Lukas et al. 2021: Data from "Diurnal Changes in Hypoxia Shape Predator-Prey Interaction in a Bird-Fish System"

Topic: In many aquatic environments, daily changes in oxygen concentrations occur frequently. When oxygen levels drop to hypoxic levels, many fishes respond with aquatic surface respiration, during which they obtain oxygen by skimming the upper, oxygenated surface layer. By increasing time at the surface, fish become more vulnerable to fish-eating birds. We explored these cascading effects in a sulfidic spring system that harbors the endemic sulfur molly (Poecilia sulphuraria) as prey species and several fish-eating bird species. Study design: All observations stem from the sulphuric spring complex ‘Baños del Azufre’ in southern Mexico. To explore whether the observed diurnal differences in fish behavior are driven by physicochemical water conditions and how they link to predator activity, we conducted two field surveys and one laboratory experiment. During one field season (subsequently termed field survey I), we first quantified how variable physicochemical water conditions were throughout the day. In another field season (subsequently termed field survey II), we investigated the link between fish’s behavior and predatory bird activity. To link observations from both surveys, they were carried out at the same location and followed the same regimen by sampling each morning (07:00 – 09:30), midday (12:00 – 14:30) and afternoon (16:00 – 18:30) for six subsequent days. Surveys were matched for season (i.e., end of dry season; I: 12–17 April 2019, II: 05–10 May 2016) and we subsequently verified that no major deviations from the temperature trend occurred between years (see DATA_FieldSurvey_SI_Temp). DATA_FieldSurvey_WaterChem: Physico-chemical water parametersWe took multiple measurements of water temperature, dissolved oxygen (DO), pH and electrical conductivity (EC), and – due to logistical constraints – one sulfide sample along a cross section of the sulfidic stream. To capture variation introduced through flow regime and/or stratification within the water column, measurements were taken according to a transect grid (i.e., one sample each meter starting at 0.5 m away from the stream bench and at depths of 0.05, 0.25, and 0.5 m as water levels would allow). With a width of 5.6 m and a depth of less than 0.8 m at the site, this resulted in 10–11 subsamples per sampling. DATA_FieldSurvey_Fish: Sulphur mollies’ diving behavior We assessed fish behavior using a focal animal sampling approach. Twenty diving fish of similar size were chosen randomly from a focal shoal and observed from the moment they initiated diving until they resurfaced to calculate the mean dive duration. DATA_FieldSurvey_Birds: Predatory bird activityWe recorded all sightings of piscivorous birds in the predefined study area within a 30-min period. For each bird, we determined species (to lowest feasible taxonomical level), entry and exit times, as well as the number of attacks launched. We calculated presence time as the mean time piscivorous birds spent in the study area (excluding mere fly-throughs, i.e., flying through the transect without landing or attacking) and attacks as the total number of bird attacks launched in a sampling period. DATA_FieldSurvey_SI_Temp: temperature monitoring Air and water temperatures were continuously monitored for the duration of the 2016 field survey. Two HOBO Pendant data loggers (Onset Computer Corp.) were placed in a shaded area at the upstream edge of the study area: one into the water at a depth of 15 centimeters, another 1.5 meters above the water. Temperatures were recorded at a ten-minute interval. For the purpose of this study, we only analyzed temperatures recorded during observational periods of birds and fish (i.e. ~1h per sampling period). DATA_LabExp_ASR: ASR tendency in response to hypoxiaWe exposed fish to dissolved oxygen concentrations ranging from near-anoxic to normoxic conditions (0.6 – 5.1 mg/L DO) in a laboratory setting. We tested a total of 27 groups (5 adult individuals each; n = 135). A trial lasted 10 minutes, but we only analyzed the last 5 minutes to ensue fish had recovered from handling and resumed swimming. Data shown here is group-pooled. We quantified the cumulative time spent at the surface by all five fish and calculated a percentage surface time. We also assessed the total number of dives and mean dive duration performed by each group. Script_Lukas2021_WaterChem All analyses were performed in R (R Core Team., 2020, version 4.0.2).