Drag on hydroid-fouled nets — An experimental approach

Abstract

The present study investigated the drag increase on aquaculture nets due to biofouling of the colonial hydroid Ectopleura larynx. It had two main parts: firstly the growth characteristics of E. larynx were investigated by use of field tests at a Norwegian aquaculture site; secondly the hydrodynamic drag on the fouled twines was studied in a towing tank by using fabricated models of net twines with artificial hydroid fouling. In the field tests, the growth of the hydroids was first measured after three weeks of immersion and then again after six weeks. During this interval, the density of hydroids and the thickness of the hydroid stem were almost constant (1.4 hydroids/mm and 0.29 mm, respectively), while the average length of the hydroids increased from 6.4 to 11.2 mm. The hydroid length followed a Rayleigh distribution, while the thickness was normal (Gaussian) distributed. Replicas of twines with three different levels of hydroid growth were made (1.5 hydroids/mm twine, hydroid length 9 mm, 16 mm and 20 mm), and the drag on these twines was measured at different towing velocities (0.1 to 1.4 m/s) and with different twine configurations. For the twine with the shortest hydroids (9 mm), the drag was from 1.5 times (Re=4000) to 2.2 times (Re=1000) the drag on a clean twine. For the longest hydroids (21 mm), the drag was 2 times and 3.8 times, respectively.

Biofouling in marine aquaculture: a review of recent research and developments

“…In addition, biofouling can increase the hydrodynamic load on nets up to 10-fold , deforming cages and adding strain to moorings. Impacts differ with species composition (Gansel et al , 2017 and organism size (Lader et al 2015), and percentage net-aperture occlusion is a good predictor of flow reduction and drag increase associated with biofouling .…”

“…It is the settlement and development of unwanted aquatic species on natural and artificial surfaces, and plagues shellfish, finfish and seaweed culture globally. The direct economic costs of managing biofouling in the aquaculture industry are estimated to be 5-10% of production costs (Lane and Willemsen 2004). However, the cost of biofouling often varies considerably between aquaculture locations, species and companies, as farmers use differing management approaches and cost accounting (Iversen et al 2016).…”

Potential environmental risks associated with biofouling management in salmon aquaculture

“…For example, in many of the main global salmon-growing regions biofouling communities on net pens are dominated by hydroids or sea anemones. In Norwegian and Irish Atlantic salmon farms, the hydroid Ectopleura larynx occurs at densities of up to 126 000 polyps m −2 of pen nets (Guenther et al 2010, Baxter et al 2012, Lader et al 2015 and its recruitment dynamics represent one of the main drivers of net cleaning operations. Densities of up to 45 000 polyps m −2 have been observed for the closely related species E. exxonia on New Zealand Chinook salmon farms , and hydroids of the genera Ectopleura, Plumelaria, Obelia and Sarsia are amongst the most abundant biofouling organisms on or around salmon farms in Tasmania and North America (Rensel & Forster 2007;B.…”

Investigations on the porous resistance coefficients for fishing net structures

“…This could be considered as a reasonable simplification for clean net, which has a very high porosity value, normally between 0.7 and 0.9. However for nets with biofouling, as shown in Lader et al (2015), the porosity could be reduced to a very low value. Then this simplification is questionable.…”