CCAMLR

This paper presents a flexible structure for a general yield model, which provides flexibility for assessing the influence of different patterns of growth, natural mortality, spawning and fishing on the estimates of yield per recruit. Also, it can be used to evaluate the performance of a stock under a specified catch regime. The model uses an adaptive Runge-Kutta procedure to evaluate stock trajectories and catch rates over a simulation period. In particular, this procedure calculates catches over each year by integrating a set of differential equations which incorporate functions that specify the rate of change or magnitude of parameters, such as growth, mortality, age dependent selectivity and seasonal patterns in fishing mortality, at specified time intervals during the year. This general approach also includes an option for a stock to experience a known catch history before the constant catch regime is introduced.

A final report of scientific observations conducted aboard the Japanese krill fishing vessel Chiyo Maru No. 2 was presented to the first meeting of WG-EMM. The WG-EMM welcomed the report but noted that the results for reporting fish bycatch were not presented in CCAMLR's standard format. This manuscript presents a brief reanalysis of the bycatch data collected onboard the Chiyo Maru No. 2; means and variances are calculated according to the methods of Aitchison (1955) and Pennington (1983). In numbers, an average of about 20 fish were caught per ton of krill, and an average of about 23 fish were caught per hour of towing. In weight, an average of about 1.5 kg of fish were caught per ton of krill, and an average of about 2 kg of fish were caught per hour of towing. Confidence intervals for mean bycatch estimates are wide.

An age-length key was prepared for Champsocephalus gunnari from the Subarea 48.3, based on the readings of otoliths collected during the R/V Holmberg survey in February 1995. Small and medium sized fish (total length 12-34 cm) predominated in the samples at South Georgia, while larger specimens (>34 cm) were scarce. Age groups 1-5 and 2-4 were well represented at South Georgia and Shag Rocks respectively. At South Georgia, a good agreement between modes in the size distribution and mean length at age was found for age groups one and two. Mean lengths at age reported here for South Georgia are in line with previous results.

About 1300 stomach contents of the icefish Champsocepha1us gunnari caught in the Subarea 48.3 in February 1995, were analyzed using the frequency of occurrence (%) method. Krill (Euphausia superba) (58%) and the amphipod hyperiid Themisto gaudichaudii (55%) occurred similarly at South Georgia and constituted the main food. At Shag Rocks, T. gaudichaudii was the main prey item (52%), followed in importance by the mysid Thysanoessa sp. (35%) and krill (24%). The picture around SG was found to be similar to that observed in the E. Holmberg survey in February/March 1994: common levels of krill availability indicated by frequency of occurrence, but, on the other hand, high proportions of empty stomachs (also at Shag Rocks), close to the values reported for years of krill scarcity. While no krill aggregations were found in the South Georgia shelf during the Holmberg Cruise in Feb-March/1994, an acoustic survey carried out in the same area in March 1995 detected such aggregations. It is suggested that krill availability for C. gunnari around South Georgia in the 1995 summer season, was somewhat below the historical years of high abundance, but above than in 1994.

Due to the detrimental effect longline fisheries have had on seabirds in other parts of the world, concern was raised as to the effect an experimental Hake longline fishery would have on Procellariiform seabirds in South Africa. Carcases of Procellariiform seabirds caught on Hake longlines during setting were retrieved to determine species, sex and stomach contents. Whitechinned petrels constituted 100% of the catch on longlines during setting despite the fact that they were not the most abundant species around fishing vessels either during the day or night. Extrapolations showed that 2 vessels were responsible for 1928 ± 1345 whitechinned petrel deaths a year in South African waters. Stepwise multiple regression suggested that light intensity was the critical factor explaining variation in the number of whitechinned petrels caught during setting. Whitechinned petrels become highly active approximately 2,5 hours before sunrise. If line shooting could be completed prior to Whitechinned Petrels becoming highly active, a great deal of mortality might be avoided. Nine great shearwaters and two pintado petrels were killed during 13 hours of hauling procedures. Hauling mortalities can be avoided by diverting offal outlets to the opposite side of the vessel to where the longline is being hauled. Larger vessels which continuously process offal seem to be more attractive to scavenging seabirds then smaller vessels. Mitigation measures must be introduced into the Hake longline fishery to reduce excessive seabird bycatch. Suggested measures include: (1) the introduction of tori lines on all vessels which catch seabirds; (2) mandatory setting of lines prior to whitechinned petrel activity peaks. Experimentation with levels of artificial lighting and with differentially weighted longlines to ensure that hooks sink immediately when placed in the water are essential to further reduce catch rates of seabirds. Without the implementation of mitigation measures population growth and long term population stability of whitechinned petrels may soon be impaired.