- Email: [email protected]
Factors determining the interaction between common bottlenose dolphins and bottom trawlers off the Balearic Archipelago (western Mediterranean Sea)
Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
Contents lists available at ScienceDirect
Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e
Factors determining the interaction between common bottlenose dolphins and bottom trawlers off the Balearic Archipelago (western Mediterranean Sea) Joan Gonzalvo a,⁎, María Valls b, Lluís Cardona a, Alex Aguilar a a b
Department of Animal Biology, Faculty of Biology, University of Barcelona, Avinguda Diagonal 645, E08028-Barcelona, Spain Centre Oceanogràfic de les Balears, Instituto Español de Oceanografía, Moll de Ponent s/n. 07080. Palma de Mallorca, Spain
a r t i c l e
i n f o
Article history: Received 9 January 2008 Received in revised form 20 August 2008 Accepted 21 August 2008 Keywords: Bottlenose dolphin Bottom trawlers Distribution Fishery interaction
a b s t r a c t Factors determining bottlenose dolphin association with bottom trawlers were studied off the Balearic Islands, western Mediterranean, by studying dolphin distribution around the islands and their interaction with fishing operations. Results showed that bottlenose dolphins avoided the upper shelf (shallower than 50 m) in the warm season, but not in the cold season and that the slope was avoided all year round. Bottlenose dolphins approached most of the trawlers surveyed in the continental shelf, but seldom interacted with those operating in the slope in the warm season. As a consequence, the average depth of trawling operations that attracted dolphins was shallower than those not attracting dolphins. No statistical difference was observed in the average catch of hauls conducted in the presence or in the absence of dolphins. However, discriminant analysis showed differences in catch composition between hauls with and without dolphin presence, but this difference was not attributed to the palatability of the catch, but to influence of trawling depth on the catch. It is concluded that depth is the main factor ruling occurrence of interaction. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Fishing has dramatically modified coastal ecosystems around the world, mainly by overfishing and ecological extinction of top predators (Jackson et al., 2001). Marine mammals are one of the groups of animals that have been most negatively affected, as they have been intensely exploited (Baker and Clapham, 2002; Romero et al., 2002) and they interact adversely with fishing operations targeting other species (Gosliner; 1999; Crespo et al., 1997; Dans et al., 1997; Crespo et al., 2000). This latter interaction is not restricted to the mortality produced by entanglement in fishing gear, but it also includes aggressions by fishermen to mitigate poaching of dolphins on commercial species and avoid the consequent damage to fishing gear (Bearzi, 2002; Reeves et al., 2001; Lauriano et al., 2004). Dolphins often been reported to follow trawling boats to take advantage of discarded fish or seize fish from the net (Leatherwood, 1975; Fertl and Leatherwood, 1997; Corkeron et al., 1990; Broadhurst, 1998; Chilvers and Corkeron, 2001). This behaviour offers access to naturally unavailable resources, but also increases risk of by-catch (Fertl and Leatherwood, 1997). The common bottlenose dolphin (Tursiops truncatus), hereafter bottlenose dolphin, is the commonest marine mammal in the continental shelf of the Mediterranean Sea, although deep-water sightings have been reported from various areas (Notarbartolo di Sciara, ⁎ Corresponding author. Present address: Tethys Research Institute, Viale G.B. Gadio 2, 20121 Milan, Italy. Tel./fax: +30 2643023861. E-mail address: [email protected] (J. Gonzalvo). 0022-0981/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2008.08.013
2002). Bottlenose dolphin populations are scattered throughout the Mediterranean and fragmented into small units, one of which is found around the Balearic Islands (Forcada et al., 2004). The near-shore distribution of the species results in being one of the main involved in interactions with trawlers (Northridge, 1984; Silvani et al., 1992; Bearzi et al., 1999; Gonzalvo and Aguilar, 2004), which is a widespread type of fishing in the western Mediterranean basin (Papathanassious and Gabrielides, 1999). Published information about interactions between bottlenose dolphins and trawlers deals with the feeding behaviour of dolphins around the fishing vessels (Corkeron et al., 1990; Broadhurst, 1998; Svane, 2005) and the potential impact of the fishery on the dolphin populations (Morizur et al.,1999; Majluf et al., 2002; Lopez et al., 2003). However, information about factors determining the interaction is non-existent despite its relevance in the design of conflict-minimizing strategies. This paper aims to document this type of interaction around the Balearic Islands and assess the significance of these factors. 2. Material and methods 2.1. Study Area The study was conducted in the Balearic Islands, an archipelago situated in the western Mediterranean and separated from the Iberian Peninsula by large geological submarine barriers and depths ranging 800-2,000 m. The Balearic platform consists of two flat shelves: the smaller Ibiza-Formentera shelf (aprox. 3480 Km2) to the west, and the larger Majorca-Minorca shelf (aprox. 12,315 Km2) to the east. Together,
48
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
both extend through about 15,800 Km2. in both cases the shelf is narrow, with rocky seashores, vast sea-grass meadows shallower than 35 m, and sand and sandy-muddy bottoms deeper than 35 m. The slope is very steep and there are no submarine canyons. The shortest distance between Majorca and the Iberian Peninsula is 93 nautical miles (170 Km). 2.2. Distribution of bottlenose dolphins Surveys in the continental shelf were conducted in June, July and September 2002, from middle March to end of July 2003, and from middle March to end of June 2004 (Fig. 1). They were carried out from a 6.8 m long keeled inflatable boat (Sacs-680 Ghost) equipped with a
Yamaha 115 HP, four strokes engine, at an average speed of 17 knots. Surveying conditions were considered acceptable whenever carried out at daylight and good visibility, sea state was ≤3 Beaufort, and at least two observers were scanning the sea surface. A group of dolphins was defined as that composed of 'dolphins observed in apparent association, moving in the same direction and often, but not always, engaged in the same activity' (Shane, 1990). Once a dolphin group was located, its geographical position, depth and the shortest distance to the coast were recorded by means of a GPS chart-plotter (Garmin® GPSMap 176). Sampling effort and dolphin sightings were assigned to three different bathymetric domains: 0-50 m (upper shelf), 50-200 m (lower shelf) and N200 m (slope).
Fig. 1. Survey effort and bottlenose dolphin sightings off Majorca and Minorca in the warm (top map) and the cold (bottom map) seasons. Grey contours depict the 50, 200 and 800 m isobaths.
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
49
Fig. 2. Distribution of the monitored trawling operations with dolphin interaction (black lines) and without interaction (grey lines). Grey contours depict the 50, 200 and 800 m isobaths.
