the sample was high. No data was available on the TL of maturity of A. cyclophora and no clear pattern of size frequency was detected neither in this species (Fig. 5d), nor in the commercial skate S. bonapartii from which most of the sample was immature (Fig. 5e).
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Fig. 5. Size frequency distributions of the most common batoids in the bycatch of Puerto Quequ´en commercial trawlers. Males (white), females (black). Arrows are sizes of TL50 except for D. tschudii (TL100 , the TL at which all specimens are mature).
Fig. 4. Mean number of specimens caught per season: (a) Discopyge tschudii, (b) P. extenta, (c) P. bergi, (d) A. cyclophora and (e) S. bonapartii in eight sampled seasons: WIN = winter, SPR = spring, SUM = summer, AUT = autumn. Periods: 1998–1999 (black), 1999–2000 (white).
4. Discussion In fisheries of the South West Atlantic, Pettovello (1999) reported that the shrimp fleet fishing at the Golfo San Jorge area captured batoid fishes in small proportion (1.6% of the
total biomass caught and 32% presence in tows). Recently, Cedrola et al. (2005) upgrade the figures to 90% presence in tows using census methodology in randomly selected tows and discusses the origin of the differences between Pettovello (1999) and their assessment. In other work, Ca˜nete et al. (1999) established that the percentage of batoid fishes as part of the total catch in the common hake fishery in 1998 was also relatively low (1.37% in freezer ships and 1.94% in ice trawlers) but the presence by tow was high (67.4% and 78.3% respectively). Compared with those fisheries, the Puerto Quequ´en coastal fishery had a higher incidence in the
L.L. Tamini et al. / Fisheries Research 77 (2006) 326–332
catch of batoids (30.37% of biomass, and 100% presence in tows) and the percentage of commercial catch of batoids (27.15%) in relation to the total commercial catch agreed with the range of batoids landings for the whole Argentinean coastal fleet (9.2–57.8%, Chiaramonte, unpublished data). The detected differences in the proportions of commercial and discarded batoids in the catch of summer 1999 and summer 2000 (Fig. 3) could be due to the change of the fishing area of the vessels (Fig. 1). The summer 2000 area exhibit a different seabed composition, with a great proportion of sand and a lesser amount of mud. Additionally, the fishing effort in this area was scarce during the studied period (Fig. 1). The minimum size is a market requirement for skates and rays because the weight of the stripped fins must be at least 100 g. Since D. tschudii, P. extenta and P. bergi never get the minimum size of the other commercial species (Fig. 5a–c), their adults are not marketable and are always discarded (Table 1). In his list of fishes of Mar del Plata (Fig. 1) area Nani (1964) included the greater part of the Family Rajidae and the electric ray as species left mainly to the fishmeal industry and some of them occasionally left for the human consumption. Noticeably, D. tschudii was registered during the 12 months of Nani’s survey. Bellisio et al. (1979) found that these species arrived to northern Patagonia (40◦ S) during the summer. In agreement with Bellisio et al. (1979), seasonal absences of D. tschudii (Fig. 4) detected in the present study could be related to the migration to southern waters. For Psammobatis spp. no reproductive or trophic migrations were pointed out in the literature (Menni and Stehmann, 2000). In this study the variability in the presence of P. extenta and the absence in winter of P. bergi could be interpreted as migratory behaviour of these species and might be related to trophics requirements in both species since Braccini and Chiaramonte (2002) and San Mart´ın et al. (2005) reported no clear seasonal patterns in the reproductive strategy of these species. The bulk of the batoids discarded were in mature condition. Garc´ıa (1984) found that the electric ray, D. tschudii, presented secondary sexual dimorphism, males attaining larger size than females. In the present work, the size frequency distribution of D. tschudii showed a clear bimodal pattern according to the aforementioned sexual dimorphism of the species (Fig. 5a). Consistent with the sexual maturity stages established by Garc´ıa (1984) and the above results, 80.8% of D. tschudii in the samples (i.e. discards) could be considered mature. According to Braccini and Chiaramonte (2002) for P. extenta and San Mart´ın et al. (2005) for P. bergi, the 49.5% and 70% of the samples could be considered mature (Fig. 5b and c). For the other skate species, the sample sizes were too small to observe any pattern in their size frequency (Fig. 5d–e). Despite that not enough data are available in the literature on the reproductive biology of A. cyclophora and based on our observations on the reproductive condition of the discarded specimens (unpublished data), most skates shown
331
in Fig. 5d were immature. The specimens of S. bonapartii sampled were also immature (Fig. 5e). The case of Puerto Quequ´en coastal bottom trawl fishery does not allow establishing a clear pattern of variation in the relative abundance of batoids. The lack of information before our study prevented any comparative study of changes in community structure of batoid fishes in the area. The present fishing pressure in this mixed-species fishery could have an important impact in the structure of the bottom coastal communities including batoid fishes and thus such an impact may be irreversible in the short time. These possibilities should be a cause for concern for the environmental and fisheries authorities as well as NGO’s and fishermen communities.
