Gilthead seabream (Sparus aurata), from the Sparidae family, is one of the most extensively farmed fish species in the Mediterranean region. During the last two decades, many studies have described aspects of the biology of the species, including reproduction and genetics (Holland et al., 1998; Almansa et al., 1999; Meiri et al., 2004; Rossi et al., 2006; Arabaci et al., 2010; Mylonas et al., 2011). Gilthead seabream is a protandrous hermaphrodite species with an asynchronous ovarian development (Zohar et al., 1995). Broodstock held in captivity under natural conditions typically start vitellogenesis in September-November, spawning begins during December-January and lasts for 3-5 months with daily spawning, leading to an annual fecundity of 2,000,000 eggs/kg (diameter<1 mm) with a fertilization rate of 80-85% (Barbaro et al., 1997; Arabaci et al., 2010; Mylonas et al., 2011). However, the reproductive behaviour of gilthead seabream has not been reported, despite an increasing need to understand the factors that influence a breeders participation in spawning in order to control the families produced from a broodstock (Gorshkov et al., 1997; Brown et al., 2005; Porta et al., 2009; Chavanne et al., 2012).

Reproductive behaviour has been described in some species of the family Sparidae, kept in captivity, including silver seabream (Chrysophrys auratus) (Smith, 1986; Mylonas et al., 2011), santer seabream (Cheimerius nufar) (Buxton & Garratt, 1990; Garratt, 1991), roman seabream (Chrysoblephus laticeps) (Buxton, 1990), silver bream (Rhabdosargus sarba) (Leu, 1994) and southern black bream (Acanthopagrus butcheri) (Mylonas et al., 2011). Although there was variation among species a general similarity was observed (see review in Mylonas et al., 2011). Spawning was usually early morning (dawn) or early evening (dusk) (06:00 and 19:00, respectively). Courtship consisted of males pursuing and nudging females, a tight circling swimming behaviour to form aggregations before spawning, which consisted of a spawning rush usually either to perform pair spawning involving a single pair (a male and a female) or group spawning a single female followed by multiple males (Smith, 1986; Buxton & Garratt, 1990; Garratt, 1991; Leu, 1994; Mylonas et al., 2011).

A number of studies have examined gilthead seabream parental contribution to spawning events as there is a need to genetically improve cultured gilthead seabream to obtain desirable traits such as faster growth that will reduce production costs (Gorshkov et al., 1997; Brown et al., 2005; Porta et al., 2009; Chavanne et al., 2012; Duncan et al., 2013). Gilthead seabream spawning success was low when held in pairs (22% success) or groups of 15 females with a single male (44% success) and gilthead seabream were difficult to strip spawn for artificial fertilisation (Gorshkov et al., 1997). Different authors have concluded that large groups of breeders are required for successful spawning of gilthead seabream (Gorshkov et al., 1997; Duncan et al., 2013) and Sparidae in general (Pankhurst, 1998; Mylonas et al., 2011). Parental assignment of progeny using microsatellites identified that although large broodstocks produce large volume spawns to which many breeders contributed the participation of breeders was variable and a proportion of breeders did not participate in spawning (Brown et al., 2005; Porta et al., 2009; Chavanne et al., 2012). Consequently, the effective spawning population size was reduced compared to the actual number of breeders in the broodstock, inbreeding was higher than expected and the families obtained were not predictable. Brown et al. (2005) and Chavanne et al. (2012) referred to gilthead seabream spawning behaviour as mass-spawning, which has been defined as “spawning that consists of the great majority to all of an aggregation spawning simultaneously, as a single unit” (Domeier & Colin, 1997).

Therefore, there is a need to study the spawning behaviour of gilthead seabream to increase the understanding of spawning in Sparidae and to enable geneticists and broodstock managers to understand the parental contributions obtained for genetic improvement programmes. The aim of the present study was to investigate and describe the particularities of reproductive behaviour of the gilthead seabream in rearing conditions.

The present study described, for the first time, the reproductive behaviour of gilthead seabream (Sparus aurata) held in captivity. The reproductive behaviour was similar to that described for other Sparidae species (Smith, 1986; Buxton, 1990; Buxton & Garratt, 1990; Garratt, 1991; Leu, 1994; Mylonas et al., 2011). Gilthead seabream were observed to form defined aggregations prior to the spawning event and females were observed to make a spawning rush with one or more males that finished with gamete liberation.