2.3. Trawler-dolphin interaction During the study, the trawling fleet of the Balearic Islands was composed of 55 trawlers, distributed as follows: 36 in Majorca, 7 in Minorca, 9 in Ibiza and 3 in Formentera (Data from the Fisheries Directorate of the Balearic Islands Autonomous Government). The regulations of the Spanish Ministry of Agriculture and Fisheries
stipulate that trawlers may operate at depths between 50 and 800 m, with a maximum engine power of 500 HP and up to 12 hours per day (5:00am-17:00pm), five days per week. Boats should always remain moored during weekends. Between May 2004 and May 2005, 79 trawling operations (41 in the warm season and 38 in the cold season) were monitored by an observer placed onboard the vessels (Fig. 2). Fishing was carried out
Fig. 3. Bottlenose dolphins following operating trawlers off Majorca. (Photos: J. Gonzalvo)
50
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
between 5:16 am (earliest set up of trawling net) and 15:49 pm (latest haul). Average tow duration was 3 h:37 min ± 2 h:04 min (range: 1 h to 8 h:37 min) and average towing speed was 2.7 ± 0.3 knots (range: 2.3 to 3.6 knots). Towing depth ranged 43-760 m. The geographical position of each tow and its depth were recorded at 20-30 min intervals using GPS (Global Positioning System) and echosounder/ fishfinder, respectively. Presence of dolphins around the trawler while in operation was recorded. The catch of each haul was examined to assess its composition (identification to the species level) and to quantify the catch per species, both of commercial (marketable) and discarded (non-marketable) species. Weight of each species was estimated from the number of boxes of known capacity in which it was stored. 2.4. Statistical Analysis The Lillefor's test indicated that data did not fit a normal distribution. Therefore, the Mann-Whitney's non-parametric test was used to compare the average depth, the average duration and the total catch of each haul in the presence and in the absence of bottlenose dolphins. The analysis was conducted independently for the 38 operations surveyed from November to April (cold season) and for the 41 operations surveyed from May to October (warm season), as the water column is wind-mixed from November to April but thermally stratified from May to October (Fernández-Puelles et al., 1997). A discriminant analysis was conducted for each season to check for differences in the composition of the trawler catch when bottlenose dolphins were absent or present. Species other than fish, crustaceans and cephalopods were excluded from the analysis. Species occurring in less than 10% of hauls were also excluded. 3. Results 3.1. Distribution of bottlenose dolphins The surveys covered 7,381 Km under acceptable conditions on 136 days. This effort resulted on 75 sightings of bottlenose dolphins. Sampling effort was much larger (6,796 Km of navigation) and sightings of bottlenose dolphins were more abundant (66 sightings) in the warm season than in the cold one (585 km of navigation and 9 sightings) (Fig. 1). Only two of these sightings were made in the slope, and both occurred in the warm season. Although most of the sightings occurred on the shelf (64 sightings in the warm season and 9 in the cold season), dolphins appeared to avoid the upper shelf in the warm season (Chi-Square test, χ2 = 15.270, d.f. = 1, p b 0.001), but not in the cold season (Chi-Square test, χ2 = 1.800, d.f. = 1, p = 0.180). 3.2. Trawler-dolphin interaction Bottlenose dolphins interacted often with trawlers (Fig. 3). In the warm season, they approached most trawlers operating on the continental shelf (27 of 32 operations surveyed) and never interacted with those operating on the slope (9 operations surveyed). In the cold season, dolphins interacted with all trawling operations on the continental shelf (23 operations surveyed) and with few of those carried out on the slope (5 of 15 operations surveyed). As a consequence, the average depth of trawling operations that attracted bottlenose dolphins (warm season: 82.92 ± 30.9 m; cold season: 125.2 ± 101.1 m) was shallower than those that did not attract dolphins (warm season: 420.65 ± 269.8 m; cold season: 483.2 ± 194 m) (warm season: Mann-Whitney's U = 62.50, p = 0.001; cold season: Mann-Whitney's U = 15.50, p b 0.001). The duration of operations attracting dolphins was also shorter (warm season: 82.9 ± 31.2 min; cold season: 154.4 ± 77.0 min) than those that did not attract dolphins (warm season: 420.7 ± 279.2 min; cold season: 362.5 ± 87 min) (warm season: MannWhitney's U = 99.50, p = 0.014; Mann-Whitney's U = 17.00; p b 0.001).
Both towing depth and tow duration were positively correlated (warm season: Spearman's Rho = 0.742, p b 0.001; cold season Spearman's Rho = 0.769, p b 0.001). No statistical difference was observed in the average catch of hauls conducted in the presence (warm season: 168.3 ± 117.72 kg; cold season: 193.79 ± 182.0 kg) or in the absence of dolphins (warm season: 137.17 ± 116.42 kg; cold season: 233.8 ± 247.1 kg) (warm season: MannWhitney's U = 148.00, p = 0.260; cold season: Mann-Whitney's U = 145.00; p = 0.910). The discriminant analysis indicated that warm season hauls in the lower shelf with dolphin presence or absence differed in catch composition and the same was true for the cold season hauls in the slope. The data from the warm season revealed that, on the lower shelf, dolphins interacted only with trawlers whose catch included large amounts of the following species: Arnoglossus sp., B. sponsalis, B. ocellaris, B. boops, C. conger, E. cirrhosa, S. canicula, and U. scaber, and some amounts of A. sphyraena, I. cointedii, L. boscii, L. cavillone, O. vulgaris, P. erithrynus, Paguridae, R. clavata, S. scrofa, and S. cabrilla. Thus, the discriminant equation (Table 1; Wilks'λ = 0.042; χ2 = 49.017, d.f.= 29, p = 0.012) classified successfully the 32 hauls included in the analysis (Fig. 4). However, the operations conducted in the presence and in the absence of dolphins did not differ in average depth (Mann-Whitney's U = 62.50; p = 0.795). The data from the cold season indicated that, on the slope, dolphins interacted only with trawlers whose catch included large amounts of Arnoglossus sp., B. boops, C. aper, C. caelorhincus, H. dactylopterus, and I. coindetii, and low amounts of A. sphyraena, C. conger, G. argenteus, and L. crocodilus; the discriminant function (Table 1; Wilks'λ = 0.003; χ2 = 43.761, d.f. = 11, p b 0.001) classified successfully the 15 hauls analysed (Fig. 4). Furthermore, the hauls
Table 1 Contribution of species to the discriminant equations calculated for the warm and the cold seasons SPECIES
Warm season
Argentina sphyraena Aristeus antennatus Arnoglossus sp. Bathypolipus sponsalis Blennius. ocellaris Boops boops Caelorhincus caelorhincus Capros aper Citharus linguatula Conger conger Dardanus arrosor Eledone cirrhosa Eledone moscata Gadiculus argenteus Helicolenus dactylopterus Ilex cointedii Lampanictus crocodilus Lepidorhombus boscii Lepidotrigla cavillone Loligo vulgaris Lophius budegassa Lophius piscatorius Mullus barbatus Octopus vulgaris Pagellus erithrynus Paguridae Raja clavata Raja miraletus Scorpaena scrofa Scyliorhinus canicula Serranus cabrilla Serranus hepatus Trachinus radiatus Uranoscopus scaber Centroid Values with dolphin interaction without dolphin interaction
2.174 - 0.845 - 2.948 - 9.933 - 2.301 - 1.662 0.000 0.000 - 0.098 - 3.466 - 0.380 - 3.333 0.925 0.000 0.000 4.902 0.000 2.368 1.055 - 0.052 - 0.292 - 0.901 0.701 1.711 2.262 2.039 4.603 - 0.669 2.113 - 2.064 5.876 0.605 - 0.190 - 1.598
Cold season - 4.093 0.970 7.140 0.000 0.000 1.461 2.660 6.188 0.000 - 0.279 0.000 0.000 0.000 - 1.482 1.841 6.068 - 1.023 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
- 1.982 10.703
24.321 - 12.156
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
Fig. 4. Classification of the operations surveyed in the warm season (top panel) and in the cold season (bottom panel) in agreement with the discriminant equations.