Acknowledgments The authors thank E. Arribalzaga, A. Averbuj, C. Boy, M. Br¨oguer, S. Gonzalez Capria, F. P´erez and A. Suarez for help provided during fieldwork. M. Braccini improves the English manuscript and made many helpful comments. Skippers and crews of fishing vessels FV Deledele, FV Angela, FV La Cruz del Sur, FV Jes´us de Nazareth, FV Felicitas, FV Eusonia, FV Angel Antonio and FV La Cruz del Sud. R. Bruno (Santa Cecilia) and J. Renaudo (La Portuguesa), and their personnel. Prefectura Naval Argentina, detachment Puerto Quequ´en. This study was supported by Fundaci´on Antorchas (H.L.C., Proyecto A-13672/1-3).
References Anderson, E.D., 1990. Estimates of large shark catches in the Western Atlantic and Gulf of Mexico, 1960–1986. In: Pratt, H.L., Gruber, S.H., Taniuchi, T. (Eds.), Elasmobranchs as Living Resources: Advances in the Biology, Ecology, Systematics, and the Status of the Fisheries, vols. 443–454. NOAA Technical Report NMFS 90. Bellisio, N.B., Lopez, R.B., Torno, A., 1979. Peces marinos patag´onicos. Publicaci´on de la Subsecretar´ıa de Pesca, Secretar´ıa de Estado de Intereses Mar´ıtimos, Ministerio de Econom´ıa. Buenos Aires, pp. 280. Bonfil, R., 1994. Overview of world elasmobranch fisheries. FAO Fisheries Technical Paper 341, pp. 119. Bonfil, R., 1998. Elasmobranch bycatch monitoring. In: Gibble, N.A., McPherson, G., Lane, B. (Eds.), Shark Management and Conservation. Proceedings from the Shark and Man Workshop of the Second Fisheries Congress, Brisbane, Australia, 2 August 1996, pp. 85–87. Braccini, J.M., Chiaramonte, G.E., 2002. Biolog´ıa de la raya Psammobatis extenta Garman, 1913. Revista Chilena de Historia Natural 75, 179–188. Brander, K., 1981. Disappearance of common skate Raia batis from Irish Sea. Nature 290, 48–49. Ca˜nete, G., Blanco, G., Marchetti, C., Brachetta, H., Buono, P., 1999. An´alisis de la captura incidental (bycatch) en la pesquer´ıa de merluza com´un en el a˜no, 1998. Instituto Nacional de Investigaci´on y Desarrollo Pesquero, Informe T´ecnico Interno 80 (Mar del Plata.). Casey, J.M., Myers, R.A., 1998. Near extinction of a large, widely distributed fish. Science 281, 690–692. Cedrola, P.V., Gonz´alez, A.M., Pettovello, A.D., 2005. Bycatch of skates (Elasmobranchii: Arhynchobatidae, Rajidae) in the Patagonian red shrimp fishery. Fish. Res. 71, 141–150.
