By Morgan Bender
With over 240 polar cod sampled, 30 weeks gone by, and 2.310 bags of precisely prepared, weighed, chopped and labeled portions of calanus fish food, the long term chronic dietary exposure experiment has come a great distance! Here is a small glimpse into what the sampling and experimental design has been like so far.
Periodic measurements on all fish in the experiment has allowed us to monitor growth throughout the reproductive development without removing the fish from the experiment. It also gave us a chance to check up on our favorite individual fish progress over the span of many months.
This cold room at Kårvika havbrukstasjon outside of Tromsø has been the experimental grounds for the duration of the project. The temperature of the water is now 3 degrees and the air isn’t much warmer, following the temperature variations of Kongsfjorden on Svalbard, where most of the fish are from.
Inside the cold room its polar night and has been since November following a 79 degree North light regime. That means that work for us is also done in the dark. A total of ten tanks in this room are used for the dietary exposure experiments and are monitored continuously by technicians at the research station.
Calanus copepods are one of the main food items for polar cod and we feed them a lot of it! The different doses of crude oil are mixed with this bright orange zooplankton and fed to fish twice weekly, the remaining food for the week is untainted calanus. Feeding is done by tank and biomass of the tank is used to calculate how much food that tank receives. This amount is adjusted based on growth and sampling every 6 weeks.
Work has already been done in the lab looking at sex steroid hormones extracted from blood plasma. This will give us indications of hormone signalling involved in reproductive development and possible endocrine disruption with chronic exposure to petroleum compounds.
Over the course of the experiment, smaller sampling points using unexposed fish have allowed us to narrow in on specific events in the reproductive process with regard to planning full sampling events at critical time points.
Samples of gonads have undergone histological analysis in pace with sampling to closely follow development. This methods require deep concentration as exemplified by Master student Libe Aranguren as she cuts 5 μm slices of tissue on a microtome using a very shape knife.
Our first results! Histological analysis of female gonad samples from early December reveal that fish are nearing spawning. Yolk gobules are forming the cytoplasm of the eggs as female fish invest more nutrients and energy into the gonadal development. Timing is most critical for females thus we are following their development more closely.