Data and code from: Effects of temperature and browning on the functional response of a freshwater top predator

This dataset contains data and code underlying a study of how temperature and water browning affect feeding rates and functional responses of a freshwater top predator, the northern pike (Esox lucius). The archive includes data from two controlled laboratory experiments quantifying pike feeding on roach across gradients of temperature, prey density, and water colour (browning), as well as year-round body temperature records from free-ranging adult pike equipped with implanted biosensors. Experiment 1 provides replicated functional-response data across three temperatures (5, 9, and 13 °C) and two visual conditions (clear and brown water) at four prey densities. Experiment 2 quantifies feeding rates at two prey densities across a browning gradient ranging from completely clear to extremely brown water. Additional files include predator body mass measurements and continuous body-temperature time series used to validate climatic simulations of winter thermal regimes. All raw data, processed variables, and fully reproducible R code used for statistical analyses, functional-response fitting, model comparison, and figure generation are included.

Methods
Experiment 1:

We quantified functional responses of pike foraging on roach at three temperatures (5°C, 9°C, and 13°C) and two levels of visual condition (clear and brown water). We performed the foraging trials in the laboratory at four prey densities (2, 5, 10, or 20 roach per 700L). Foraging trials were performed in six tanks with an interior dimension of 121cm x 104 cm (length x width) filled to 56cm with filtered and aerated water (total water volume: 704L). Visual conditions were manipulated by adding 3.5g humic acid (Humintech GmbH, Germany) to three tanks. The clear water treatment had an absorbance (Abs420/5) of 0.020±0.007, and the brown water treatment 0.097±0.012. Each of the 24 treatment combinations was replicated eight times. The trials lasted 23.5h with a 15-minute acclimation period. Pike were starved for 4 days at 9°C between trials to standardize hunger levels and motivation. A total of 24 pike were used (mean wet weight 51.8±12.8g (mean ±1SD), n=23, range: 34.5g – 79.2g). Pike were used for up to 10 foraging trials (average=8). Roach were used as prey (wet weight = 0.91±0.34g (mean ±1SD), n=47).

Experiment 2:

We performed foraging trials at two prey densities (5 and 10 roach) and five visual levels. As before, visual conditions were modified by adding humic acid. The absorbance in the browning treatments was 0.02±0.01, 0.08±0.02, 0.24±0.04, 0.84±0.10 and 3.06±0.22 (mean Abs420/5 ± 1 SD; 7 weekly measurements per treatment). The temperature was kept constant at 9°C. Pike were fasted for 12 days between Experiment 1 and 2. Foraging trials were performed exactly as in Experiment 1.

Body temperature:

We equipped six adult pike with biosensors and recorded their body temperature for 12 months in experimental ponds. Pike were captured by electrofishing and transported to three experimental ponds (70m x 30m, depth 1.5m) where they were placed in keep nets (3 x 3 x 1.5 m, 1.5 cm mesh size; Pokorný Sítě, Brloh, Czech republic) for acclimation. Pike were surgically implanted with Centi-HRT biosensors (Star-Oddi, Garðabær, Iceland) set to record body temperatures at 30 minute intervals and released into the ponds on November 23, 2021. In early October 2022, all pike were recaptured by electrofishing, biosensors retrieved, and body temperature data downloaded.

All experiments were conducted under permission from the Malmö/Lund authority for ethics of animal experimentation (Licence 5.8.18-00783/2022).