Hydrography
Source: EU FP5 MERAMED project field handbook
Hydrography is one of the key features used in site selection, as it indicates: a) flux of good quality water via the finfish or shellfish gear (water quality, animal welfare); b) spatial dispersal of effluents, which may spread the load over sufficiently large area to reduce benthic impact on seafloor (environmental health), c) supply of planktonic food to shellfish (enhanced growth). Hydrographic characterization needs to be done before the intensive WP5 site study as it will serve as the basis for pre-selecting sampling stations (see below).
For microtidal sites (e.g. Mediterranean) current meter records should be as long as possible; at the very least 15 days, but preferably months of data. For macrotidal sites (e.g. France, Atlantic coast), 15 days may be sufficient to capture the tidal cycle.
The most common types of instruments used for hydrographic measurements around fish farms are rotary, electromagnetic and acoustic doppler current profilers (ADCP). The first two instrument types measure current speed and direction at a discrete depth, the third type profiles the water column with some limitations at surface and bed.
A sampling interval of 10 minutes is recommended for long-term deployments (e.g. up to 1 month). Short-term deployments (i.e. of a few days) should use a 5-minute sampling interval. Where acoustic profilers are being used, care should be taken to ensure that the standard deviation of the measurements is sufficiently small. Total battery life and memory capacity should be taken into account when setting up the instrument.
Select a mooring type depending on local site conditions (i.e. level of exposure of site, local fishing activities), intended mooring location, instrument type and length of deployment. The mooring should be located outside the shadowing effect of the cages. U-shaped mooring arrangement is recommended for long-term deployments as it allows three methods for recovery in the event of the surface buoys being lost. Where a surface marker is lost, recovery can take place from the opposite end. Where both surface markers are lost, recovery can take place by snagging the groundline with a grapnel.
A conductivity, temperature and depth (CTD) profile is useful at the deployment and recovery stages of the hydrographic instruments. This gives information on the depth of any water column features in relation to instrument depth. The depth and length of deployment of each instrument depends on the objectives of the current meter study. It also relates to any modelling intended for the survey site, in which case the modelling guidelines can be consulted.
The following recommendations are made with respect to current meter deployment depth:
- if three instruments are available – deploy surface, mid-water and near-bed
- if one instrument is available – deploy mid-water or below the depth of the cage bottom
- if two instruments are available, then consideration should be given to the level of resuspension expected at the site before deciding on instrument deployment depths.
Near-bed current speed exceeding 10 cm s-1 causes resupension of fish farm material and so near-bed measurements are crucial at sites where these conditions exist. Therefore, deploy surface and near-bed if greater than 5 % of near-bed current speeds are expected to exceed 10 cm s-1, otherwise deploy surface and mid–water.
If profiler is available – deploy either ‘looking down’ or ‘looking up’ but consideration must be given to the effect of surrounding fish farm equipment and the limitations of current measurement at the surface and near-bed. Actual deployment depths will depend on the total depth of the water column, but measurement of surface current and current below the bottom of the net pen is desirable. In the case where a counter current to the surface flows exists at depth, this should be measured if possible.
A deployment of 1 month is recommended for studies aiming at measuring the general hydrodynamic features of a site. The effect of season must also be taken into account where possible, as seasonal wind patterns are likely to change hydrographic patterns. Measurement of current in the same season as any intensive benthic sampling is ideal, but this may not be practical. Need to select one sampling station which should avoid shadowing effect of cages but should be sufficiently close to cages to sample general flow conditions in the area of expected deposition.
