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. In order to
maintain tanks with approximately equal sized shoals, tanks with fish
from the same size class and same habitat were combined after periodic
sampling. 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 TDF 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. During 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.