Hill, S.L., K. Reid, S.E. Thorpe, J. Hinke and G.M. Watters (2007), A compilation of parameters for ecosystem dynamics models of the Scotia Sea – Antarctic Peninsula region

Expansion of the krill fishery in the Scotia Sea–Antarctic Peninsula region beyond the current operational catch limit requires the development and assessment of methods for subdividing the precautionary catch limit amongst smaller spatial units. This paper compiles parameters for use in the ecosystem dynamic models that are needed to assess these methods. These parameters include life history and krill consumption parameters for the fish, whale, penguin and seal species that feed on krill in this region. Maximum krill transport rates are also derived from the OCCAM global ocean circulation model. This parameter set, like most others, is associated with considerable uncertainty, which must be taken into account when it is used. The sources, assumptions and calculations at every stage of the compilation process are therefore detailed, and plausible limits for parameter values are provided where possible. The results suggest that fish are the major krill consumers in all SSMUs, with perciform fish taking as much krill as whales, penguins and fur seals combined and myctophid fish taking double that amount. However, estimates of krill consumption per unit predator biomass suggest that this is an order of magnitude higher in penguins and seals than in whales and fish.

Saunders, R.A., A.S. Brierley, J.L. Watkins, K. Reid, E.J. Murphy, P. Enderlein and D.G. Bone (2007), Intra-annual variability in the density of Antarctic krill (Euphausia superba) at South Georgia, 2002–2005: within-year variation provides a new framework for interpreting previous ‘annual’ estimates of krill density

Upward-looking acoustic Doppler current profilers (300 kHz) and echo sounders (125 kHz) were deployed on moorings on- and off-shelf to the northwest of South Georgia between 14 October 2002 and 29 December 2005 to measure density of Antarctic krill and environmental parameters continuously. A distinct seasonal pattern in krill density, recurring consistently over all three years, was detected. Krill densities in winter were predominantly low (mean = 18.7 g m^–2, SD = 24.3), but had risen substantially by summer in each year (mean = 89.5 g m^–2, SD = 64.2). A sinusoidal regression model (period = 52 weeks) with time as the independent variable explained 64% of the observed week-to-week variation. Estimates of krill density from moored instruments were not statistically
different (P > 0.05) from estimates derived from standard ship-based krill surveys in adjacent time periods, suggesting that the point estimates from moored instruments were representative of krill density in a wider spatial context (ship surveys cover c. 100 x 100 km). Data from moored instruments were used to explore whether high-frequency temporal variation (i.e. within-year) could have led to the perceived between-year variation in krill density arising from previous summer surveys in the South Georgia western core box region between 1990 and 2005. Comparison of these ‘snap-shot’ ship survey estimates with the observed pattern of within-year variability showed that some of the apparent ‘year-to-year’ variation could simply be attributed to sampling on different dates of the year (e.g. November cf. February). However, there were some survey estimates that were signifi cantly different (P < 0.01) from the regression-predicted within-year variation. Years that stand out for markedly low krill density (i.e. densities below the range expected due to intra-annual variation) were 1993/94, 1998/99 and 1999/2000. Moored instruments provide valuable data that could be important for ecosystem-based management at South Georgia because, for example, they will enable predator–prey functional responses to be explored there for the first time at appropriate temporal scales, and will enable hypotheses relating variation in krill abundance to physical oceanographic variability to be tested.

Candy, S.G., A.J. Constable, T. Lamb and R. Williams (2007), A von Bertalanffy growth model for toothfish at Heard Island fitted to length-at-age data and compared to observed growth from mark–recapture studies

Length-at-age data for Patagonian toothfish (Dissostichus eleginoides) at Heard Island (Division 58.5.2) were fitted using a von Bertalanffy (VB) growth model taking into account response-biased sampling of fish that were aged. Subsampling of random length frequency (LF) data used to obtain the samples of fish for ageing used length-bin sampling involving a fixed sample size per bin. Estimation of the VB parameters used a definition of the likelihood function based on variable probability (VP) sampling due to the prespecified length-dependent selectivity function for trawl fishing and the additional effect of length-bin sampling on sampling probabilities. The VB curve fitted to the length-at-age data, ignoring VP sampling and assuming normal errors with constant coefficient of variation using iteratively weighted least squares (IWLS), predicted substantially lower mean length-at-age for older ages compared to the VB curve fitted using VP maximum likelihood (MLP) with length-bin relative probabilities defined using fishing selectivity alone. This was due to the feature of the selectivity function of a sharp decline from ‘full’ selection at 1 000 mm length down to 1% selection for a length of 1 600 mm. When length-bin sampling frequencies were also included in defining relative probabilities, the VP maximum likelihood (MLPLB) and IWLS-estimated curves were more similar.

