Fishers’ and scientific histories: an example of consensus from an inland fishery
Juliana Strieder Philippsen A F , Carolina Viviana Minte-Vera B A , Edson Kiyoshi Okada C , Adriana Rosa Carvalho D and Ronaldo Angelini E
+ Author Affiliations
- Author Affiliations
A Pós-Graduação em Ecologia de Ambientes Aquáticos Continentais, Universidade Estadual de Maringá, Avenida Colombo 5790, Maringá, PR, CEP 87020-900, Brazil.
B Inter-American Tropical Tuna Commission, 8901 La Jolla Shores Drive, La Jolla, CA 92037-1509, USA.
C Núcleo de Pesquisas em Limnologia, Ictiologia e Aquicultura (Nupélia), Universidade Estadual de Maringá, Avenida Colombo 5790, Maringá, PR, CEP 87020-900, Brazil.
D Departamento de Ecologia, Universidade Federal do Rio Grande do Norte (UFRN), Campus Universitário, Natal, RN, CEP 59072-970, Brazil.
E Departamento de Engenharia Civil, Centro de Tecnologia, Universidade Federal do Rio Grande do Norte (UFRN), Campus Universitário, Natal, RN, CEP 59072-970, Brazil.
F Corresponding author. Email: striederjp@gmail.com
Marine and Freshwater Research 68(5) 980-992 https://doi.org/10.1071/MF16053
Submitted: 23 February 2016 Accepted: 9 June 2016 Published: 3 August 2016
Abstract
In this study we tested the basic premise that fishers’ knowledge provides similar perceptions about exploited stocks to data gathered by standardised monitoring in a small-scale commercial fishery. To that end, we compared temporal trends in catch and individual size for fish species between data obtained from interviews with fishers and a 25-year dataset from a landing monitoring program in the Itaipu Reservoir (Brazil). Fishers described with accuracy changes in species composition of the catches, from large migratory species of high commercial value (common before impoundment) to migratory species of low commercial value and sedentary species following damming. Temporal trends in catches reported by fishers and documented in the landing data matched for most species. Histories diverge when absolute values are involved, such as when fishers were asked to recall their largest catch. Fishers were homogeneous in their reports, indicating that instead of individual opinions, they revealed knowledge resulting from their observation and fishery experience. Fishers can be a reliable source of information for detecting changes in catches over time, especially when large-scale habitat changes have occurred within the time scale of a fisher career. Fishers may be key partners to be considered by managers in information gathering for effective management.
Additional keywords: Bayesian analysis, data-limited fisheries, interview, local knowledge, reservoir, river.
References
Agostinho, A. A., Okada, E. K., and Gregoris, J. (1999). A pesca no reservatório de Itaipu: aspectos socioeconômicos e impactos do represamento. In ‘Ecologia de reservatórios: estrutura, função e aspectos sociais’. (Ed. R. Henry.) pp. 281–319. (FUNDIBIO: Botucatu.)Agostinho, A. A., Gomes, L. C., and Pelicice, F. M. (2007). ‘Ecologia e manejo de recursos pesqueiros em reservatórios do Brasil’, 1st edn. (Eduem: Maringá.)
Agostinho, A. A., Pelicice, F. M., and Gomes, L. C. (2008). Dams and the fish fauna of the Neotropical region: impacts and management related to diversity and fisheries. Brazilian Journal of Biology 68, 1119–1132.
| Dams and the fish fauna of the Neotropical region: impacts and management related to diversity and fisheries.Crossref | GoogleScholarGoogle Scholar | 1:STN:280:DC%2BD1M7ltVSqsg%3D%3D&md5=86b49b192765f3d69ce63fafa1ce5752CAS |
Agostinho, C. S., Marques, E. E., Oliveira, R. J., and Braz, P. (2009). Feeding ecology of Pterodoras granulosus (Siluriformes, Doradidae) in the Lajeado Reservoir, Tocantins, Brazil. Iheringia. Série Zoologia 99, 301–306.
Ainsworth, C. H., and Pitcher, T. J. (2005). Using local ecological knowledge in ecosystem models. In ‘Fisheries Assessment and Management in Data-limited Situations’. (Eds G. H. Kruse, V. F. Galluci, D. E. Hay, R. I. Perry, R. M. Peterman, T. C. Shirley, P. D. Spencer, B. Wilson, and D. Woodby.) pp. 289–304. (Alaska Sea Grant College Program: Fairbanks, AK, USA.)
