The diet composition and feeding intensity of mackerel ice fish (Champsocephalus gunnari) around South Georgia in January/February 1991 was analyzed in 3877 stomachs collected on 44 stations. Both, the proportion of krill (Euphausia superba) in the stomachs and the feeding intensity were uncommonly low in a period when energy-rich food is needed for the final maturation of the gonads.
Abstract:
An adaptive approach was used to assess the Champsocephalus. gunnari stock in Area 48.3. Values of catch/effort by age group and by year of fishing were used in conjunction with abundance indices from trawl surveys in tuning the VPA.
The C. gunnari stock at the start of the 1990/91 season is estimated at 184 thousand tonnes.
Calculation of TAC was carded out keeping in mind the change to the fishing regime brought about by a change in mesh size (90 mm).
A TAC of 59.4 thousand tonnes for C. gunnari is advised for an optimal regime of fishing operations at a level of F0.1 = 0.65.
Abstract:
An adaptive approach was used to assess the Notothenia gibberifrons stock in Area 48.3. It is demonstrated that when tuning the VPA using abundance indices from trawl surveys it is imperative to estimate the catchability coefficient. This parameter was equal to 0.326 to 0.635.
An adaptive approach in this case involved using a target function in the form of the sum of square deviations in logarithms of observed and calculated biomass indices and resulted in an estimate of from 21.5 to 42.1 thousand tonnes for N. gibberifrons biomass in the 1989/90 season. It is thus possible to suggest a TAC of 1.9 to 2.7 thousand tonnes.
A similar approach is recommended for assessing the stock of other commercial species in this area.
Abstract:
The method of refining the natural mortality coefficient is based on the notion that the free component of the regression equation between the mean-weighted coefficient of natural mortality as defined by the VPA and fishing effort must be equal to zero if the natural mortality coefficient has been correctly identified and something other than zero if it has not been correctly chosen.
As a result of this mathematical modelling study using C. gunnari in Area 48.3 as an example, it was shown that the proposed method facilitates such a refinement. When a mistake is made in the choice of natural mortality coefficient, the estimate of the free component in the equation will be other than zero. A bias in the estimate can be positive if the assumed natural mortality coefficient is less than the “true" value and close to zero or slightly greater than zero if the assumed value is greater than the "true" one
Abstract:
Shnyute’s generalised cohort analysis of size composition in catches was used to estimate the Dissostichus eleginoides stock in Area 48.3. This made it possible to ignore the hypothesis of equilibrium in the stock and to examine the dynamics of its abundance and biomass.
At the start of the 1990/91 season the D. eleginoides stock is estimated to be 91.5 thousand tonnes.
It has been determined that for optimal results, fishing operations should be conducted at a level of F0.1. In accordance with such a policy, a TAC of 8.8 thousand tonnes is proposed for the 1991/92 season.
A sensitivity analysis demonstrated that the stock size and TAC were determined with a high degree of robustness
Abstract:
The seventh cruise of the RV Atlantida was a continuation of regular AtlantNIRO research into the status of Chaenichthyidae and Nototheniidae fish in Statistical Subarea 48.3 (South Georgia, Shag Rocks and Black Rocks). The minimum biomass of commercial fish species was calculated from the results of a census trawl survey carded out in April-May 1991. This paper contains data on distribution, biology and size composition of commercial fish species.