The Seasonal and Interannual Patterns of Larvaceans and Pteropods in the Coastal Gulf of Alaska, and Their Relationship to Pink Salmon Survival
Investigator
University of Alaska Fairbanks
Student
- Ayla Doubleday, MS program
Synopsis
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.
Overview
The issue
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.
Objectives
Determine composition, abundance, and biomass of larvaceans and pteropods using research collections (2001–2009) made by the Gulf of Alaska Global Ocean Ecosystem Dynamics (GLOBEC) program.
Estimate the potential production of these groups using laboratory determined rates.
Compare the variable rates of pink salmon returns observed during the GLOBEC period to the availability and production of the larvaceans and pteropods.
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.
Research collaborators
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.
Results
What researchers learned
Larvaceans (appendicularians) and the pteropod Limacina helicina were studied each May and late summer over an 11 year period (2001 to 2011), along the Seward Line transect that crosses the continental shelf of the subarctic Gulf of Alaska. Collection with 53 µm plankton nets allowed the identification of larvaceans to species: five occurred in the study area. Temperature was the driving variable in determining larvacean community composition, yielding pronounced differences between spring and late summer, while individual species were also affected differentially by salinity and chlorophyll a concentration. Oikopleura labradoriensis and Fritillaria borealis were present at nearly all stations during the spring; abundances averaged 124±63 individuals per m3 and 210±110 ind. per m3, respectively (with 95% confidence intervals). O. dioica (245±145 ind. per m3) dominated numerically at nearshore stations during the late summer, and F. borealis (125±71 ind. per m3) occurred at outer stations with the occasional presence of F. pellucida during warmer temperatures. Biomass of larvaceans consisted predominantly of O. labradoriensis in the spring, when total larvaceans averaged 0.72±0.31 mg dry weight per m3. O. dioica dominated the biomass in the late summer, when total larvaceans averaged 0.25±0.11 mg DW per m3. L. helicina spring abundance was positively correlated to spring temperature. When L. helicina biomass was pooled by year, the spring biomass (0.83±0.22 mg DW per m3) was not significantly different from the late summer biomass (1.11±0.32 mg DW per m3) and abundance displayed a similar pattern: 217±78 ind. per m3 and 478±215 ind. per m3, respectively. L. helicina abundance determined with 505 µm nets in nearby Prince William Sound was positively correlated to percent survival of pink salmon; however, no significant correlations existed to larvacean or L. helicina abundances from the Gulf of Alaska.
Research outcomes
Publications
Li, K., A.J. Doubleday, M.D. Galbraith, and R.R. Hopcroft. 2016. High abundance of salps in the coastal Gulf of Alaska during 2011: A first record of bloom occurrence for the northern Gulf. Deep Sea Research II 132:136-145. https://doi.org/10.1016/j.dsr2.2016.04.009
Howes, E.L., N. Bednarsek, J. Budenbender, S. Comeau, A. Doubleday, S.M. Gallager, R. Hopcroft, S. Lischka, A.E. Maas, J. Bijma, and J. Gattuso. 2014. Sink and swim: a status review of thecosome pteropod culture techniques. Journal of Plankton Research 36(2):299–315. https://doi.org/10.1093/plankt/fbu002
Doubleday, A.J., and R.R. Hopcroft. 2014. Interannual patterns during spring and late summer of larvaceans and pteropods in the coastal Gulf of Alaska, and their relationship to pink salmon survival. Journal of Plankton Research 37(1):134–150. https://doi.org/10.1093/plankt/fbu092
Doubleday, A. 2013. Seasonal and interannual patterns of larvaceans and pteropods in the coastal Gulf of Alaska, and their relationship to pink salmon survival. Master's thesis, University of Alaska Fairbanks, SGT-13-02, 65 pp.