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Cousseau, M.B., Figueroa, D., D´ıaz de Astarloa, J.M., 2000. Clave de identificaci´on de las rayas del litoral mar´ıtimo de Argentina y Uruguay (Chondricthyes, Familia Rajidae). Instituto Nacional de Investigaci´on y Desarrollo Pesquero (Mar del Plata.). Dulvy, N.K., Metcalfe, J.D., Glanville, J., Pawson, M.G., Reynolds, J.D., 2000. Fishery stability, local extinctions and shifts in community structure in skates. Conserv. Biol. 14, 283–293. Garc´ıa, M.L., 1984. Sobre la biolog´ıa de Discopyge tschudii (Chondrictyes, Narcinidae). Physis (Buenos Aires), sec. A, 42 (103), 101–112. Hall, M.A., 1996. On bycatches. Rev. Fish Biol. Fish. 6, 319–352. Holden, M.J., 1973. Are long-term sustainable fisheries for elasmobranch possible? Rapports et Proc`es–Verbaux des R´eunions du Conseil International pour l’Exploration de la Mer 164, 360–367. Holden, M.J., 1974. Problems in the rational exploitation of elasmobranch populations and some suggested solutions. In: Harden-Jones, F.R. (Ed.), Sea Fisheries Research. John Wiley and Sons, New York, pp. 117–137. Holden, M.J., 1977. Elasmobranchs. In: Gulland, J.A. (Ed.), Fish Populations Dynamics. John Wiley, London, pp. 187–216. Mabraga˜na, E., Lucifora, L.O., Massa, A.M., 2002. The reproductive ecology and abundance of Sympterygia bonapartii endemic to the southwest Atlantic. J. Fish Biol. 60, 951–967.
Menni, R.C., Ringuelet, R.A., Aramburu, R.H., 1984. Peces marinos de la Argentina y Uruguay. Ed. Hemisferio Sur, Bs. As., pp. 359. Menni, R.C., Stehmann, M.F.W., 2000. Distribution, environment and biology of batoid fishes off Argentina, Uruguay and Brazil: a review. Revista del Museo Argentino de Ciencias Naturales (nueva serie) 2, 69–109. Nani, A., 1964. Variaciones estacionales de la fauna ´ıctica del a´ rea de pesca de Mar del Plata. CARPAS/2/Documento T´ecnico No. 20, pp. 21. Pettovello, A., 1999. By-catch in the Patagonian red shrimp (Pleoticus muelleri) fishery. Mar. Freshwater Res. 50, 123–127. San Mart´ın, M.J., Perez, J.E., Chiaramonte, G.E., 2005. Reproductive biology of the South West Atlantic marbled skate Psammobatis bergi Marini, 1932 (Elasmobranchii, Rajidae). J. Appl. Ichthyol. 21, 504–510. Stevens, J.D., Bonfil, R., Dulvy, N.K., Walker, P.A., 2000. The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems. ICES J. Mar. Sci. 57, 476–494. Walker, T.I., 1998. Can shark resources be harvested sustainably? A question revisited with a review of shark fisheries. Mar. Freshwater Res. 49, 553–572. Zar, J.H., 1996. Biostatistical Analysis. Prentice Hall, New Jersey.
