In recent times, there has been relatively little discussion of the risks posed by the fishery to the krill population itself. This lack of attention reflects a view that catches up to the trigger level could not have a measurable impact on the krill population because they represent only a small fraction of overall biomass ca.
However, this view is challenged by the high levels of variability observed in available indices of krill abundance, which typically span two to three orders of magnitude 15 , 20 , 21 and in the increasing spatial focus of the fishery Fig. The locations of the main oceanographic fronts are marked in purple , This paper highlights areas that we, SKAG, believe warrant most pressing attention for the management of the krill fishery.
Understanding the distribution and abundance of new recruits and the mechanisms that determine a successful recruitment is critical information in fisheries management. Eggs sink to deep water before hatching, then early stage larvae ascend toward the surface, and continue their development into juveniles through winter Recruitment for krill is defined as the surviving juveniles joining the population the following spring For successful reproduction, female krill need food of sufficient quality e.
During development, the first feeding larval stage must find sufficient food within ca. For successful recruitment, larvae must be transported by the currents to areas where they can overwinter successfully and where the newly recruited juveniles are in proximity to productive regions in spring for rapid growth Krill recruitment fluctuates greatly between years, with typically only one or two strong recruitments occurring per decade.
While the exact dynamics vary between regions, there is some evidence of cyclicity of interconnections between regions 30 and of cyclicity, especially in the Western Antarctic Peninsula region 31 , 32 , 33 and at South Georgia 21 , However, while recruitment as a proportion of the total stock size shows inter-annual, in some cases cyclical, variability, the absolute density of recruits measured during surveys is not sufficient to support the long-term average of the measured density of the adult population The apparent mismatch between field observations of recruit numbers and expectations from population dynamics theory needs to be reconciled in order to quantify the relationship between krill recruitment indices and absolute recruitment.
Environmental conditions correlated with enhanced recruitment e. Recent modeling studies suggest that the seasonal location of sea ice is the main limiting factor for successful larval recruitment 39 , Early seasonal sea ice formation and extensive coverage in late autumn and during winter is hypothesized to promote survival by spatially separating developing larvae from the adult population, reducing ontogenetic competition for food and minimizing cannibalistic predation on larval krill by post-larvae While such hypotheses offer potential explanations of the relationship between recruitment and sea ice conditions, the mechanisms behind them are still poorly understood.
In the SW Atlantic, spawning and early larval development takes place in association with oceanic frontal zones 41 , at the Weddell-Scotia Confluence and at the shelf break This suggests that off-shelf migration and subsequent spawning off-shelf is a crucial component of the life cycle, notwithstanding the fact that successful reproduction can occur in some shelf areas A compilation of krill abundance and distribution data adapted from Perry et al.
In late season January to April , larval krill appear along shelf-slope and adjacent areas in the same region Fig. The question as to whether a small portion of the population can sustainably replenish the entire Area 48 krill population remains a major knowledge gap.
Spawning areas further south, including in the Weddell Sea and Bellingshausen Sea are often suggested as source areas, and may contribute to the population in the region, but data are sparse Quantifying the contribution of these seas to overall recruitment is a priority for future research. The risk of fishing on the potential spawning stock in spring and summer is presently slight because the fishery has shifted its main period of activity from mid-summer toward autumn and winter 8 , 9.
Nonetheless protection of spawning areas, using measures, such as seasonal closures, may be necessary to ensure that the numerically-limited spawning stock does not decrease below critical levels 45 in the event of future changes in catch distribution. Such measures may be relatively easy to agree to under present circumstances where there is no immediate conflict with the operation of the fishery.
The Antarctic Peninsula is an important spawning ground and recruitment there fluctuates nearly synchronously across its large geographic area 31 , However, production of larvae along the Peninsula is uncorrelated between regions 47 , with different temporal patterns being evident at the northern and western Antarctic Peninsula NAP and WAP, respectively. Recent patterns of recruitment are correlated solely with larval production from the WAP, suggesting that krill production is presently being driven by upstream sources.
High numbers of early larval krill stages calyptopis have been observed off Marguerite Bay half-way down the WAP, as well as near the tip of the NAP Trajectories of near-surface satellite-tracked, ocean drifters 49 indicate that larvae found along the Antarctic Peninsula and Scotia Arc 42 , 47 may come from both local and more remote locations along the Antarctic Peninsula itself Fig. Larval krill distribution Fig. The main fronts of the Antarctic Circumpolar Current are marked in purple , In addition to the potential influx of larvae from the Antarctic Peninsula, there is interest in understanding the relative contribution of Weddell Sea larvae to recruitment through advection and migration, at the NAP, Bransfield Strait, Elephant Island, and the South Orkney Islands.
