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Antarctic Krill (Euphausia Superba)

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Antarctic Krill (Euphausia Superba) Antarctic Krill (Euphausia superba) Stephen Nicol and So Kawaguchi General information Antarctic Krill (Euphausia superba) are one of the larger species of krill (Euphausiids – free swimming shrimp-like crustaceans) reaching a maximum length of 60 mm and a weight of 2 g when mature. They live exclusively in the Southern Ocean and have a very wide distribution over a range of habitats. They are a swarming (or schooling species) and much of the krill biomass is to be found in large, dense aggregations that can extend for tens of kilometers. Krill can be found in the surface layer, in mid water and near the ocean floor, and can undertake daily vertical migrations. Antarctic Krill have a complicated life history, changing size, shape and habitat as they grow (Nicol 2006). They mature at two years old and can live for up to 11 years. Adult krill are capable of living anywhere in the Southern Ocean – from the very surface layer to the seafloor, and from inshore areas to the deep open ocean. Larval and juvenile krill are associated with sea ice and feed on algae that grow on the underside. There is a lack of clarity on whether there are distinct populations of krill around the Antarctic or whether there is regular interchange between centers of distribution. The krill population is large and their production rate is high consequently they are preyed upon by a range of vertebrates including seals, penguins and baleen whales as well as by invertebrates such as squid. Range The range of krill is estimated to be 19 x 106 km2 (Atkinson et al. 2009). See figure 1. THE IUCN RED LIST OF THREATENED SPECIES™ Figure 1. Map generated from the KRILLBASE data base* derived from a historical compilation of scientific net hauls from 1926 onwards. Green dots indicate the presence of Antarctic Krill in the catch, red dots indicate absence. The blue area encompasses the range of krill: for a given longitude, the extent is taken as the northernmost latitude of any krill observation in a 30-degree sector centred on that given longitude. The three major frontal systems in the Southern Ocean are indicated. *Krill Base data (version March 2014) has been provided with by the major data contributors to the database (A. Atkinson, S. Hill, V. Siegel, E. Pakhomov, C. Reiss, S. Kawaguchi). Krill abundance The overall mean abundance of krill estimated from scientific net surveys conducted between1926-2004 was estimated to be 379 million tons (Atkinson et al. 2009). Atkinson et al. (2009) estimated the krill biomass for January-February 2000 to be 133 million tons. This estimate was based on an acoustic estimate of biomass derived from the CCAMLR2000 survey of the South Atlantic of 37.3 million tons which was estimated to be 28% of the global krill biomass. CCAMLR has subsequently revised this biomass estimate (SC-CCAMLR 2010 para. 3.29) to 60.3 million tons. Applying the process used in Atkinson et al. (2009) to this new estimate of biomass results in a global krill biomass of 215 million tons in the year 2000. Generation length The generation length of Antarctic krill has been calculated to be 5 years using the standard IUCN methodology and the following inputs: • Survival up to 12 month=0.03: Based on average number of juveniles survived from eggs in Nagoya Aquarium (Hirano et al. 2003). • Survival from 1st to 2nd year = 0.1: Based on Brinton and Townsend (1984). • Survival from 2nd year onwards = 0.4: Based on widely accepted krill post-larval mortality rate (M=0.8) which equals to survival rate of 0.4. • Life span = lives until end of its 6th year (Siegel 1987). • Annual fecundity, 12,000 for age 3+ onwards based on Tarling et al. (2007). Annual fecundity of 6,000 was used for age 2+ krill (spawning during 3rd their summer) due to their smaller body size. Trends in krill biomass. Although a decline in krill density in the 1970s and 1980s has been reported from analysis of net survey data in the Southwest Atlantic sector (Atkinson et al. 2004) the magnitude of any such decline is difficult to ascertain. For the purposes of this current assessment the trend in krill biomass over the last 15 years (3generation times) has been used. Two time series of acoustic data (the standard method of biomass assessment used by CCAMLR) exist within the range of Antarctic krill – one for South Georgia (Fielding et al. 2014) and one for the Elephant Island area Cossio et al. 2011). Both of these time series are from the South Atlantic region where 28% of the krill biomass is thought to reside (Atkinson et al. 2009). There is considerable inter-annual (and