Batoids in a coastal trawl fishery of Argentina L.L. Tamini, G.E. Chiaramonte ∗ , J.E. Perez, H.L. Cappozzo Divisi´on Ictiolog´ıa and Estaci´on Hidrobiol´ogica de Puerto Quequ´en, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Angel Gallardo 470, C1405DJR Buenos Aires, Argentina Received 23 February 2005; received in revised form 16 August 2005; accepted 20 August 2005
Abstract Elasmobranchs have become an important element of the bycatch in the fisheries worldwide and an important resource for most of Argentine trawl fisheries. This work is the first analysis of the diversity of batoid fishes in the bycatch of the Argentine coastal fisheries. Between July 1998 and June 2000, an onboard observer program was carried out and 89 tows were monitored. A total of 1354 batoid specimens were obtained from the bycatch and 11 different species were identified: Atlantoraja castelnaui, A. cyclophora, Dasyatis pastinaca, Dipturus chilensis, Discopyge tschudii, Myliobatis goodei, Psammobatis bergi, P. extenta, Rioraja agassizi, Sympterygia bonapartii and Zapteryx brevirostris. The electric ray D. tschudii and the sand skate P. extenta were the most abundant species and were always discarded. The commercial capture of batoids was estimated at 33.6 kg h−1 and showed no clear pattern of temporal variation. © 2005 Elsevier B.V. All rights reserved. Keywords: Elasmobranchs; Commercial catch; Bycatch; Discard; Fishing effects; South West Atlantic
1. Introduction Bycatch has been defined by Hall (1996) as “is that portion of the capture that is discarded at sea dead (or injured to an extent that death is the most likely outcome) because it has little or no economic value or because its retention is prohibited by law”. Sharks, skates and rays (Elasmobranchii) typically exhibit rapid declines in catch rates (‘boom and bust’ yields), with fisheries collapsing soon after initiation of heavy exploitation (Holden, 1973, 1977; Anderson, 1990; Bonfil, 1994). Although there is some evidence that some elasmobranch species could be exploited sustainably at low fishing pressure, elasmobranchs taken as bycatch in fisheries targeting other species could be extirpated long before appropriate management policies could be implemented (Walker, 1998). Skates and rays are considered to be particularly vulnerable to over-exploitation by fisheries as a result of their K-selected life-history strategy (Stevens et al., 2000). The ∗ Corresponding author. Tel.: +54 11 4982 9410x219; fax: +54 11 4982 5243. E-mail address: [email protected] (G.E. Chiaramonte).
0165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2005.08.013
biology of most skate species (low fecundity and high age and size at maturity) makes most of them susceptible to increased mortality, for example by fisheries (Holden, 1973, 1974, 1977). Examining fisheries catch trends often assessed the status of rays and skates but these data are not generally recorded on a species-specific level (Dulvy et al., 2000) and in most fisheries, the discard of rays and skates is not recorded. Some cases of local extinctions of skates have been recently recorded (e.g. the disappearance of the common skate Dipturus batis by the trawling fishery operating in the Irish Sea (Brander, 1981) and the close-to-extinction status of the barndoor skate Raja laevis in northwest Atlantic (Casey and Myers, 1998)). The negative impact of fisheries highlights the need to assess not just the conservation status of target species but also the status of the bycatch, particularly rays and skates discarded in trawl fisheries of coastal areas. Overall, high-seas fisheries have more complete elasmobranch bycatch data than coastal fisheries, probably because coastal fisheries are much more diverse and complex (Bonfil, 1998). In the multispecies bottom trawl coastal fishery of Puerto Quequ´en, Argentina (38◦ 37 S, 58◦ 50 W; Fig. 1), Tamini (unpublished data) found an important bycatch
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Fig. 1. Puerto Quequ´en area in the South West Atlantic. Shaded squares are fishing grounds of bottom trawlers with onboard observers. Wi98 = winter 1998; Sp98 = spring 1998; Su99 = summer 1999; Au99 = autumn 1999; Wi99 = winter 1999; Sp99 = spring 1999; Su00 = summer 2000; Au00 = autumn 2000.
biomass which fluctuated seasonally between 44.5 and 67.5% of the total capture. In this fishery commercial batoid fishes were the largest and most abundant species within the elasmobranch group. The non-commercial batoid species and undersized specimens of commercial species were incorporated to the bycatch, from which 44.8% were elasmobranchs. In the two years period of this study the national fisheries authorities reported 1757 t of skates landed at Puerto Quequ´en (5.3% of the total landed skates in Argentina). In order to assess the status and the fishing impact on batoid fishes in the coast of south Buenos Aires province, this paper describes the catches taken by bottom trawl vessels, calculates the seasonal yield of commercial batoids and examines the relative abundance of the main discarded species by season during a period of two years and analyses the data in a wider context of South West Atlantic fisheries.
