Zooplankton

 Herring stocks in Prince William Sound collapsed in the years following the Exxon Valdez oil spill.  There has been considerable debate as to what precipitated the decline; the 1989 Exxon Valdez oil spill, disease, fishing pressure and ecosystem-level changes in the forage base have all been invoked as possible reasons (and none of these factors are mutually exclusive). PWSSC researcher Dr. Richard Thorne has been involved in resolving this debate.  Herring stocks have remained at very low levels, and there is considerable interest in learning what has prevented recovery of the stocks.

One suggestion is that the amount of food available to herring, particularly young herring, is at least partially responsible for the lack of recovery.  Without sufficient food, herring will more likely to succumb to disease or predation.  As well, herring must store sufficient energetic reserves within their body to carry them through the winter, when there is very little food available.

There are numerous types of zooplankton present in the Sound, and the type present and their abundance depends on many things, including the time of year and oceanographic conditions.  Plankton must over-winter as the herring do, and many emerge in the spring as light levels increase and their phytoplankton food begins to grow.  Copepods of the genus Neocalanus, the most common medium-sized zooplankton in the Gulf of Alaska Spring, exhibit such an emergence, and they are a very important prey item for herring.  The Alaska Coastal Current, a current traveling along the periphery of the Gulf of Alaska, also sometimes brings plankton into the Sound, and may “subsidize” local production.

In 2007, we began a plankton monitoring program in the Sound, supported by a grant from the Exxon Valdez Oil Spill Trustee Council as part of a comprehensive herring recovery program.  Cruises in Prince William Sound and the coastal Gulf of Alaska were done in May and September, in order to survey plankton populations in the spring (as herring juveniles recover from the winter and also when herring larvae start feeding), and autumn (prior to over-wintering).

We use a Hydro-Bios Multinet to sample plankton during our surveys (photo 1). The Multinet has 5 nets that may be opened and closed by commands sent from the ship through a conducting tow cable (photo 2), so that multiple samples can be collected in a single deployment (photo 3).  

Photo 1: Deploying the Multinet from F/V Kyle David, 2007. Ms. River Gates is steadying the Multinet while Dr. Rob Campbell is lifting a tow body that will hang below it during the horizontal deployment used for our zooplankton surveys. The wire leading to the winch is running through the orange block located in front of Dr. Campbell. Photo taken by Dr. Tom Kline while operating the winch.

Photo 2: Controlling the Multinet from the bridge of the F/V Kyle David. Following deployment on the deck, Photo 1, the Multinet is controlled from within the bridge of the deployment vessel, the F/V Kyle David. Dr. Rob Campbell is controlling the winch using a Morse telegraph control mounted on a wooden box. He is keeping an eye out on the cable though the window while monitoring the electronics and computers in front of him. These are used to control the Multinet and upload data. The laptop computer displays the depth and other parameters being collected such as temperature, salinity, and chlorophyll. The five nets are switched by pushing the ‘fire’ button located on the electronic control box (light grey unit in front of Dr. Campbell) after a given amount of time or net sample volume, which is also displayed on the computer. To the left of the laptop computer used for the Multinet is another one. This one is associated with hydro-acoustics data being collected simultaneously. The white-topped jars in front of the acoustics electronics will be used to preserve the samples being collected at the time this picture was taken. Their labels match the field notebook located in front of the Multinet electronics. Photo by Dr. Tom Kline.

Photo 3: Sample recovery following Multinet deployment. Zooplankton that caught in the nets end up in the orange cod-ends. The cod-ends are taken off the net, rinsed and the contents put in jars (Photo 1). Jars containing zooplankton are either frozen or preserved by adding formalin. The typical ‘destination’ for the five cod-ends is that two samples are frozen for energy (calorimetric) analysis, two samples are preserved for species identification, and one sample is sorted by species and frozen individually or in aggregate for stable isotope analysis. The microscopes seen in Photo 1 are used during species sorting.

Figure 1: Preliminary analysis confirms that low carbon isotope values (more negative) were diagnostic of oceanic carbon in 2007. Carbon isotope values measured in zooplankton sampled in herring nursery bays, other areas in Prince William Sound, and in the adjacent Gulf of Alaska are compared as box and whisker plots.

Figure 2: Pelagic forage fishes throughout Prince William Sound shifted to low carbon isotope values in late 1995 suggesting a systemic role for oceanic carbon subsidies. Herring shifted to a greater extent than other species suggesting a relatively greater dependence on oceanic carbon subsidies. Figure adapted from Kline (2007).