Ankei, Y. (1982). Folk-knowledge of the fish among the Songola and the Bwari: comparative ethnoichthyology of the Zaire River and Lake Tanganyica fishermen. Africa Kenkyu 1982, 1–56.
| Folk-knowledge of the fish among the Songola and the Bwari: comparative ethnoichthyology of the Zaire River and Lake Tanganyica fishermen.Crossref | GoogleScholarGoogle Scholar |
Barros, L. C., Santos, U., Zanuncio, J. C., and Dergam, J. A. (2012). Plagioscion squamosissimus (Sciaenidae) and Parachromis managuensis (Cichlidae): a threat to native fishes of the Doce River in Minas Gerais, Brazil. PLoS One 7, e39138.
| Plagioscion squamosissimus (Sciaenidae) and Parachromis managuensis (Cichlidae): a threat to native fishes of the Doce River in Minas Gerais, Brazil.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC38XovF2jtrY%3D&md5=ab1917c7f9cdccf0f9c84072d47b6363CAS | 22720053PubMed |
Bender, M. G., Machado, G. R., Silva, P. J. A., Floeter, S. R., Monteiro-Netto, C., Luiz, O. J., and Ferreira, C. E. L. (2014). Local ecological knowledge and scientific data reveal overexploitation by multigear artesanal fisheries in the Southwestern Atlantic. PLoS One 9, e110332.
| Local ecological knowledge and scientific data reveal overexploitation by multigear artesanal fisheries in the Southwestern Atlantic.Crossref | GoogleScholarGoogle Scholar | 25333661PubMed |
Berkes, F., Mahon, R., McConney, P., Pollnac, R., and Pomeroy, R. (2001). ‘Managing Small-scale Fisheries: Alternative Directions and Methods.’ (International Development Research Centre: Ottawa, ON, Canada.)
Berkson, J., and Thorson, J. T. (2015). The determination of data-poor catch limits in the United States: is there a better way? ICES Journal of Marine Science 72, 237–242.
| The determination of data-poor catch limits in the United States: is there a better way?Crossref | GoogleScholarGoogle Scholar |
Bevilacqua, A. H. V., Carvalho, A. R., Angelini, R., and Christensen, V. (2016). More than anecdotes: fishers’ ecological knowledge can fill gaps for ecosystem modeling. PLoS One 11, e0155655.
| More than anecdotes: fishers’ ecological knowledge can fill gaps for ecosystem modeling.Crossref | GoogleScholarGoogle Scholar |
Carvalho, A. R., Williams, S., January, M., and Sowman, M. (2009). Reliability of community-based data monitoring in the Olifants River estuary (South Africa). Fisheries Research 96, 119–128.
| Reliability of community-based data monitoring in the Olifants River estuary (South Africa).Crossref | GoogleScholarGoogle Scholar |
Castello, L., Viana, J. P., Watkins, G., Pinedo-Vasquez, M., and Luzadis, V. A. (2009). Lessons from integrating fishers of arapaima in small-scale fisheries management at the Mamirauá Reserve, Amazon. Environmental Management 43, 197–209.
| Lessons from integrating fishers of arapaima in small-scale fisheries management at the Mamirauá Reserve, Amazon.Crossref | GoogleScholarGoogle Scholar | 18946698PubMed |
Cochrane, K., and Garcia, S. (2009). ‘A Fishery Manager’s Guidebook’, 2nd edn. (FAO/Wiley-Blackwell: London.)
Coll, M., Carreras, M., Ciércoles, C., Cornax, M. J., Gorelli, G., Morote, E., and Sáez, R. (2014). Assessing fishing and marine biodiversity changes using fishers’ perceptions: the Spanish Mediterranean and Gulf of Cadiz case study. PLoS One 9, e85670.
| Assessing fishing and marine biodiversity changes using fishers’ perceptions: the Spanish Mediterranean and Gulf of Cadiz case study.Crossref | GoogleScholarGoogle Scholar | 24465644PubMed |
Damasio, L. M. A., Lopes, P. F. M., Guariento, R. D., and Carvalho, A. R. (2015). Matching fishers’ knowledge and landing data to overcome data missing in small-scale fisheries. PLoS One 10, e0133122.