2. Materials and methods Data were collected seasonally by on board observers at Puerto Quequ´en bottom trawlers during 22 trips (89 tows;
∼0.64% of the fleet tows) between July 1998 and June 2000 and samples of capture were obtained in only 73 tows. The variation in the distribution of the observed fishing grounds by season is shown in Fig. 1. Random sampling was not attempted. We decided to work with all available hauls, since observers had no option to choose in which commercial vessel or trip to work on. Bottom trawlers ranged from 12.25 to 17.25 m long with engines of 120–240 HP. Nets with footropes of length 25 m were used, while codend mesh size was 38 mm (stretched) and trawling speeds were between two to three knots. Position, time, duration and depth were recorded for each trawl. One sample crate (∼5% of the capture of each tow) was taken from the middle of the right deck when the codend was opened. After the crew opened the footrope of the net and once the catch was released on the deck, the identities and numbers of the commercial batoids species from the crate (∼50 kg; 44 cm × 65 cm × 21 cm) were recorded and then, left in the commercial crates. Commercial and bycatch specimens were determined using the crew’s criteria. Each specimen was identified to species level following Menni et al. (1984) and reassessed using Cousseau et al. (2000), sexed and the disc width (DW) was measured to the nearest mm.
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For a comparison with other studies the DW–total length (TL) relationship of the five most common species (Discopyge tschudii, Psammobatis extenta, Psammobatis bergi, Atlantoraja cyclophora and Sympterygia bonapartii) was calculated by a regression analysis or obtained from the literature. For these five species DW–TL relationships, maximum DW (DWmax ), maximum TL (TLmax ), DW of 50% of maturity (DW50 ) and TL of 50% of maturity (TL50 ) were estimated from our and published data. Total landed weight of commercial batoid fishes was calculated using an estimated weight of crates landed for the last six of the eight analysed seasons. Thus, yield by seasons (kg of batoids per hour of trawl) was estimated. Fishery statistics were obtained from published reports by the national fisheries authorities (Direcci´on Nacional de Pesca, SAGyP). The seasonal alternating abundance between the two most common species (D. tschudii and P. extenta) was analysed by a Spearman rank correlation coefficient (rs ) corrected for ties (Zar, 1996) using the number of specimens catch per hour.
was 2.457 t, and of these, 1.658 t were discarded while 0.799 t were landed. The sampling batoid capture was 0.746 t, and of these, 0.529 t were discarded while 0.217 t were landed. A total of 1354 specimens of batoid fishes were identified during the study (average 18 batoid fishes per sample). These batoid fishes were classified into five families and 11 species (Table 1). The DW–TL relationship, DWmax , TLmax , DW50 and TL50 are shown for the five most frequent species in the bycatch (Table 2). The lowest yield of commercial skates was found during 1999-summer season (9.5 kg h−1 ) and highest in 2000summer season (62.1 kg h−1 ) with an average of 33.6 kg h−1 . Yield seemed to increase for the second half of the study but the sampled period was not extended enough to draw further conclusions (Fig. 2). In five of six seasons when we were able to calculate the commercial–bycatch relationship, the batoid fishes discards exceeded 50% of the total batoid fishes caught and reached the 48% of the capture in one season (Fig. 3). However, during summer 2000 when the discards of batoid fishes were 21%, we found out that five of the seven samples of that season came from a fishing ground different than the usual (Fig. 1). This also agreed with the highest fishing yield during the studied period (Fig. 2). Seasonally, of the sampling batoid capture for the last six seasons, commercial batoids were 1.8–19.6% (in weight, Fig. 3), while the batoids bycatch comprised 5.0–48.0% (in weight, Fig. 3). Winter 1999 showed that a great proportion of the capture (48% was batoids bycatch). Summer 2000 was the only season when commercial landings of batoids were higher than batoid bycatch (Fig. 3).
