Plankton Identification

Plankton are tiny organisms that either move, drift or float about in the water. To classify plankton we can separate it into two groups: Zooplankton and Phytoplankton. Zooplankton eat other things, they are considered animal plankton. Phytoplankton on the other hand are photosynthetic and are considered plant plankton. We are going to focus on Phytoplankton as it is the most cause for concern to a fish farmer.
Phytoplankton can cause harm and create havoc on a fish farm in three major ways.
The first would be Mechanical Irritation.
As the fish breathe through their gills, any plankton in that water will also be filtered through these very sensitive organs. Certain species of phytoplankton like Chaetoceros Concavicorne and Chaetoceros Convolutus have spines that get stuck in the gill tissue and cause irritation. This causes the fish to create mucus which in turn causes the fish to suffocate.
Second major way is Production of Toxins.
Many species of phytoplankton can produce toxins that harm fish. Some of the species would be Heterosigma Akashiwo, Chattonella Marina, Dictyocha Speculum as well as Alexandrium Catenella which causes paralytic shellfish poisoning.
Third major way would be Low Dissolved Oxygen.
Fish start dying if the DO (dissolved oxygen) drops below 4 ppm. Low DO's can be associated with plankton blooms when a thick bloom starts to die off bacteria breakdown the cell material. Bacteria use the oxygen and produce Co2.
Here is a list of 10 different Phytoplankton that a farmer off the east coast of Vancouver Island should be familiar with and has to watch for every single day of the farm stocks life:
Chaetoceros Concavicorne , Chaetoceros Convolutus , Corethron Hystrix , Rhizosolenia Setigera , Heterosigma Akashiwo , Chattonella Marina , Dictyocha Speculum , Alexandrium Catenella , Cochlodinium Polykrikoides , Gymnodinium Mikimotoi.
We take water samples from the 1 meter depth, the 5 meter depth and the 10 meter depth. Each sample is then prepared on a rafter slide and scanned through a microscope. Usually done every morning before the day is started. Once any of the above Phytoplankton are seen its time to be diligent and maybe do your samples after lunch as well. We do environmental samples using a hach do meter. It gives us the DO and Temperature. We then use a secchi disk to tell us the visibility or also known as Turbidity. Then you can use your plankton samples to get the salinity using a refractometer. So to recap, before feeding starts you should know whats in the water at 1m,5m,10m. You should also know the Temperature, DO and Salinity at the three depths and know how far the visisbility is. If all is well, continue on knowing that your fish are safe.