Escape panels in trawls: does placement matter when every individual contacting the panel can escape?

Escape panels are one of the bycatch reduction devices most used in trawl fisheries but their efficiency rely on fish actively contacting the panel to escape. To investigate if contact behaviour changes at different panel placements, we tested a 300 mm square mesh panel placed in the upper panel of the codend at 3, 4 and 7 m from the codline. Seven competing models of contact probability were fitted to the empirical data. Based on the results, we inferred that panel placement significantly affects escape efficiency due to a change in type of contact behaviour. Cod (Gadus morhua) showed a contact increasing with length when the panel was closest to the codline, while contact probability decreased with length at the other placements. Similarly, contact probability for plaice (Pleuronectes platessa) was found to increase with length at 3 and 4 m, whereas a length-independent contact best represented the data at 7 m. Finally, Nephrops (Nephrops norvegicus) had in general low contact probability. The results provide new knowledge regarding species and placement-dependent panel escape

Simulations of super-structure domain walls in two dimensional assemblies of magnetic nanoparticles

We simulate the formation of domain walls in two-dimensional assemblies of magnetic nanoparticles. Particle parameters are chosen to match recent electron holography and Lorentz microscopy studies of almost monodisperse cobalt nanoparticles assembled into regular, elongated lattices. As the particles are small enough to consist of a single magnetic domain each, their magnetic interactions can be described by a spin model in which each particle is assigned a macroscopic "superspin." Thus, the magnetic behaviour of these lattices may be compared to magnetic crystals with nanoparticle superspins taking the role of the atomic spins. The coupling is, however, different. The superspins interact only by dipolar interactions as exchange coupling between individual nanoparticles may be neglected due to interparticle spacing. We observe that it is energetically favorable to introduce domain walls oriented along the long dimension of nanoparticle assemblies rather than along the short dimension. This is unlike what is typically observed in continuous magnetic materials, where the exchange interaction introduces an energetic cost proportional to the area of the domain walls. Structural disorder, which will always be present in realistic assemblies, pins longitudinal domain walls when the external field is reversed, and makes a gradual reversal of the magnetization by migration of longitudinal domain walls possible, in agreement with previous experimental results. (C) 2015 AIP Publishing LLC