attracting dolphins were carried out in waters significantly shallower than those not attracting them (Mann-Whitney's U = 6.000; p = 0.020). 4. Discussion Bottlenose dolphins are euriphagous feeders that consume a range of fish and cephalopods but consistently avoid crustaceans (Corkeron et al., 1990). Fishing may reduce food availability by decreasing the size and abundance of prey (Trites et al., 2006) but may also increase food availability if dolphins learn how to get fish entangled in nets or discarded from fishing vessels. Bottlenose dolphins off Australia are known to interact with trawlers to forage on the fish discarded (Corkeron et al., 1990; Svane, 2005) and also to manipulate the codend to have an access to the catch (Broadhurst, 1998). Here, we report that such a foraging interaction between bottlenose dolphins and trawlers is also common in the western Mediterranean. Off the Balearic Islands, bottlenose dolphins approach operating trawlers while towing, hauling and discarding, but only a fraction of the group approach the trawlers once the net is hauled or during the release of discarded fish. Thus, different dolphins from a same group may differ in the resources they use. Total catch weight, indicative of the amount of food available, was a poor predictor of dolphin presence. Bottlenose dolphins are known to forage selectively on discarded fish (Corkeron et al., 1990; Svane, 2005), so the composition of the catch could be said to be more relevant to attract dolphins than the overall size of the catch. Apparently confirming this, catch composition was found to be a good predictor of dolphin occurrence both in the shelf and in the slope. However, a more likely
51
reason justifying the association between dolphins and prey is that prey discriminates between lower shelf and slope assemblages, and therefore its distribution is strongly ruled by depth (Massutí et al., 1996; Massutí and Reñones, 2005). Boat surveys showed that bottlenose dolphins avoided the shallowest part of the continental shelf and also the slope, at least in the warm season, and this distribution pattern is shared by most of the prey species associated with dolphin occurrence (Massutí et al., 1996; Massutí and Reñones, 2005). Indeed, the isotope signal of bottlenose dolphins from Majorca and Minorca indicate that these animals do not prey on species from the upper shelf (Cardona et al., 2007a). Available knowledge about the feeding habits of the bottlenose dolphins in the Spanish Mediterranean coast also support such a scenario, as they appear to prey mainly on hake (Merluccius merluccius) (Blanco et al., 2001), a typical inhabitant of the middle and lower parts of the shelf (Relini et al., 2002). The distribution of bottlenose dolphins is almost always ruled by food availability (Hastie et al., 2004). In the Balearic archipelago, bottlenose dolphins and the local trawling fleet may be seen to behave as two sympatric species, where dolphins play a parasitic role over the fishing activity. A trawler might be considered a moving patch of food source and, by moving with it, dolphins presumably consume less time and energy to forage. As a consequence, this food source provides a higher than usual caloric value (Fertl and Leatherwood, 1997). However, interacting with trawlers might be risky. Fertl and Leatherwood (1997) reported that at least two mysticete species, 23 odontocete species, and several unidentified cetaceans (including beaked whales) have been incidentally caught in trawls. However, in the Mediterranean Sea, with the exception of the Israeli coast (Goffmann et al., 1995), cetacean bycatch in trawling nets appears to be a relatively uncommon occurrence. In the Balearic Islands, during our study, no incidental capture was recorded by observers or reported to us by over 50 interviewed fishermen (Gonzalvo unpublished data). Also, Massutí (unpublished data) monitored 460 commercial trawling operations off Majorca from 2001 to 2004 without reporting a single incidental catch of bottlenose dolphins. Hence, the only negative impact of trawling on the bottlenose dolphin population off the Balearic Archipelago appears to be the alteration of the sea bottom and the reduction in food availability caused by overexploitation, a general problem occurring all over the Mediterranean (Bearzi, 2002). Dolphins were observed preferably interacting with trawling operations that were shallow and short. These two variables were correlated because, when trawls are deployed at deeper depths, boats tend to tow longer to compensate decrease in catch per unit effort with depth (Massutí and Reñones, 2005). As a consequence, depth appears to be the main factor explaining variability in the dolphintrawlers interaction: bottlenose dolphins preferred the lower shelf, and therefore interacted more frequently with vessels operating there. Several studies have shown that local physiography can play a significant role on odontocete distribution, being depth one of the variables with the strongest influence (Hui, 1985; Ingram and Rogan, 2002; Cañadas et al., 2002; Yen et al., 2004). However, bottlenose dolphins off the Balearic Islands avoided the upper shelf in the warm season. The design of the study here reported does not allow identifying the precise reason for such a pattern, but increased boat traffic and human presence during the summer tourist season might be involved. Tourism is the main industry in Majorca and Minorca, which in 2005 they together received 9.3 million visitors and registered about 35,000 leisure boats, this is, 1 boat per 25 m of coastline (GOB 2002; http://www.gobmallorca.com/mar/index.htm). Also, recreational boat-fishing, targeting fish species living in the upper shelf, is widespread in the islands (Morales-Nin et al., 2005; Cardona et al., 2007b). All these activities peak markedly during the summer, and short-term displacement has been reported as a common response of cetaceans to boat traffic when interactions become intrusive or too lengthy to avoid acoustic disturbance and risk of collisions (Bejder et al., 1999; Nowacek et al., 2001; Williams et al.,
52
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
2002; Lusseau, 2003). Thus, in a similar scenario in Milford Sound, New Zealand, Lusseau (2005) has shown that the benefits for bottlenose dolphins of using a certain area are largely overcome by the costs of interaction with boats. Nevertheless, further research is needed to asses the actual relevance of boat traffic on dolphin distribution in the shelf of the Balearic Islands, as differences in water stratification may confound the results of seasonal comparisons. Acknowledgements This study was funded by the Spanish Ministry of the Environment, EU-LIFE project NAT/E/7303, IEO-DEMO and IEO-MEGA-2 projects. The authors wish to thank Irene Alvarez, Maite Lijalad, Vanessa Martin, Monica Revelles, Fabiana Saporiti and Silvia Scali for their help with data collection during boat surveys, and also the skippers of the fishing vessels “Antonia Munar II”, “Moralti nou”, “Joven Mirmer”, “Bellver”, “El Montgó”, “Nou Capdepera” and “Villa Sóller segundo”. [SS] References Baker, C.S., Clapham, P.J., 2002. Marine mammal exploitation: whales and whaling. In: Douglas, I. (Ed.), Encyclopaedia of global environmental change. Causes and consequences of global environmental change, vol. 3. John Wiley and Sons, New York, pp. 446–450. Bearzi, G., 2002. Interactions between cetacean and fisheries in the Mediterranean Sea. In: Notarbartolo di Sciara, G. (Ed.), Cetaceans of the Mediterranean and Black Seas: state of knowledge and conservation strategies. A report to the ACCOBAMS Secretariat, Monaco, February 2002. Section 9, 20 pp. Bearzi, G., Politi, E., Notarbartolo di Sciara, G., 1999. Diurnal behavior of free-ranging bottlenose dolphins in the Kvarneric (northern Adriatic Sea). Mar. Mamm. Sci. 15 (4), 1065–1097. Bejder, L., Dawson, S.M., Harraway, J.A., 1999. Responses by Hector's dolphins to boats and swimmers in Porpoise Bay, New Zealand. Mar. Mamm. Sci. 15 (3), 738–750. Blanco, C., Salomón, O., Raga, J.A., 2001. Diet of the bottlenose dolphin (Tursiops truncatus) in the Western Mediterranean Sea. J. Mar. Biol. Assoc. U.K. 81, 1053–1058. Broadhurst, M.K., 1998. Bottlenose dolphin, Tursiops truncatus, removing bycatch from prawn-trawl codends during fishing in New South Wales, Australia. Mar. Fish. Rev. 60 (3), 9–14. Cañadas, A., Sagarminaga, R., García-Tiscar, S., 2002. Cetacean distribution related with depth and slope in the Mediterranean waters off southern Spain. Deep-sea Res., Part 1, Oceanogr. Res. Pap. 49 (11), 2053–2073. Cardona, L., Revelles, M., Sales, M., Aguilar, A., Borrell, A., 2007a. Meadows of the seagrass Posidonia oceanica are a significant source of organic matter for adjoining ecosystems. Mar. Ecol. Prog. Ser. 335, 123–131. Cardona, L., López, D., Sales, M., de Caralt, S., Díez, I., 2007b. Effects of recreational fishing on three fish species from the Posidonia oceanica meadows off Minorca (Balearic archipelago, western Mediterranean). Sci. Mar. 71, 811–820. Chilvers, B.L., Corkeron, P.J., 2001. Trawling and bottlenose dolphins' social structure. Proc. R. Soc. Lond., B 268, 1901–1905. Corkeron, P.J., Bryden, M.M., Hedstrom, K.E., 1990. Feeding by bottlenose dolphins in association with trawling operations in Moreton Bay, Australia. In: Leatherwood, S., Reeves, R.R. (Eds.), The bottlenose dolphin. Acad. Press, San Diego, Calif., pp. 329–336. Crespo, E.A., Pedraza, S.N., Dans, S.L., Koen Alonso, M., Reyes, L.M., García, N.A., Coscarella, M., Schiavini, A.C.M., 1997. Direct and indirect effects of the high seas fisheries on the marine mammal populations in the northern and central Patagonian coast. J. Northwest Atl. Fish. Sci. 22, 189–207. Crespo, E.A., Koen Alonso, M., Dans, S.L., García, N.A., Pedraza, S.N., Coscarella, M., González, R., 2000. Incidental catches of dolphins in mid-water trawls for Argentine anchovy (Engraulis anchoita) off the Argentine shelf. J. Cetacean Res. Manag. 2, 11–16. Dans, S.L., Crespo, E.A., García, N.A., Reyes, L.M., Pedraza, S.N., Alonso, M.K., 1997. Incidental mortality of Patagonian dusky dolphins in mid-water trawling: Retrospective effects from the early 1980s. Rep. Int. Whal. Comm. 47, 699–703. Fernández-Puelles, M.L., Jansa, J., Gomis, C., Gras, D., Amengual, B., 1997. Variación anual de las principales variables oceanográficas y planctónicas en una estación nerítica del mar Balear. Bol. Inst. Esp. Oceanogr. 13, 13–33 (in Spanish). Fertl, D., Leatherwood, S., 1997. Cetacean interactions with trawlers: a preliminary review. J. Northwest Atl. Fish. Sci. 22, 219–248. Forcada, J., Gazo, M., Aguilar, A., Gonzalvo, J., Fernández-Contreras, M., 2004. Bottlenose dolphin abundance in the NW Mediterranean: addressing heterogeneity in distribution. Mar. Ecol. Prog. Ser. 275, 275–287. Goffmann, O., Kerem, D., Spanier, E., 1995. Dolphin interactions with fishing-trawlers off the Mediterranean coast of Israel. Abstract. 11th Biennial Conference on the Biology of Marine Mammals, Orlando, FL. 14-18 December 1995. Gonzalvo, J., Aguilar, A., 2004. Photoidentification of bottlenose dolphins (Tursiops truncatus) in the Balearic Islands. Book of Abstracts of 18th Annual Conference of the European Cetacean Society, Kolmarden, Sweden, 28–31 March, 2004. Gosliner, M.L., 1999. The tuna-dolphin controversy. In: TwissJr. Jr., J.R., Reeves, R.R. (Eds.), Conservation and Management of Marine Mammals. Smithsonian Institution Press, Washington, D.C., pp. 120–155.
