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16 Science Manual – Using Video Images for Fisheries Monitoring

Table 3.1: Summary specification of the lighting equipment

LED strip (449 mm) Infrared illuminator Infrared LED lamp

Voltage 12 24 12

Power requirement 260 mA per strip 2 amps 490 mA

Wavelength (nm) 625 850 850

Size (mm) 449 mm long 125 x 175 x 100 70 (D) x 90 (L)

Cost (£) 7 600 90

3.3.5 Suspended sediment

Suspended sediment attenuates light and reduces the visual range in a body of

water. A measurement of the optical attributes of suspended matter (Nephelometric

Turbidity Units or NTUs of a turbidity meter) is more useful than its mass

concentration. But turbidity is only a relative measure of scattering set against an

arbitrary standard. To assess the impact this will have on a visual fish counting

system, a measurement of visual clarity (measured as Secchi or black disc visibility)

is more useful. This will vary between sites and types of suspended sediment,

making a table of turbidity measurements versus light penetration potentially

misleading.

Visual clarity measurements are relatively non-subjective. It is recommended that a

library of images gathered by each visual system is collated over a range of

environmental conditions, together with the Secchi or black disc reading.

3.3.6 Algaecide and cleaning

Algae will build up on cameras and lighting placed in a river, reducing the

effectiveness of the system. The rate at which this builds will depend on light levels,

water velocity and site-specific conditions. If you know of a non-environmentally

damaging algaecide that is transparent and can be sprayed or painted on the surface

of equipment, please pass it on to the contacts listed in the Contacts Section.

Meanwhile, your camera lens and lighting source will need regular brushing to clean

it (once every two to four weeks in the summer months).

3.4 Cameras

The type of underwater camera you go for will depend on your budget and the site. If

you have a site where equipment is likely to be damaged by debris or vandals, it is

best to go for cheap (£100) small underwater cameras (Figure 3.6) which can be

easily replaced if the worst happens. When choosing a camera, make sure that the

cable length is sufficient to reach your recording equipment. Cable extensions can

be added but any cable joins will need to be kept out of the water. Some underwater

cameras have integral LED lights which are supposed to be useful in low light

conditions, but in reality the lights reflect off debris in the water close to the lens and

aren’t helpful. Try and avoid these. All of these cameras operate from a 12-volt

supply and are fairly robust.

Most of the fish images in this manual have been gathered using cheap underwater

cameras. For information on higher specification cameras, see the Contacts Section.

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