CCAMLR

The CEMP Standard Methods for monitoring foraging trip duration in Macaroni Penguins recommend using only males. However, males attend the chicks at the nest during most of the brooding period, while the females make frequent foraging trips. We analysed data from 17 males instrumented with VHF-transmitters in the middle of the brooding period (chick age 12-18 days) and 17 females instrumented at the start of the creching period (chick age 25-30 days). Males made their first foraging trips at the start of the creching period. The first trip usually lasted several nights, whereas the subsequent trips were shorter (average 21.8 h). Female foraging trips averaged 27.2 h. No apparent changes in the foraging trip duration of either sex as a function of date were recorded during the creching period. Female attendance periods in the colony where shorter (average 11.9 h) than those of males (average 21.4 h), and females made more foraging trips than males. These results suggest that females might be more appropriate than males as the subjects when monitoring foraging trip duration. In addition, the fact that females spent more time foraging and conducted more foraging trips than males might suggest that female foraging trip duration is more sensitive to changes in prey availability.

The impact of midwater trawl selectivity and catchability upon TS and TS 1 kg estimates in echosurveys is discussed. It is known that krill abundance and biomass estimation should be considered as two independent tasks with different requirements to midwater trawl utilization in echosurveys. Application of the proposed method of mean weighted TS 1 kg estimation provides the biomass estimation determined actually only by MSBS (or MVBSS) acoustic parameter distribution and independent on the midwater trawl selectivity and catchability. Krill abundance estimation related to TS target strength values requires another approach to echosurveys, since two actually independent task should be solved in that case: assessment of MSBS (MVBS) acoustic parameter distribution and krill length (weight) distribution in the study area. The latter requires to settle a complex of problems which allow to consider the trawl as a measuring system. However, the information obtained in that case provides not only total krill abundance value but also krill biomass and abundance by length classes as well as other demographic researches of krill.

The relation between mid-water trawl catchability and its construction, trawling regime, krill aggregations distribution in the study area is discussed. It is shown that while the trawl selectivity affects krill length composition in each trawl sample, the trawl catchability affects the estimation of total krill lengths distribution, obtained on the basis of trawl samples from different catched in the study area. Several approaches to trawl samples processing are discussed. It is shown that the total distribution of krill length, obtained using weighting of trawl samples (usually 200 ind.) against the catch, may considerably differ (statistically significantly) from that calculated on the basis of trawl samples weighting against the catch corrected taking in account the trawl catchability for each trawling. The example of trawl samples processing for the study area off Elephant Island is presented. Difference of total krill lengths distribution, calculated using different methods, results in respective difference of density estimates by 48%.

Comparative analysis of krill length distribution in the echosurvey area, obtained with trawl and acoustic methods is presented. Besides, the acoustic estimate of krill length composition was obtained on the basis of the empiric model by Greene et al. (1991) using estimates of target strength in situ at the frequency of 120 kHz as measured with echosounder EK-500 (split-beam) principle). Krill was caught with the commercial trawl RT 741448. It was shown that differences between krill length distribution predicted from the acoustic model by Greene et al. (1991) and those from trawl catches were considerable (statistically significant). Besides, krill length estimates, obtained with acoustic method, were significantly lower (20-25%) those obtained from catch analysis. Utilization of trawl and acoustic methods of krill length estimation in echosurveys implemented in practice as target strength (TS) in situ measurement with EK-500 and calculation of TS talc. on the basis of krill length composition in catches stipulates significant difference of krill density estimates obtained for the same population. In some cases such difference reached 200%. Significance of trawl samples in assessment of krill length composition during echosurveys was shown.