Hastie, G.D., Wilson, B., Wilson, L.J., Parsons, K.M., Thompson, P.M., 2004. Functional mechanisms underlying cetacean distribution patterns: hotspots for bottlenose dolphins are linked to foraging. Mar. Biol. 144, 397–403. Hui, C.A., 1985. Undersea topography and the comparative distributions of two pelagic cetaceans. U.S. Fish. Bull. 83, 472–475. Ingram, S.N., Rogan, E., 2002. Identifying critical areas and habitat preferences of bottlenose dolphins Tursiops truncatus. Mar. Ecol. Prog. Ser. 44, 247–255. Jackson, J.B.C., Kirby, M.X., Berger, W.H., Bjorndal, K.A., Botsford, L.W., Bourque, B.J., Bradbury, R.H., Cooke, R., Erlandson, J., Estes, J.A., Hughes, T.P., Kidwell, S., Lange, C.B., Lenihan, H.S., Pandolfi, J.M., Peterson, C.H., Steneck, R.S., Tegner, M.J., Warner, R.R., 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293, 629–638. Lauriano, G., Fortuna, C.M., Moltedo, G., Notarbartolo di Sciara, G., 2004. Interactions between common bottlenose dolphins (Tursiops truncatus) and the artisanal fishery in Asinara Island National Park (Sardinia): assessment of catch damage and economic loss. J. Cetacean Res. Manag. 6 (2), 165–173. Leatherwood, S., 1975. Some observations of feeding behavior of bottle-nosed dolphins (Tursiops truncatus) in the northern Gulf of Mexico and Tursiops (cf T. gilli) off southern California, Baja California, and Nayarit, Mexico. Mar. Fish. Rev. 37 (9), 10–16. Lopez, A., Pierce, G.J., Santos, M.B., Gracia, J., Guerra, A., 2003. Fishery by-catches of marine mammals in Galician waters: results from on-board observations and an interview survey of fishermen. Biol. Conserv. 111, 25–40. Lusseau, D., 2003. Effects of tour boats on the behavior of bottlenose dolphins: Using Markov chains to model anthropogenic impacts. Conserv. Biol. 17 (6), 1785–1793. Lusseau, D., 2005. The residency pattern of bottlenose dolphins (Tursiops spp.) in Milford Sound, New Zealand, is related to boat traffic. Mar. Ecol. Prog. Ser. 295, 265–272. Majluf, P., Babcock, E., Riveros, J., Schreiber, M., Alderete, W., 2002. Catch and Bycatch of Sea Birds and Marine Mammals in the Small-Scale Fishery of Punta San Juan, Peru. Conserv. Biol. 16 (5), 1333–1343. Massutí, E., Reñones, O., 2005. Demersal resource assemblages in the trawl fishing grounds off the Balearic Islands (western Mediterranean). Sci. Mar. 69 (1), 167–181. Massutí, E., Reñones, O., Carbonell, A., Oliver, P., 1996. Demersal fish communities exploited on the continental shelf and slope off Majorca (Balearic Islands, NW Mediterranean). Vie Milieu 46 (1), 45–55. Morales-Nin, B., Moranta, J., Garcia, C., Tugores, M.P., Grau, A.M., Riera, F., Cerda, M., 2005. The recreational fishery off Majorca Island (western Mediterranean): some implications for coastal resource management. ICES J. Mar. Sci. 62, 727–739. Morizur, Y., Berrow, S.D., Tregenza, N.J.C., Couperus, A.S., Pouvreau, S., 1999. Incidental catches of marine mammals in pelagic trawl fisheries of the northeast Atlantic. Fish. Res. 41, 297–307. Nowacek, S.M., Wells, R.S., Solow, A.R., 2001. Short-term effects of boat traffic on bottlenose dolphins, Tursiops truncatus, in Sarasota Bay, Florida. Mar. Mamm. Sci. 17, 673–688. Northridge, S., 1984. World review of interactions between marine mammals and fisheries. FAO Fisheries Technical Paper, vol. 251. FAO, Rome, Italy. 190 pp. Notarbartolo di Sciara, G., 2002. Cetacean species occurring in the Mediterranean and Black Seas. In: Notarbartolo di Sciara, G. (Ed.), Cetaceans of the Mediterranean and Black Seas: state of knowledge and conservation strategies. A report to the ACCOBAMS Secretariat, Monaco, February 2002. Section 3, 17 pp. Papathanassious, E., Gabrielides, G.P., 1999. State and pressures of the marine and coastal Mediterranean environment. European Environment Agency. Reeves, R., Read, A., Notarbartolo di Sciara, G., 2001. Report of the Workshop on Interactions between Dolphins and Fisheries in the Mediterranean: Evaluation of Mitigation Alternatives. ICRAM, Rome, Italy. Relini, L.O., Papaconstantinou, C., Jukic-Peladic, S., Souplet, A., De Sola, L.G., Piccinetti, C., Kavadas, S., Rossi, M., 2002. Distribution of the Mediterranean hake populations (Merluccius merluccius smiridus Rafinesque, 1810) (Osteichthyes: Gadiformes) based on six years monitoring by trawl-surveys: some implications for management. Sci. Mar. 66 (2), 19–38. Romero, A., Baker, R., Creswell, J.E., Singh, A., McKie, A., Manna, M., 2002. Environmental history of marine mammal exploitation in Trinidad and Tobago, W.I. and its ecological impact. Environ. Hist. 8 (3), 255–274. Silvani, L., Raich, J., Aguilar, A., 1992. Bottle-nosed dolphins, Tursiops truncatus, interacting with fisheries in the Balearic Islands, Spain. Eur. Res. Cetaceans 6, 32–34. Shane, S.H., 1990. Behavior and ecology of the bottlenose dolphin at Sanibel Island, Florida. In: Leatherwood, S., Reeves, R.R. (Eds.), The bottlenose dolphin. Academic Press, San Diego, Calif., pp. 245–265. Svane, I., 2005. Occurrence of dolphins and seabirds and their consumption of by-catch during prawn trawling in Spencer Gulf, South Australia. Fish. Res. 76, 317–327. Trites, A.W., Christensen, V., Pauly, D., 2006. Effects of fisheries on ecosystems: just another top predator? In: Boyd, I.L., Wanless, S., Camphuysen, C.J. (Eds.), Top Predators in Marine Ecosystems. Cambridge University Press, pp. 11–27. Williams, R., Trites, A.W., Bain, D.E., 2002. Behavioural responses of killer whales (Orcinus orca) to whale-watching boats: opportunistic observations and experimental approaches. J. Zool. 256, 255–270. Yen, P.P.W., Sydeman, W.J., Hyrenbach, K.D., 2004. Marine bird and cetacean associations with bathymetric habitats and shallow-water topographies: Implications for trophic transfer and conservation. J. Mar. Syst. 50, 79–99.