| Matching fishers’ knowledge and landing data to overcome data missing in small-scale fisheries.Crossref | GoogleScholarGoogle Scholar |
Davis, A., and Wagner, J. R. (2003). Who knows? On the importance of identifying experts when researching local ecological knowledge. Human Ecology 31, 463–489.
| Who knows? On the importance of identifying experts when researching local ecological knowledge.Crossref | GoogleScholarGoogle Scholar |
Daw, T. M., Robinson, J., and Graham, N. A. J. (2011). Perceptions of trends in Seychelles artisanal trap fisheries: comparing catch monitoring, underwater visual census and fishers’ knowledge. Environmental Conservation 38, 75–88.
| Perceptions of trends in Seychelles artisanal trap fisheries: comparing catch monitoring, underwater visual census and fishers’ knowledge.Crossref | GoogleScholarGoogle Scholar |
De Graaf, G., Bartley, D. M., Jorgensen, J. V., and Marmulla, G. (2015). The scale of inland fisheries, can we do better? Alternative approaches. Fisheries Management and Ecology 22, 64–70.
| The scale of inland fisheries, can we do better? Alternative approaches.Crossref | GoogleScholarGoogle Scholar |
Diegues, A. C. S. (2004). ‘A pesca construindo sociedades: leituras em antropologia marítima e pesqueira.’ (NUPAUB/USP: São Paulo.)
Eddy, T. D., Gardner, J. P. A., and Perez-Matus, A. (2010). Applying fishers’ ecological knowledge to construct past and future lobster stocks in the Juan Fernandez Archipelago, Chile. PLoS One 5, e13670.
| Applying fishers’ ecological knowledge to construct past and future lobster stocks in the Juan Fernandez Archipelago, Chile.Crossref | GoogleScholarGoogle Scholar | 21079761PubMed |
FAO (2003). Fisheries Management. The Ecosystem Approach to Fisheries. FAO Technical Guidelines for Responsible Fisheries, 4. FAO, Rome.
Fischer, J., Jorgensen, J., Josupeit, H., Kalikoski, D., and Lucas, C. M. (Eds) (2015). Fishers’ knowledge and the ecosystem approach to fisheries: applications, experiences and lessons in Latin America. FAO Fisheries and Aquaculture Technical Paper number 591, FAO, Rome.
García-Allut, A., Freire, J., Barreiro, A., and Losada, D. E. (2007). Methodology for integration of fisher’s ecological knowledge in fisheries biology and management using knowledge representation (artificial intelligence). In ‘The Value of Anecdote, Fishers’ Knowledge in Fisheries Science and Management’. (Eds N. Haggan, B. Neis, and I. G. Baird.) pp. 227–237. (UNESCO Publishing: Paris.)
García-Quijano, C. G., and Pizzini, M. V. (2015). Ecosystem-based knowledge and reasoning in tropical, multispecies, small-scale fishers’ LEK: what can fishers’ LEK contribute to coastal ecological science and management. In ‘Fishers’ Knowledge and the Ecosystem Approach to Fisheries: Applications, Experiences and Lessons in Latin America’. (Eds J. Fischer, J. Jorgensen, H. Josupeit, D. Kalikoski, and C. M. Lucas.) pp. 19–40. (FAO: Rome.)
Gelman, A., Carlin, J. B., Stern, H. S., and Rubin, D. B. (2004). ‘Bayesian Data Analysis’, 2nd edn. (Chapman & Hall/CRC: New York.)
Goodman, L. (1961). Snowball sampling. Annals of Mathematical Statistics 32, 148–170.
| Snowball sampling.Crossref | GoogleScholarGoogle Scholar |
Grant, S., and Berkes, F. (2007). Fisher knowledge as expert system: a case from the longline fishery of Grenada, the eastern Caribbean. Fisheries Research 84, 162–170.
| Fisher knowledge as expert system: a case from the longline fishery of Grenada, the eastern Caribbean.Crossref | GoogleScholarGoogle Scholar |
Hallwass, G., Lopes, P. F., Juras, A. A., and Silvano, R. A. M. (2013). Fishers’ knowledge identifies environmental changes and fish abundance trends in impounded tropical rivers. Ecological Applications 23, 392–407.