3. Results The vessels operated in depths of ca. 35–56 m and trawling lasted for 1.5–3.0 h, both during the day and night. The main target species were the volute Zidona dufresnei (Neogastropoda: Volutidae), flatfishes (Osteicthyes: Paralichthyidae) and the smoothound Mustelus schmitti (Chondrichthyes: Triakidae); in addition, other 20 fish species of less economic value were commonly landed. The sampling total capture
Table 1 Status, number, percentage number and weight in relationship to the total sampling number and weight, and percentage of occurrence of batoid species in the catch from 73 commercial bottom trawls off Puerto Quequ´en, July 1998–June 2000 Taxon
Status
Common name
Common name in Argentina
Number
% Number
Torpedinidae D. tschudii
B
Electric ray
Torpedo
730
53.9
53.9
70.0
70.0
64.4
B B B/C B/C B/C B/C
Skate Skate Skate Skate Skate Skate Skate
Rayita de orlas Raya reticulada Raya de c´ırculos Raya marmorada Raya pintada Raya lisa
410 90 50 38 6 4 6
30.3 6.6 3.7 2.8 0.4 0.3 0.4
84.2 90.8 94.5 97.3 97.8 98.1 98.5
5.3 8.4 4.0 6.9 0.4 0.1 0.6
75.3 83.7 87.7 94.6 95.0 95.1 95.7
76.7 43.8 34.2 21.9 2.7 2.7 6.8
Rajidae D. chilensis
B/C
Skate
Raya hocicuda
2
0.1
98.6
0.3
96.0
2.7
Rhinobatidae Zapteryx brevirostris
B
Small guitarfish
Guitarra chica
16
1.2
99.8
2.7
98.7
17.8
Myliobatidae Myliobatis goodie
B
Eagle ray
Chucho
1
0.1
99.9
0.6
99.3
1.4
Dasyatidae Dasyatis pastinaca
B
Eagle ray
Chucho
1
0.1
100.0
0.7
100.0
1.4
Arhynchobatidae P. extenta P. bergi A. cyclophora S. bonapartii Atlantoraja castelnaui R. agassizi Arhynchobatidae Unid.
B: bycatch. C: commercial.
Cumulative number
wt.%
Cumulative weight
% Occurrence
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Table 2 Relationship between disc width (DW) and total length (TL); maximum sizes (DWmax and TLmax ) and sizes at 50% of maturity (DW50 and TL50 ) of the five more common batoids species in the discards from bottom trawlers off Puerto Quequ´en Species
DW (mm)–TL (mm)
DWmax –TLmax (mm)
DW50 –TL50 (mm)
References
D. tschudii
Sex
DW = 0.44552 TL + 16.46582 DW = 0.65093 TL − 22.87371
257–540 231–390
139–2751 185–3201
Garc´ıa (1984) 1. TL100 (is the TL at which all specimens are mature)
P. extenta
DW = 0.5882 TL − 2.47162 DW = 0.5767 TL − 0.85752
182–3133 180–3133
144–2493 150–2623
2. This work 3. Braccini and Chiaramonte (2002)
P. bergi
DW = 0.6123 TL − 15.806 DW = 0.592 TL − 8.5194
326–558 350–605
255–442 263–458
San Mart´ın et al. (2005)
A. cyclophora
DW = 0.7811 TL + 5.96264 DW = 0.7714 TL + 7.65434
539–6825 544–6955
– –
4. This work 5. Cousseau et al. (2000)
S. bonapartii
DW = 0.6911 TL − 5.9626 DW = 0.6711 TL − 0.5228
552–808 500–746
434–636 436–651
Modified from Mabraga˜na et al. (2002)
The DW–TL relationships estimated for P. extenta and A. cyclophora were significant at P < 0.05.
