CCAMLR

Data on Antarctic petrel Thalassoica antarctica population and demography monitoring at Svarthamaren, Dronning Maud Land from five field seasons between 1991/92 and 1996/97 is presented. The results of the population monitoring show that the number of pairs with chick or egg varied between 55,387-178,240 (mean 119,000). Assuming that about 10 % of the colony is situated in inaccessible parts outside the area covered by the monitoring system the mean number of pairs producing a chick each year is about 132,000. On average 52% of the reproductive fraction of the population breeds each year indicating that the total number of reproductive birds attached to the colony is approximately 510,000 individuals. Annual survival rates varied between 0.871-0.954 (mean 0.924). Our results suggest that a monitoring programme for breeding Antarctic Petrels should include estimation of mortality rates and breeding frequency in addition to the numbers of birds breeding.

Target strength measurements of free swimming krill at 120kHz were made using a single beam monostatic system in a 10 m3 laboratory tank. Krill (grouped according to length classes) swam freely in the tank triggering the data acquisition system when generating a back scattered signal larger than a threshold, determined by the system noise level. Dorsal and ventral target strength estimates were calculated indirectly by deconvolution of the cumulative probability function of echo ensembles of single animal insonifications. For mean length classes in the range [29.6, 36.2] mm the median single-animal target strengths are in the range [-76.7,-71.8] dB. Monte Carlo computer simulations were used to evaluate the effects of varying the ratio of largest to smallest echo amplitudes for a given ensemble, thus enabling the estimation of threshold induced bias in the target strength estimates. The threshold induced bias was then determined for each ensemble of experimental data and used to determine corrections which were in the range of [-0.84, -0.33] dB. An error analysis of the target strength estimates detailing the components due to measurement accuracy and precision, and the indirect signal processing techniques used is also presented.

The results of acoustic surveys, especially in low signal-to-noise regimes, are sensitive to the method used to exclude the contribution of background noise from the acoustic data. The use of thresholds to reject weak signals during data collection is inappropriate because the noise contribution to the above-threshold-signal cannot later be removed in post processing. The only solution is to log unthresholded data and to correct for noise in post processing. Integrated data collected at 120 kHz with a Simrad EK500 are presented to demonstrate the highly variable nature of background noise. A dynamic method to determine background noise for individual integration intervals was found to be better than several methods that apply a single background noise level to a data set. The dynamic technique requires that logged data have high resolution in both depth and distance. It can be applied so long as some integration layers in each integration intervals include only noise. Further, it is recommended that the physically meaningful quantity "noise power", being the apparent received power (dB/ 1 W) at the transducer due to noise, is used to describe background noise levels.

Krill (Euphausia superba) provide a direct link between primary producers and higher trophic levels in the Antarctic marine food web. The pelagic tunicate Salpa thompsoni can also be important during spring and summer through the formation of extensive and dense blooms. Although salps are not a major dietary item for Antarctic vertebrate predators, their blooms can affect adult krill reproduction and survival of krill larvae. Here we provide data from 1995 and 1996 that support hypothesized relationships between krill, salps and region-wide sea-ice conditions. We have assessed salp consumption as a proportion of net primary production, and found correlations between herbivore densities and integrated chlorophyll-a that indicate that there is a degree of competition between krill and salps. Our analysis of the relationship between annual sea-ice cover and a longer time series of air temperature measurements indicates a decreased frequency of winters with extensive sea-ice development over the last five decades. Our data suggest that decreased krill availability may affect the levels of their vertebrate predators. Regional warming and reduced krill abundance therefore affect the marine food web and krill resource management.