Contents lists available at ScienceDirect
Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e
Factors determining the interaction between common bottlenose dolphins and bottom trawlers off the Balearic Archipelago (western Mediterranean Sea) Joan Gonzalvo a,⁎, María Valls b, Lluís Cardona a, Alex Aguilar a a b
Department of Animal Biology, Faculty of Biology, University of Barcelona, Avinguda Diagonal 645, E08028-Barcelona, Spain Centre Oceanogràfic de les Balears, Instituto Español de Oceanografía, Moll de Ponent s/n. 07080. Palma de Mallorca, Spain
a r t i c l e
i n f o
Article history: Received 9 January 2008 Received in revised form 20 August 2008 Accepted 21 August 2008 Keywords: Bottlenose dolphin Bottom trawlers Distribution Fishery interaction
a b s t r a c t Factors determining bottlenose dolphin association with bottom trawlers were studied off the Balearic Islands, western Mediterranean, by studying dolphin distribution around the islands and their interaction with fishing operations. Results showed that bottlenose dolphins avoided the upper shelf (shallower than 50 m) in the warm season, but not in the cold season and that the slope was avoided all year round. Bottlenose dolphins approached most of the trawlers surveyed in the continental shelf, but seldom interacted with those operating in the slope in the warm season. As a consequence, the average depth of trawling operations that attracted dolphins was shallower than those not attracting dolphins. No statistical difference was observed in the average catch of hauls conducted in the presence or in the absence of dolphins. However, discriminant analysis showed differences in catch composition between hauls with and without dolphin presence, but this difference was not attributed to the palatability of the catch, but to influence of trawling depth on the catch. It is concluded that depth is the main factor ruling occurrence of interaction. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Fishing has dramatically modified coastal ecosystems around the world, mainly by overfishing and ecological extinction of top predators (Jackson et al., 2001). Marine mammals are one of the groups of animals that have been most negatively affected, as they have been intensely exploited (Baker and Clapham, 2002; Romero et al., 2002) and they interact adversely with fishing operations targeting other species (Gosliner; 1999; Crespo et al., 1997; Dans et al., 1997; Crespo et al., 2000). This latter interaction is not restricted to the mortality produced by entanglement in fishing gear, but it also includes aggressions by fishermen to mitigate poaching of dolphins on commercial species and avoid the consequent damage to fishing gear (Bearzi, 2002; Reeves et al., 2001; Lauriano et al., 2004). Dolphins often been reported to follow trawling boats to take advantage of discarded fish or seize fish from the net (Leatherwood, 1975; Fertl and Leatherwood, 1997; Corkeron et al., 1990; Broadhurst, 1998; Chilvers and Corkeron, 2001). This behaviour offers access to naturally unavailable resources, but also increases risk of by-catch (Fertl and Leatherwood, 1997). The common bottlenose dolphin (Tursiops truncatus), hereafter bottlenose dolphin, is the commonest marine mammal in the continental shelf of the Mediterranean Sea, although deep-water sightings have been reported from various areas (Notarbartolo di Sciara, ⁎ Corresponding author. Present address: Tethys Research Institute, Viale G.B. Gadio 2, 20121 Milan, Italy. Tel./fax: +30 2643023861. E-mail address: [email protected] (J. Gonzalvo). 0022-0981/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2008.08.013
2002). Bottlenose dolphin populations are scattered throughout the Mediterranean and fragmented into small units, one of which is found around the Balearic Islands (Forcada et al., 2004). The near-shore distribution of the species results in being one of the main involved in interactions with trawlers (Northridge, 1984; Silvani et al., 1992; Bearzi et al., 1999; Gonzalvo and Aguilar, 2004), which is a widespread type of fishing in the western Mediterranean basin (Papathanassious and Gabrielides, 1999). Published information about interactions between bottlenose dolphins and trawlers deals with the feeding behaviour of dolphins around the fishing vessels (Corkeron et al., 1990; Broadhurst, 1998; Svane, 2005) and the potential impact of the fishery on the dolphin populations (Morizur et al.,1999; Majluf et al., 2002; Lopez et al., 2003). However, information about factors determining the interaction is non-existent despite its relevance in the design of conflict-minimizing strategies. This paper aims to document this type of interaction around the Balearic Islands and assess the significance of these factors. 2. Material and methods 2.1. Study Area The study was conducted in the Balearic Islands, an archipelago situated in the western Mediterranean and separated from the Iberian Peninsula by large geological submarine barriers and depths ranging 800-2,000 m. The Balearic platform consists of two flat shelves: the smaller Ibiza-Formentera shelf (aprox. 3480 Km2) to the west, and the larger Majorca-Minorca shelf (aprox. 12,315 Km2) to the east. Together,
48
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
both extend through about 15,800 Km2. in both cases the shelf is narrow, with rocky seashores, vast sea-grass meadows shallower than 35 m, and sand and sandy-muddy bottoms deeper than 35 m. The slope is very steep and there are no submarine canyons. The shortest distance between Majorca and the Iberian Peninsula is 93 nautical miles (170 Km). 2.2. Distribution of bottlenose dolphins Surveys in the continental shelf were conducted in June, July and September 2002, from middle March to end of July 2003, and from middle March to end of June 2004 (Fig. 1). They were carried out from a 6.8 m long keeled inflatable boat (Sacs-680 Ghost) equipped with a
Yamaha 115 HP, four strokes engine, at an average speed of 17 knots. Surveying conditions were considered acceptable whenever carried out at daylight and good visibility, sea state was ≤3 Beaufort, and at least two observers were scanning the sea surface. A group of dolphins was defined as that composed of 'dolphins observed in apparent association, moving in the same direction and often, but not always, engaged in the same activity' (Shane, 1990). Once a dolphin group was located, its geographical position, depth and the shortest distance to the coast were recorded by means of a GPS chart-plotter (Garmin® GPSMap 176). Sampling effort and dolphin sightings were assigned to three different bathymetric domains: 0-50 m (upper shelf), 50-200 m (lower shelf) and N200 m (slope).
Fig. 1. Survey effort and bottlenose dolphin sightings off Majorca and Minorca in the warm (top map) and the cold (bottom map) seasons. Grey contours depict the 50, 200 and 800 m isobaths.
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
49
Fig. 2. Distribution of the monitored trawling operations with dolphin interaction (black lines) and without interaction (grey lines). Grey contours depict the 50, 200 and 800 m isobaths.