Predicted and observed values of annual growth rate (AGR) for lengths measured at release and first recapture in mark–recapture studies were compared where predictions used the VB parameter estimates obtained from the length-at-age data and the Fabens (1965) form of the VB growth model. Formulae for adjusting predictions for bias imparted by the use of the Fabens model were developed and showed that the bias is relatively small for the range of release lengths in the data. Predictions of AGR using the MLPLB-estimated VB parameters were closer to, but still substantially higher than, the mean trend in observed AGR values with release length compared with those obtained using IWLS-estimated parameters. A young-age adjustment (less than 5 years old) to the VB model is also given in order to give more realistic predictions of mean length-at-age for young fish.

Roche, C., C. Guinet, N. Gasco and G. Duhamel (2007), Marine mammals and demersal longline fishery interactions in Crozet and Kerguelen Exclusive Economic Zones: an assessment of depredation levels

Interactions between killer whales (Orcinus orca), sperm whales (Physeter macrocephalus), fur seals (Arctocephalus spp.) and longline fishing operations were reported by observers on board fishing vessels targeting Patagonian toothfish (Dissostichus eleginoides) in the Crozet and Kerguelen Islands Exclusive Economic Zones (EEZs) between 2003 and 2005. In the Crozet EEZ, the reported interactions involved killer whales and sperm whales. These two species, alone or in co-occurrence with each other, were observed in 71% of the 1 308 longlines set. In the Kerguelen EEZ, the reported interactions involved sperm whales and fur seals. These two species, alone or in co-occurrence with each other, were observed in 54% of the 6 262 longlines monitored. Interactions were observed in all fishing areas. The effect of depredation was assessed by comparing catch-per-unit-effort (CPUE) (fish/weight/hook) of each longline set in the absence/presence of marine mammal species alone or in co-occurrence. In the Crozet EEZ, CPUE was found to be reduced by 22.5% in the presence of killer whales, 12.1% by sperm whales, and 42.5% when both species were present together. An extensive photo-identifi cation effort, primarily focussing on killer whales, allowed a total of 103 individual whales to be identifi ed. The analysis of photoidentification indicated that a small number of individual killer whales were responsible for most of the interactions with the fishery. In the Kerguelen EEZ only sperm whales, alone or in co-occurrence with fur seals, were found to impact negatively on CPUE.

Barrera-Oro, E.R. and E.R. Marschoff (2007), Information on the status of fjord Notothenia rossii, Gobionotothen gibberifrons and Notothenia coriiceps in the lower South Shetland Islands, derived from the 2000–2006 monitoring program at Potter Cove

The long-term monitoring program of demersal fish at inshore sites of the South Shetland Islands has continued at Potter Cove from 2000 to 2006, extending a continuous sampling period of 24 years which began in 1983, and at Harmony Cove in the summers of 2001to 2003. The decline in trammel net catches of fjord Notothenia rossii and Gobionotothen gibberifrons in relation to the non-commercially fished Notothenia coriiceps is still evident. At Potter Cove, despite an overall increasing trend of N. rossii catches from 1991 to 2006, the actual levels are half of those found in the early 1980s, while those of G. gibberifrons have further declined and remain close to zero. At Harmony Cove, the relative abundance of N. rossii showed an increase in 2002 and 2003, whereas G. gibberifrons was absent in the catches. These trends are consistent with those observed in scientific cruises studying the offshore populations in a similar period. No recovery of the stocks of N. rossii and G. gibberifrons was observed, more than two decades after the end of the commercial fishery.