| Fishers’ knowledge identifies environmental changes and fish abundance trends in impounded tropical rivers.Crossref | GoogleScholarGoogle Scholar | 23634590PubMed |
Heidelberger, P., and Welch, P. D. (1983). Simulation run length control in the presence of an initial transient. Operations Research 31, 1109–1144.
| Simulation run length control in the presence of an initial transient.Crossref | GoogleScholarGoogle Scholar |
Henry, B., Moffitt, T. E., Caspi, A., Langley, J., and Silva, P. A. (1994). On the ‘remembrance of things past’: a longitudinal evaluation of the retrospective method. Psychological Assessment 6, 92–101.
| On the ‘remembrance of things past’: a longitudinal evaluation of the retrospective method.Crossref | GoogleScholarGoogle Scholar |
Herbst, D. F., and Hanazaki, N. (2014). Local ecological knowledge of fishers about the life cycle and temporal patterns in the migration of mullet (Mugil liza) in Southern Brazil. Neotropical Ichthyology 12, 879–890.
| Local ecological knowledge of fishers about the life cycle and temporal patterns in the migration of mullet (Mugil liza) in Southern Brazil.Crossref | GoogleScholarGoogle Scholar |
Heyman, W. D., and Granados-Dieseldorff, P. (2012). The voice of the fishermen of the Gulf of Honduras: improving regional fisheries management through fisher participation. Fisheries Research 125–126, 129–148.
| The voice of the fishermen of the Gulf of Honduras: improving regional fisheries management through fisher participation.Crossref | GoogleScholarGoogle Scholar |
Hoeinghaus, D. J., Agostinho, A. A., Gomes, L. C., Pelicice, F. M., Okada, E. K., Latini, J. D., Kashiwaqui, E. A. L., and Winemiller, K. O. (2009). Effects of river impoundment on ecosystem services of large tropical rivers: embodied energy and market value of artisanal fisheries. Conservation Biology 23, 1222–1231.
| Effects of river impoundment on ecosystem services of large tropical rivers: embodied energy and market value of artisanal fisheries.Crossref | GoogleScholarGoogle Scholar | 19459891PubMed |
Johannes, R. E., and Neis, B. (2007). The value of anecdote. In ‘The Value of Anecdote, Fishers’ Knowledge in Fisheries Science and Management’. (Eds N. Haggan, B. Neis, and I. G. Baird.) pp. 41–58. (UNESCO Publishing: Paris.)
Johannes, R. E., Freeman, M. M. R., and Hamilton, R. J. (2000). Ignores fishers’ knowledge and miss the boat. Fish and Fisheries 1, 257–271.
| Ignores fishers’ knowledge and miss the boat.Crossref | GoogleScholarGoogle Scholar |
Junk, W. J. (2007). Freshwater fishes of South America: their biodiversity, fisheries, and habitats – a synthesis. Aquatic Ecosystem Health & Management 10, 228–242.
| Freshwater fishes of South America: their biodiversity, fisheries, and habitats – a synthesis.Crossref | GoogleScholarGoogle Scholar |
Kinas, P. G., and Andrade, H. A. (2010). ‘Introdução à análise bayesiana (com R)’, 1st edn. (maisQnada: Porto Alegre.)
Leite, M. C. F., and Gasalla, M. A. (2013). A method for assessing fishers’ ecological knowledge as a pratical tool for ecosystem-based fisheries management: seeking consensus in southeastern Brazil. Fisheries Research 145, 43–53.
| A method for assessing fishers’ ecological knowledge as a pratical tool for ecosystem-based fisheries management: seeking consensus in southeastern Brazil.Crossref | GoogleScholarGoogle Scholar |
Lunn, K. E., and Dearden, P. (2006). Monitoring small-scale marine fisheries: an example from Thailand’s Ko Chang archipelago. Fisheries Research 77, 60–71.
| Monitoring small-scale marine fisheries: an example from Thailand’s Ko Chang archipelago.Crossref | GoogleScholarGoogle Scholar |
Medeiros, R. P., McConney, P., Foppa, C. C., and Faraco, L. F. (2015). Fishers and their knowledge in Brazil: from extractive uses to collaborative exchanges. In ‘Fishers’ Knowledge and the Ecosystem Approach to Fisheries: Applications, Experiences and Lessons in Latin America’. (Eds J. Fischer, J. Jorgensen, H. Josupeit, D. Kalikoski, and C. M. Lucas.) pp. 191–208. (FAO: Rome.)