Fig. 2. Mean yield (batoids caught per hour of trawl) in 85 commercial tows for eight successive seasons. Bars: standards deviation. Numbers: sample sizes. Wi98 = winter 1998, Sp98 = spring 1998, Su99 = summer 1999, Au99 = autumn 1999, Wi99 = winter 1999, Sp99 = spring 1999, Su00 = summer 2000, Au00 = autumn 2000.
Six out of 11 batoid species were always discarded and five species—A. cyclophora, S. bonapartii, A. castelnaui, Rioraja agassizi and Dipturus chilensis—were discarded or kept as commercial species depending on their size (Table 1).
Fig. 3. Captured batoids (wt.%) in six sampled seasons. Commercial batoids (black), discarded batoids (gray) and remained capture (white).
Two species were dominant in the capture, the torpedo D. tschudii and the small sand skate P. extenta. D. tschudii and P. extenta were present in 47 and 56 trawls, respectively, accounting for 84.2% in number of the discarded batoids (Table 1). From the analysis of the seasonal distribution abundance of the five more abundant species in the area we can see that D. tschudii and P. extenta showed strong changes in their occurrence in the bycatch (Fig. 4). The former was not present in the summer samples and was found in a high proportion in winter 1998–1999 and spring 1998, whilst P. extenta was present during all sampled seasons with a peak in summers. For these two species a seasonal alternating negative correlation (rs = −0.396, P < 0.001) was established. P. bergi was absent both years only in winter, A. cyclophora was more abundant in autumn. Although, no pattern was detected for S. bonapartii, it seems a little more abundant in winter and spring (Fig. 4). The DW–TL relationships estimated for P. extenta and A. cyclophora were significant at P < 0.05 (Table 2). In order to allow further comparative studies, the latter relationship (plus DWmax and TLmax , DW50 and TL50 ) was resumed for the more common batoids species in the bycatch from bottom trawlers off Puerto Quequ´en (Table 2). Fig. 5 shows the size frequency distribution by sex of those five species. The bulk of D. tschudii specimens sampled (80.8% of the sample) were mature (Fig. 5a) and showed a bimodal size frequency distribution corresponding with sex. P. extenta (Fig. 5b) did not show sexual differences in their size frequency distribution and only half of the sample (49.5%) was mature animals. P. bergi (Fig. 5c) had a bimodal distribution by sex and the number of mature specimens in the sample was high. No data was available on the TL of maturity of A. cyclophora and no clear pattern of size frequency was detected neither in this species (Fig. 5d), nor in the commercial skate S. bonapartii from which most of the sample was immature (Fig. 5e).
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Fig. 5. Size frequency distributions of the most common batoids in the bycatch of Puerto Quequ´en commercial trawlers. Males (white), females (black). Arrows are sizes of TL50 except for D. tschudii (TL100 , the TL at which all specimens are mature).
Fig. 4. Mean number of specimens caught per season: (a) Discopyge tschudii, (b) P. extenta, (c) P. bergi, (d) A. cyclophora and (e) S. bonapartii in eight sampled seasons: WIN = winter, SPR = spring, SUM = summer, AUT = autumn. Periods: 1998–1999 (black), 1999–2000 (white).
