Experimental preference set-ups are hard nuts to crack, but we managed to do some decent work on yellow perch salinity preference
Tagged brackish water perch from last year return to overwintering site. Anglers report size, ID, and photos online while I’m busy at the lab. For determination of population size, growth, and migration patterns of perch in the Baltic Sea
I was so lucky to be invited on board a research cruise to southwestern Greenland catching the worlds oldest vertebrate, the Greenland shark. The cruise was a part of great project at University of Copenhagen, lead by my supervisor, Prof. John F. Steffensen (see more on http://bioold.science.ku.dk/jfsteffensen/OldAndCold/). On the picture above, Professors Steffensen and Bushnell are releasing one of the many monstrous sharks we caught.
I collected tissue samples for RNA analysis, blood chemistry analysis (plasma osmolality, TMAO and urea) and muscle chemistry (TMAO and urea). While not directly related to my PhD project, the parameters measured are concerning the counteracting of salinity stress in cartilaginous fish, which is a closely related subject and thus a great contribution to my PhD education. On the picture to the left I am drawing a blood sample from a 4.3 m shark. Amazing animals!
The overall results are not ready yet, but we got good data from 27 individuals between 2.5 and 4.4 m, the biggest ones weighing more than 1000 kg!
Approximately 300 fish have been tagged so far, and the tagging will continue at least until end April, where all fish have probably left the delta of Ishøj to venture into brackish water over the summer.
The tagging of perch in Ishøj is being reported at a high rate, which is due to the commitment of the local fishermen. To date, 20 % individuals have been reported via this homepage, and of those, 3 have been caugth more than two times. Coupling this with water parameters (www.saltnfish.dk/vandmaaler) and a population estimate will be a great contribution to my PhD thesis, which is mainly focused on labexperiments.
The first publication of the project is called Plasma osmolality and oxygen consumption of perch Perca fluviatilis in response to different salinities and temperatures, and has been published in Journal of Fish Biology (http://onlinelibrary.wiley.com/doi/10.1111/jfb.13200/full). It shows how aerobic scope of perch changes with salinity and temperature, and suggests that seasonal migrations between brackish water and fresh water is, in part, a behavior of optimising metabolism.
The harbor of Ishøj (DK) is the winter residence for a peculiar population of brackishwater perch that stays in the area from around October through April. They spawn in the brackish water of the harbor at salinities of up to 10, yet evacuate the area after spawning and seek the adjecent brackish water bay, Køge Bugt, during the summer months. The specimens are insanely large for perch, with individuals above 50 cm and 2 kg, and the population is also presumably large. This has not gone past the attention of recreational fishermen, and every winter the area around Ishøj Harbor is packed with anglers enjoying an entertaining fishery. This, of cause, puts a large fishing pressure in the population, and as a consequence the management has put an unconditional catch and release policy on perch caught in the area.
The combination of dedicated fishermen as well as unconditional catch and release makes Ishøj Harbor an ideal place to conduct a mark-recapture study in a setup where the locals report catches online. The fish are being tagged with external identification tags, and scale samples are collected in order to determine the age of the individuals. In addition, a device measuring oxygen level, salinity, temperature and depth has been setup in the area. These data have been made available for the local fishermen for them to assess the water conditions before planning a fishing trip.
The joining forces with the locals gives us an opportunity to estimate the population size, determine importance of physical/chemical factors on migration patterns, estimate growth rates, etc.
A productive stay in Miami is over, and I owe a big thanks to all the help and advice I got from Martin Grosell and the people in his lab. I feel save to say that I got some interesting results on the response to salinity of Yellow perch, and the data is going to be a nice reference to the European perch.
Back at the Mother Ship (that is, Marine Biological station in Helsingør, the first picture in this post) I’m preparing the next round of experiments, which is going to be on a freshwater population of European perch. The specimens are already caught as you can see in the video below. Let the fun begin!
I currently have three swimming respirometers at my disposal, which I can run at the same time. The upper video is showing the setup.
Perch (yellow perch and European perch) are said to be poor swimmers, but I decided to give them a go anyway. The lower video shows a 14 g yellow perch at 22 ºC at a velocity of 3.5 body lenghts per second (42 cm per second). I would say that it is not too bad a swimmer after all: It almost swam 1 body lenghts per second faster than this before it gave up (30 min periods, 0.5 body lenghts per second increments). The lighting was kept dim to stress the fish minimally, and I put some of the plastic water plants I keep in their holding tanks on top of the tunnel for shelter.