Nina Pirker Dalsgaard, Masters student

Thesis title: Selection for faster growth rate in juvenile Atlantic cod

Having a bigger size and faster growth can be a big advantage for juvenile fish, to be able to survive into adulthood and being able to reproduce. The development in the early life stages are important for the dynamics of recruitment in a fish population. This influence on recruitment makes it possible to predict the abundance of a cohort by analyzing it in its early stages of life. Takasuka, Yoshioki et al. (2004) proposes three different hypothesis compiling evidence to explain the advantage of faster growth and larger size. One of them being “bigger is better” hypothesis. The advantage of having a bigger body size relates to the fact that by being big, you often have fewer predators. This will then increase your chance of survival. Another aspect of fast growth is the advantage spending as short time as possible in dangerous life stages, such as the danger of being prey in the early life stages. This means that individuals with fast growth rate, can skip though life stages with high mortality risk. The final hypothesis is “growth-selective predation.” This hypothesis states that faster growing fish larvae may be les vulnerable to predation than slower growing larvae of the same size. This may be because fast growth corelates with being harder to catch and they themselves taking fewer chances to catch prey. These tree hypothesis gives a rough idea of the potential of having faster growth, which is the foundation of this master’s project.

In this master’s project the selection on growth rate and size will be investigated in an Atlantic cod population. This will be done by using cod collected during the spring and autumn of 2019 at Buøyodden and Langerumpa off the coast of Norway. Twenty fish from each of the seven samplings spread out across the six months will be used. At the beginning 4 evenly spread samplings across the time period will be done. If time, the gabs will be filled by the additional samplings. To analyses the growth rate of the Atlantic cod, Sagittae otoliths will be used from each fish by filling it down, investigating and photographing it under the microscope. Using the photographs, based on the width of the daily increments for each otolith, growth rates and size-at-age for each fish can be calculated and compared. Two different types of analyses will be made on the data collected from the cod. One will be modeling change in size over time, based on otolith back-calculating. This is to see the difference in growth over time between fish caught early/before selection and fish caught later/after selection. The other analyses that will be made is focused on comparing the different dates and the possible selection event. By using Anderson (1995) cross-sectional studies, i.e. ‘before–after’ comparisons. This means that by having datapoint before and after the possible selection event, one can pinpoint a possible timing of the selection in the time period selected. This will be done by first making a relative frequency distribution of fish caught at different times. Then each of the different frequency models will be compared to each other to make a relation between relative fitness and fish length. This will give a curve describing the relative fitness for a given fish length in each time period.

By being able to pinpoint the placement of a possible selection for fast growth rate, in this population of Atlantic cod, we may be able to predict an upcoming collapse or fall in numbers by using the growth rate of early stages of juvenile individuals.