Does abundance of Northeast Arctic cod (Gadus morhua) influence the schooling dynamics of capelin (Mallotus villosus)?

• Main Author: Hough, Jessica Anne
• Format: Master Thesis
• Language: English
• Published: 2021
• Subjects: Arctic; Barents Sea; Arctic cod; Gadus morhua; Northeast Arctic cod
• Online Access: http://hdl.handle.net/10852/91299; http://urn.nb.no/URN:NBN:no-93906

Cod (Gadus morhua ) and capelin (Mallotus villosus) play an important ecological and commercial role in the Barents Sea. Capelin provide high lipid energy to many of its predators in the Barents Sea including cod, which is capelin's main predator. Exploring cod and capelin's interaction is instrumental when assessing the stocks and provides improved understanding about the Barents Sea Ecosystem. Studies concerning their interaction and overlap, have been conducted on a large scale, using survey data collected over the course of several years, but few studies have addressed their interaction on a finer scale. Therefore, the main aim of this thesis was to assess small scale interactions by investigating whether abundance of cod affects capelin's schooling dynamics, namely packing density and minimum distance to bottom. To do this, one of the objectives of this study was to establish a method to enable the use of acoustic data to evaluate capelin school features. Acoustic data was collected onboard the Barents Sea Ecosystem Survey in August/September, when cod and capelin have undergone migrations to the northern Barents Sea. The original interpretations of acoustic data were made on board by scientific personnel and I further examined the capelin school data using the program Large Scale Survey System (LSSS) and its multi-extension toolbox Korona, enabling for school detection. Through sensitivity testing I found that the Korona school detection settings acousitc threshold in particular, but also minimum school length, strongly influenced the capelin school detection. I found a setting which was successful in detecting and delineating capelin schools and this setting can be utilized for future studies on capelin schooling dynamics, for instance using time-series data. I found that abundance of cod had no direct impact on capelin's packing density and minimum distance to bottom. However, previous studies has demonstrated that light and bottom depth play an integral role in fish schooling dynamics and were therefore considered here. Testing a set of linear mixed models, the best model, showed that bottom depth and sun altitude contributed to significant variations in schooling dynamics. Packing density increased with increasing sun altitude and was higher at shallow banks, whereas minimum distance to bottom increased with increasing bottom depth and decreased with increasing sun altitude. Although abundance of cod did not directly impact capelin's schooling dynamics, the results suggests that capelin's schooling dynamics is rooted in a trade-off between engaging in activities such as feeding and avoiding predators such as cod. The increase in packing density along shallow banks during daytime suggests that capelin may have been responding to cod that are typically associated with shallow banks when feeding. The results from this study indicates that capelin adjusts their position in the water column to a depth where light gradients provide enough light to detect prey, but also avoid predation.