Settlement and post-settlement survival rates of the white seabream (Diplodus sargus) in the western Mediterranean Sea

Abstract

Survival during the settlement window is a limiting variable for recruitment. The survival is believed to be strongly determined by biological interactions and sea conditions, however it has been poorly investigated. We examined the settlement patterns related to relevant biotic and abiotic factors (i.e. Density-dependence, wind stress, wave height and coastal current velocity) potentially determining post-settler survival rates of a coastal necto-benthic fish of wide distribution in the Mediterranean and eastern Atlantic, the white seabream (Diplodus sargus). An observational study of the demography of juveniles of this species was carried out at six coves in Menorca Island (Balearic Islands, western Mediterranean). Three of the coves were located in the northern and wind exposed coast, at the Northeast (NE) side; while the other three were found along the southern and sheltered coast, at the Southwest (SW) side of the island. The settlement period extended from early May to late June and maximum juvenile densities at the sampling sites varied between 5 and 11 ind. m-1 with maximum values observed in late May simultaneously occurring in the two coasts. Our analysis of juvenile survival, based on the interpretation of the observed patters using an individual based model (IBM), revealed two stages in the size-mortality relationships. An initial density-dependent stage was observed for juveniles up to 20 mm TL, followed by a density independent stage when other factors dominated the survival at sizes > 20 mm TL. No significant environmental effects were observed for the small size class (<20mm TL). Different significant environmental effects affecting NE and SW coves were observed for the medium (20-30mm TL) and large (>30mm TL) size class. In the NE, the wind stress consistently affected the density of fish of 20–30 mm and >30 mm TL with a dome-shape effect with higher densities at intermediate values of wind stress and negative effect at the extremes. The best models applied in the SW coves showed a significant non-linear negative effect on fish density that was also consistent for both groups 20–30 mm and >30 mm TL. Higher densities were observed at low values of wave height in the two groups. Because of these variations, the number of juveniles present at the end of the period was unrelated to their initial density and average survival varied among locations. In consequence, recruitment was (1) primarily limited by denso-dependient procedures at settlement stage, and (2) by sea conditions at post-settlement, where extreme wave conditions depleted juveniles. Accordingly, regional hydrodynamic conditions during the settlement season produced significant impacts on the juvenile densities depending on their size and with contrasted effects in respectto cove orientation. The similar strength in larval supply between coves, in addition to the similar mean phenology for settlers in the north and south of the Island, suggests that all fish may come from the same parental reproductive pool. These factors should be taken...

Aggressive Predation Drives Assembly of Adriatic Fish Communities

“…The Diplodus species are known to coexist in pristine sites through habitat or depth partitioning (e.g., D. vulgaris and D. sargus), but also through differential prey use within the same habitats and depths (e.g., D. vulgaris and D. puntazzo) [60]. As juveniles they all settle within rocky reefs, where they are well protected from physical stresses and mainly threatened by density dependent competition with each other [61][62][63]. The decline of D. sargus and D. puntazzo due to overfishing may have released D. vulgaris from its competitive restrictions and it may in turn contribute to the further decline of the other congeners through direct competition within the adults and juvenile cohorts or by increased intraguild predation on juveniles of D. sargus and D. puntazzo [53].…”

Fish nursery value of algae habitats in temperate coastal reefs

“…Scenarios are imaginable in which, due to a strong settlement pulse and fishery-depleted predators, there are many juveniles in a habitat. However, because of the lack of food provided by the habitat, fish may have low long-term survival potential (e.g., Macpherson et al, 1997; Planes et al, 1999; Cuadros et al, 2018). Using abundance indices alone as a tool to assess habitat quality has limitations, as these do not consider the effect of different habitat conditions on the individual fish, i.e., how a habitat affects parameters such e.g., growth and survival potential.…”

“…The survival of recently settled juveniles within a habitat, through to their recruitment to the adult population, depends to a large extent on the environmental conditions encountered at the site of settlement (Beck et al, 2001). Intolerance to physical extremes, starvation and predation are among the major causes of juvenile mortality in these habitats (Sogard, 1997; Guidetti, 2001; Thiriet et al, 2016; Cuadros et al, 2018). High nursery value is thus conferred through a combination of factors that provide adequate physical conditions, refuge space and a sufficient food supply.…”

Typology of fish nurseries in shallow Mediterranean coastal zones: all habitats matter

“…On the other hand, juvenile fishes' abundance 56 , growth 101 and mortality 102 vary considerably in space and time due to natural stochastic processes linked to both environmental conditions (currents, winds, hydrological parameters) 44,53 and the success of adult reproduction 103 , being high or low at one place from one year to another. The same nursery site can therefore perform as a 'good' nursery site one year and not the following one 56 .…”