2.3. Trawler-dolphin interaction During the study, the trawling fleet of the Balearic Islands was composed of 55 trawlers, distributed as follows: 36 in Majorca, 7 in Minorca, 9 in Ibiza and 3 in Formentera (Data from the Fisheries Directorate of the Balearic Islands Autonomous Government). The regulations of the Spanish Ministry of Agriculture and Fisheries
stipulate that trawlers may operate at depths between 50 and 800 m, with a maximum engine power of 500 HP and up to 12 hours per day (5:00am-17:00pm), five days per week. Boats should always remain moored during weekends. Between May 2004 and May 2005, 79 trawling operations (41 in the warm season and 38 in the cold season) were monitored by an observer placed onboard the vessels (Fig. 2). Fishing was carried out
Fig. 3. Bottlenose dolphins following operating trawlers off Majorca. (Photos: J. Gonzalvo)
50
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
between 5:16 am (earliest set up of trawling net) and 15:49 pm (latest haul). Average tow duration was 3 h:37 min ± 2 h:04 min (range: 1 h to 8 h:37 min) and average towing speed was 2.7 ± 0.3 knots (range: 2.3 to 3.6 knots). Towing depth ranged 43-760 m. The geographical position of each tow and its depth were recorded at 20-30 min intervals using GPS (Global Positioning System) and echosounder/ fishfinder, respectively. Presence of dolphins around the trawler while in operation was recorded. The catch of each haul was examined to assess its composition (identification to the species level) and to quantify the catch per species, both of commercial (marketable) and discarded (non-marketable) species. Weight of each species was estimated from the number of boxes of known capacity in which it was stored. 2.4. Statistical Analysis The Lillefor's test indicated that data did not fit a normal distribution. Therefore, the Mann-Whitney's non-parametric test was used to compare the average depth, the average duration and the total catch of each haul in the presence and in the absence of bottlenose dolphins. The analysis was conducted independently for the 38 operations surveyed from November to April (cold season) and for the 41 operations surveyed from May to October (warm season), as the water column is wind-mixed from November to April but thermally stratified from May to October (Fernández-Puelles et al., 1997). A discriminant analysis was conducted for each season to check for differences in the composition of the trawler catch when bottlenose dolphins were absent or present. Species other than fish, crustaceans and cephalopods were excluded from the analysis. Species occurring in less than 10% of hauls were also excluded. 3. Results 3.1. Distribution of bottlenose dolphins The surveys covered 7,381 Km under acceptable conditions on 136 days. This effort resulted on 75 sightings of bottlenose dolphins. Sampling effort was much larger (6,796 Km of navigation) and sightings of bottlenose dolphins were more abundant (66 sightings) in the warm season than in the cold one (585 km of navigation and 9 sightings) (Fig. 1). Only two of these sightings were made in the slope, and both occurred in the warm season. Although most of the sightings occurred on the shelf (64 sightings in the warm season and 9 in the cold season), dolphins appeared to avoid the upper shelf in the warm season (Chi-Square test, χ2 = 15.270, d.f. = 1, p b 0.001), but not in the cold season (Chi-Square test, χ2 = 1.800, d.f. = 1, p = 0.180). 3.2. Trawler-dolphin interaction Bottlenose dolphins interacted often with trawlers (Fig. 3). In the warm season, they approached most trawlers operating on the continental shelf (27 of 32 operations surveyed) and never interacted with those operating on the slope (9 operations surveyed). In the cold season, dolphins interacted with all trawling operations on the continental shelf (23 operations surveyed) and with few of those carried out on the slope (5 of 15 operations surveyed). As a consequence, the average depth of trawling operations that attracted bottlenose dolphins (warm season: 82.92 ± 30.9 m; cold season: 125.2 ± 101.1 m) was shallower than those that did not attract dolphins (warm season: 420.65 ± 269.8 m; cold season: 483.2 ± 194 m) (warm season: Mann-Whitney's U = 62.50, p = 0.001; cold season: Mann-Whitney's U = 15.50, p b 0.001). The duration of operations attracting dolphins was also shorter (warm season: 82.9 ± 31.2 min; cold season: 154.4 ± 77.0 min) than those that did not attract dolphins (warm season: 420.7 ± 279.2 min; cold season: 362.5 ± 87 min) (warm season: MannWhitney's U = 99.50, p = 0.014; Mann-Whitney's U = 17.00; p b 0.001).
Both towing depth and tow duration were positively correlated (warm season: Spearman's Rho = 0.742, p b 0.001; cold season Spearman's Rho = 0.769, p b 0.001). No statistical difference was observed in the average catch of hauls conducted in the presence (warm season: 168.3 ± 117.72 kg; cold season: 193.79 ± 182.0 kg) or in the absence of dolphins (warm season: 137.17 ± 116.42 kg; cold season: 233.8 ± 247.1 kg) (warm season: MannWhitney's U = 148.00, p = 0.260; cold season: Mann-Whitney's U = 145.00; p = 0.910). The discriminant analysis indicated that warm season hauls in the lower shelf with dolphin presence or absence differed in catch composition and the same was true for the cold season hauls in the slope. The data from the warm season revealed that, on the lower shelf, dolphins interacted only with trawlers whose catch included large amounts of the following species: Arnoglossus sp., B. sponsalis, B. ocellaris, B. boops, C. conger, E. cirrhosa, S. canicula, and U. scaber, and some amounts of A. sphyraena, I. cointedii, L. boscii, L. cavillone, O. vulgaris, P. erithrynus, Paguridae, R. clavata, S. scrofa, and S. cabrilla. Thus, the discriminant equation (Table 1; Wilks'λ = 0.042; χ2 = 49.017, d.f.= 29, p = 0.012) classified successfully the 32 hauls included in the analysis (Fig. 4). However, the operations conducted in the presence and in the absence of dolphins did not differ in average depth (Mann-Whitney's U = 62.50; p = 0.795). The data from the cold season indicated that, on the slope, dolphins interacted only with trawlers whose catch included large amounts of Arnoglossus sp., B. boops, C. aper, C. caelorhincus, H. dactylopterus, and I. coindetii, and low amounts of A. sphyraena, C. conger, G. argenteus, and L. crocodilus; the discriminant function (Table 1; Wilks'λ = 0.003; χ2 = 43.761, d.f. = 11, p b 0.001) classified successfully the 15 hauls analysed (Fig. 4). Furthermore, the hauls
Table 1 Contribution of species to the discriminant equations calculated for the warm and the cold seasons SPECIES
Warm season
Argentina sphyraena Aristeus antennatus Arnoglossus sp. Bathypolipus sponsalis Blennius. ocellaris Boops boops Caelorhincus caelorhincus Capros aper Citharus linguatula Conger conger Dardanus arrosor Eledone cirrhosa Eledone moscata Gadiculus argenteus Helicolenus dactylopterus Ilex cointedii Lampanictus crocodilus Lepidorhombus boscii Lepidotrigla cavillone Loligo vulgaris Lophius budegassa Lophius piscatorius Mullus barbatus Octopus vulgaris Pagellus erithrynus Paguridae Raja clavata Raja miraletus Scorpaena scrofa Scyliorhinus canicula Serranus cabrilla Serranus hepatus Trachinus radiatus Uranoscopus scaber Centroid Values with dolphin interaction without dolphin interaction
2.174 - 0.845 - 2.948 - 9.933 - 2.301 - 1.662 0.000 0.000 - 0.098 - 3.466 - 0.380 - 3.333 0.925 0.000 0.000 4.902 0.000 2.368 1.055 - 0.052 - 0.292 - 0.901 0.701 1.711 2.262 2.039 4.603 - 0.669 2.113 - 2.064 5.876 0.605 - 0.190 - 1.598
Cold season - 4.093 0.970 7.140 0.000 0.000 1.461 2.660 6.188 0.000 - 0.279 0.000 0.000 0.000 - 1.482 1.841 6.068 - 1.023 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
- 1.982 10.703
24.321 - 12.156
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
Fig. 4. Classification of the operations surveyed in the warm season (top panel) and in the cold season (bottom panel) in agreement with the discriminant equations.
