Agonistic behaviour and shoal composition of juvenile Diplodus sargus : First field observations

Gonçalves, Ana R.; Silva, Matheus Augusto Farias da; Vendrell, Catarina L.; Almada, Vítor Carvalho

http://hdl.handle.net/10400.12/3553

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Authors

Gonçalves, Ana R.

Silva, Matheus Augusto Farias da

Vendrell, Catarina L.

Almada, Vítor Carvalho

Abstract(s)

Behavioural observations of juvenile Diplodus sargus in natural habitats were conducted to test whether agonistic interactions occur in natural circumstances and, if they do, in which context they happen. The level of aggression and the shoals composition were also compared between distinct natural habitats. Behavioural observations were also conducted in captivity, and the level of aggression was comparedwith the level recorded in natural conditions to investigate which factors may promote aggressive behaviours. The observations were performed at two sites in the Atlantic Portuguese shore: São Pedro do Estoril (38°41′N, 9°22′W) and Cabo Raso (38°42′N, 9°29′W) from early July to late August 2010 and from late July to early October 2011, respectively. In nature agonistic behaviours do occur, with a higher frequency in pools than in open areas. However, even the highest values observed in nature are significantly lower than those reported for captive conditions. Shoal composition also changed in different habitats, with more members and aggregates more compact in open areas.We suggested that the high levels of aggression observed in captive groups may be an artefact caused by very high densities in confined spaces and eventually by the accumulation of chemicals that are known to be released by stressed fish and can be detected by other conspecifics. The confinement imposed in captivity may also affect shoal composition limiting the number of members and inter-individual distances in each group.