Sampling protocol
Samples from the before time point were obtained from the
archives of preserved fish specimens (usually in 10% formalin)
deposited in the Lee Kong Chian Natural History Museum of the National
University of Singapore. We filtered specimens by locality and taxonomy,
selecting individuals from genera of predatory fishes we encountered at
the respective catchments in the after time point. Fin tissue was
excised from suitable specimens for subsequent laboratory analyses. We
also measured the standard length of individuals sample. We collected a
total of 57 specimens from the before time point, comprising 13
individuals (one species) from Danum, 10 individuals (six species in
three genera) from Endau, 12 individuals (four species in two genera)
from Nee Soon, and 22 individuals (four species in three genera) from
NSPSF (Supplementary Table 1).
Samples from the after time point were collected during field
excursions to each of the four river catchments between 2015 and 2018.
We used a combination of backpack electrofishing (Bretschneider
Spezialelektronik EFGI-650), cast nets (2.75 m diameter; 1 cm mesh) and
rigid-frame push nets (60 × 45 cm; 2 mm mesh) to maximise our
catch-rate. Fin tissue was excised from all predatory fish taxa
encountered and kept in 70% ethanol or 10% formalin (= 3.7%
formaldehyde) solutions for subsequent laboratory analyses. Fish
specimens were also measured to determine their standard length. We
collected a total of 39 specimens, comprising three individuals (one
species) from Danum, 18 individuals (four species in three genera) from
Endau, seven individuals (three species in two genera) from Nee Soon,
and 11 individuals (five species in three genera) from NSPSF
(Supplementary Table 1).