CCAMLR

Swarming is a fundamental part of the life of Euphausia superba, yet we still know very little about what drives the considerable variability in swarm shape, size and biomass. We examined swarms across the Scotia Sea in January and February 2003 using a Simrad EK60 (38 kHz, 120 kHz) echosounder, concurrent with net sampling. The acoustic data were analysed through applying a swarm-identification algorithm and then filtering out all non-krill targets. The area, length, height, depth, packing-concentration and inter-swarm distance of 4525 swarms was derived by this method. Hierarchical clustering revealed 2 principal swarm types, which differed in both their dimensions and packing-concentrations. Type 1 swarms were generally small (< 50 m long) and were not very tightly packed (< 10 ind m-3) whereas type 2 swarms were an order of magnitude larger and had packing concentrations up to 10 times greater. Further sub-divisions of these types identified small and standard swarms within the type 1 group and large and superswarms within the type 2 group. A minor group (swarm type 3) was also found, containing swarms that were isolated (>100 km away from the next swarm). The distribution of swarm types over the survey grid was examined with respect to a number of potential explanatory variables describing both the environment and the internal-state of krill (namely maturity, body length, body condition). Most variables were spatially averaged over scales of ~100 km and so mainly had a mesoscale perspective. The exception was the level of light (photosynthetically active radiation, PAR) for which measurements were specific to each swarm. A binary logistic model was constructed from four variables found to have significant explanatory power (P<0.05): surface fluorescence, PAR, krill maturity and krill body length. Larger (type 2) swarms were more commonly found during nighttime or when it was overcast during the day, when surface fluorescence was low, and when the krill were small and immature. A strong pattern of diel vertical migration was not observed although the larger and denser swarms tended to occur more often at night than during the day. The vast majority of krill were contained within a minor fraction of the total number of swarms. These krill-rich swarms were more common in areas dominated by small and immature krill. We propose that, at the mesoscale level, the structure of swarms switches from being predominantly large and tightly-packed to smaller and more diffuse as krill grow and mature. This pattern is further modulated according to feeding conditions and then level of light. (Deep-Sea Res. I, 56 (2009): 1994–2012)

We studied the relationship between the proximity of land and the distribution and swarming characteristics of Antarctic krill across the Scotia Sea in January and February 2003. Krill swarms identified with a Simrad EK60 (38 kHz, 120 kHz) echosounder were grouped into 4 categories according to distance from shoreline: 0 to 50 km, 50 to 100 km, 100 to 200 km and 200 to 500 km. Cross-sectional areas of swarms were significantly larger inshore, with a mean value of 120 m² in the 0 to 50 km zone compared to <80 m² further offshore. The packing concentration of krill within inshore swarms was also significantly greater, with an average density of 95 ind. M–3 compared to between 24 and 31 ind. M–3 elsewhere. A large proportion of the biomass was concentrated into a small number of large, dense swarms throughout the survey area, and this trend increased with decreasing distance from shore. The highest median number of swarms per km and krill acoustic biomass per km was found in the 50 to 100 km zone. However, a significantly greater number of large, biomass-rich swarms occurred in the 0 to 50 km zone compared to all other zones. Swarms in the 0 to 50 km zone were also significantly further apart. The majority of swarms were located in the upper 50 m during the daytime although they were marginally deeper in the night in offshore regions. Krill are likely to move between inshore and offshore environments continuously over their lifetimes. The change in krill behaviour between environments could be a response to local predatory threats over short spatial and temporal scales. (Mar. Ecol. Prog. Ser., (in press))

The South Georgia region supports a large biomass of krill that is subject to high interannual variability. The apparent lack of a locally self-maintaining krill population at South Georgia means that understanding the mechanism underlying these observed population characteristics is essential to successful ecosystem-based management of krill fishery in the region. Krill acoustic-density data from surveys conducted in the early, middle and late period of the summers of 2001 to 2005, together with krill population size structure over the same period from predator diet data, were used with a krill population dynamics model to evaluate potential mechanisms behind the observed changes in krill biomass. Krill abundance was highest during the middle of the summer in 3 years and in the late period in 2 years; in the latter there was evidence that krill recruitment was delayed by several months. A model scenario that included empirically derived estimates of both the magnitude and timing of recruitment in each year showed the greatest correlation with the acoustic series. The results are consistent with a krill population with allochthonous recruitment entering a retained adult population; i.e. oceanic transport of adult krill does not appear to be the major factor determining the dynamics of the adult population. The results highlight the importance of the timing of recruitment, especially where this could introduce a mismatch between the peak of krill abundance and the peak demand from predators, which may exacerbate the effects of changes in krill populations arising from commercial harvesting and/or climate change. (Mar. Ecol. Prog. Ser., 399 (2010): 243–252)