4. Discussion In fisheries of the South West Atlantic, Pettovello (1999) reported that the shrimp fleet fishing at the Golfo San Jorge area captured batoid fishes in small proportion (1.6% of the
total biomass caught and 32% presence in tows). Recently, Cedrola et al. (2005) upgrade the figures to 90% presence in tows using census methodology in randomly selected tows and discusses the origin of the differences between Pettovello (1999) and their assessment. In other work, Ca˜nete et al. (1999) established that the percentage of batoid fishes as part of the total catch in the common hake fishery in 1998 was also relatively low (1.37% in freezer ships and 1.94% in ice trawlers) but the presence by tow was high (67.4% and 78.3% respectively). Compared with those fisheries, the Puerto Quequ´en coastal fishery had a higher incidence in the
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catch of batoids (30.37% of biomass, and 100% presence in tows) and the percentage of commercial catch of batoids (27.15%) in relation to the total commercial catch agreed with the range of batoids landings for the whole Argentinean coastal fleet (9.2–57.8%, Chiaramonte, unpublished data). The detected differences in the proportions of commercial and discarded batoids in the catch of summer 1999 and summer 2000 (Fig. 3) could be due to the change of the fishing area of the vessels (Fig. 1). The summer 2000 area exhibit a different seabed composition, with a great proportion of sand and a lesser amount of mud. Additionally, the fishing effort in this area was scarce during the studied period (Fig. 1). The minimum size is a market requirement for skates and rays because the weight of the stripped fins must be at least 100 g. Since D. tschudii, P. extenta and P. bergi never get the minimum size of the other commercial species (Fig. 5a–c), their adults are not marketable and are always discarded (Table 1). In his list of fishes of Mar del Plata (Fig. 1) area Nani (1964) included the greater part of the Family Rajidae and the electric ray as species left mainly to the fishmeal industry and some of them occasionally left for the human consumption. Noticeably, D. tschudii was registered during the 12 months of Nani’s survey. Bellisio et al. (1979) found that these species arrived to northern Patagonia (40◦ S) during the summer. In agreement with Bellisio et al. (1979), seasonal absences of D. tschudii (Fig. 4) detected in the present study could be related to the migration to southern waters. For Psammobatis spp. no reproductive or trophic migrations were pointed out in the literature (Menni and Stehmann, 2000). In this study the variability in the presence of P. extenta and the absence in winter of P. bergi could be interpreted as migratory behaviour of these species and might be related to trophics requirements in both species since Braccini and Chiaramonte (2002) and San Mart´ın et al. (2005) reported no clear seasonal patterns in the reproductive strategy of these species. The bulk of the batoids discarded were in mature condition. Garc´ıa (1984) found that the electric ray, D. tschudii, presented secondary sexual dimorphism, males attaining larger size than females. In the present work, the size frequency distribution of D. tschudii showed a clear bimodal pattern according to the aforementioned sexual dimorphism of the species (Fig. 5a). Consistent with the sexual maturity stages established by Garc´ıa (1984) and the above results, 80.8% of D. tschudii in the samples (i.e. discards) could be considered mature. According to Braccini and Chiaramonte (2002) for P. extenta and San Mart´ın et al. (2005) for P. bergi, the 49.5% and 70% of the samples could be considered mature (Fig. 5b and c). For the other skate species, the sample sizes were too small to observe any pattern in their size frequency (Fig. 5d–e). Despite that not enough data are available in the literature on the reproductive biology of A. cyclophora and based on our observations on the reproductive condition of the discarded specimens (unpublished data), most skates shown
331
in Fig. 5d were immature. The specimens of S. bonapartii sampled were also immature (Fig. 5e). The case of Puerto Quequ´en coastal bottom trawl fishery does not allow establishing a clear pattern of variation in the relative abundance of batoids. The lack of information before our study prevented any comparative study of changes in community structure of batoid fishes in the area. The present fishing pressure in this mixed-species fishery could have an important impact in the structure of the bottom coastal communities including batoid fishes and thus such an impact may be irreversible in the short time. These possibilities should be a cause for concern for the environmental and fisheries authorities as well as NGO’s and fishermen communities.
Acknowledgments The authors thank E. Arribalzaga, A. Averbuj, C. Boy, M. Br¨oguer, S. Gonzalez Capria, F. P´erez and A. Suarez for help provided during fieldwork. M. Braccini improves the English manuscript and made many helpful comments. Skippers and crews of fishing vessels FV Deledele, FV Angela, FV La Cruz del Sur, FV Jes´us de Nazareth, FV Felicitas, FV Eusonia, FV Angel Antonio and FV La Cruz del Sud. R. Bruno (Santa Cecilia) and J. Renaudo (La Portuguesa), and their personnel. Prefectura Naval Argentina, detachment Puerto Quequ´en. This study was supported by Fundaci´on Antorchas (H.L.C., Proyecto A-13672/1-3).
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