attracting dolphins were carried out in waters significantly shallower than those not attracting them (Mann-Whitney's U = 6.000; p = 0.020). 4. Discussion Bottlenose dolphins are euriphagous feeders that consume a range of fish and cephalopods but consistently avoid crustaceans (Corkeron et al., 1990). Fishing may reduce food availability by decreasing the size and abundance of prey (Trites et al., 2006) but may also increase food availability if dolphins learn how to get fish entangled in nets or discarded from fishing vessels. Bottlenose dolphins off Australia are known to interact with trawlers to forage on the fish discarded (Corkeron et al., 1990; Svane, 2005) and also to manipulate the codend to have an access to the catch (Broadhurst, 1998). Here, we report that such a foraging interaction between bottlenose dolphins and trawlers is also common in the western Mediterranean. Off the Balearic Islands, bottlenose dolphins approach operating trawlers while towing, hauling and discarding, but only a fraction of the group approach the trawlers once the net is hauled or during the release of discarded fish. Thus, different dolphins from a same group may differ in the resources they use. Total catch weight, indicative of the amount of food available, was a poor predictor of dolphin presence. Bottlenose dolphins are known to forage selectively on discarded fish (Corkeron et al., 1990; Svane, 2005), so the composition of the catch could be said to be more relevant to attract dolphins than the overall size of the catch. Apparently confirming this, catch composition was found to be a good predictor of dolphin occurrence both in the shelf and in the slope. However, a more likely
51
reason justifying the association between dolphins and prey is that prey discriminates between lower shelf and slope assemblages, and therefore its distribution is strongly ruled by depth (Massutí et al., 1996; Massutí and Reñones, 2005). Boat surveys showed that bottlenose dolphins avoided the shallowest part of the continental shelf and also the slope, at least in the warm season, and this distribution pattern is shared by most of the prey species associated with dolphin occurrence (Massutí et al., 1996; Massutí and Reñones, 2005). Indeed, the isotope signal of bottlenose dolphins from Majorca and Minorca indicate that these animals do not prey on species from the upper shelf (Cardona et al., 2007a). Available knowledge about the feeding habits of the bottlenose dolphins in the Spanish Mediterranean coast also support such a scenario, as they appear to prey mainly on hake (Merluccius merluccius) (Blanco et al., 2001), a typical inhabitant of the middle and lower parts of the shelf (Relini et al., 2002). The distribution of bottlenose dolphins is almost always ruled by food availability (Hastie et al., 2004). In the Balearic archipelago, bottlenose dolphins and the local trawling fleet may be seen to behave as two sympatric species, where dolphins play a parasitic role over the fishing activity. A trawler might be considered a moving patch of food source and, by moving with it, dolphins presumably consume less time and energy to forage. As a consequence, this food source provides a higher than usual caloric value (Fertl and Leatherwood, 1997). However, interacting with trawlers might be risky. Fertl and Leatherwood (1997) reported that at least two mysticete species, 23 odontocete species, and several unidentified cetaceans (including beaked whales) have been incidentally caught in trawls. However, in the Mediterranean Sea, with the exception of the Israeli coast (Goffmann et al., 1995), cetacean bycatch in trawling nets appears to be a relatively uncommon occurrence. In the Balearic Islands, during our study, no incidental capture was recorded by observers or reported to us by over 50 interviewed fishermen (Gonzalvo unpublished data). Also, Massutí (unpublished data) monitored 460 commercial trawling operations off Majorca from 2001 to 2004 without reporting a single incidental catch of bottlenose dolphins. Hence, the only negative impact of trawling on the bottlenose dolphin population off the Balearic Archipelago appears to be the alteration of the sea bottom and the reduction in food availability caused by overexploitation, a general problem occurring all over the Mediterranean (Bearzi, 2002). Dolphins were observed preferably interacting with trawling operations that were shallow and short. These two variables were correlated because, when trawls are deployed at deeper depths, boats tend to tow longer to compensate decrease in catch per unit effort with depth (Massutí and Reñones, 2005). As a consequence, depth appears to be the main factor explaining variability in the dolphintrawlers interaction: bottlenose dolphins preferred the lower shelf, and therefore interacted more frequently with vessels operating there. Several studies have shown that local physiography can play a significant role on odontocete distribution, being depth one of the variables with the strongest influence (Hui, 1985; Ingram and Rogan, 2002; Cañadas et al., 2002; Yen et al., 2004). However, bottlenose dolphins off the Balearic Islands avoided the upper shelf in the warm season. The design of the study here reported does not allow identifying the precise reason for such a pattern, but increased boat traffic and human presence during the summer tourist season might be involved. Tourism is the main industry in Majorca and Minorca, which in 2005 they together received 9.3 million visitors and registered about 35,000 leisure boats, this is, 1 boat per 25 m of coastline (GOB 2002; http://www.gobmallorca.com/mar/index.htm). Also, recreational boat-fishing, targeting fish species living in the upper shelf, is widespread in the islands (Morales-Nin et al., 2005; Cardona et al., 2007b). All these activities peak markedly during the summer, and short-term displacement has been reported as a common response of cetaceans to boat traffic when interactions become intrusive or too lengthy to avoid acoustic disturbance and risk of collisions (Bejder et al., 1999; Nowacek et al., 2001; Williams et al.,
52
J. Gonzalvo et al. / Journal of Experimental Marine Biology and Ecology 367 (2008) 47–52
2002; Lusseau, 2003). Thus, in a similar scenario in Milford Sound, New Zealand, Lusseau (2005) has shown that the benefits for bottlenose dolphins of using a certain area are largely overcome by the costs of interaction with boats. Nevertheless, further research is needed to asses the actual relevance of boat traffic on dolphin distribution in the shelf of the Balearic Islands, as differences in water stratification may confound the results of seasonal comparisons. Acknowledgements This study was funded by the Spanish Ministry of the Environment, EU-LIFE project NAT/E/7303, IEO-DEMO and IEO-MEGA-2 projects. The authors wish to thank Irene Alvarez, Maite Lijalad, Vanessa Martin, Monica Revelles, Fabiana Saporiti and Silvia Scali for their help with data collection during boat surveys, and also the skippers of the fishing vessels “Antonia Munar II”, “Moralti nou”, “Joven Mirmer”, “Bellver”, “El Montgó”, “Nou Capdepera” and “Villa Sóller segundo”. [SS] References Baker, C.S., Clapham, P.J., 2002. Marine mammal exploitation: whales and whaling. In: Douglas, I. (Ed.), Encyclopaedia of global environmental change. Causes and consequences of global environmental change, vol. 3. John Wiley and Sons, New York, pp. 446–450. Bearzi, G., 2002. Interactions between cetacean and fisheries in the Mediterranean Sea. In: Notarbartolo di Sciara, G. (Ed.), Cetaceans of the Mediterranean and Black Seas: state of knowledge and conservation strategies. A report to the ACCOBAMS Secretariat, Monaco, February 2002. Section 9, 20 pp. Bearzi, G., Politi, E., Notarbartolo di Sciara, G., 1999. Diurnal behavior of free-ranging bottlenose dolphins in the Kvarneric (northern Adriatic Sea). Mar. Mamm. Sci. 15 (4), 1065–1097. Bejder, L., Dawson, S.M., Harraway, J.A., 1999. Responses by Hector's dolphins to boats and swimmers in Porpoise Bay, New Zealand. Mar. Mamm. Sci. 15 (3), 738–750. Blanco, C., Salomón, O., Raga, J.A., 2001. Diet of the bottlenose dolphin (Tursiops truncatus) in the Western Mediterranean Sea. J. Mar. Biol. Assoc. U.K. 81, 1053–1058. Broadhurst, M.K., 1998. Bottlenose dolphin, Tursiops truncatus, removing bycatch from prawn-trawl codends during fishing in New South Wales, Australia. Mar. Fish. Rev. 60 (3), 9–14. Cañadas, A., Sagarminaga, R., García-Tiscar, S., 2002. Cetacean distribution related with depth and slope in the Mediterranean waters off southern Spain. Deep-sea Res., Part 1, Oceanogr. Res. Pap. 49 (11), 2053–2073. Cardona, L., Revelles, M., Sales, M., Aguilar, A., Borrell, A., 2007a. Meadows of the seagrass Posidonia oceanica are a significant source of organic matter for adjoining ecosystems. Mar. Ecol. Prog. Ser. 335, 123–131. Cardona, L., López, D., Sales, M., de Caralt, S., Díez, I., 2007b. Effects of recreational fishing on three fish species from the Posidonia oceanica meadows off Minorca (Balearic archipelago, western Mediterranean). Sci. Mar. 71, 811–820. Chilvers, B.L., Corkeron, P.J., 2001. Trawling and bottlenose dolphins' social structure. Proc. R. Soc. Lond., B 268, 1901–1905. Corkeron, P.J., Bryden, M.M., Hedstrom, K.E., 1990. Feeding by bottlenose dolphins in association with trawling operations in Moreton Bay, Australia. In: Leatherwood, S., Reeves, R.R. (Eds.), The bottlenose dolphin. Acad. Press, San Diego, Calif., pp. 329–336. Crespo, E.A., Pedraza, S.N., Dans, S.L., Koen Alonso, M., Reyes, L.M., García, N.A., Coscarella, M., Schiavini, A.C.M., 1997. Direct and indirect effects of the high seas fisheries on the marine mammal populations in the northern and central Patagonian coast. J. Northwest Atl. Fish. Sci. 22, 189–207. Crespo, E.A., Koen Alonso, M., Dans, S.L., García, N.A., Pedraza, S.N., Coscarella, M., González, R., 2000. Incidental catches of dolphins in mid-water trawls for Argentine anchovy (Engraulis anchoita) off the Argentine shelf. J. Cetacean Res. Manag. 2, 11–16. Dans, S.L., Crespo, E.A., García, N.A., Reyes, L.M., Pedraza, S.N., Alonso, M.K., 1997. Incidental mortality of Patagonian dusky dolphins in mid-water trawling: Retrospective effects from the early 1980s. Rep. Int. Whal. Comm. 47, 699–703. Fernández-Puelles, M.L., Jansa, J., Gomis, C., Gras, D., Amengual, B., 1997. Variación anual de las principales variables oceanográficas y planctónicas en una estación nerítica del mar Balear. Bol. Inst. Esp. Oceanogr. 13, 13–33 (in Spanish). Fertl, D., Leatherwood, S., 1997. Cetacean interactions with trawlers: a preliminary review. J. Northwest Atl. Fish. Sci. 22, 219–248. Forcada, J., Gazo, M., Aguilar, A., Gonzalvo, J., Fernández-Contreras, M., 2004. Bottlenose dolphin abundance in the NW Mediterranean: addressing heterogeneity in distribution. Mar. Ecol. Prog. Ser. 275, 275–287. Goffmann, O., Kerem, D., Spanier, E., 1995. Dolphin interactions with fishing-trawlers off the Mediterranean coast of Israel. Abstract. 11th Biennial Conference on the Biology of Marine Mammals, Orlando, FL. 14-18 December 1995. Gonzalvo, J., Aguilar, A., 2004. Photoidentification of bottlenose dolphins (Tursiops truncatus) in the Balearic Islands. Book of Abstracts of 18th Annual Conference of the European Cetacean Society, Kolmarden, Sweden, 28–31 March, 2004. Gosliner, M.L., 1999. The tuna-dolphin controversy. In: TwissJr. Jr., J.R., Reeves, R.R. (Eds.), Conservation and Management of Marine Mammals. Smithsonian Institution Press, Washington, D.C., pp. 120–155.