Considerable numbers of krill larvae and postlarvae can be associated with the sea ice edge in the Weddell Sea, and models have suggested that production in the northeastern Antarctic Peninsula region northwestern Weddell Sea could be an important source of larvae for the Scotia Sea population 29 , Answering the question of whether the biomass of the successful spawning stock is smaller than that of the entire spawning stock is of critical importance to fishery management.
This requires identification of the key regions of krill larvae production, and quantification of the flux of larvae among those regions, to areas of high krill recruitment and to areas of interest to the fishery.
A major part of krill life history is thought to be the seasonal offshore-onshore migration of the spawning stock along the Antarctic Peninsula 26 , 50 , In this region in spring, at the onset of the spawning season, krill show a distinct spatial separation by maturity stage, although some overlap is evident Fig. In general, smaller, immature krill inhabit coastal waters, while the distribution of large, gravid, and spawning adults extends to the continental slope in oceanic waters.
This spatial segregation of developmental stages in summer may be explained by active offshore migration of adults 50 , such that spawned eggs encounter waters sufficiently deep for successful development The y -axis left shows the distance of life stages from land across seasons 26 , whereas the x -axis right shows the relative abundance of post-larval krill in relation to fishery captured krill.
Superimposed onto background levels of all krill stages across time and space are bulk movements of the adult population from inshore-shelf regions during winter to shelf-beak and off-shelf regions during summer.
The dashed red line indicates the relative abundances of post-larval krill within the present-day fishing region. The blue shaded region indicates captured krill during the present-day krill fishing season and is based on catch data from CCAMLR note that fishing gear optimally retains immature and adult krill In autumn and winter, the distribution of the adult population shifts from its predominately off-shelf summer distribution to on-shelf, and moves deeper in the water column While there are several possible explanations for this, including retention near spawning grounds 54 , shifts in food distribution, and behavioral response to changes in predator distributions, the exact reason is unclear.
Importantly, the present fishing effort overlaps the winter krill distribution, which is more concentrated and deeper in autumn and winter Other hypotheses hold that seasonal changes in krill distribution may be related to predator—prey interactions For example, seasonal changes in predator demand might result in differential consumption of mature krill 57 nearshore during the summer, skewing spatial patterns of krill length composition determined from diet samples.
The recovery of cetacean populations in Subareas In winter, the sea-ice environment provides krill with some refuge from air-breathing predators, and migration of baleen whales to lower latitudes reduces predation pressure. It is important to note that these predator—prey hypotheses, while plausible, remain untested. Two factors may impact the validity of these assumptions: 1 the successful spawning stock the biomass of reproductively mature krill that are responsible for recruitment to the adult population is much smaller than the total adult biomass i.
If we assume, according to the krill distribution pattern shown in Fig. At present, there is no explicit prohibition of fishing on the spawning stock, suggesting that the importance of the second factor, disproportionate fishing on the successful spawning stock cannot be discounted. The first is the catch limit for Subareas The process for setting the 5. However, long-term abundance and distribution data suggests that the successful spawning stock is restricted to shelf slope areas Fig.
Here we illustrate the potential impact of fishing when the successful spawning stock represents different percentages of the total biomass, which fluctuates between years 20 , The figure shows the exploitation rate on the successful spawning stock for each of these scenarios based on the interim catch limit of , tonnes. The dashed black line depicts an illustrative safe ER of 9. While the krill habitat in the SW Atlantic sector has clearly undergone rapid climatic change, with warming 0.
Concomitantly, there is considerable debate and uncertainty over trends in krill population size in the SW Atlantic sector during the last half-century. In recent years, there have been two large-scale Area level krill biomass surveys ca. Variability in the krill stock size has been studied using a variety of proxy indices at regional spatial scales. Some analyses of the data from these regional studies suggest the absence of directional change 21 , 35 , One interpretation is that a general, long-term trend at the larger scale is masked by high levels of interannual variability and nonlinearity between indices and scales 38 , 79 , This interpretation is consistent with strong evidence that krill is sensitive to climatic modes such as the SAM and the ENSO 33 , 37 and that climatic conditions in the SW Atlantic have become increasingly unfavorable for recruitment An alternative interpretation is that the average density of the population is stable amid rapid climate change 73 , Understanding the past dynamics of krill is necessary for reliable projections of future scenarios.
Therefore, there is a clear need to better characterize the uncertainties associated with the various indices and to develop a scientific consensus on interpretation. Climate change impacts on the krill population are not explicitly included in krill fisheries management, where catch limits do not change from year to year However, given the far-reaching implications of any possible climate change-related declines in krill stocks for the Antarctic marine ecosystem, there is a need to incorporate environmental variability into the management framework explicitly by, for example, adjusting catch limits based on environmental predictors or in response to variations in stock size as is common practice in other fisheries Recent field investigations of environmental changes 83 and studies investigating the long-term trends in environmental variables such as water temperature, sea-ice and water-column production, and climate indices such as ENSO and the SAM, suggest they will negatively impact krill 33 , 37 , Importantly, projections of environmental variables that are correlated with recruitment 38 , 85 , 86 suggest that climate change is likely to have a negative impact on future recruitment.
