Effect of Cutting Ratio and Catch on Drag Characteristics and Fluttering Motions of Midwater Trawl Codend

The codend of a trawl net is the rearmost and crucial part of the net for selective fish catch and juvenile escape. To ensure efficient and sustainable midwater trawl fisheries, it is essential to better understand the drag characteristics and fluttering motions of a midwater trawl codend. These are generally affected by catch, cutting ratio, mesh size, and twine diameter. In this study, six nylon codend models with different cutting ratios (no cutting, 6:1, 5:1, 4:1, 7:2, and 3:1) were designed and tested in a professional flume tank under two conditions (empty codends and codends with catch) and five current speeds to obtain the drag force, spatial geometry, and movement trend. As the cutting ratio of empty codends decreased, the drag force decreased, and the drag coefficient increased. The unfolding degree of codend netting and the height of empty codends were found to be directly proportional to the current speed and inversely proportional to the cutting ratio. The positional amplitude of codend with cutting ratio 4:1 was the smallest for catch. The drag force of codends with catch increased as the current speed increased, and first decreased and then increased as the cutting ratio decreased. To ensure the best stability and minimum drag force of the codend, it is recommended to use the 4:1 cutting ratio codend.

Fewer and Farther between: Changes in the Timing of Longfin Smelt (Spirinchus thaleichthys) Movements in the San Francisco Estuary

Abundance of estuarine fish species has declined globally. In the San Francisco Estuary (SFE), long-term monitoring documented declines of many species including the anadromous species Longfin Smelt (Spirinchus thaleichthys). To improve management and recovery planning, we identified patterns in the timing, seasonal occupancy, and distribution of Longfin Smelt in a monitoring study (San Francisco Bay Study) for five regions of the SFE using a generalized additive model. We then investigated the year-to-year variability in the shape of the seasonal relationships using functional data analysis (FDA). FDA separated the variability due to population size from variability due to differences in occupancy timing. We found that Longfin Smelt have a consistent seasonal distribution pattern, that two trawl types were needed to accurately describe the pattern, and that the pattern is largely consistent with the hypothesized conceptual model. After accounting for variability in occupancy due to year-class strength, the timing of occupancy has shifted in three regions. The most variable period for the upstream regions Suisun Bay and Confluence was age-0 summer and for the downstream region Central Bay, was age-0 late fall. This manifested as a recent delay in the typical fall re-occupation of upstream regions, reducing Longfin Smelt abundance as calculated by another monitoring study (Fall Midwater Trawl); thus, a portion of recent reductions in Fall Midwater Trawl abundance of Longfin Smelt result from changes in behavior rather than a decline in abundance. The presence of multiple monitoring surveys allowed analysis of distribution from one data set to interpret patterns in abundance of another. Future investigations will examine environmental conditions as covariates during these periods and could improve our understanding of what conditions contribute to the shifting occupancy timing of Longfin Smelt, and possibly provide insight into the long-term quality of the San Francisco Estuary as habitat.

PROBLEMS OF CONTROL OF MIDWATER TRAWL COMPLEXES USING EQUATIONS OF ELECTRIC AND MECHANICAL DRIVES OF TRAWL WINCHES

The paper touches upon the problems of transition to advanced digital, intelligent man-ufacturing technologies, robotic systems, new materials and design methods, the creation of systems for processing large amounts of data, machine learning and artificial intelligence. Automation of fishing process requires an interdisciplinary approach using modern information technologies. The possibility of using artificial intelligence technologies for solving the problems of predictive modeling of the behavior of a trawl system while fishing on a self-learning neural network has been proved. The equations of electric and mechanical drives of trawl winches for controlling the shape-changing design of a midwater trawl are given. The question of improving the control characteristics of a midwater trawl system by introducing a control architecture adapted for the trawl system taking into account the industrial requirements and by developing a mathematical model of the trawl system, in-cluding an accurate model of hydrodynamic forces on the trawl flaps is considered.

