Effects of salinity on strontium:calcium ratios in the otoliths of the West African black-chinned tilapia Sarotherodon melanotheron in a hypersaline estuary

Abstract

The biology and the behaviour of fish populations in hypersaline environments are poorly known. The habitat occupation strategy of the tilapia Sarotherodon melanotheron has been studied along a salinity gradient in the Saloum hypersaline estuary in Senegal (salinity between 32 and 100). The individual migratory behaviour has been analysed from otolith strontium-to-calcium concentration ratios from fish sampled in five locations during the 2003 wet season and the 2004 dry season. In the upper part of the estuary (salinities > 50), the Sr:Ca ratio in the otolith showed high variations, from 2.51 to 33.30 · 10 -3 . These maximum observed values have never been reported in the literature. The individual mean of Sr:Ca ratios increased according to the salinity gradient in the estuary, with significantly higher values in the upper part (Sr:Ca mean = 16 · 10 -3 ) than in the lower part (salinity < 50, Sr:Ca mean = 12 · 10 -3 ). No significant difference in the Sr:Ca mean was observed between locations with comparable salinities. Sr concentration in the water at the different locations was positively correlated with ambient salinity. The mean of Sr:Ca in the otoliths was then in relationship with the level of Sr in the water. Thus, the Sr:Ca ratios in the otolith of S. melanotheron allowed to discriminate the populations of this species and to hypothesize that they did not undertake large scale movements within the estuary.

Hypersaline waters pose new challenges for reconstructing environmental histories of fish based on otolith chemistry

“…Otolith Ba : Ca generally shows a negative relationship with salinity (Elsdon and Gillanders 2005a;Dorval et al 2007) and has been suggested as a tracer of salinity. Likewise, Sr : Ca has often been used as a tracer of salinity because of the positive relationship between otolith Sr : Ca and salinity (see review by Secor and Rooker 2000), although experimental laboratory studies report mixed results, which likely reflect underlying differences in water chemistry (Elsdon et al 2008).…”

“…Several studies have investigated stock structure of fish based on stable isotopes and/or Sr : Ca ratios in otoliths, including at sites that were slightly hypersaline (with a maximum salinity of between 42.5 and 65 [Dufour et al 1998;Edmonds et al 1999;Bastow et al 2002]). A single study has investigated the relationship between Sr : Ca ratios of otoliths and salinity for fish collected from Saloum estuary (Senegal, West Africa), where salinities ranged from 32 to 58 during the rainy season and from 40 to 100 during the dry season (Diouf et al 2006). Sr concentration in the water and its relationship to salinity was also investigated within the same estuary, but samples were collected 2 yr after the fish; therefore, no direct comparison could be made between otolith Sr : Ca and water Sr (Diouf et al 2006).…”

“…A single study has investigated the relationship between Sr : Ca ratios of otoliths and salinity for fish collected from Saloum estuary (Senegal, West Africa), where salinities ranged from 32 to 58 during the rainy season and from 40 to 100 during the dry season (Diouf et al 2006). Sr concentration in the water and its relationship to salinity was also investigated within the same estuary, but samples were collected 2 yr after the fish; therefore, no direct comparison could be made between otolith Sr : Ca and water Sr (Diouf et al 2006). No studies (that we are aware of) have investigated the relationship between otolith chemistry, water chemistry, and salinity of hypersaline waters for elements other than Sr.…”

Body size mediated effects of multiple abiotic stressors on the growth and social behaviour of an estuarine fish, Australian Bass (Macquaria novemaculeata)

“…The spatial distribution of fish along estuaries can indicate the tolerable salinity range of different species (Cardona 2000;Diouf et al 2006;Bachman and Rand 2008;Macdonald and Crook 2010), however future changes in salinity may cause conditions to move towards, or away from the optimum conditions for each species (Labonne et al 2009). For example, large increases in salinity may have negative impacts on freshwater and estuarine resident species through physiological stress (Elliott et al 2007), particularly where upstream estuarine areas have become hypersaline Pratchett et al 2011).…”

Egg production in the euryhaline tilapia,Sarotherodon melanotheron heudelotii, experimentally maintained in fresh, sea and hypersaline waters

“…Similar trends have been observed in fishes living in adverse environmental conditions, as regards salinity, oxygen, food availability or predation pressure, and are interpreted as trade-offs in the context of life history strategies (mosquitofish Gambusia spp., Brown-Peterson and Peterson 1990; Alcaraz and García-Berthou 2007; Caspian roach Rutilus rutilus caspicus, Naddafi et al 2005; for cichlids: Chapman 2008, 2012;Russell et al 2012). Populations of S. m. heudelotii exhibit a marked genetic structure (Ndiaye 2012) and studies of their mobility indicate that this species is rather sedentary (Diouf et al 2006). Henceforth, it is uncertain whether the aforementioned variations in the life history traits of S. m. heudelotii in environments with contrasting salinities reflect genetic differences between populations or phenotypic plasticity in response to salinity or to associated environmental factors (food, predation, oxygen).…”