CCAMLR

Estimates of the number of seabirds taken incidentally in longline fisheries on Patagonian toothfish (Dissostichus eleginoides) are still based on limited information. We report here on observations on the by-catch of seabirds during a cruise of the Chilean longliner 'Puerto Ballena' in Statistical Subarea 48.3. A total of 118 dead black-browed albatrosses, 3 dead grey-headed albatrosses, 46 dead white-chinned petrels were reported by the observer and crew members. In addition 21 black-browed albatrosses, 4 giant petrels, 1 white-chinned petrel and 1 unidentified penguin became hooked alive and were released alive. Our preliminary analysis confirmed results from last year that black-browed albatrosses are particularly at risk when longlines are set during the day and/or the streamer line is malfunctioning. If setting of lines had been restricted to nighttime as required in order to comply with Conservation Measure 29/XIV(CCAMLR, 1995) the number of black-browed albatrosses and white-chinned petrels taken incidentally would have been substantially lower. The by-catch of both black-browed albatrosses and white-chinned petrels appeared to decline towards the end of April/begriming of May. The estimated total catch for all observed sets was 312 black-browed albatrosses and 213 white-chinned petrels. This estimate should be regarded as tentative as there rue still problems associated with extrapolating observed numbers of dead seabirds from a subsample of hooks to the total number of hooks in a set which warrant further investigation. However, even if we consider the number of dead birds recorded by the observer and by crew members as a minimum estimate, these figures appear by far too high to be sustainable by both black-browed albatrosses and white-chinned petrel populations in the area.

Results of the analysis of three consecutive cruises to subarea 48.3 are analyzed. A significant variance component associated with the formation of clusters of stations was found as well as a general trend for increasing densities since 1994. The significant variance component obtained for clusters formed with stations at 8.0 nautical miles implies that considering all stations as independent (as in the estimation of the variance of the mean density) might result in a gross underestimation of the variance. The spatial pattern of density across years might be applied to construct a model of density variability to minimize the variance of estimates of regional parameters.

A diet analysis of the patagonian toothfish Diissostichus eleginoides in the Subarea 48.3 in March/April 1996 was carried out by frequency of occurrence (F%) and coefficient "Q" (%) methods. The samples consisted chiefly of immature specimens, with predominant length ranges of 30-70 cm (TL). Fish was by far the main food at SR and SG (63 % as sole prey). Krill appeared as secondary food, although its importance was overestimated by the frequency of occurrence method. Cephalopods and mysids occurred scarcely in the stomachs and only at SR and SG respectively. Lepidonotothen kempi, Champsocephalus gunnari and Chaenocephalus aceratus constituted the main fish prey and their variability between SR and SG depended on their local abundance. The large proportion of fish exhibiting stomachs full or close to fullness (together 62 %), suggests that feeding intensity of the species was high.

The diet composition of the icefish Champsocephalus gumari caught in the Subarea 48.3 in March/April 1996 was analyzed using Frequency of occurrence (F%) and coefficient "Q" (%) methods. Krill was by far the main food around South Georgia and Shag Rocks (F= 85%, Q= 99%). Other prey items, known as an important alternative food in years of krill scarcity, were ingested only occasionally (Themisto gaudichaudii, F= 1 8%, Q= 1 % at SG; mysids, F= 11 %, Q= 1.1 % at SR) or in negligible amounts (Thysanoessa sp.). Many empty stomachs (56%) were found in the sampling area, in levels even higher than those reported for the 1991 and 1994 years of krill shortage. The analysis of the ovarian maturation stages showed that a high proportion of the mature fish sampled (79 %) were 1996 spawners, a fact which may be related with a large availability of food (= krill). It is suggested that krill was present in the region in levels equivalent to the historical years of high abundance.

The mackerel icefish (Champsocephalus gunnari) is a prominent member of the coastal fish fauna of the Seasonal Pack-ice Zone and the islands north of it. Separated into a number of stocks, its distribution ranges from the Scotia Arc region, namely South Georgia, in the Atlantic Ocean sector to the Kerguelen-Heard Plateau in the Indian Ocean sector. Mackerel icefish have been heavily exploited since the beginning of the 1970's with reported annual catches exceeding 50- 100 000 tomes in some years.
Champsocephalus gunnari has many characteristics typical of Antarctic fish species with respect to life history characteristics, such as egg size, fecundity and growth. These fall well within the range of other sympatric red-blooded notothenioids: Eggs are large and yolky. Egg diameter, egg production per gram body weight, and growth performance at South Georgia and the Kerguelen Islands was comparable to similar sized nototheniids and channichthyids. Stocks of C. gunnari have a number of biological characteristics in common, such as feeding mode and food, early life history, and growth in the first years of life. However, stocks in Sub-Antarctic waters differ in reproductive characteristics, such as length and age at first spawning, natural mortality and life expectancy from those on more southerly grounds. They may grow to more than 60cm and may become 13-15 years old. However, in the vicinity of South Georgia and the Kerguelen Islands, fish apparently do not exhaust this growth potential fully. Their ability to reproduce at an early age coupled with a comparatively high fecundity and growth performance may mean that the stocks of C. gunnari in Sub-Antarctic waters have far more resilience and a greater capacity to rebuild than species with the slower reproductive rates common in other notothenioids.

Scientific observation under the auspices of the CCAMLR Scheme of International Scientific Observatio was conducted onboard the Chilean longliner 'Puerto Ballena' fishing for Patagonian toothfish (Dissostichus eleginoides) in Subarea 48.3 from March to May 1996. Details of the cruise are provided. Biological investigations incorporated length, sex, maturity and weight measurements of D. eleginoides, the estimation of the finfish by-catch, the estimation of a conversion factor of product to fresh weight for D. eleginoides and the assessment of incidental mortality of seabirds associated with the longline operations. The catch of D. eleginoides was mainly comprised of specimens of 70-130 cm whith the proportion of females in the catch exceeding those of males in fish larger than 1 10 cm. Length at sexual maturity and length at first spawning differed considerably between males and females with males becoming mature at a much smaller length. Black-browed albatrosses and white-chinned petrels were the most abundant species in the vicinity of the vessel when longlines were set. About 44 % of the longlines were set during daylight hours. Consequently, black-browed albatrosses made up a considerable portion of the seabird by-catch. A by-catch of 159 dead seabirds was reported by the scientific observer and the crew with black-browed albatrosses and white-chinned petrels forming the bulk of the by-catch. In addition a number of albatrosses, giant petrels and white-chinned petrels were hooked alive during hauling and released alive. Sperm whales and Orcas were the most common marine mammals observed in the vicinity of the vessel. Orcas in some cases caused a considerable loss of fishes from the lines.