Techniques

Weather permitting, weekly cruises head out to sample the Bay from January through May. A suite of sampling techniques can be used during a field cruise to collect data. To collect oceanographic information – the physical aspects of water – a CTD (Conductivity, Temperature, Depth recorder) is employed. The CTD carries a computer chip that processes and stores information regarding depth, salinity, ambient light and temperature every second. All of these factors affect plankton: every species of plankton has differing tolerances for temperature, salinity, depth and light.

netow

Plankton net towed from the stern; orange colored plankton caught in sample bottle.

One of the simplest and most time-honored methods for collecting plankton is the plankton tow. A fine meshed, cone shaped net can be towed through the water, concentrating plankton in a sample bottle at the back end. The net can be towed to sample a distance of surface water (a horizontal tow), or the net can be sunk and raised to sample the plankton throughout the water column (a vertical tow). The use of the plankton tow can create broad scale image of the planktonic community but is limited in that it does not collect environmental data to relate to the samples. Two other techniques can bring the biological and environmental samples together.

Vertical sampling creates an image of the habitat from the Bay floor to the surface waters using the CTD, a motorized pump and a long spool of hose. The hose, attached to the CTD is set overboard and sunk to the bottom. As it is raised, it is stopped every two meters to sample ten gallons of water. The pump draws water through the CTD (collecting environmental data) and sent, via the hose, to the deck. Onboard the same ten gallons of water that was analyzed by the CTD is sent through small, PVC collecting bottles that strain out a sample of the plankton. This detailed picture of the water column takes into account plankton and the physical properties of the water that sustain them.

sampling

Seawater can be drawn through a hose by a pump (for vertical sampling) or as the boat travels (for horizontal sampling of the near surface). Short PVC sections outfitted with fine mesh can be held into the flow of the hose at intervals. A zooplankton sample is caught by the mesh and can be transferred to bottles and preserved for counting in the lab.

To look at the surface community over a long distance, horizontal sampling methods are used. A tall pipe, mounted off the stern of the boat reaches into the first meter of surface water. As the boat runs a transect across a portion of the Bay, water enters the pipe and is pushed onboard through PVC collecting bottles to sample for plankton. The water then passes over the CTD to collect data about temperature and salinity. In this way large areas of the surface of the Bay can be sampled.

All of these techniques can be used if whales are present (“positive data”) and with no whales present (“negative data”) to understand how the habitat changes over time and space: over hours, over weeks, over years, from the shallow waters, to deep water, from north to south.

At the end of the cruise the difficult and tedious process of counting begins. Hundreds of samples may be gathered in a season and each sample must be analyzed. Using microscopes and species guides each sample is examined for species and numbers of individuals within the species. All of this information can then be related to the sightings of right whales to create an impression of the habitat.