Other studies suggest that there has been a long-term southward contraction of krill distribution in the SW Atlantic sector, with populations becoming concentrated toward the Antarctic continental shelves 38 , 87 as a consequence of climatic-driven ecosystem changes.
Whether or not krill will track their thermal niche southward is complicated because of the interaction with other habitat requirements that are important for krill, including water depth, ice regimes, and quantity of primary production While time-series analyses of recruitment, distribution patterns, or future projections from models have revealed population-level responses to particular climate indices 33 , 37 , 86 , the mechanisms underlying these responses are unclear.
Despite 90 years of krill research, we have only limited knowledge of the adaptive capability of krill to a range of environmental factors including temperature and ocean pH. Increasing our efforts toward a mechanistic understanding of responses by krill to climate change and the incorporation of these data into krill population models will enable more robust projection of krill stock dynamics in the future.
Our synthesis of present knowledge on E. Although krill is one of the most studied pelagic species, the gaps in the present knowledge are major. Over the past 30 years, considerable efforts have been made in the field and laboratory to understand the biology and ecology of krill through a series of scientific studies.
During that time, the general conception of krill biology and its life history has changed dramatically. Years of data have been amassed, including annual acoustic data, net-based population density and demographic data, data from predator diets, and data from process studies in different seasons and locations in the SW Atlantic sector.
However, there remains much debate and controversy on population trends, overwintering, migration and other key aspects of krill biology. The main obstacle to a better understanding of krill biology lies in the difficulties associated with the logistics, operation, and planning of scientific studies. The majority of krill surveys have been conducted during the summer months, but recent investigations outside that time window have shown that studies in the other seasons are essential to understand the biology and ecology of krill, as well as their function in the Antarctic ecosystem.
Examples include the on-shore migration of a large part of the population in winter 53 , and the overwintering diets of early life stages Ship-based net and acoustic surveys have been the main contributor to our knowledge of the population dynamics of krill, and time-series analyses of these data have revealed various trends in the population. Multi-ship, large scale krill biomass surveys provide the baseline biomass estimates for krill management and require large planning effort, often over years, resulting in a single biomass estimate specific to one season.
The critical point here is that these surveys are unable to give answers about population trends or why they occur. At the same time, modern fisheries, which operate almost year-round and are more concentrated than ever before in the SW Atlantic sector, are expanding by using new, more efficient technologies to find krill horizontal sonars and catch them continuous pumping systems These changes in catch techniques require new ways of thinking about the impact of the commercial krill fishery and the effort-based indices needed to manage krill populations.
In order to increase our knowledge of krill, we must go beyond correlative studies toward a mechanistic understanding of their life history.
Combining biomass surveys with process-oriented studies, at different times of the year, either on vessels or at Antarctic field stations should continue.
However, research vessel time is becoming increasingly difficult to obtain, and field stations that can be used to understand the details of the biology of krill are limited due to logistical and facility constraints of catching and maintaining krill for experimental studies. It is, therefore, crucial that we begin to coordinate international research efforts and resources.
The new generations of fishing vessels and their potential year-round operation could provide new opportunities to fill the gaps in knowledge that have been identified. Cooperative research with the commercial fishery might release national scientific research vessel time to conduct studies in other areas that are potentially important for krill, and yet are not sampled sufficiently to understand their importance to krill life history in the SO.
For example, the fishery routinely takes samples from krill aggregations multiple times each day. Such data provide vital information over long periods of time on the demography of krill in different seasons and regions. These demographic data could help quantify the impact of the fishery on the krill winter population and the potential level of spawning in the coming spring.
In addition, commercial vessels could provide high-resolution seasonal krill sampling for studies on the physiological adaptability of krill to environmental changes. Ship acoustic systems, on fishing and research vessels, that record krill behavior patterns on small scales, could be combined with acoustic biomass data from gliders operating on larger scales to better understand the mechanism of seasonal changes in krill biomass and distribution.
In addition, autonomous sampling instruments installed on the fishing vessels, such as the continuous plankton recorder CPR 95 can provide the distribution pattern of juvenile krill. Installed FerryBoxes 96 , collecting basic environmental data, such as water temperature, salinity, primary production in terms of chlorophyll-a concentration and particulate organic carbon could provide information continuously on food availability and thus survival potential of larval krill in autumn and winter.