Moura, G. G. M., Kalikoski, D. C., and Diegues, A. C. S. (2013). A resource management scenario for traditional and scientific management of pink shrimp (Farfantepenaeus paulensis) in the Patos Lagoon estuary (RS), Brazil. Journal of Ethnobiology and Ethnomedicine 9, 6.
| A resource management scenario for traditional and scientific management of pink shrimp (Farfantepenaeus paulensis) in the Patos Lagoon estuary (RS), Brazil.Crossref | GoogleScholarGoogle Scholar |
O’Donnell, K. P., Moolloy, P. P., and Vincent, A. C. (2012). Comparing fisher interviews, logbooks, and catch landings estimates of extraction rates in a small-scale fishery. Coastal Management 40, 594–611.
| Comparing fisher interviews, logbooks, and catch landings estimates of extraction rates in a small-scale fishery.Crossref | GoogleScholarGoogle Scholar |
Okada, E. K., Agostinho, A. A., and Gomes, L. C. (2005). Spatial and temporal gradients in artisanal fisheries of a large Neotropical reservoir, the Itaipu Reservoir, Brazil. Canadian Journal of Fisheries and Aquatic Sciences 62, 714–724.
| Spatial and temporal gradients in artisanal fisheries of a large Neotropical reservoir, the Itaipu Reservoir, Brazil.Crossref | GoogleScholarGoogle Scholar |
Okada, E. K., Agostinho, A. A., Ambrósio, A. M., Suzuki, H. I., Galdioli, E. M., and Souza, S. R. (2012). ‘Monitoramento do rendimento e da socioeconomia da pesca no reservatório de Itaipu. Relatório Geral Científico.’ (UEM- NUPELIA: Maringá.)
Petrere, M., and Agostinho, A. A. (1993). La pesca en el tramo brasileño del río Paraná. In ‘Comisión de Pesca Continental para América Latina. Informe de la sexta reunión del Grupo de Trabajo sobre Recursos Pesqueros, Montevideo, Uruguai, 10-13 de Mayo 1993'. (Eds I. Vila and R. Welcomme) FAO Informe de Pesca, number 490, pp. 52–72. (FAO: Rome.)
Plummer, M., Best, N., Cowles, K., and Vines, K. (2006). CODA: convergence diagnosis and output analysis for MCMC. R News 6, 7–11.
Poizat, G., and Baran, E. (1997). Fishermen’s knowledge as background information in tropical fish ecology: a quantitative comparison with fish sample results. Environmental Biology of Fishes 50, 435–449.
| Fishermen’s knowledge as background information in tropical fish ecology: a quantitative comparison with fish sample results.Crossref | GoogleScholarGoogle Scholar |
Ramires, M., Clauzet, M., Barrella, W., Rotundo, M. M., Silvano, R. A. M., and Begossi, A. (2015). Fishers’ knowledge about fish trophic interactions in the southeastern Brazilian coast. Journal of Ethnobiology and Ethnomedicine 11, 19.
| Fishers’ knowledge about fish trophic interactions in the southeastern Brazilian coast.Crossref | GoogleScholarGoogle Scholar | 25889226PubMed |
Reis-Filho, J. A., Freitas, R. H. A., Loiola, M., Leite, L., Soeiro, G., Oliveira, H. H. Q., Sampaio, C. L. S., Nunes, J. A. C. C., and Leduc, A. O. H. (2016). Traditional fisher perceptions on the regional disappearance of the largetooth sawfish Pristis pristis from the central coast of Brazil. Endangered Species Research 29, 189–200.
| Traditional fisher perceptions on the regional disappearance of the largetooth sawfish Pristis pristis from the central coast of Brazil.Crossref | GoogleScholarGoogle Scholar |
Rivera, V. L., Rivera, A. M., and Borrás, M. F. (2015). Integrating traditional and scientific knowledge for the management of small scale fisheries: an example from Costa Rica. In ‘Fishers’ Knowledge and the Ecosystem Approach to Fisheries: Applications, Experiences and Lessons in Latin America’. (Eds J. Fischer, J. Jorgensen, H. Josupeit, D. Kalikoski, and C. M. Lucas.) pp. 179–190. (FAO: Rome.)
