Bridle efficiency of a survey trawl for flatfish: measuring the length of the bridles in contact with the bottom

A bottom trawl catch of flatfish is composed of fish that were initially in the path of the net and fish that were initially in the path of the bridles and were subsequently herded into the net path. Bridle efficiency (i.e. the proportion of fish in the bridle path herded into the net path) can be estimated by fitting a model to catch and trawl measurement data collected in a field experiment in which the bridle path is varied in width by varying bridle length. Since flatfish are herded by the direct contact or close approach of the bridles with the bottom, part of the necessary field work is estimating the length of the bridles in contact with the bottom. Previously, bottom contact of the bridles has been determined using video cameras, but this is not possible when the bridles are entirely obscured by the mud clouds generated by the trawl doors. As an alternative approach, a bridle-mounted bottom contact sensor (BCS) was developed to allow measurement of the off-bottom distance of a bridle and the percentage of time it is in contact with the bottom. For the survey trawl studied, approximately 43% of the lower bridle was sufficiently close to the bottom to presumably elicit a herding response; however, the middle of the three bridles sagged to such an extent that it touched the bottom ahead of the lower bridle and extended the length to 51%.     

拖网的网袖结构对其性能的影响

文章以国内较典型的单船底拖网 (72 ◇×18 m) 为研究对象,基于L₉(3⁴) 正交试验表,设计了网具总长度与网口周长之比 (L/C)、浮沉子纲长度之比 (S₁/S₂)、网身长度与网具总长度之比 (L_b/L)“三因素三水平”的正交试验,并利用平均值和极差分析方法检测3种主要结构参数对网具阻力、网口垂直扩张和能耗系数的影响趋势,旨在对大网目底拖网结构进行优化设计。结果表明,不同水平扩张下,3种因素对网具阻力的影响由主到次均为L/C> S₁/S₂>L_b/L,对网口垂直扩张的影响由主到次均为S₁/S₂>L/C>L_b/L,对能耗系数的影响由主到次均为L/C>S₁/S₂>L_b/L。拖网L/C相对于小网目拖网应偏小,且比值越小阻力越小,但L/C=0.2是网口垂直扩张和能耗系数作为指标时的最佳水平;S₁/S₂相对于小网目应更大,且比值越大网口垂直扩张越好;S₁/S₂在试验范围 (0.83~0.99) 内对阻力影响不显著,而S₁/S₂=0.99是能耗系数作为指标时的最佳水平;L_b/L随着网目尺寸增大而减小,但减小幅度随之逐渐降低,L_b/L=0.65应更适应于中低速拖网,L_b/L=0.75更适应于高速快拖。     

The influence of warp length on trawl dimension and catch of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> in a bottom trawl survey

ABSTRACT:   Variations in trawl dimension, bottom contact, and catch with differing warp lengths during walleye pollock surveys conducted by the Fisheries Research Agency were examined. The ratio of warp length to fishing depth (scope ratio) was set at 2.5, 3.0 and 3.5 at depths of 110 m. At a 2.5 scope ratio, the net mouth shape and footgear contact fluctuated, as the trawl door did not contact the bottom. Footgear contact was complete when the scope ratio was 3.0 or more. Walleye pollock, sculpins and flatfishes were the main catch in all tows, and the catch increased with scope ratio. There was no difference in the length frequency of walleye pollock ( n  = 300) captured at 2.5 and 3.0 scope ratio. However, the length frequency at 3.5 was significantly different from that of other scope ratios. These results suggest that at a scope ratio of 3.0 or more, the trawl door will not leave the bottom at any depths. To compliment echo surveys for walleye pollock, a 3.0 scope ratio would be optimal, since the catch data for a 3.5 scope ratio was dissimilar from that of scope ratios.     

Model of trawlable area using benthic terrain and oceanographic variables—Informing survey design and habitat maps in the Gulf of Alaska

Bottom trawl surveys provide fishery‐independent data on relative abundance and life history parameters for a wide range of marine taxa. Survey data are used to assess species distribution, biological interactions, and ecosystem structure and to manage marine resources. Not all bottom types or oceanographic conditions accommodate this survey method. We applied National Ocean Service hydrographic smooth sheets to evaluate physical attributes associated with habitat available to surveys. Random forests were used to evaluate the relative influence of benthic terrain and oceanographic predictors in determining accessibility to bottom trawl gear. We examined the marginal importance of each predictor, quantified the response gradient, and applied piecewise regression to determine threshold breakpoint values. Thresholds were used to develop predictive maps and distinguish untrawlable habitat at the scale of discrete towpaths and survey stations. Untrawlable habitat was associated with increased complexity in terrain, roughness, slope, surface curvature, substrate coarseness, current, and aspect. Maps of critical thresholds suggest different variables constrain the probability of a successful trawl in the nearshore, shelf, and continental slope. Overlay analysis of the model projection demonstrates the utility of archived smooth sheet data and identifies areas where higher resolution data might improve results. The model and maps produced in this analysis might be used to identify habitats available to and impacted by commercial trawl fisheries, inform the relative availability of various species and habitat types to bottom trawl surveys, evaluate bias in assessment indices and ecosystem metrics derived from survey data, and advance habitat‐specific biomass estimates.     

Effects of environmental variables on survey catch rates and distribution by size of shallow‐ and deep‐water Cape hakes,Merluccius capensis and Merluccius paradoxus off Namibia

In order to study the effects of temperature, oxygen, salinity and time of day on survey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also be highly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions.     

Inhibition of embryo development of the Mytilus galloprovincialis marine mussel by organic pollutants; assessment of risk for its extensive culture in the Galician Rias

This study evaluates the risk posed by selected organic pollutants on the culture of the marine mussel in the Galician coast (NW Iberian Peninsula), which depends on collection of natural seed in densely populated coastal areas. With this aim toxicity tests were carried out with embryos of the Mytilus galloprovincialis mussel, and the toxicity of the surfactant sodium dodecyl sulphate (SDS) and the biocides TBT, chlorpyrifos and lindane was quantified in terms of median effective concentration (EC₅₀) and toxicity threshold (EC₁₀ and LOEC), using embryogenesis success as end point. The EC₁₀ and EC₅₀ values were 161 and 377 ng/L for TBT, 79 and 154 µg/L for chlorpyrifos, 495 and 2353 µg/L for SDS, 1.41 and 1.99 mg/L for lindane. Toxicity thresholds for mussel embryos from this study and crustacean larvae from the literature were compared to environmental concentrations in coastal waters, either directly measured or estimated from mussel bioaccumulation data, in an attempt to evaluate the risk posed by those pollutants to these commercial species. It was concluded that SDS and, especially, chlorpyrifos and lindane, do not pose a threat to these commercial resources. In contrast, TBT risk quotients derived either from actual seawater measurements or mussel bioaccumulation data were both unacceptably high. TBT pollution represents thus a potential threat to natural availability of spat, the basis of extensive mussel culture. At the light of the data presented, current TBT seawater quality criterion seems underprotective and it should be revised.