CCAMLR

A revision of the calculation of total international catch at age of Champsocephalus gunnari over the period 1976/77 to 1990/91 is presented. This uses iterative application of two age length keys to obtain age distributions, according to a method described by Kimura and Chikuni (1987). It is proposed that the revised catch at age be used for future assessment of the C. gunnari fishery at South Georgia using VPA.

Ovarian maturation in Notothenia coriiceps is known to include a period during which yolk deposition begins to take place. This 'adolescent' phase was thought originally to last for about one year. The results from a two year study at Signy Island indicate that this adolescent phase lasts for about four years. There is also some evidence to indicate that not all sexually mature fish spawn each season. Two reproductive strategies are considered, one where adverse conditions are accompanied by a failure to spawn and an alternative where fecundity is controlled by the number of small oocytes which ripen.

The Lepidonotothen squamifrons group has been described to be comprised of 3 species: L squamifrons (Günther, 1880). L kempi (Norman, 1937) and L macrophthalma (Norman, 1937). All 3 species closely resemble each other and morphological and meristic characters utilized to distinguish between species are overlapping. Enzyme electrophoresis of enzyme polymorph isms has been applied to specimens morphologically recognized as L squamifrons and L kempi from various localities in the Scotia Arc region and the Kerguelen Islands to assist in clarifying their taxonomic status. Results suggest that both 'species' represent only populations of one species. Evidence is presented that the third species of the group, L macrophthalma, may also be identical with L squamifrons and that the L squamifrons 'group' is comprised of only one species, Lepidonotothen squamifrons (Günther, 1880). We found no statistically significant difference in allele frequencies between specimens from geographically isolated shelf areas of the Scotia Arc. This does not necessarily mean that the Scotia Arc region is inhabited by a single population but suggests some gene flow between shelf areas via larval drift for which evidence exists.

The diet composition and feeding intensity of mackerel icefish ( Champsocephalus gunnari) around Shag Rocks and the mainland of South Georgia was analyzed from c. 8,700 stomachs collected in January/February 1985, January/February 1991 and January 1992. Main prey items were krill (Euphausia superba), the hyperiid Themisto gaudichaudii, mysids (primarily Antarctomysis maxima) and in 1985 also Thysanoessa species. The proportion of krill and Themisto in the diet varied considerably between the three seasons, whereas the proportion of mysids in the diet remained fairly constant. Krill which has a high energy content appears to be the prefered diet. In years of krill shortage, such as in 1991, krill is replace d by the ubiquitous Themisto gaudichaudii. The occurrence of krill in the diet in 1991 was one of the lowest within a 28 year period of investigation. Variation in food composition between sampling sites was high. This high variation can be primarily attributed to differences in prey availability, but was much less influenced by prey size selectivity due to differences in length composition of fish between sampling sites. Feeding intensity varied considerably between seasons. Both, the proportion of empty stomachs was uncommonly high and stomach content weight was uncommonly low in 1991 in a period when energy-rich food is needed for the final maturation of the gonads. It was highest in 1992. At the same time, an unusually high proportion of sexually mature fish showed no signs of the gonad development necessary leading up to spawning in that season. It was hypothesized that like in other, non-Antarctic fish species the shortage of suitable food, such as krill, may have forced the fish to sacrifice gonad maturation in order to maintain body size.

Four production models were fit to a time series of daily catch per unit effort (CPUE) data from the 1991/92 fishery for Paralomis spinosissima around South Georgia Island. The four models considered recruitment in different ways. Model 1 contained a linear recruitment function; Model 2 had constant recruitment; Model 3 contained a Beverton-Holt recruitment function, and Model 4 used a Ricker recruitment function. The best fitting model was Model 1. Model 1 had three parameters: an estimate of initial abundance (N0), an estimate of the scaling coefficient relating abundance to CPUE (q), and a recruitment parameter (a). The generalized likelihood ratio was used to place 95% confidence bounds around the parameter estimates from Model 1. These confidence bounds were very precise: Pr(240928 ≤ N0 ≤ 255374) ≈ 0.95; Pr(8.56 x 10-7 ≤ q ≤ 9.49 x 10-7) ≈ 0.95; and Pr(0.00804 ≤ a ≤ 0.00890) ≈ 0.95. Assuming that fishery removals should not be greater than the number of crabs that recruit to the fishery during the course of a fishing season, Bayesian statistics were used to evaluate alternative levels of a Total Allowable Catch (TAC) for the 1993/94 crab fishery. An optimal TAC was determined to be about 300 t. However, this TAC was conditional on the 1991/92 fishery data, and this data was limited to small temporal and spatial scales (about 4 mo. and 3600 n. mi.2). To extrapolate the estimated TAC to longer time periods information about the frequency and duration of the molting/mating event is required. To extrapolate the estimated TAC to larger areas (i.e. to estimate a TAC for all of South Georgia), it was necessary to determine whether growth or movement was predominantly responsible for recruitment. Monthly length frequency histograms were constructed and showed that growth was probably not the primary recruitment mechanism. Since movement may be important to the recruitment process, a TAC for all of South Georgia cannot be estimated by simple multiplication.

An experimental management strategy is proposed for the Paralomis spinosissima fishery around South Georgia. The strategy is designed to answer specific, a priori questions about the population dynamics of P. spinosissima, and consists of three phases to be conducted over a period of two years/fishing seasons. Phase 1 occurs at the start of the first fishing season and is a "survey" regime where fishing effort is artificially distributed over a large area. After Phase 1, normal fishing operations continue until the TAC for the first fishing season is attained. Phase 2 starts at the beginning of the second fishing season and is a series of three depletion experiments conducted in local areas. After Phase 2 normal fishing operations are conducted until CCAMLR initiates closure of the second fishing season. Phase 3 commences just prior to the close of the second fishing season. In this phase, fishing effort is redirected to the three local areas depleted during Phase 2. The experimental management strategy is designed for application on a per vessel basis (i.e. vessels may not "cooperate" to complete phases -- each vessel must complete all three phases on its own). The strategy is more powerful when multiple vessels participate in the fishery, but valuable information can still be obtained if only one vessel participates.