Prøveforelesning og disputas - MSc in International Fisheries Management Nadine Jacques

Later, at 12.15 she will defend her thesis entitled: "Understanding size selectivity and fish behaviour in relation to rigid sorting grids in the Barents Sea demersal trawl fishery"

Professor Monica Alterskjær Sundset at the Deparment of Arctic and Marine Biology will lead the disputation.

Popular Science Summary
Optimal size selectivity is a fundamental aspect for sustainable fisheries. In recent years, it has become clear that the mandatory sorting grid system used in the Barents Sea demersal trawl fishery is not functioning optimally. Specifically, the sorting grid releases significant quantities of marketable sizes of target species, while also retaining bycatch species and undersized target species. This results in fishermen requiring additional time at sea to fish their quotas, unnecessary fishing pressure on the bycatch species caught and detrimental impacts on the economic and ecological viability of the fishery in the future. Therefore, this study addresses how behavioural and morphological factors influence the size selectivity of fish sorting grids in demersal trawls, and how these can be investigated using indirect and direct approaches.

As a fish moves through a trawl, the sorting grid is usually its first size selection opportunity. To be released between the grid bars, two conditions need to be fulfilled. First, the fish must contact the grid in a way that provides it with a size-dependent chance of passing between the bars. Second, as the fish contacts the grid, it must be small enough to successfully pass between the sorting grid bars and escape. The first condition is related to the behaviour, and the second is related to the morphology of the fish. Therefore, these two elements and how they influence the grid size selection process were investigated within the Barents Sea demersal trawl fishery, targeting cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens) and redfish (Sebastes spp.) as a case study.

First, an indirect modelling approach was used (Paper 1). A set of five different structural models were applied to describe the size selectivity exhibited during sea trials. The models each made different assumptions concerning the underlying behavioural and morphological factors influencing the grid size selectivity. A bootstrap model selection approach was applied and is presented which enabled the uncertainty of choosing a particular model over another to be accounted for. This study identified multiple competing explanations regarding the potential behaviours giving rise to size selectivity. It highlighted the complexity of fish behaviour in relation to the sorting grid and warned against ignoring model selection uncertainty in future studies (Paper 1).

Paper I suggested size selection to be more complex than initially thought. Therefore, laboratory-based experiments were conducted to emulate grid size selection and investigate different potential selection processes in more detail (Paper II). The emulated behaviours were able to reproduce the size selectivity measured at sea for almost all length classes of fish. This provided significant detail about how various processes involving different behaviours and morphology interact to produce the grid size selectivity. However, without directly observing the processes involved in size selection, it was not possible to fully determine which of these proposed processes indeed occur. Therefore, direct analysis using video observation was conducted (Paper 3). This analysis systematically quantified fish behaviour and how it relates to fish morphology during the grid size selection process. An artificial light source was required to attain the video observations in Paper 3 due to the low light conditions of trawling in this area. Thus, the final study investigated the effect of artificial light on fish behaviour in the grid section (Paper 4).

Evaluation Committee

• Professor Pingguo He, School for Marine Science and Technology, University of Darthmouth, USA (1. opponent)
• Professor Barry O’Neill, Technical University of Denmark (2. opponent)
• Associate Professor Laurene Pecuchet, NFH (internal member and leader of the committee)

Supervisors

• Associate Professor Jesse Brinkhof, Norwegian College of Fishery Science (NFH)
• Researcher Dr. Manu Sistiaga, NFH
• Professor Bent Herrmann NFH

Streaming

Both the trial lecture and defense will be streamed and recorded:

• Trial lecture
• Defense

Thesis

 The thesis is available through Nasjonalt vitenarkiv