Moorings 97 , gliders 98 , and sail buoys 99 can be used to gather regular biomass data at small scales in different regions. These operations can also be carried out in close cooperation with the krill fishery that might support the deployment and recovery of these instruments. These new efforts could provide the improved understanding of krill biology and ecology necessary to manage krill at appropriate time and space scales and can be summarized as four key activities:. Increasing laboratory capacity on Antarctic stations to conduct krill process studies in the field.
Focusing scientific research surveys on krill biology in areas and times where the fishery is absent, and where autonomous instruments cannot operate. Such a combined effort by the krill research community and fishery would provide the opportunity to integrate the generated data into novel models that consider both individual krill and their predators in their biological and physical environment, enabling a holistic approach to ecosystem change. Unravel the controls on krill recruitment: The recruitment term is central to population models used by fishery managers.
Correlation and model studies have suggested which environmental conditions and krill behaviors favor recruitment. However, surveyed numbers of juvenile krill are far below those required to explain the patterns in abundance of adults. Recommendations: Use continuous plankton recorders CPR 95 and ferry boxes 96 on year-round operating fishing vessels to estimate the abundance and distribution of juvenile krill and to estimate food availability, respectively.
These data, added to efforts to rescue and compile existing krill data, will build the long spatial-temporal time series essential to understand the crucial early life stage. Combine these efforts with remote sensing information on sea ice and new ocean color products, to reveal the physical environmental factors, driving krill recruitment. Resolve the debate over whether krill populations have declined: Krill fishery management does not explicitly factor in the effects of long-term warming of the SW Atlantic sector and projected adverse consequences for krill.
Recent debate around estimates of krill population trends has led to a lack of clarity for all stakeholders. Understanding whether krill have been impacted by, or are resilient to, the known ecosystem changes in the last 90 years is necessary to inform future projections. Addressing this issue will identify knowledge gaps and direct research efforts. Recommendations: Workshop-based dialogue among krill researchers on what the various time series methods and models tell us about change in the krill-based food web.
This is needed to provide a clear synthesis of diverging opinions on krill trends in the context of differences, for example, in time series length and location, and indices based on biomass, abundance, population structure or predators.
Moving forward, the scientific community needs to provide expert opinion on how modern krill sampling methods, such as moored echosounders, gliders and the fishery itself, inform us about trends and harmonize the needs of science and spatially-resolved fisheries management.
Given that the fishery is now concentrated on the shelf during autumn and winter, it may mean that fishing pressure on the spawning portion of the population is higher than previously thought.
Recommendations: Pursue a holistic analysis of krill maturity stage data from the fishery collected by observers , land-based krill predators and scientific research cruises. Krill-dependent predators seals, penguins attached with depth sensors and GPS instruments as well as cameras and accelerometers , , , along with stomach content analyses, can provide information on the location of mature krill females.
Data focused on resolving where and when spawning hotspots occur can be used to assess the fishery risk to the krill stock. Identify seasonal overlaps between the fishery and successful spawning stock: Adult krill distribution shifts from mainly off-shelf in summer to on-shelf in winter, where the present-day fishery effort is focused.
However, the timing of this migration and the mechanisms behind it, e. Understanding the mechanism of this behavior and what portions of the population are involved, are critical to understanding the impact of the on-shelf fishing on this winter krill population. Therefore, interim catch limits especially in Subarea Recommendations: Workshop-based dialogue among krill researchers, funding agencies and fishing industry to coordinate existing research and sampling platforms by national programs and fishing industry to enable year-round operations of acoustic equipment on moorings 97 and gliders 98 to estimate the biomass and migration of krill between regions and seasons, and use fishery derived demographics of the krill catch to understand what part of the population is removed by fishing.
Future-proof fishery management for climate change: Model projections suggest future reductions in conditions that are favorable to krill, and contractions of suitable habitat.
In particular we need to ensure that catch limits remain appropriate even in years of climatic extremes or step-changes, since these are projected to increase. Trathan, P. Siegel, V. Atkinson, A. A re-appraisal of the total biomass and annual production of Antarctic krill. Deep Sea Res. However, small but biologically important fisheries for some fish, such as the toothfish, still exist.
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All resources. What is being done to ensure the sustainable use of the Southern Ocean, especially in the Convention Area? Look at the page on this site about The Antarctic Treaty system. Commercial fishing targets nototheniids, laternfish and icefish. Bycatch: bird species such as albatross drown when caught in the lines. Krill Fishing for krill began in the s. Management of the Conservation Area The aims of CCAMLR are to: Conserve marine life of the Southern Ocean to maintain the ecological balance between harvested and other marine populations and restore depleted populations.
Conservation measures CCAMLR has introduced a number of conservation methods to try and monitor and preserve fish stocks. Here are some of the main methods used: Boats need to be licensed to operate in the convention area.
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