Antarctic fur seals Arctocephalus gazella and macaroni penguins Eudyptes chrysolophus are the two main land-based krill Euphausia superba consumers in the northern Scotia Sea. Using a combination of concurrent at sea (predator observations, net hauls and multi-frequency acoustics), and land-based (animal tracking and diet analysis) techniques, we examined variability in the foraging ecology of these sympatric top predators during the austral summer and autumn of 2004. Krill availability derived from acoustic surveys was low during summer, increasing in autumn. During the breeding season, krill occurred in 80% of fur seal diet samples, with fish remains in 37% of samples. Penguin diets contained the highest proportion of fish in over 20 years of routine monitoring (46% by mass; particularly the myctophid Electrona antarctica), with krill (33%) and amphipods (Themisto gaudichaudii; 21%) also occurring. When constrained by the need to return and feed their offspring both predator species foraged to the northwest of South Georgia, consistent with an area of high macrozooplankton biomass, but fur seals were apparently more successful at exploiting krill. When unconstrained by chick-rearing (during March) penguins foraged close to the Shag Rocks shelf-break, probably exploiting the high daytime biomass of fish in this area. Penguins and seals are able to respond differently to periods of reduced krill abundance (in terms of variability in diet and foraging behaviour), without detriment to the breeding success of either species. This highlights the importance of myctophid fish as an alternative trophic pathway for land-based predators in the Scotia Sea ecosystem. (Mar. Biol., 157 (1) (2009): 99–112)

The horizontal and vertical distributions of larvae and reproductive timing of euphausiids were investigated in the Ross Sea and its adjacent waters during austral summer 2004–2005. Occurrences of larvae of Euphausia frigida and E. triacantha were confined to the northern oceanic area where the Upper Circumpolar Deep Water prevailed, although their juvenile and adult stages extended the distribution further southward to the area where cooler Lower Circumpolar Deep Water prevailed. Larvae of Thysanoessa spp. were widely distributed within the oceanic to slope areas but did not occur on the shelf as juvenile or adult stages. Eggs and larvae of E. superba occurred with gravid females along the slope, but no juveniles occurred concurrently. Thus the slope does not appear to be a nursery ground for this species. The distribution of E. crystallorophias larvae was mostly confined to the shelf in the juvenile and adult stages. The onset of deepening was from early and later frucilia stages onward for E. triacantha and E. frigida, respectively. However, Thysanoessa spp. were concentrated increasingly within the surface layers from furcilia I onward. The onset of recruitment to calyptopis I appeared to be earlier in the more northern species with the exception of E. crystallorophias, which recruited before E. superba. The intense spawning of E. crystallorophias and E. superb coincided with a period of development of a coastal polynya. Generally, the onset of spawning of euphausiids appeared to be related to the latitudinal distribution and timing of sea-ice melting. Relationships between surface temperatures and development and recruitment of larvae of euphausiids are discussed. (Plankton and Benthos Res., 4 (4) (2009): 135–146)

Breeding numbers of Laridae and other surface-nesting seabirds have been monitored at subantarctic Marion Island since 1996/97 and counts of breeding birds were made at nearby Prince Edward Island in December 2001 and December 2008. Four species are regular breeders at the islands: subantarctic skua Catharacta antarctica, kelp gull Larus dominicanus, Antarctic tern Sterna vittata and Kerguelen tern S. virgata. The latter three species currently each have populations of below 150 breeding pairs at the islands. Kelp gull numbers appear to be relatively stable though they may have decreased since the 1980s. Kerguelen tern numbers decreased and then recovered at Marion Island but numbers of the species have declined at Prince Edward Island and it maintains a tenuous foothold there. The small Antarctic tern population appears to be stable. (Afr. J. Mar. Sci, 31 (3) (2009): 439–444)

