Age at length data for larval African halfbeak Hyporhamphus picarti (Hemiramphidae) in the Sine Saloum estuary, Senegal

The African halfbeak Hyporhamphus picarti (Hemiramphidae) is one of the most abundant species within the ichthyoplankton community of the Sine Saloum estuary (Senegal). A year-round occurrence of larvae suggests that the Sine Saloum is an important spawning habitat for this species. Annual fluctuations in water temperature, however, can have severe impacts on the survival probabilities of marine fish larvae. To determine whether temperature has an effect on the growth of H. picarti during its larval development, larval age at length and somatic growth rates were investigated for two contrasting spawning seasons in 2014: February (cold season, 20.8°C) and June (warm season, 26.4°C). H. picarti larvae were sampled at two stations inside the Saloum River using two vertically connected neuston nets (30 × 15 cm, 500 µm mesh size). The nets were attached to a catamaran (Hobie Cat 15), which was towed by a motorboat at a speed of two to three knots. Fish larvae were fixed immediately after the haul with a 30% ethanol/sea water solution and stored in an electric cooling box. At the laboratory, fish larvae were successively transferred to 50% and 70% ethanol. Water temperatures (°C) and salinities (practical salinity scale) were determined at each station with a multiprobe (WTW 3430). Larval standard lengths (SL ± 0.01 mm) were measured using a calibrated stereomicroscope micrometre (Stemi SV 11 and Stemi 2000-C, Zeiss). The sagittal otoliths were dissected and stored in immersion oil for several days. In order to assess the age of H. picarti larvae at a given length, otolith daily growth increments were counted (core to edge) along the otolith's largest diameter using a stereo microscope (Axioskop, Zeiss) at 200-500× magnification. Increments were counted three times by the same reader, and counter read once by an additional reader. If the results of the counts of both persons deviated by more than one increment, the respective otolith was discarded. Otolith diameter was measured using ImageJ 1.49 (Rasband, WS, US National Institutes of Health, Bethesda, MD, http://imagej.nih.gov/ij/).