Observatories based on cable technology give opportunities for efficient monitoring of marine life, but also represent a new generation research infrastructure that provides new insight into regulation mechanisms of marine production, stock abundance, behavior, and migrations. A priority of the biological NOON is to have a fjord observatory established. Besides being an important research infrastructure such a laboratory will enable cost effective tests of sensors as well as training and capacity building. The Norwegian fjords represent a unique opportunity. Fjords are sheltered inshore locations where weather independent underwater operations can be undertaken year around. The large depth and the oceanic fauna, however, make these environments excellent models for ocean areas. Another area of high priority is to establish an observatory along a section from the coast affected by Norwegian Coastal Water to deep offshore areas with Atlantic Water. Such sections will cover biological oceanographic variations that are expected to reveal mechanisms for the biological production along the Norwegian coast and its changes through space and time. Unique pilot tests that have been conducted in Norwegian fjords suggest that the combination of acoustical and optical registrations will provide new insight into habitat quality and behavior and migration mechanisms in zooplankton and fish. Consequently systems that support such registration additionally to standard oceanographic measurements will have high priority.
Preliminary fjord observatories
The Universities of Oslo and Bergen apply moorings consisting of echo sounders and environmental sensors deployed at the bottom of Norwegian fjords. The systems are cabled to land for electricity and transfer of data, and acoustic data are displayed in real time on a Webpage. The continuous records are accompanied by intense sampling campaigns identifying the acoustic targets. Fjords are deep, with an oceanic fauna, and the close proximity to land means simple logistics and low operation costs. Fjord observatories can therefore be used for obtaining immediate, inexpensive, high quality data, and as experimental sites for competence building. As the use of cabled observatories increases there is a need for scientists who know the possibilities and limitations of such new infrastructure, who can phrase the right questions to take full benefit of the laboratories and who are able to handle the vast amount of data collected. We advocate the use of fjord laboratories as training sites.
Stationary echo sounders provide information on abundance, vertical distribution, size and individual behaviour of both plankton and fish. They may scan the entire water column more than once per second, for - in principle - endless observation periods. Our work has been focused on fjords which each provide unique opportunities for addressing particular scientific questions. We exemplify by three case studies.
Mesopelagic fish (Masfjorden)
A 38 kHz echosounder being deployed at 400 m depth in Masfjorden, Norway. The electronic parts (transceiver) are housed in a pressure proof glass sphere next to a pressure proof transducer, mounted in a frame with gimbal couplings to ensure horizontal orientation of the transducer surface. The echosounder is connected to shore with 1200 m of cable, providing electricity and transmitting data to a PC on shore.
During a 15 months study we addressed the diel vertical migration (DVM) of the light fish Maurolicus muelleri and the northern laternfish Benthosema glaciale at a 400 m deep site in Masfjorden. We observed marked seasonal variations and previously undescribed behavioral patterns in both species. To illustrate, the behavior of lightfish comprised a) normal DVM with the entire population ascending at night, b) migrations mainly in the morning and c) limited inverse migration with part of the population ascending at day. The deeper-living lanternfish displayed both normal, and reversed DVM as well as no migration.
The DVM pattern of the lightfish Maurolicus muelleri and the northern lanternfish Benthosema glaciale in different seasons, as measured from a bottom mounted echosounder, cabled to shore. Time for sunset and sunrise depicted with blue arrow. During summer, the entire population of M. muelleri ascends at night; in autumn most of the adult population ascends only in the morning, and during winter a part of the population ascends slightly at day. Juvenile M. muelleri (the shallowest scattering layer) always carry out normal DVM. B. glaciale carries out normal DVM in mid-summer, later in summer the population splits into a normally migrating and reversely migrating part, and in winter only reversed DVM is recorded. Color scale refers to acoustic backscatter (Sv), with reddish-brown representing the strongest echoes (densest concentrations of fish).
Photo of catches with samples of M. muelleri and B. glaciale
A Periphylla periphylla at 50 m depth, photographed by a drifting time-lapse camera
Lurefjorden is an ecosystem being “taken over” by the deep-sea jellyfish Periphylla periphylla at depths where mesopelagic fish usually reigns. Jellyfish are weak acoustic targets, but can nevertheless be recorded acoustically, particularly in habitats with few strong targets. We conducted a 4 months study of Periphylla periphylla from an echo sounder deployed at nearly 300 m depth, our shore-based acoustic laboratory consisting of a shack on a quay.
The population of jellyfish appeared to be segregated into population modes with different behavior. The shallowest mode inhabited the upper 100 m, carried out diel vertical migrations with limited amplitude. A mid-water group of jellyfish inhabited waters at about 200 m during the day, ascending at dusk and descending at dawn. Individuals of a third and deeper-living component of the population were continuously migrating up or down at vertical speeds of 1-2 cm s-1 regardless of time of day, yet the range of their asynchronous vertical migrations encompassed the entire water column at night. A last group of primarily large jellyfish remained motionless for prolonged periods, intermittently changing their vertical distribution slightly. The acoustic study from the mooring unveiled much more diverse behavior among the jellyfish than previously found using other methods.
The vertical distribution and diel vertical migration behaviour of different “modes” (numbered 1-4) of the jellyfish Periphylla periphylla in Lurefjorden, Norway, as measured from a bottom mounted 38 kHz echosounder, cabled to shore.
Echosounders can also be used for studying individual swimming behavior as plankton or fish move through the acoustic beam. We illustrate by results on the krill Meganyctiphanes norvegica at a 150 m location in Oslofjorden. The inner part of this fjord becomes ice-covered in winter, so in the example provided the cabled observatory facilitated studies on under-ice behavior.
Photo from a webcamera located in our acoustic laboratory, providing hourly updates on conditions like ice-coverage of the fjord. While anglers enjoy the winter days fishing on the ice, an echosounder (120 kHz) cabled to shore provided continuous information on under-ice biology.
The results showed that the krill was swimming at velocities corresponding to ~1 body length s-1 the first part of winter. The krill changed their behaviour as a more oxygenated water mass intruded the hypoxic fjord branch in mid-winter. Swimming speed increased, particularly at night, when the krill was swimming at about 3 body lengths s-1. Unveiling such diel variation in swimming speed was made possible by measurements from an observatory at a stable substrate in a calm fjord branch.
Swimming speed of krill obtained through 5 months of records from an echosounder (120 kHz) cabled to shore, unveiling marked diel variations with maximal swimming speed at night.
Fjord observatories have proven their potential through initial studies of individuals, populations and ecosystems. Essential infrastructure is already in place so that fjord observatories are ready to be used at a broader scale for providing new scientific results and to train young scientists in parallel with establishment of large-scale oceanic laboratories. With easy access to oceanic fauna, central research questions can be addressed and solved with limited logistic challenges and at low costs. Results can be disseminated in real-time via internet and presentations made for public outreach. We have the facilities, experience and knowledge to establish Norwegian fjord laboratories at short notice.