In accordance with Domeier & Colin (1997) the aggregation behaviour performed by fish was defined as a group of conspecific fish that gathered for the purpose of spawning, with fish densities or numbers significantly higher than those found in the area during the non reproductive period. In the present study, gilthead seabream aggregations were well defined, included the participation of all the stock and were clearly associated with spawning. The courtship behaviour of gilthead seabream was mostly brief and characterized by rapid forward swimming by females followed by one or more males. In addition, males displayed two characteristics: a colour change to become slightly darker and nudging and rubbing the female´s bellies close to the oviduct. The formation of aggregations and the courtship (changes in swimming speed, colour changes and nudging) appeared to offer the opportunity for mate selection and brought all the available individuals together for mate selection. Dichromatism (ability to take on one of two different colours patterns separately) was suggested to be a motivational factor for females to select males with better physical condition and social status (Kodric-Brown, 1998; Okumura et al., 2002; Kline et al., 2011). The action of rubbing and nudging was hypothesized to help males to perceive female pheromones, trigger the ovulation and induce the oocytes liberation (Bond, 1996; Domeier & Colin, 1997; Heyman et al., 2005; Stacey & Sorensen, 2008). In the present study, obvious behaviours associated with gaining dominance were not observed between males or males and females. However, a passive process of selection between males and females can be suggested as both observations of behavioural and morphological aspects appeared to offer opportunities for females to accept or reject advances from males. These indications that presented opportunities consisted of: a) spawning was often in a pair indicating the pair could select each other, b) aggregations brought all the fish together for close contact to aid selection and spawning was often soon after an aggregation, c) males followed females perhaps seeking selection, d) males nudged females to possibly stimulate selection, e) females were observed to swim away from advances from males and f) males changed colour changing appearance to perhaps aid selection by the female. Aggregations and/or courtship behaviours similar to the present study have been described in other species of Sparidae including silver seabream (Chrysophrys auratus) (Smith, 1986; Mylonas et al., 2011), santer seabream (Cheimerius nufar) (Buxton & Garratt, 1990; Garratt, 1991), roman seabream (Chrysoblephus laticeps) (Buxton, 1990), silver bream (Rhabdosargus sarba) (Leu, 1994) and southern black bream (Acanthopagrus butcheri) (Mylonas et al., 2011) and non-Sparidae such as the spotted sand bass (Paralabrax maculatofasciatus) (Miller & Allen, 2006), yellowtail amberjack (Seriola lalandi) (Moran et al., 2007), dusky grouper (Epinephelus marginatus) (Zabala et al., 1997), cubera snapper (Lutjanus cyanopterus) (Heyman et al., 2005) and white seabass (Atractoscion nobilis) (Aalbers & Drawbridge, 2008).

However, in the present study, aggregations were not always observed immediately prior to gilthead seabream spawning and no inter-individual dominances were observed. Liberation of gametes was observed both in gilthead seabream coming from an aggregation (with or without courtship) and fish that had not participated in aggregation (or courtship) behaviour immediately prior to spawning. However, the importance of these social interactions (aggregations and courtship) during the spawning period should not be lessened by these observations. Gilthead seabream spawning success was low when held in pairs (Gorshkov et al., 1997; N. Duncan, pers. obs.) or groups of 15 females with a single male (Gorshkov et al., 1997). Holding gilthead seabream in pairs or 15 females with a single male would be too few fish or the wrong sex ratios to enable the social interactions (aggregations and courtship) observed in the present study and this may explain the poor spawning success observed in gilthead seabream held in pairs or small groups (Gorshkov et al., 1997; Duncan et al., 2013). Various authors have suggested large groups of breeders were required for successful spawning of gilthead seabream (Gorshkov et al., 1997; Duncan et al., 2013) and Sparidae in general (Pankhurst, 1998; Mylonas et al., 2011).

In the present study, gilthead seabream made a spawning rush with a preference to rush and spawn as a pair and 71.6% of total spawns were observed to be between a single female and male. However, gilthead seabream were also observed to group spawn when one female spawned with several males: two (22.5%) or three males (4.9%). Species from the Sparidae family all presented a spawning rush and different species presented pair or group or both types of spawning. The silver seabream (Smith, 1986; Mylonas et al., 2011) and santer seabream (Buxton & Garratt, 1990; Garratt, 1991), like the gilthead seabream presented both pair and group spawning. However, silver seabream were predominantly group spawners with one female being followed by many males (Smith, 1986; Mylonas et al., 2011), but pair spawning was observed on one occasion (Smith, 1986). Santer seabream pair spawned (Buxton & Garratt, 1990; Garratt, 1991) and the dominant male was aggressive towards other males, however, on occasions a “streaker” or “sneaker” male was observed to successfully participate in spawns by keeping to the opposite side of the female to the dominate male (Garratt, 1991). In the present study, no evidence of sneaker males was observed in gilthead seabream, although, when group spawning was observed there was always a lead male closest to the female followed by a second and less frequently a third male. The roman seabream (Buxton, 1990) and silver bream (Leu, 1994) were only observed to pair spawn and the southern black bream was only observed to group spawn (Mylonas et al., 2011). To date no Sparidae species has been observed to mass spawn and the observed pair and/or group spawning preceded by social interactions related to mate selection were characteristic of gilthead seabream and other Sparidae species.