intra-annual) fluctuation in krill density (either measured acoustically or using nets) at a fixed location; however, in neither of these two acoustic time series is there a significant trend in the data. Figure 2. (a) Frequency distribution of log10(krill density+1) for each annual mid- season survey. The median (white circles), mean (black crosses), and inter-quartile range (black bar) is shown. (b) The mean WCB krill density from 1997 to 2013 following Jolly and Hampton (1990). Figure 3. Acoustic estimates based on updated methods from ASAM 2010 of Antarctic Krill biomass (tons) between 1996 and 2011 for the AMLR study area during the January-February (A) and February-March (B) legs. Biomass for each area is calculated if the area was sampled. In several recent years no second leg has been conducted. The krill fishery: The krill fishery has been operating for more than over 40 years. Catches peaked in the early 1980s with Japanese and Soviet vessels catching over half a million tons a year (see below). 600 500 400 300 200 catch thousands in of tonnes 100 0 1973 1979 1985 1991 1997 2003 2009 2015 year ending Figure 4. Catches of krill in the Southern Ocean from 1973 to 2014 (data from the CCAMLR Statistical Bulletin). Today, about 300,000 tons are caught from the South West Atlantic, largely by Norwegian vessels, producing high-end aquaculture feed and krill oil supplements for human consumption (Foster et al., 2011). When the krill fishery was established there was concern that it might cause irreversible damage to the Antarctic ecosystem, so a unique international treaty was signed to ensure it would be managed using an approach that took into account the needs of the entire ecosystem. This treaty was the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR). The management of the krill fishery has been guided by its principles since the early 1980s. The fishery is regulated through a series of measures that specify how much can be caught, where it can be caught, acceptable levels of by-catch and other operational restrictions. The amount of krill that can be caught in any one year is set through “Precautionary Catch Limits (PCL)”, which is more conservative than normal fishery quotas because of the sensitivity of the Antarctic krill biomass, the animals that depend on it as a food source, and the unique environment in which they live (Nicol and Foster 2003). Catch limits are calculated for a particular area by working out how much krill is in that area and by determining the proportion of the krill stock in that area that can be harvested without irreversibly impacting the ecosystem in the long-term. Currently, only 9.3% of the krill stock is available to the fishery. Precautionary Catch Limits have been set for several large areas of the Southern Ocean, totaling more than 8 million tons per year. In the South Atlantic, where the krill fishery currently operates, there is a Precautionary Catch Limit of 5.6 million tons. As an added element of precaution, CCAMLR has applied a “Trigger Level” of 620,000 tons throughout the main fishing grounds – a level of catch that cannot be exceeded until more advanced management procedures are in place. Climate change and krill. The life history and population dynamics of Antarctic krill are likely to be impacted by the climate change due to increasing levels of CO2 in the atmosphere (Flores et al. 2012), and may result in a reduced habitat range (Hill et al. 2013). The reproductive output and recruitment success of krill has been related to the extent, timing and duration of winter sea ice cover. The underside structure of sea ice provides a nursery ground for overwintering krill larvae and a substrate for algae which are their food. Extensive winter sea ice promotes strong spring phytoplankton bloom when retreating in spring which fuels the adults’ reproductive output for their summer spawning season (Quetin and Ross 2001). Krill growth has also been observed to decrease above a temperature optimum of 0.5°C (Atkinson et al. 2006). Their early developmental stage of krill is vulnerable to increased levels of CO2 projected within their habitat range in year 2100 and beyond (Kawaguchi et al. 2013). Overall, the cumulative impact of climate change is most likely to be negative. References Atkinson, A., Siegel, V., Pakhomov, E. and Rothery, P. (2004). Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature. 432(7013): 100-103. Atkinson, A., Shreeve, R.S., Hirst, A.G., Rothery, P., Tarling, G.A., Pond, DW., Korb, R.E., Murphy, E.J. and Watkins, J.L. (2006). Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage. Limnology and Oceanography. 51(2): 973-987. Atkinson, A., Siegel, V., Pakhomov, E.A., Jessopp, M.J.
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