Coincident Mass Occurrence of Gelatinous Zooplankton in Northern Norway

Abstract

In autumn 2015, several sources reported observations of large amounts of gelatinous material in a large north Norwegian fjord system, either caught when trawling for other organisms or fouling fishing gear. The responsible organism was identified as a physonect siphonophore, Nanomia cara, while a ctenophore, Beroe cucumis, and a hydromedusa, Modeeria rotunda, were also registered in high abundances on a couple of occasions. To document the phenomena, we have compiled a variety of data from concurrent fisheries surveys and local fishermen, including physical samples, trawl catch, and acoustic data, photo and video evidence, and environmental data. Because of the gas-filled pneumatophore, characteristic for these types of siphonophores, acoustics provided detailed and unique insight to the horizontal and vertical distribution and potential abundances (∼0.2-20 colonies•m −3) of N. cara with the highest concentrations observed in the near bottom region at ∼320 m depth in the study area. This suggests that these animals were retained and accumulated in the deep basins of the fjord system possibly blooming here because of favorable environmental conditions and potentially higher prey availability compared to the shallower shelf areas to the north. Few cues as to the origin and onset of the bloom were found, but it may have originated from locally resident siphonophores. The characteristics of the deep-water masses in the fjord basins were different compared to the deep water outside the fjord system, suggesting no recent deep-water import to the fjords. However, water-masses containing siphonophores (not necessarily very abundant), may have been additionally introduced to the fjords at intermediate depths, with the animals subsequently trapped in the deeper fjord basins. The simultaneous observations of abundant siphonophores, hydromedusae, and ctenophores in the Lyngen-Kvaenangen fjord system are intriguing, but difficult to provide a unified explanation for, as the organisms differ in their biology and ecology. Nanomia and Beroe spp. are holopelagic, while M. rotunda has a benthic hydroid stage. The species also have different trophic ecologies and dietary preferences. Only by combining information from acoustics, trawling, genetics, and local fishermen, were the identity, abundance, and the vertical and horizontal distribution of the physonect siphonophore, N. cara, established.

Macroscale abundance patterns of hydromedusae in the temperate Southwestern Atlantic (27°–56° S)

“…Furthermore, reliable baseline data and long-term time series to understand their population dynamics are scarce [7,8]. To elicit information concerning a community structure, distribution and abundance patterns are essential to evaluate the evolution of a given community in order to detect changes over time and space together with their potential causes and the underlying processes [9–11]. Fundamental research in jellyfish ecology is required to define baselines that will allow researchers to make retrospective analyses and to identify trends at different spatial and temporal scales.…”

Mesopelagic Sound Scattering Layers of the High Arctic: Seasonal Variations in Biomass, Species Assemblage, and Trophic Relationships

“…Midwater trawls are inefficient at sampling siphonophores because these fragile organisms are easily destroyed. Only one siphonophore was sampled in January 2016, but their abundance was likely higher as they can be abundant at high latitudes (Knutsen et al, 2018). Siphonophores have gas-filled pneumatophores that resonate at similar frequencies as swimbladdered fish, and it is possible that some of the signal classified as fish was in fact coming from siphonophores, thus resulting in an overestimation of the fish biomass.…”

Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video

“…The largest source of bias and uncertainty however likely resided in the biological sampling, due to avoidance [13] and fragile organism destruction by the nets issues [57]. Biological sampling errors were however not included in our uncertainty analysis, due to difficulties to quantify them.…”