CCAMLR

In response to concern about rajid bycatch in CCAMLR finfish fisheries, we conducted a preliminary analysis of data collected by UK scientific observers on vessels fishing toothfish in Subarea 48.3 from April to July 1999. The overall average catch rate of rays was 0.7/thousand hooks, compared with 34.7/thousand hooks for toothfish and 2.2/thousand hooks for grenadier species. Generalised linear models demonstrated that there are significant differences between the catch rates of rays for different vessels, areas, and depths in Subarea 48.3. Some vessels, fishing on the northern shelf edges of both Shag Rocks and South Georgia, achieved average catch rates of over 1 ray/thousand hooks, and 20-30 rays/ thousand toothfish. Catch rates were lowest to the south and east of South Georgia. The two species most frequently found on hooks were R. georgiana and B. murrayi. R. georgiana was found in all areas, B. murrayi was not found in this study to the east or south of South Georgia. Modal length of R. georgiana caught was 65-69cm for males and 80-84cm for females, for B. murrayi was 65-69cm and 70-74cm for males and females respectively.

We tracked by satellite the foraging trips of males and females of the two sibling species of giant petrels, Macronectes halli and M.giganteus, breeding sympatrically at Bird Island (South Georgia, Antarctica), during the incubation period (November-December). Size of the activity range as well as speed and distance covered on foraging trips were similar between species but lower for males than females in both species. Sex differences agree with previous observations on diet and on attendance at seal carcasses, suggesting that females mainly forage at sea, whereas males mainly scavenge on the coast. Overall, however, foraging ecology of both species seems very similar. Inter-specific competition may be reduced by the limited overlap in the activity range, with southern giant petrels foraging further south than northern giant petrels, suggesting some spatial partitioning in foraging areas. Male northern giant petrels foraged almost exclusively on the South Georgia coast; their strong dependence during the breeding season on fur seals, which have increased exponentially in recent years, may be ref1ected in their recent population increase at South Georgia. Foraging areas of giant petrels overlapped extensively with longline fishery distribution, confirming their susceptibility to being caught on longline hooks. Females were at higher risk during the study period since they made longer trips and further west than males, into areas where local longline fisheries are more active.

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.