Hastie, G.D., Wilson, B., Wilson, L.J., Parsons, K.M., Thompson, P.M., 2004. Functional mechanisms underlying cetacean distribution patterns: hotspots for bottlenose dolphins are linked to foraging. Mar. Biol. 144, 397–403. Hui, C.A., 1985. Undersea topography and the comparative distributions of two pelagic cetaceans. U.S. Fish. Bull. 83, 472–475. Ingram, S.N., Rogan, E., 2002. Identifying critical areas and habitat preferences of bottlenose dolphins Tursiops truncatus. Mar. Ecol. Prog. Ser. 44, 247–255. Jackson, J.B.C., Kirby, M.X., Berger, W.H., Bjorndal, K.A., Botsford, L.W., Bourque, B.J., Bradbury, R.H., Cooke, R., Erlandson, J., Estes, J.A., Hughes, T.P., Kidwell, S., Lange, C.B., Lenihan, H.S., Pandolfi, J.M., Peterson, C.H., Steneck, R.S., Tegner, M.J., Warner, R.R., 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293, 629–638. Lauriano, G., Fortuna, C.M., Moltedo, G., Notarbartolo di Sciara, G., 2004. Interactions between common bottlenose dolphins (Tursiops truncatus) and the artisanal fishery in Asinara Island National Park (Sardinia): assessment of catch damage and economic loss. J. Cetacean Res. Manag. 6 (2), 165–173. Leatherwood, S., 1975. Some observations of feeding behavior of bottle-nosed dolphins (Tursiops truncatus) in the northern Gulf of Mexico and Tursiops (cf T. gilli) off southern California, Baja California, and Nayarit, Mexico. Mar. Fish. Rev. 37 (9), 10–16. Lopez, A., Pierce, G.J., Santos, M.B., Gracia, J., Guerra, A., 2003. Fishery by-catches of marine mammals in Galician waters: results from on-board observations and an interview survey of fishermen. Biol. Conserv. 111, 25–40. Lusseau, D., 2003. Effects of tour boats on the behavior of bottlenose dolphins: Using Markov chains to model anthropogenic impacts. Conserv. Biol. 17 (6), 1785–1793. Lusseau, D., 2005. The residency pattern of bottlenose dolphins (Tursiops spp.) in Milford Sound, New Zealand, is related to boat traffic. Mar. Ecol. Prog. Ser. 295, 265–272. Majluf, P., Babcock, E., Riveros, J., Schreiber, M., Alderete, W., 2002. Catch and Bycatch of Sea Birds and Marine Mammals in the Small-Scale Fishery of Punta San Juan, Peru. Conserv. Biol. 16 (5), 1333–1343. Massutí, E., Reñones, O., 2005. Demersal resource assemblages in the trawl fishing grounds off the Balearic Islands (western Mediterranean). Sci. Mar. 69 (1), 167–181. Massutí, E., Reñones, O., Carbonell, A., Oliver, P., 1996. Demersal fish communities exploited on the continental shelf and slope off Majorca (Balearic Islands, NW Mediterranean). Vie Milieu 46 (1), 45–55. Morales-Nin, B., Moranta, J., Garcia, C., Tugores, M.P., Grau, A.M., Riera, F., Cerda, M., 2005. The recreational fishery off Majorca Island (western Mediterranean): some implications for coastal resource management. ICES J. Mar. Sci. 62, 727–739. Morizur, Y., Berrow, S.D., Tregenza, N.J.C., Couperus, A.S., Pouvreau, S., 1999. Incidental catches of marine mammals in pelagic trawl fisheries of the northeast Atlantic. Fish. Res. 41, 297–307. Nowacek, S.M., Wells, R.S., Solow, A.R., 2001. Short-term effects of boat traffic on bottlenose dolphins, Tursiops truncatus, in Sarasota Bay, Florida. Mar. Mamm. Sci. 17, 673–688. Northridge, S., 1984. World review of interactions between marine mammals and fisheries. FAO Fisheries Technical Paper, vol. 251. FAO, Rome, Italy. 190 pp. Notarbartolo di Sciara, G., 2002. Cetacean species occurring in the Mediterranean and Black Seas. In: Notarbartolo di Sciara, G. (Ed.), Cetaceans of the Mediterranean and Black Seas: state of knowledge and conservation strategies. A report to the ACCOBAMS Secretariat, Monaco, February 2002. Section 3, 17 pp. Papathanassious, E., Gabrielides, G.P., 1999. State and pressures of the marine and coastal Mediterranean environment. European Environment Agency. Reeves, R., Read, A., Notarbartolo di Sciara, G., 2001. Report of the Workshop on Interactions between Dolphins and Fisheries in the Mediterranean: Evaluation of Mitigation Alternatives. ICRAM, Rome, Italy. Relini, L.O., Papaconstantinou, C., Jukic-Peladic, S., Souplet, A., De Sola, L.G., Piccinetti, C., Kavadas, S., Rossi, M., 2002. Distribution of the Mediterranean hake populations (Merluccius merluccius smiridus Rafinesque, 1810) (Osteichthyes: Gadiformes) based on six years monitoring by trawl-surveys: some implications for management. Sci. Mar. 66 (2), 19–38. Romero, A., Baker, R., Creswell, J.E., Singh, A., McKie, A., Manna, M., 2002. Environmental history of marine mammal exploitation in Trinidad and Tobago, W.I. and its ecological impact. Environ. Hist. 8 (3), 255–274. Silvani, L., Raich, J., Aguilar, A., 1992. Bottle-nosed dolphins, Tursiops truncatus, interacting with fisheries in the Balearic Islands, Spain. Eur. Res. Cetaceans 6, 32–34. Shane, S.H., 1990. Behavior and ecology of the bottlenose dolphin at Sanibel Island, Florida. In: Leatherwood, S., Reeves, R.R. (Eds.), The bottlenose dolphin. Academic Press, San Diego, Calif., pp. 245–265. Svane, I., 2005. Occurrence of dolphins and seabirds and their consumption of by-catch during prawn trawling in Spencer Gulf, South Australia. Fish. Res. 76, 317–327. Trites, A.W., Christensen, V., Pauly, D., 2006. Effects of fisheries on ecosystems: just another top predator? In: Boyd, I.L., Wanless, S., Camphuysen, C.J. (Eds.), Top Predators in Marine Ecosystems. Cambridge University Press, pp. 11–27. Williams, R., Trites, A.W., Bain, D.E., 2002. Behavioural responses of killer whales (Orcinus orca) to whale-watching boats: opportunistic observations and experimental approaches. J. Zool. 256, 255–270. Yen, P.P.W., Sydeman, W.J., Hyrenbach, K.D., 2004. Marine bird and cetacean associations with bathymetric habitats and shallow-water topographies: Implications for trophic transfer and conservation. J. Mar. Syst. 50, 79–99.