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.
Abstract:
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.
There is no abstract available for this document.
Abstract:
The factors affecting the number and the mortality of seabirds attending long-liners and trawlers fishing in the Kerguelen area were studied during four successive seasons (1994-1997), based on observations carried out onboard by dedicated observers. Twenty-four species of seabirds were observed attending fishing vessels representing 591 birds / census. The total numbers attending varied mainly according to the year, the cloud cover and the presence of offal for long-liners. The dumping of offal increased the numbers of birds attending the vessel, especially when offals can be handled easily by birds. The activity of the vessels also affected the numbers attending, birds being more abundant during line setting and during trawl hauling. The white-chinned petrel was the most abundant ship-following seabird followed by Black-browed albatrosses, giant petrels and cape pigeon. The number of white-chinned petrels, black-browed and grey headed albatrosses attending fishing vessels increased through the season whereas it was the reverse situation for giant petrels and cape petrels. Four species of birds were caught by fishing gears mainly by long-lines, in order of importance white-chinned petrels, black-browed, grey-headed and wandering albatrosses. Taking into account the number of birds from each species attending long-liners and known to be potential by-catch, it appears that some species appear to be more susceptible of being caught than others. White-chinned and grey-headed albatrosses appear to be caught in much larger proportion than the number of potential by-catch present, whereas black-browed are caught in lower numbers. Giant petrels are abundant around long-liners but were never caught. In long-liners, most birds were killed when the lines were set during the day or when the deployment of the scaring device was not successful with an overall figure of 0.47 birds / 1.000 hooks. No albatross except one was caught when the lines were set during the night. The white-chinned petrel represented 92.2 % of all birds killed by long-liners. The number of birds caught varied significantly between months and between years. The type of bait used also affected the catch rate. The catch rate was related to the number of birds attending the long-liner only for black-browed albatrosses. Most birds killed by trawlers were caught by the netsonde cable. The efficiency of mitigation measures in order to reduce seabird mortality is discussed and it is stressed that night setting is the most efficient method way to reduce mortality and should be enforced everywhere when possible. However further methods should be developed to reduce the mortality of species active at night, especially the white-chinned petrel whose populations in the Indian ocean may by threatened by long-line fisheries.
Abstract:
Grey-headed mollymawks Thalassarche chrysostoma are killed by longline fishing operations in the southern Indian Ocean. We studied the foraging ecology of breeding grey-headed mollymawks at Marion Island, by tracking their foraging trips and sampling their diets. During the incubation period, birds made longer foraging trips, mostly towards the subtropical convergence and Subantarctic zone. This brought them into contact with areas of intense Southern Blue-fin Tuna Thunnus maccoyii longline fishing. During the early post-guard phase, foraging trips were shorter and to the southwest of the island, in the Polar frontal and Antarctic zones. Short foraging trips (
Abstract:
The effect of an intensive sampling programme on an inshore population of Notothenia coriiceps was studied at Potter Cove, South Shetland Islands, by comparing catch data taken in successive summers of 1992/93 to 1994/95 at one specific zone (site 1) with those taken in the same last summer at two close but not previously sampled zones (sites 2 and 3). The fish were caught with trammel nets under similar sampling conditions (depth, net measurements, bottom type). In site 1, a marked decrease in length (TL) of the fish was observed throughout the whole period. The fish from sites 2 (x=32.4 cm) and 3 (x=31.8 cm) exhibited no significant differences in mean length. They were significantly larger than those from site 1 caught in the summers of 1994/95 (x=28.8 cm) and 1993/94 (x=30.2 cm), but were similar in size to those sampled in the summer of 1992/93 (x=31.7 cm), just when the sampling programme started in site 1. Present results show that the size variations of N. coriiceps observed at Potter Cove were not due to a natural decrease of the proportion of larger fish in the population, but related to an intensive sampling effort carried out at one specific site.
There is no abstract available for this document.
There is no abstract available for this document.
Abstract:
The foraging range and principal feeding areas of White-chinned Petrels breeding at South Georgia were determined using satellite telemetry. Foraging trips during incubation lasted 12-15 days and covered 3,000-8,000 km and 2-11 days and 1,100-5,900 km during chick-rearing. Adults covered less distance per day during chick-rearing (71 km) than during incubation (91 km) but the proportion covered at night (47%) was the same. Mean (31 - 34 km/h) and maximum (80 km/h) flight velocities were similar during both periods of the breeding season and during day and night. Between incubation shifts, White-chinned Petrels travelled to the Patagonian shelf; during chick-rearing they forage more extensively. Most locations were between 30° to 55 °W and 52 °to 60 0W around South Georgia/Shag Rocks and south to the South Orkney Islands. Diet samples from known foraging locations suggested birds fed mainly on krill and squid. They caught the squid Brachioteuthis ? picta and Galiteuthis glacialis, around Shag Rocks/South Georgia and also at sites close to the South Orkney Islands; Illex argentinus on the Patagonian shelf. Dispersal of adults after breeding failure was south to the South Orkney Islands then west to the Falkland Islands. This study confirms that breeding White-chinned Petrels are amongst the widest-ranging of seabirds; they may minimise competition with other Procellariiformes in the South Atlantic by their more extensive foraging range. The nature and extent of their range also brings substantial risk of high mortality rate in South Atlantic long-line fisheries.
Abstract:
We develop a new approach to quantifying habitat use within the foraging ranges of satellite-tracked seabirds. We applied kernel estimation techniques to 167 days (3738 locations) of data from black-browed and grey-headed albatrosses (Diomedea melanophris and D. chrysostoma) during the chick rearing period of the breeding cycle at South Georgia. At this time the activity range of these two species covers an estimated 440,000 and 640,000 km2 respectively, with very substantial overlap. In contrast, kernel estimation reveals that the main foraging areas of these two sympatric, congeneric species are very distinct. Based on location density categories accounting for 50 % of locations, the foraging areas cover c.81,500 and c.119,700 km2 respectively, with 42 % and 50 % of the range of one species overlapping with that of the other.