CCAMLR

Giant petrels (Macronectes spp) are the most sexually dimorphic of all seabirds. We used satellite-tracking and mass change during incubation to investigate the influence of sexual size dimorphism, in terms of the intersexual food competition hypothesis, on foraging and fasting strategies of northern giant petrels at South Georgia. Females foraged at sea whereas males foraged mainly on the South Georgia coast, scavenging on seal and penguin carcasses. Foraging effort (flight speed, distance covered, duration of foraging trips) was greater for females than for males. In contrast, foraging efficiency (proportionate daily mass gain while foraging) was significantly greater for males than for females. Females were significantly closer to the desertion mass threshold than ales and could not compensate for the mass loss during the incubation fast while foraging, suggesting greater incubation costs for females than for males. Both sexes regulated the duration and food intake of foraging trips depending on the depletion of the body reserves. In males the total mass gain was best explained by mass at departure and body size. We suggest that sexual segregation of foraging strategies arose from size-related dominance at carcasses, promoting sexual size dimorphism. Our results indicate that sex-specific differences in fasting endurance, contest competition over food and flight metabolic rates are key elements in maintenance of sexual size dimorphism, segregating foraging strategies and presumably reducing competition between sexes.

New Zealand conducted Longline weighting trials during the 1998/99 fishery for Dissostichus spp. in Sub-area 88.1. Daylight setting was allowed south of 65°S in Sub-area 88.1 under special provisions in Conservation Measure 169/XVII. The provisions included strictly applied line-weighting regimes and rigorous monitoring of line sink rates.
In the trials, two autoline vessels set weighted longlines in Statistical Sub-area 88.1 between 30th December 1998 and 27th February 1999. Time depth recorders were used to record the actual sink rate achieved on 30% of all sets. For all sets combined the average sink rate achieved was greater than the minimum required standard of 0.3 m/sec (mean = 0.37 m/sec, 1 s.d. = 0.06, n = 74). Observer coverage was 31% of all hooks hauled and 100% of all sets during the trials. During the trials, zero seabird mortalities were recorded. The need for further line-weighting trials and the development of new line-weighting technologies is recommended.

A revised bathymetric map of the South Orkney Islands (Subarea 48.2) was generated using several integrated bathymetric databases and newly available acoustic seafloor data. The region extends from 60.2ºS-62.2ºS latitude and 42.5º-47.5º longitude. From the integrated data sets, areas of seabed within the 500 m isobath were computed for 50 m depth intervals. Areas were calculated based on interpolated surface area of seabed incorporating seafloor slope. These results were compared to the previously reported estimates of Everson (1997) for the 50-150, 150-250, and 250 to 500 m depth intervals. The updated estimates are about 1,424 (20%) nautical miles larger in area within the 50-500 m isobaths than Everson’s estimates, though changes in area are specific to the depth interval. There is a corresponding change in estimated biomass within strata when these areas are incorporated into swept area trawl survey models, though not for total estimated biomass. Of the nine species examined, the point estimate of total biomass increased from 5%-30% for eight species and decreased 20% for one.

Stocks of finfish around the South Orkney Islands (Subarea 48.2) suffered substantial declines during the period the fishery was open from split year 1977/78 through 1989/90. Scientific bottom trawl surveys of finfish biomass within the 500 m isobath of the South Orkney Islands have been conducted by the Federal Republic of Germany in 1985, Spain in 1991, and the United States in 1999. Estimates of total stock biomass were computed for eight species that comprised 98% of survey nominal catch. Biomass levels in March 1999 were compared to previous trawl surveys conducted in Feb. 1985, and Jan-Feb 1991. Species examined were Gobionotothen gibberifrons, Lepidonotothen squamifrons, Pseudochaenichthys georgianus, Champsocephalus gunnari, Chaenocephalus aceratus, Chionodraco rastrospinosus, Notothenia rossii and Lepidonotothen larseni. Although there is substantial variability in point estimates, biomass levels of most species appear to be unchanged or may have declined slightly since 1991. The stock of C. gunnari is currently extremely low, while there appears to be a strong signal of recovery for N. rossii. However, overall levels of biomass indicate very little potential for commercial exploitation at this time.

