Effect of seasonal temperature changes on the escape behaviour of haddock, Melanogrammus aeglefinus, from the codend

Williams, K., Punt, A. E., Wilson, C. D., and Horne, J. K. 2011. Length-selective retention of walleye pollock, Theragra chalcogramma, by midwater trawls. – ICES Journal of Marine Science, 68: 119–129.Midwater trawls are commonly used during acoustic surveys of fish abundance to determine species and length compositions of acoustically sampled fish aggregations. As trawls are selective samplers, catches can be unrepresentative of sampled populations and lead to biased abundance estimates. Length-dependent retention of walleye pollock was estimated using small recapture nets, so-called pocket nets, attached to the outside of the trawl. Experimental haul sets comprising eight hauls each were conducted in the Gulf of Alaska in 2007 and 2008 and in the eastern Bering Sea (EBS) in 2007. Pocket-net catches were then modelled by fitting parameters for selectivity and escapement location along the trawl. Within- and between-haul variability was jointly estimated using hierarchical Bayesian methods. There was significant undersampling of juvenile (<25 cm) pollock, with the length-at-50%-retention (L 50 ) estimated between 13.5 and 26.1 cm among haul sets. In the EBS set, L 50 values were correlated with light level, escapement being greater at night. Trawl selectivity may be a significant source of error in acoustic-survey estimates of the abundance of pollock.

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Length-selective retention of walleye pollock, Theragra chalcogramma, by midwater trawls

Influence of water temperature and fish length on the maximum swimming speed of sand flathead, Platycephalus bassensis: Implications for trawl selectivity

Walleye pollock (Theragra chalcogramma) behavior in midwater trawls

A comparative study of diploid versus triploid Atlantic salmon (Salmo salar L.). The effects of rearing temperatures (5, 10 and 15 °C) on raw material characteristics and storage quality

The muscle twitch contraction time of Walleye pollock Theragra chalcogramma was measured in relation to fish size and muscle temperature, on board a Northern trawler in the Okhotsk Sea. The contraction time was defined as the time period from the electric pulse of 2ms wide and 5-20V, to the peak of muscle twitch contraction. The scale effect was observed as the muscle con-traction increased from 60-80ms for fish 20-30cm long, to 90-120ms for fish 40-50cm long, at the water temperature of 2°C. According to the results, the maximum swimming speed of the fish was estimated as 1.0-1.6m/s, which was much lower than the towing speed of the trawl net. When muscle temperature was elevated, the contraction time was shortened. It implies that more chances are given, at higher temperature, for fish to escape from the net due to the higher swim-ming speed.

Muscle Contraction Time of Captured Walleye Pollock Theragra chalcogramma.

Structured porous surface for drag reduction and wake attenuation of cylinder flow

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric. 

Modification of subcritical cylinder flow with an upstream rod

In this study, a novel flow control scheme characterized by active suction through the structured porous surfaces on the side-walls of a square cylinder is introduced and experimentally investigated to control the shear-layer separation and dynamic wake flows. The Reynolds number (Re) is about 1.05 × 104, given the incoming flow speed and diameter of the square cylinder test model. 55 pores are regularly arranged on the top and bottom side-walls of the square cylinder to form communicating channels for the distributed suction flow. In the wind tunnel experiment, a high-speed particle image velocimetry (PIV) system is used to measure the flow field, and then a proper orthogonal decomposition (POD) method is adopted to obtain dynamic patterns of the wake flow. The experimental results demonstrated that the control effects are closely related with the dimensionless suction momentum coefficient Cμ. As Cμ increases to 0.0197 and 0.0338, the unsteady wake of the square cylinder is significantly attenuated and the process of flow separation is notably restrained. It is shown that distributed suction through side-walls effectively controls the unsteady shear-layer separation and wake flows of the square cylinder. 

Xu Chang

Papers

Muscle Contraction Time of Captured Walleye Pollock Theragra chalcogramma.

Structured porous surface for drag reduction and wake attenuation of cylinder flow

Modification of subcritical cylinder flow with an upstream rod

Effects of distributed suction through the porous side-walls of a square cylinder on separation and wake flows

Dynamic wake behind a dandelion pappus: PIV and smoke-wire visualization