Growth and distribution of Baltic Cod recaptured in the 1960 and 1970

Thünen institute and origin of the data

Johann Heinrich von Thünen Institute is a German institute that researches field, forest and seas. The institute has 14 specialist institutes, my professional practice would have been in the institute of Baltic Sea Fisheries that is located in Rostock.

There are five different fields of activity each Living marine resources; Reproductive biology; Fisheries and surveys technology; fisheries management; Fisheries and environment Baltic sea and German marine angler program with its own different ongoing projects. I worked on the TABACOD project in the Living marine resources activity field.

“The aim of TABACOD (Tagging Baltic Sea Cod) is to Improve the management of eastern Baltic cod by providing new information on growth and mortality patterns, and develop a validated method for deriving this information from historic and future samples.” - http://www.tabacod.dtu.dk

However, because of the epidemic I have unfortunately not been able to go to the institute itself. So, during my professional practice I communicated with my supervisor trough mail and online meeting platforms.

During the 1960 and 70 a scientist named Berner did several expeditions where she tagged and released Cod in the Baltic sea the tag was Colourless, transparent plastic tubes with an inlaid, rolled paper strip with the necessary information in German, Polish and Swedish were used for marking. . They were attached with a silver wire eyelet at the end of the 1st dorsal lap between the fin beam carriers. The data she required during these times was sent to me so that I could do initial analyses, growth and distribution of the released and recaptured cod which then could be compared with more recent data.

If you want more information about the Thünen institute or the TABACOD project here are some links

Thünen institute – https://www.thuenen.de/en/

Thünen institute Of Baltic sea Fisheries – https://www.thuenen.de/en/of/

TABACOD project – http://www.tabacod.dtu.dk


Initial data analysis

For the initial data analysis, I used Microsoft Excel.

I made an overview where I calculated

  • number of released and recaptured cod

  • the mean, standard deviation and range of; the days at liberty (DAL) and length at release and recaptured

DAL = recapture date - release date

Further I also calculated the growth (G) per year, which I needed for the growth analyses later on.

G= (length recaptured- length released)/DAL*365

The plots I made during the initial analysis I made with R 3.6.2

Cleaning of the data

After the initial data analysis, I needed to clean the data before I started with the growth analyses. Days at liberty and measurements precision can influence the growth estimates so the following approach was followed to minimise impact.

I excluded every fish that has a DAL < 50 and implied a maximum annual growth threshold of (29,74 cm/year), so I removed every fish with a growth higher than 29,74 cm/year. The maximum annual growth threshold was decided by calculating the 97 percentile of the remaining data. To balance the data, the same percentile of fish with extreme negative growth was also removed.

At the beginning I started with 545 fish that had information on length released, length recaptured and DAL in the end there 348 fish that would be used for the growth analyses.

Growth analyses

In the growth analyses I wanted to know if the release length of the Cod has an influence on the growth rate of set Cod. To know this I made a general linear model. Before the analyses however the release length was mean centred for an easier interpretation of the analyses.

L*released= Lreleased - ∑(Lreleased/n)

With L*released the length released mean centred, Lreleased the length at release and ∑(Lreleased/n) the mean of length at release

The result showed a significant negative relationship between L*released and annual growth rate which means that the length has a negative influence on the growth (bigger fish have a smaller growth rate than smaller fish)

Distribution

The Baltic sea is divided by ICES (International Council for the Exploration of the Sea) called subdivisions (SB) (map). The cod was released in SD 22, SD24 and SD25. Most of the cod was recaptured in those SB but some has travelled to SD21, SD23,SD26 and SD28. To know far the cod has travelled I calculated the distance travelled in R by using the coordinates and categorised them.

  • Stationary 0-50 km

  • Medium far 50-100 km

  • Far 100-200km

  • Very far >200km

The maps that I made with this information were made using qGIS 3.10. However because I had problems with the coordinates I also learned how to make a map in R version 3.6.3

My thoughts

My professional practice was a wonderful experience. I learned a lot mainly due to the focus lay on two weak points of mine; communication and advanced data analyses. Because of this I had a lot of pressure on myself to work hard and make no mistakes. Which clearly didn't happen.

At the end I learned a lot by making those mistakes and gained confidence in what I already could do.

Acknowledgements

I really want to thank my supervisors Uwe Krumme and Kate McQueen. They were phenomenal and very supportive and above all they were motived to come up with a good remotely professional practice where I could learn, make mistakes and grow.

References

Mion, Monica, Annelie Hilvarsson, Karin Hüssy, Uwe Krumme, Maria Krüger-johnsen, Kate Mcqueen, Esha Mohamed, et al. 2020. “Historical Growth of Eastern Baltic Cod ( Gadus Morhua ): Setting a Baseline with International Tagging Data.” Fisheries Research 223 (June 2019): 105442. https://doi.org/10.1016/j.fishres.2019.105442.