In 1998, an offshore scientific trawl survey of bottom fish sampling within the 50-500 m isobath of the lower South Shetlands Islands (King George Island to Low Island) was conducted. In addition, the abundance of two commercially important Antarctic fish, Notothenia rossii and Gobionotothen gibberifrons has been monitored relative to another potentially exploitable fish, Notothenia coriiceps, in the lower South Shetlands Islands from inshore sites mainly at Potter Cove from 1983 to 1999. These studies have been conducted using trammel nets sampling bottom depths from 5 to 50 m. Information from this trawl survey was compared to the 1998 Potter Cove data to examine potential inshore-offshore relationships for these three species. In general, these two data sets are complementary, and demonstrate well-defined changes in size that take place between inshore and offshore sampling. By combining samples collected from a single year, the sample size and regression range available for length-weight relationships can be increased. However, the value of these data as a combined singular data set for one year of sampling is limited. A future offshore survey coupled with inshore sampling would provide substantially more information, and allow a more direct comparison of the two data sets to be realised, particularly with respect to trends between the three species.

A monitoring program of demersal fish in inshore sites of the South Shetland Islands has continued in Potter Cove from 1991 to 1999, covering a continuous sampling period of sixteen years and in Harmony Cove, Nelson Island, in the austral summer 1995/96. The decline in trammel net catches of fjord fishes of the species Notothenia rossii and Gobionotothen gibberifrons in relation to the non commercially fished Notothenia coriiceps, which was already reported for the period 1983-1990 in a previous study, is still evident. An increasing trend of N. rossii catches was observed, but the actual levels of relative abundance of this species and G. gibberifrons are well below those found in the early 80´s. These results are supported by our knowledge on the diet of the piscivorous Antarctic shag Phalacrocorax bransfieldensis in the South Shetland/Antarctic Peninsula area in this decade. The most likely explanation for the decrease in recruitment to the inshore sub-populations of N. rossii and G. gibberifrons in the last sixteen years is the effect of the offshore commercial fishery in the area in the late 1970s. This interpretation is consistent with the information on the historical offshore commercial fishing and with the results of scientific surveys in the area.

We used data collected by CCAMLR International Scientific Observers in 1997 and 1998 to examine potential relationships between seabird incidental mortality rates on longline vessels fishing for Dissostichus eleginoides and various factors, including the nature and use of mitigating measures as well as environmental variables such as time of day, time of year.
Out of 3283 longline sets analysed only 311 caught birds (9.4%). Data conformed most closely to a Delta distribution (many zero values and log-normal distribution of non-zero values) and were analysed using two Generalised Linear Models, a binomial model for presence/absence of seabird catches and a Gamma model for the magnitude of non-zero catches.
Sparsity of data precluded analysis of seabirds at a taxon level more detailed than albatrosses and petrels combined. Other analytical difficulties, particularly in using generalised linear models, related to the large number of potentially important factors, the lack of overlap between factors and the fact that fishing has purposely avoided making catches of seabirds. There are, for instance, only three records in the entire dataset where none of the mitigation measures have been used.
The only factors consistently significant were time of year (very few birds caught after April) and use of streamer lines but the effects of most other factors cannot be fully analysed with the present data. Even vessels using streamer lines and setting at night were found to catch albatrosses occasionally. The “residual” mortality associated with using all prescribed mitigation measures provides a more useful indication of the success of mitigation measures than general bird bycatch rates.
Given the difficulties of analysing this dataset, especially the problem of very low numbers of hauls not using mitigation measures and hauls catching birds, experimental approaches to identifying effective mitigation measures may be preferable to post-hoc analysis of observer data.