Domeier & Colin (1997) defined a mass spawning as “a form of group spawning that consists of the great majority to all of an aggregation spawning simultaneously, as a single unit”. Studies on parental assignment of progeny (Brown et al., 2005; Chavanne et al., 2012) have referred to gilthead seabream spawning behaviour as mass spawning. However, the present study found that gilthead seabream only participated in pair and group spawning in agreement with other studies on Sparidae species. Nevertheless, all these observations were made on fish held in captivity and no reports have been published on the reproductive behaviour of wild populations of Sparidae. Although to date no study on a Sparidae species has observed mass spawning this spawning type cannot be discounted as a possible spawning behaviour in Sparidae and gilthead seabream. Mass spawning reproductive behaviour has been documented in several marine fish species such as the Lutjanus cyanopterus (Heyman et al., 2005) and the Dermatolepis dermatolepis (Erisman et al., 2009). Both species were observed in natural conditions and fish were described to release a massive cloud of gametes into the water column that made observation difficult. Females were, however, observed to exit from the mass spawning aggregations with accompanying males in examples of simultaneous group spawning. Therefore, the group spawning observed in Sparidae and the gilthead seabream could form part of mass spawning in different conditions. Domeier & Colin (1997) in an extensive review of aggregations and spawning type observed that species change spawning type in relation to the situation, with pair spawning more common in the absence of an aggregation and group spawning more common in aggregations and mass spawning was observed in some species to involve many incidents of simultaneous group spawning (as mentioned above). However, caution should be used in referring to a species such as gilthead seabream as mass spawning when only pair and group spawning has been observed.

Parental assignment of progeny also identified that the participation of gilthead seabream breeders was variable with a proportion of breeders that did not participate in spawning (Brown et al., 2005; Porta et al., 2009; Chavanne et al., 2012) and this variation or dominance by certain fish was particularly clear amongst male breeders (Brown et al., 2005). A similar situation was observed in the parental assignment of male cod breeders to progeny (Bekkevold et al., 2002) and this coupled with observations of cod reproductive behaviour (Brawn, 1961; Hutchings et al., 1999) suggested that cod males had reproductive hierarchies that explained the dominance of progeny by certain males (Bekkevold et al., 2002). A similar coupling of the present study on gilthead seabream spawning behaviour with studies on parental assignment of gilthead seabream progeny (Brown et al., 2005; Porta et al., 2009; Chavanne et al., 2012) also suggested the hypothesis that gilthead seabream had reproductive hierarchies that resulted in the dominance of progeny by certain breeders particularly amongst males. Chavanne et al. (2012) concluded that further research was required to understand the spawning kinetics of gilthead seabream. The present study, highlights that such studies need to also focus on spawning behaviour to understand why certain fish dominate spawning in relation to the spawning environment considering both physical (tank design, size) and social (characteristics of individuals, sex ratios, density) aspects. This, the first description of gilthead seabream spawning behaviour provides an important bases for these studies and for the first time researchers and broodstock managers can have a clear idea of the spawning behaviour when considering physical and social manipulations to increase parental contribution for breeding programs.

In the present study, the spawning activity took place midmorning, which was actually initiated 42 ± 8 min after the lights switched on and can be considered similar to previous studies that established that gilthead seabream and others sparid fish such as silver seabream (Sparus sarba), Pacific seabream (Acanthopagrus pacificus), yellowfin bream (Acanthopagrus australis), red seabream (Pagrus major) and black bream (Acanthopagrus butcheri) tend to spawn at sunset or early in the morning (Pollock, 1982; Matsuyama et al., 1988; Mihelakakis & Kitajima, 1995; Haddy & Pankhurst, 1998; Meseguer et al., 2008; Sheaves & Molony, 2013). In the present study, spawning was successfully and regularly obtained and presented a prolonged spawning season (up to 5 months). Spawning was close to every day in both tanks. These observations were characteristic of this species, and in accordance with Zohar et al. (1995), Barbaro et al. (1997) and Arabaci et al. (2010).

The present study demonstrated that gilthead seabream spawning behaviour was similar to other sparids. In most occasions, spawns were observed to initiate in the morning hours and presented the characteristic to be associated with aggregation behaviour, followed by the spawning rush performed by a single female pursued by a male or, less common, by two or three males. Aggregation and courtship behaviour appeared to be an essential part of the spawning behaviour probably related to mate selection, highlighting the need to have a group of breeders and not single pairs. These findings described for the first time the characteristics of gilthead seabream reproductive behaviour and that many spawning events during a short space of time were involved in the production of a “spawn”. Altogether the study provided valuable information that may explain the uneven participation of breeders in studies that determined paternity of progeny with microsatellites and provides a solid basis for future work to increase parental contributions to breeding programs.