Description of Index
Jellyfish sampling was incorporated aboard the BASIS (Bering Aleutian Salmon International Surveys) vessels beginning in 2004 and will continue through 2013. All jellyfish medusae caught in the surface trawl (top 18-20 m of the water column) are sorted by species and subsampled for bell diameter and wet weight. Six species are commonly caught with the surface trawl: Aequorea sp., Chrysaora melanaster,Cyanea capillata, Aurelia labiata, Phacellocephora camtschatica, and Staurophora mertensi. Biomass is calculated for each species and compared across species, and oceanographic domains on the Bering Sea shelf (Inner Domain <50m; Middle Domain 50m-100m; Outer Domain = >100m). Yearly distributions throughout the sample grid for all species have been patchy. Despite uneven distributions throughout oceanographic domains, highest concentrations of all species were found to occur in the Middle Shelf Domain. Of the six species sampled, Chrysaora melanaster had the highest weight per unit effort (kg) for all years.
Status and Trends
Figure 1. Total annual jellyfish biomass (1,000 metric tons (t)) split by region. Includes combined species caught in surface trawls in the eastern Bering Sea during August-October. Biomass was calculated using average effort per survey area in km2 by year.
Figure 2. BASIS surface trawl Biomass (1,000 t) by genus for 2004-12 in the eastern Bering Sea during August -October. Biomass was calculated using average effort per survey area in km2 by year.
In 2012 total jellyfish biomass more than doubled compared to 2011 and was the highest recorded biomass year for our survey (Fig. 1). One station in the southern Bering Sea portion of our grid during 2012 was responsible for half the total catch for the entire survey. During 2010, combined jellyfish species biomass also nearly doubled compared to the previous highs of 2004 and 2005. Unlike in 2012, half the total catch did not come from a single station but was spread out over the sampling grid. Starting in 2006, notable declines in jellyfish species compostion were observed for all taxa except C. melanaster and continued through 2012 (Fig. 2). The dominant species, C. melanaster continued to increase in 2010, nearly tripling its biomass compared to 2009. In 2008 our station grid was significantly reduced. However, comparisions with past years using the same survey area as 2008 indicate similar trends in species composition and distribution patterns. During 2006-09, biomass of all other species remained low in comparison to 2004 and 2005, suggesting the trend for the region has shifted from multiple species to a single species dominant catch.
Factors Causing Trends The cause for these shifts in biomass and distribution do not seem to rely solely on physical ocean factors (temperature and salinity). These shifts could also be a result of environmental forcing earlier in the growing season or during an earlier life history stage (polyp), which may influence large medusae biomasses and abundances.
Implications Significant increases in jellyfish biomass may redirect energy pathways in the eastern Bering Sea food web through jellyfish predation on zooplankton and larval fish, and could result in limiting carbon transfer to higher trophic levels.
By Kristin Cieciel, Jeanette Gann, and Lisa Eisner