Development of seabed friendly bottom trawls

Nguyen, Truong X. (2016) Development of seabed friendly bottom trawls. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Concerns over the impacts of fishing practices, especially bottom trawling, on the ocean environment have been expressed at the local, national and international scale. While physical alterations of the seabed by bottom trawling are known to occur, the biological effects on benthic communities and their recovery rates depend on substrate types, depth, and natural disturbance in the fishing area, as well as how trawl gears are designed and operated. In this thesis, I investigate different key research aspects regarding the subject of development of seabed friendly bottom trawls, in particular shrimp trawling in Newfoundland and Labrador, Canada. The complementary use of different research approaches (e.g., underwater video observations, numerical modeling and simulation, flume tank testing, and at-sea experiments) were applied for each of the research questions. First, I investigated the behavioural interactions of individual snow crab in response to the rockhopper footgear of a traditional inshore shrimp trawl used in Newfoundland and Labrador, Canada. I found that snow crab were quickly overtaken under the footgear of the approaching trawl and over half of the snow crab (i.e., 54%) observed experienced an encounter with the rockhopper footgear components. The majority of the snow crab observed appeared to be aware of the trawl and were actively responding and/or reacting to the approaching threat. Second, the strengths and limitations of different commercially available trawl simulation software (i.e., DynamiT, SimuTrawl, and Trawl Vision PRO) in terms of design capability, simulation capability, and reliability of results, were investigated and interpreted. The study provides valuable knowledge and reference for stakeholders (e.g., gear designers, researchers, and educators) who are considering using numerical simulation methods to optimize their gear design concepts during the early stages of development of seabed friendly bottom trawls (e.g., predict expected mechanical stresses of trawl components on the seabed). Next, I addressed the question of how well computer simulation and flume tank testing of scale engineering models actually predict full-scale at-sea performance of bottom trawls. The results demonstrated that the complementary use of two or three methods should be encouraged for assisting the gear development cycle given their own weakness and merits. For instance, the flume tank testing method was successfully utilized to estimate the percentage of contact area made by trawl footgear with the seabed, while at-sea experiments were not designed to measure such impacts. Moreover, I clarified that the precision and accuracy of the predictions depends on many factors. Thus, thoroughness and care must be emphasized in order to reduce bias in predicted performance. Finally, I examined the effectiveness of a reduced seabed impact footgear (i.e., drop chain) over a traditional rockhopper footgear on identical bottom trawls targeting northern shrimp (Pandalus borealis) in Newfoundland and Labrador, Canada. The results demonstrated that seabed impacts of shrimp trawling can be reduced if the trawl footgear is made lighter and/or designed to have less contact with the seabed. In particular, it was revealed that with the experimental drop chain footgear trawl we are able to reduce the interaction or encounter of snow crab. In summary, the knowledge presented in this thesis is believed to significantly contribute to the research and development of low-impact bottom trawls both in theoretical and practical aspects. While the potential impact of bottom trawling activities on habitats and benthic communities is not easy to predict and characterize for various reasons, I do believe that further development and application of fishing gears and techniques that reduce impacts on seabed habitats and associated benthic communities will be essential to achieve ecosystem objectives.

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