The Seasonal and Interannual Patterns of Larvaceans and Pteropods in the Coastal Gulf of Alaska, and Their Relationship to Pink Salmon Survival
- Russell Hopcroft, Institute of Marine Science, University of Alaska Fairbanks
- Ayla Doubleday
Research stemming from the Gulf of Alaska Global GLOBEC program found that juvenile pink salmon preferentially feed on two understudied zooplankton groups, larvaceans and thecosome pteropods, and that this diet may be tied to salmon survival and adult returns. In this study, scientists will examine zooplankton collected during the GLOBEC program that are appropriate for assessing these groups, in order to estimate composition, abundance, biomass, and production of larvaceans and pteropods. Additionally, researchers will undertake experimental work to determine the growth rates of the major larvacean and pteropod species, so that their availability to higher trophic levels can be calculated. Finally, they will explore relationships between the production of these groups and salmon survival.
Emerging science indicates that salmon may preferentially feed upon two understudied zooplankton groups, larvaceans (appendicularians) and the “shelled” thecosome pteropods, and that this diet may be tied to salmon juvenile survival and adult returns. Little is known about the composition, abundance, biomass, and production of the larvaceans and pteropods. Researchers will address this deficiency by examining samples collected during the GLOBEC program that are appropriate for assessing these groups.
Why is this an Alaska Sea Grant project?
One of Alaska Sea Grant's six key goals outlined in the 2009–2013 Strategic Plan is sustained, well-managed, and healthy marine, coastal, and watershed ecosystems in Alaska. The program pursues this goal through support of research that provides decision-makers with science-based information that can be used to craft well-informed policies governing the use and conservation of Alaska's marine and coastal resources.
How will researchers conduct their study?
Researchers will examine GLOBEC samples collected between 2001 and 2009 from the Gulf of Alaska that have already been processed for the numerically dominant taxa (i.e., crustaceans), and are currently archived at the University of Alaska Fairbanks. Researchers will use appropriate techniques to resample for larvacean and pteropod species to determine composition, abundance, biomass, production, and availability to high trophic levels, such as juvenile feeding salmon.
To determine larvacean growth rates, researchers will lab-culture two species of Oikopleura expected to be most heavily exploited by salmon. Oikopleura labradoriensis is a large subpolar species for which no estimates of growth rate exist. Oikopleura dioica is a smaller temperate-tropical species for which growth rates exist, but not at the full range of temperatures experienced in the Gulf of Alaska.
Culturing of larvaceans is more complicated than other zooplankton because the animal pumps water through an elaborate external food-concentrating filter that must not be damaged in culture. Researchers will employ a culturing system that has been successfully used and modified by a number of researchers for grazing and growth rate studies of temperate and tropical species. This system uses a large paddle that rotates slowly to keep the larvaceans and their food suspended within the incubation cylinder. Cultures will be started from eggs spawned by field-collected adults. They will be fed a mixture of Isochrysis galbana, Chaetoceros calcitrans, and Rhinomonas reticulata. Handling of animals will be facilitated using specialized pipettes.
Growth of the larvaceans will be assessed as the change in trunk length and number of external mucus-feeding filters produced over time. Fecundity will be determined by counting eggs in mature females. Researchers anticipate that during the project it will also be possible to estimate growth rates using shipboard techniques. This approach has proven successful in preliminary experiments with the polar/subpolar species Fritillaria borealis typica and O. vanhoeffeni in the Bering Sea (see Community composition and production of larvaceans in the Northern Bering Sea [PDF; 936 KB]).
For pteropods, there have been no documented culturing attempts, likely due to the more fragile nature of their mucus-feeding structures. Recent communication with Scott Gallagar at Woods Hole Oceanographic Institution, however, indicates he has successfully cultured Limacina in 1.5 m tall tanks, feeding them the dinoflagellate Ceratium longipes. Researchers will employ an enlarged version of the larvacean culturing system, and variations of the system developed by Gallager, to address growth rates for Limacina helicina. They also will employ a wider mixture of algae and protists during pteropod culturing, as they may consume a larger size range of particles than larvaceans. In addition to tracking change in mean shell size, researchers will experiment with use of the stain Alizarin red, which is specific to the calcium carbonate (aragonite) that composes the pteropod shell. Several hours of exposure to moderate concentration of this stain in seawater will hopefully provide enough of a mark on the shell that subsequent deposition of new (unstained) calcium will provide a clear indication of growth in individual animals. Although others have successfully estimated growth rates by measuring the incorporation of 45Ca, this isotope is considered relatively hazardous and expensive compared to most commonly used radioisotopes. Alternatively, researchers will see if growth can be estimated simply from change in mean shell size between GLOBEC sampling cruises, provided growth is as slow as implied by the literature. Researchers have already determined that no sensible Limacina growth rates can be obtained using mean weights from sequential cruises during the GLOBEC years based on the current data quality.
Production for larvaceans and pteropods will be calculated as the product of their biomass and the growth rates determined during this study. These rates will be compared to those of the crustacean zooplankton. Finally, researchers will search for relationships among the production of the mucus-net feeders, the crustaceans, and salmon survival with simple correlation to the relevant recruitment year.
Continued interaction will occur with other GLOBEC and PICES (North Pacific Marine Science Organization) principal investigators seeking to understand ecosystem processes in the North Pacific.
Expected outcomes will include the following:
Provide the first detailed characterization of larvacean and pteropod species' seasonal and interannual patterns in the Gulf of Alaska.
Develop techniques to estimate pteropod and cold-water larvacean growth rates.
Improve understanding of the role of larvaceans and pteropods as prey species that influence juvenile salmon survival in the Gulf of Alaska for scientists, fishermen, industry, and agencies.
Train graduate student.
Present results at national and international meetings. Publish results in peer-reviewed journals and through the Web.