The second mid-summer survey of surface-nesting seabirds at the Prince Edward Island group (Marion and Prince Edward islands) was conducted during December 2008, seven years after the initial mid-summer survey. Wandering albatrosses Diomedea exulans may have decreased slightly at Prince Edward Island, mirroring a decrease of roughly 2% per year at Marion Island from 1998 to 2005, a decline that has since reversed. Numbers of grey-headed albatrosses Thalassarche chrysostoma on Marion Island have remained stable, whereas the population on Prince Edward Island decreased by 20% from 2001 to 2008 (3% per year). The estimate of Indian yellow-nosed albatrosses T. carteri at Prince Edward Island was similar in 2001 and 2008. Counts of both sooty albatrosses Phoebetria spp. were substantially higher at Prince Edward Island in 2008, possibly as a result of better coverage compared to 2001. Dark-mantled sooty albatrosses P. fusca on Marion Island have decreased by almost 2% per year since 1996, continuing a negative trend from the early 1980s, whereas light-mantled sooty albatrosses P. palpebrata have increased by almost 6% per year at Marion Island since 1996. Counts of both giant petrels increased at Prince Edward Island (northern Macronectes halli by 44%; southern M. giganteus by 28%), whereas their numbers have remained stable at Marion Island. Current best estimates for annual breeding populations (pairs) at the two islands are 3 650 wandering albatrosses, 9 500 grey-headed albatrosses, 7 000 Indian yellow-nosed albatrosses, 2 900 dark-mantled sooty albatrosses, 800 lightmantled sooty albatrosses, 750 northern giant petrels and 2 800 southern giant petrels, confirming the global importance of the Prince Edward Islands for these seven species. Apart from the dark-mantled sooty albatross, their populations are reasonably healthy despite fishing mortality. (Afr. J. Mar. Sci., 31 (3) (2009): 409–417)

Subantarctic skuas Catharacta antarctica are key predators of burrowing petrels at sub-Antarctic islands, and can be used to monitor the health of burrowing petrel populations. A survey of skuas at the Prince Edward Islands was conducted during December 2008, repeating a previous survey in December 2001. Prince Edward Island (46 km²) remains free of introduced mammals, whereas Marion Island (290 km²) had a feral population of cats from the 1950s to 1980s, and still supports a large population of introduced house mice Mus musculus. Breeding skuas were more widespread, occurred at greater densities and extended to higher elevations at Prince Edward Island than Marion Island. Prince Edward also supported twice as many non-breeding birds. Burrowing petrels comprised 96% of prey in skua middens at Prince Edward Island compared to only 22% on Marion Island where penguins are more important. The numbers of breeding pairs at Prince Edward Island increased from 2001 to 2008, probably as a result of better coverage in 2008, whereas the number of skua nests on Marion Island was barely half that counted in 2001, continuing an apparent decrease in this species at Marion Island since the 1980s. There is no evidence that removal of cats from Marion Island in the early 1990s has benefited the major native predator of burrowing petrels. (Afr. J. Mar. Sci., 31 (3) (2009): 431–437)

Numbers of Crozet shags Phalacrocorax [atriceps] melanogenis breeding at Marion Island decreased by more than 70% from 840 pairs in 1994/95 to 220 pairs in 2003/04 and then increased to some 500 pairs in 2008/09. The trends are thought to have been influenced by breeding success, which averaged 0.30 and 0.66 chicks per pair per year from 1998/99–2002/03 and 2003/04–2008/09, respectively. There were similar trends in numbers breeding and breeding success of gentoo penguins Pygoscelis papua, which at Marion Island have a similar diet to Crozet shags, suggesting that both species may have been influenced by food availability. Numbers of Crozet shags breeding at Prince Edward Island approximately doubled between the summers of 2001/02 and 2008/09. In 2008/09, some 600 pairs of Crozet shags were breeding at the Prince Edward Islands. (Afr. J. Mar. Sci., 31 (3) (2009): 427–430)

Four species of penguin breed regularly at South Africa’s Prince Edward Islands: king penguin Aptenodytes patagonicus, gentoo penguin Pygoscelis papua, macaroni penguin Eudyptes chrysolophus and southern rockhopper penguin E. chrysocome. In December 2008 it was estimated that some 65 000 pairs of king penguins were incubating eggs at Marion Island, the larger of the two islands in the group, and 2000 pairs at Prince Edward Island. At Marion Island from 1987–2008, there was no long-term trend in numbers of king penguin chicks that survived to the end of the winter period, but there was considerable fluctuation in chick production in the 1990s. It was roughly estimated that on average 88% of king penguin chicks survived the winter period (from April to September/October). Numbers of gentoo penguins at Marion Island decreased from more than 1300 pairs in the mid 1990s to fewer than 800 pairs in 2003, and then increased to almost 1100 pairs in 2008 as breeding success improved. Between 1994/95 and 2008/09 numbers of macaroni and southern rockhopper penguins at Marion Island decreased by about 30% and 70%, respectively. In 2008/09, some 290 000 pairs of macaroni penguins bred at this island, mostly in two large colonies where there was a progressive decrease in the density of nests. At both these colonies decreases in numbers breeding followed outbreaks of disease. Inadequate breeding success has influenced the decreases of macaroni and rockhopper penguins. In 2008/09, some 42 000 pairs of southern rockhopper penguins bred at Marion Island and 12 000 pairs of macaroni penguins and 38 000 pairs of southern rockhopper penguins at Prince Edward Island. (Afr. J. Mar. Sci., 31 (3) (2009): 419–426)