Rochet, M. J., Prigent, M., Bertrand, J. A., Carpentier, A., Coppin, F., Delpech, J. P., Fontenelle, G., Foucher, E., Mahé, K., Rostiaux, E., and Trenkel, V. M. (2008). Ecosystem trends: evidence for agreement between fishers’ perceptions and scientific information. ICES Journal of Marine Science 65, 1057–1068.
| Ecosystem trends: evidence for agreement between fishers’ perceptions and scientific information.Crossref | GoogleScholarGoogle Scholar |
Rosa, R., Carvalho, A. R., and Angelini, R. (2014). Integrating fishermen knowledge and scientific analysis to assess changes in fish diversity and food web structure. Ocean and Coastal Management 102, 258–268.
| Integrating fishermen knowledge and scientific analysis to assess changes in fish diversity and food web structure.Crossref | GoogleScholarGoogle Scholar |
Rossi, R. M. (2011). ‘Introdução aos métodos Bayesianos na análise de dados zootécnicos com uso do WinBUGS e R.’ 1st edn. (Eduem: Maringá.)
Saavedra-Díaz, L. M., Rosenberg, A. A., and Pomeroy, R. (2015). Why Colombian marine fishers’ knowledge is a fundamental tool for marine resource management and assessment. In ‘Fishers’ Knowledge and the Ecosystem Approach to Fisheries: Applications, Experiences and Lessons in Latin America’. (Eds J. Fischer, J. Jorgensen, H. Josupeit, D. Kalikoski, and C. M. Lucas.) pp. 89–106. (FAO: Rome.)
Sáenz-Arroyo, A., Roberts, C. M., Torre, J., and Cariño-Olvera, M. (2005). Using fishers’ anecdotes, naturalists’ observations and grey literature to reassess marine species at risk: the case of the Gulf grouper in the Gulf of California, Mexico. Fish and Fisheries 6, 121–133.
| Using fishers’ anecdotes, naturalists’ observations and grey literature to reassess marine species at risk: the case of the Gulf grouper in the Gulf of California, Mexico.Crossref | GoogleScholarGoogle Scholar |
Silvano, R. A. M., and Begossi, A. (2002). Ethonoichthyology and fish conservation in the Piracicaba River (Brazil). Journal of Ethnobiology 22, 285–306.
Silvano, R. A. M., Silva, A. L., Ceroni, M., and Begossi, A. (2008). Contributions of ethnobiology to the conservation of tropical rivers and streams. Aquatic Conservation: Marine and Freswater Ecosystems 18, 241–260.
| Contributions of ethnobiology to the conservation of tropical rivers and streams.Crossref | GoogleScholarGoogle Scholar |
Tesfamichael, D., Pitcher, T. J., and Pauly, D. (2014). Assessing changes in fisheries using fishers’ knowledge to generate long time series of catch rates: a case study from the Red Sea. Ecology and Society 19, 18.
| Assessing changes in fisheries using fishers’ knowledge to generate long time series of catch rates: a case study from the Red Sea.Crossref | GoogleScholarGoogle Scholar |
Thomas, A., O’Hara, B., Ligges, U., and Sturtz, S. (2006). Making BUGS open. R News 6, 12–17.
Valbo-Jørgensen, J., and Poulsen, A. F. (2000). Using local knowledge as a research tool in the study of river fish biology: experiences from the Mekong. Environment, Development and Sustainability 2, 253–276.
| Using local knowledge as a research tool in the study of river fish biology: experiences from the Mekong.Crossref | GoogleScholarGoogle Scholar |
van Densen, W. L. T., and McCay, B. J. (2007). Improving communication from managers to fishers in Europe and the US. ICES Journal of Marine Science 64, 811–817.
| Improving communication from managers to fishers in Europe and the US.Crossref | GoogleScholarGoogle Scholar |
Welcomme, R. L., Cowx, I. G., Coates, D., Béné, C., Funge-Smith, S., Halls, A., and Lorenzen, K. (2010). Inland capture fisheries. Philosophical Transactions of the Royal Society of London – B. Biological Sciences 365, 2881–2896.
| Inland capture fisheries.Crossref | GoogleScholarGoogle Scholar | 20713391PubMed |
Wickham, H. (2009). ggplot2: elegant graphics for data analysis. Available at http://had.co.nz/ggplot2/book [Verified 20 October 2013].