A study of approach to assessing a functional relationship between solidity ratios for the rope/net part of midwater trawl

In this paper we have analyzed an approach to assessing the relationship between some characteristics of the net panels forming the rope/net cone of a fishing trawl. Using the mathematical relationship of solidity ratios for the rope/net panels, it is possible to analyze existing structures and, furthermore, reduce the time for selecting the prototype trawl. Such mathematical relationship allows faster calculation of new trawl designs which takes into account the production specifics of an enterprise manufacturing the trawl fishing gear. The use of mathematical relationships provides the opportunity to accelerate the process of trawl construction.

Examining Retention-at-Length of Pelagic Fishes Caught in the Fall Midwater Trawl Survey

The Fall Midwater Trawl Survey has provided data on aquatic organisms in the San Francisco Estuary for over five decades. In 2014–2015, a study was conducted to investigate and quantify the efficiency of this trawl for catching the endangered fish species Delta Smelt (Hypomesus transpacificus). In an analysis based on that study, we calculated retention probability—the probability that a Delta Smelt is retained in the cod end of the trawl—as a function of fish length and fit a selectivity curve reflecting the relationship between size and retention. Here we return to the same gear efficiency study and further utilize the data set by (1) fitting selectivity curves for three additional pelagic fish species: Threadfin Shad (Dorosoma petenense), American Shad (Alosa sapidissima), and Mississippi Silverside (Menidia beryllina), (2) refitting the selectivity curve for Delta Smelt to incorporate between-haul variability, and (3) calculating the lengths of 50% and 95% retention in order to characterize and compare the resulting selectivity curves. We also present retention data on age-0 Striped Bass (Morone saxatilis), all of which were retained in the cod end. We found that Threadfin Shad, American Shad, and Delta Smelt are 95% retained at 45, 49, and 61 mm fork length, respectively. Because data were limited for Mississippi Silverside, American Shad, and age-0 Striped Bass, we used body shape, in conjunction with retention data, to develop hypotheses about selectivity based on whether each species’ body shape resembles that of Threadfin Shad, which are more deep-bodied and laterally compressed, or Delta Smelt, which are more fusiform. We also found that retention-at-length was more variable for Delta Smelt than for Threadfin Shad, potentially because length is a good predictor of retention in deep-bodied, laterally compressed fish whereas maximum girth is a better predictor of retention in fusiform fish.

DETERMINING OF OPERATIONAL PARAMETERS FOR TRAWL SYSTEM TAKING INTO ACCOUNT EXPERIMENTAL DATA ON SHAPE OF THE ROPE-NET SHELL OF MIDWATER TRAWLS

Calculating of horizontal opening for a trawl mouth using the mathematical model of midwater trawl system developed by V.I. Gabryuk gives the errors 25.4 and 32.9 %, on average, for the trawls RT 57/360 m and RT 80/396 m, respectively. The significant errors cause doubts about correctness of this model equations application for calculation of the catch zone that is necessary for assessment of abundance and biomass of marine biological resources. New empirical equations are proposed on the basis of experimental data on shape of the rope-net shell of a midwater trawl, which allow to calculate the horizontal opening for its certain sections. The errors of the horizontal opening calculation with these new equations are 13.7 and 6.1 %, on average, for the trawls RT 57/360 m and RT 80/396 m, respectively, that is satisfactory for using them for calculating operational parameters of a midwater trawl system. The operational parameters of the midwater trawls RT 57/360 m and RT 80/396 m are calculated using Baranov’s approach with these new empirical equations, and their good adequacy is shown in the experiment with hydroacoustic measurements of the trawls mouth horizontal opening and the distance between the trawl doors conducted aboard RV TINRO and RV Professor Kaganovsky in the Okhotsk Sea in 2012–2015.

Problem scientific basis equal strengthe construction of cable-net part of the midwater trawl

The article outlined the R&D activities that would create a theory and methodology for designing the uniform strength cable-net mid-water trawls. To manage the mentioned problems a large volume of research is needed, however, the transition to cable-net design of trawls' parts will provide significant economic benefits. It therefore seems rather relevant and important.