Fish collection, husbandry and experimental design
Between August 21st and 28th 2018, we sampled perch
from the lake Erken (59°50’09.6”N, 18°37’52.3”E) in Central Sweden by
angling from littoral and pelagic habitats. We caught additional
young-of-the year juveniles by beach seining. Using cooled and aerated
boxes, we brought the fish to the aquarium facility at Uppsala
University.
Fish were anesthetized using 60 mg L-1 benzocaine and
weighted to the nearest 0.01g. We grouped the fish according to their
habitat of origin and the body weight at the beginning of the experiment
into weight classes: <20 g (juveniles of approximately 4 g),
20-30 g, 30-40 g, and 40-50 g. Perch of the weight class 20-30 g were
caught in both littoral and pelagic habitats. Due to potential
differences, we analyzed this weight class habitat-specifically. 3-9
perch individuals were placed together in 30l –aquaria (50 x 25 x 25cm)
with the bottom covered by a 3cm thick layer of sand. Room temperature
of the facility was set at 15 C°. Aquaria were equipped with a
flow-through system of fresh tap water. As the tap water became colder
during the winter months (November to March), we noticed a drop in the
tank water temperatures from 18 C° to 14 °C. We fed the fish daily with
commercially available Chironomids, with an amount corresponding to
approximately 15% of the individual wet weight –day.
Unfortunately, we noticed strong variation in isotopic signatures of
Chironomids and were forced to switch suppliers to obtain more stable
signatures in perch diet (final food: δ13C: -30.6 ‰ ±
0.5, δ15N: 14.0 ‰ ± 3.7; average ± SD, respectively).
Using the new food, we started the feeding experiment to estimate TDFs
in perch on October 10th 2018 (day 0). We ended the
trials between July 12th (day 275) and
25th 2019 (day 288), i.e. the experiment ran slightly
longer than 9 months. At this time, perch had approximately doubled
their original body mass on average (132.7 % weight gain ± 89.4, Table
1). This is an approximate value as we did not track the weight increase
individually, but set initial weights as the average values of the
respective weight class. We assumed perch to be in isotopic equilibrium
with the Chironomid diet based on the equations for isotopic half-life
of vertebrate muscle tissue and liver reported in Vander Zanden et al.
(2015) and the predictive turnover equations presented by Thomas and
Crowther (2015). For both predictions, we assumed isotopic equilibrium
in 4-5 times the half-live (Thomas and Crowther 2015). Before the fish
were sacrificed at the end of the experiment with an overdose of
benzocaine, we conducted metabolic trials to obtain individual SMR (see
section below). We weighed the sacrificed fish to the nearest 0.01g and
measured total length to the nearest mm. We dissected a sample of the
dorsal muscle tissue and the entire liver for stable isotope analyses
and dried the tissue in a drying oven at 60°C for 48 hours.
Over the course of the experiment, we sacrificed a subset of 5
individuals every 6-10 weeks to observe the development of the TEFs over
time. In total, we analyzed the Δ13C and
Δ15N in muscle and liver for 48 and 47 individuals
respectively. In addition to the fish sacrificed over the course of the
experiment, 28 individuals were maintained for the entire 9-month period
and SMR measurements were taken for 26 of them.
The study was approved by the Uppsala Animal Ethic Committee with permit
number: C59/15.