New research investigating the oils or ‘lipids’ in Antarctic krill will provide scientists and the krill fishing industry with improved tools for measuring the health of krill populations and their environment.
Antarctic krill (Euphausia superba) are a textbook example of the phrase ‘you are what you eat’. As these shrimp-like creatures graze the tiny phytoplankton and zooplankton (marine plants and animals) with which they share the water column and sea ice environment, they assimilate the long-chain, omega-3 fatty acids (see ‘Unravelling the fatty acid chain’ below), produced by these organisms, into their bodies.
These individual fatty acids are incorporated into lipids in the krills’ bodies, where they are important for cell membrane structure and function and as an energy reserve for growth, reproduction and survival over the harsh winter months.
Among these fatty acids are those essential to human brain and heart health, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oil and increasingly krill oil supplements supply a growing market for these essential fatty acids. But the quantity and quality of krill oil extracted from a catch – the amount of EPA and DHA and the ratio of the two – varies from season to season, most likely reflecting changes in krills’ environment, and particularly diet.
Now, an innovative collaboration between Antarctic scientists and the krill fishing industry aims to better understand the reasons for this variable oil quantity and quality. Project Chief Investigator and Australian expert on krill lipids, Dr Patti Virtue, from the Institute for Marine and Antarctic Studies, along with scientists from the Australian Antarctic Division, CSIRO, Griffith University and the University of Tasmania, have teamed up with Norwegian krill fishing company Aker Biomarine – a manufacturer of krill oil supplements and other krill products.
Using Aker Biomarine’s considerable krill catching capacity, the team will be able to investigate research questions at a scale unprecedented in traditional scientific research. The work will enhance the quality control of commercial krill products and collect data essential to understanding krill and managing the fishery.
‘Over the past 20 years scientists on research vessels have only been able to collect small krill samples in a limited time, and while we’ve done some good research, we really only have a snapshot of what’s going on in the wild and very little information about what happens in winter,’ Dr Virtue said.
‘This means that key questions of the basic biology and ecology of krill remain unanswered – in particular, what is the relationship between seasonal dietary changes and krill growth, condition, maturity, reproduction and the recruitment of young krill into the adult population?
‘This information is critical for estimating krill production to assist the Commission for the Conservation of Antarctic Marine Living Resource (CCAMLR) to set sustainable catch limits and to understand how environmental change may affect krill populations.’
Over three years, Aker Biomarine will collect daily krill samples from the South West Atlantic fishing grounds, which encompasses waters near the Antarctic Peninsula, South Orkney Islands and South Georgia.
Scientific advisor to Aker Biomarine and a former krill researcher at the Australian Antarctic Division, Dr Steve Nicol, said the Marine Stewardship Council-certified company uses a specially designed pumping system that excludes bycatch and delivers krill to the ship alive and in pristine condition. Scientific samples will be snap frozen in a -80°C freezer onboard the ship.
‘This will allow us to look at lipid changes on a daily, weekly and seasonal basis,’ Dr Nicol said.
Aker Biomarine will also collect oceanographic data (sea surface temperature, currents) and bathymetric data (seafloor structure). The ship’s acoustic data will be used to analyse the density and vertical distribution of krill, patterns of daily migration (day/night) and swarm aggregation behaviour. Scientists will also collect phytoplankton samples from onboard the fishing vessel, and satellite information on phytoplankton production and abundance.
‘We’ll be able to match all this information with the results of our lipid studies, to relate the physical condition of krill at different times of year with what’s happening in their environment,’ Dr Virtue said.
‘This will allow us to identify the factors governing the lipid content and composition in krill.’
This knowledge could help commercial fishers to better plan their fishing strategy. Most of the 300 000 tonnes of krill caught in the fishery (from a total allowable catch of 620 000 tonnes for the South West Atlantic – about 0.3% of the total estimated biomass of krill), is used for aquaculture. About 10% of this catch is processed for oil.
‘If krill are predicted to have a favourable lipid profile at a particular time of year, for example, fishers could target krill specifically for their oil, based on that knowledge, and ensure a standardised product,’ Dr Nicol said.
To understand how and why lipid content and composition changes, Dr Virtue, CSIRO’s Dr Peter Nichols, and Aker BioMarine’s Dr Nils Hoem, will study ‘signature’ lipids. Like a human fingerprint, these lipids are characteristic of the organisms that produced them – the krills’ food source.
‘Krill primarily feed on larger phytoplankton – the diatoms and dinoflagellates – which are both microalgae,’ Dr Virtue said.
