Submarine groundwater discharge (SGD) is an ubiquitous pathway for nutrient rich freshwater to coastal ecosystems, demonstrably altering their hydrography, hydrochemistry, and primary productivity. Yet only little is known about the effects of fresh SGD on the fitness of higher trophic levels such as teleost fish. We examined feeding intensities, somatic conditions, and growth rates in juvenile grey demoiselle Chrysiptera glauca sampled between October 2017 and January 2018 in two contrasting tide pools in Mauritius: One strictly marine tide pool at Flic en Flac and another one influenced by the influx of SGD (Albion).
Specimens of C. glauca were caught with hand nets and immediately preserved in 30% ethanol/seawater solution. In the laboratory, fish were gradually transferred to 50% and 70% ethanol/freshwater solution. Further, fish were measured by standard length (SL, nearest mm), eviscerated, and weighed (EM, ± 0.001 g). Additionally, for each fish, stomach wet mass (SM, ± 0.001 g) was determined to calculate feeding intensity (FI): FI = SM × [E]^(-1) × 100. Further, a condition index (CI) was calculated for each individual by using b of the length-weight relationship: CI = EM × [SL]^(-b) × 100. We used the frequency of occurrence method for stomach content analysis (0: absence and 1: presence). Food organisms were identified to class level.
The sagittae and lapilli otoliths were dissected under a stereomicroscope, cleaned with deionized water, and stored dry in FEMA-cells (26 x 76 mm). Sagittae otoliths were weighed (± 0.001 mg). Lapilli otoliths were fixed on glass slides using 2-component adhesive (Araldite 2020/A and Araldite 2020/B) and ground sequentially on glass plates by silicon carbide (SiC) powder with grit sizes of 400 and 800. After polishing with waterproof silicon carbide grinding paper (grain size of 5 µm), otoliths in immersion oil were examined with a digital microscope (Keyence VHX-5000) using transmitted light at a magnification of 400x - 800x. With the digital microscope it was possible to compose depth-stitchings of different focus levels. Increment counts were conducted and OR was measured using ImageJ 1.49 (Rasband, WS, US National Institutes of Health, Bethesda, MD, http://imagej.nih.gov/ij/). Daily deposition of increments on the lapillus otolith has been validated in a number of species from this family and it was therefore assumed that increments on the lapilli were deposited daily. The increment closest to the core of the otolith was assumed to be formed at the day of hatching, as is the case in many other species from the family Pomacentridae. The number of increments was determined from 3 replicate increment counts. The settlement mark was identified as the first increment of a transition zone, characterized by a rapid narrowing in increment width.
