A rationale for when to steam back to station

Last edited: 13 July 1995

What constitutes a station location and when should one steam back to station before continuing a sampling program at a station?

In the broad-scale surveys, Georges Bank and environs are "covered" by 38 primary standard station locations. Weather and time permitting additional stations will be occupied to enable better definition of fish larval patches encountered on the initial survey. Each standard station represents an area (volume) defined by a polygon created by drawing a line halfway to the adjacent stations and splitting the distance for any gaps that might be left by the process. When biological data are ultimately used to make contour plots or compute statistical properties of an area involving more than one station, the implicit assumption is that the observation made at that station location is representative of the entire area (volume) defined by the station polygon. It follows that any single sample taken anywhere in the polygon is as representative of that area (volume) as any other sample taken somewhere else in the polygon. From the sampling point of view, therefore, the need is to have all of the observations made with the various sampling devices (CTD, Pump, MOCNESS, Bongo, etc) fall into the polygon. Deployment of these sampling systems may take several hours during which time the ships position in the polygon will change. From the biological perspective, there is no need to steam back to a single set of coordinates (i.e. station center point) station unless the ship is beyond the polygon boundary or that in the course of sampling, the ship would leave the polygon.

For hydrographic data, the assumption cannot be made that any observation within the polygon is as representative as any other. The fields of physical water properties are fairly regular and smoothly varying, and suitable for interpolation between observations. One value of having standard station locations is that the properties at a location can be compared through time. For example, the bottom salinity at a station on the southern flank of the Bank may be an indicator of the encroachment of Slope Water up onto the Bank, which could be compared each survey. If station locations could change by 10 miles between surveys (half way between neighboring stations), such comparisons could not be done reliably. Choosing a radius of 3- 4 miles could relieve the vessel of returning to station in most cases, while assuring a reasonable ability to create time series of water properties at stations.

A simple rule that would assist in making a decisions about whether to steam back towards the central "Standard Station location" is:

Steam back if the ship is more than halfway to the next station. Otherwise, make the observations where the ship ends up after the last instrument has been brought on board.

It might be argued that there is a need to have all of the observations from the different instruments made "at the same place". Intercomparisons between biological and physical data are internally consistent because most biological instrument systems carry CTD's. There is not a need to have the CTD profile at exactly the same location in the station area as net tow or pump collections, but only within the sampling polygon.

Opposing Viewpoint

Ted and Ann Durbin

Ann and I strongly disagree with the premise that any sample within a polygon surrounding a station is equivalent to any other sample within the polygon. The 1 m2 MOCNESS and zooplankton pump samples will be looked at in many different ways, not only to make contour plots of distributions on the bank. For comparisons of stations we would want something better than "less than halfway to the next station" as the station location. The present policy has been to steam back to the station coordinates between gear deployments. Although this takes extra time, all of the planned work on the survey cruises has been completed on the recent cruises, so time should not be an issue. Any change from the present policy which is to return to the station between gear deployments would be unacceptable to us, and for the pump and MOCNESS sampling for which we are responsible we would insist that they be taken at the station.

Follow Up Comment to Opposing Viewpoint

While the argument is valid that up to half way to the next station is no longer on station, the fact is that the water one is sampling has moved too during the time that the ship has been taking samples. So, while the exact location of the station can be pinpointed, and occupied, the water mass is no longer the same. The affect of this water mass change will be influenced by the sample water depth, time of year, weather conditions and other physical conditions. Indeed, this affect is one such area of study. This could imply that the decision on whether to return to the exact station location may depend on the particular experiment's goals.(rcg)