‘Diatoms tend to be dominated by EPA and dinoflagellates by DHA. However, krill are also omnivores, depending on the season and the food source available, and we have seen lipid signatures for bacteria, detritus and copepods.’
Dr Nichols said there would be a staged approach to lipid analysis.
‘The first thing we’ll do is look at the lipid content of the whole animal or individual tissues, such as the ovaries. This will give an indication of the condition of animals in different seasons,’ he said.
‘Then we’ll look at the lipid classes – so whether they’re structural or storage lipids. This will tell us whether they’re starving or not. Then we’ll identify the individual fatty acids that make up the lipids and compare these to our database of algal lipids to see what they’ve been eating.’
The work will also provide insights into the role of lipids in krill during periods of limited food supply (particularly over winter) and whether krill exploit alternate winter food sources for growth and survival.
‘This will allow us to develop a seasonal nutritional model to predict variability in spawning and recruitment,’ Dr Nichols said.
Alongside the signature lipid research, Australian Antarctic Division krill scientist Dr So Kawaguchi, will conduct experiments on live krill in the Antarctic Division’s krill aquarium, to look at the effect of dietary changes on lipid content and composition, as well as the effects of climate change.
‘We’ll be able to look at environmental changes, such as temperature and carbon dioxide levels, on growth, reproductive potential and other metabolic processes that affect lipid accumulation and profiles,’ Dr Kawaguchi said.
Previous research at the Australian Antarctic Division by Dr Andrew Davidson (see Sweating the small stuff) also suggests that warmer ocean waters and ocean acidification (due to increasing amounts of carbon dioxide dissolving in seawater) could favour smaller phytoplankton species over larger diatoms (Australian Antarctic Magazine 24: 22-23, 2013).
‘Many of these smaller phytoplankton are less nutritious for krill than the larger diatoms, and changes in their abundance or composition could alter the types and amount of fatty acids available for krill growth, reproduction and recruitment into the fishery,’ Dr Kawaguchi said.
‘These changes will have implications for krill fishery management into the future.’
The research team will also analyse field-caught krill for persistent organic pollutants (POPs), to provide clues to krill population distribution and movement.
POPs are chemicals found in common household items which persist in the environment (see Household pollutants detected around Antarctic stations) and accumulate in the lipids of plants and animals. Therefore, populations of krill found in the vicinity of local contamination sources will carry distinctive POP ‘signatures’, which will be reflected in their tissues.
By examining these signatures, scientists hope to determine whether krill populations intermingle or remain separate.
‘Recent research suggests that Antarctic krill do not move passively on ocean currents but may be capable of maintaining distinct population distributions,’ Dr Virtue said.
‘The question as to whether krill are one or many populations has implications for management, so we will use POPs to determine whether fishing operations are targeting one or many krill populations.’
Ultimately, the research aims to provide win-win outcomes for scientists, the krill fishery, the krill population and the predators that feed on them.
‘Our current view of krill is all built on the capacity of scientific research vessels,’ Dr Kawaguchi said.
‘Now we have a chance to use a completely different way of sampling to understand what’s happening in the wild.’
Source: Australian Antarctic Division
Unravelling the fatty acid chain
Krill assimilate ‘long chain’ omega-3 fatty acids into their bodies from their food. These fatty acids have 20 or more carbon atoms in their structure (hence the term ‘long chain’), in contrast to other shorter chain (18 carbon) omega-3 fatty acids such as those found in chia or flax seeds. Krill are a good source of health-beneficial eicosapentaenoic acid (EPA) and and docosahexaenoic acid (DHA). EPA has 20 carbon atoms and five double bonds in the chain (20:5w3), with the first double bond beginning at the ‘tail’ or omega end of the chain (w3). DHA has 22 carbon atoms and six double bonds (22:6w3).
A sustainable krill fishery
Despite reports of unsustainable and even illegal krill harvesting, illegal harvesting does not occur and would not be cost-effective in the current market. Krill is one of the few marine resources with the potential for a large but sustainable increase in catches, if an appropriate management scheme is in place. CCAMLR sets and regularly reviews and revises precautionary catch limits based on the best available science. The krill fishery is currently operating only in the South Atlantic sector with an annual catch of up to about 300 thousand tonnes. The sum of the precautionary catch limit established for the Antarctic krill fishery is presently 5.6 million tonnes in the South Atlantic Ocean and 3 million tonnes for the South Indian Ocean.