CCAMLR

Assessments of long-term sustainable yield using the Generalised Yield Model and the setting of total allowable catches (TAC) for Patagonian Toothfish, Dissostichus eleginoides, for the Heard Island Plateau region (Division 58.5.2) are based on data obtained from annual random stratified trawl surveys (RSTS) and from targeted commercial fishing operations via an observer program. The current RSTS design for Division 58.5.2 is reviewed and estimates of optimal distribution of trawl stations among strata provided for estimates of abundance of individual pre-recruitment age classes and total abundance for a range of levels of confidence. The results indicate that while the current and previous survey stratifications appear appropriate, the current levels of survey effort are not sufficient to provide reasonable levels of confidence in the estimates of abundance for all but the youngest year class. It is recommended that the implications of these results need to be examined as part of a full evaluation of the current management procedure for D.eleginoides in Division 58.5.2 in order to determine the potential implications for meeting long-term yield and conservation objectives. An approach to progressing such evaluations in the context of the elements of the current monitoring and assessment methods is outlined.

We investigated local depletions of toothfish under concentrated fishing effort as a means of estimating toothfish density at South Georgia. Over the four years 2000-2003, we found 25 areas of 12’ by 12’ where there had been forty or more hauls in any particular year. There were 62 individual vessel fishing events in these areas over this period, 40 of which appeared to be depletions. Applying a Leslie method we derived toothfish densities in these areas which were linearly correlated with the initial CPUE experienced by the fishing vessels. Using this correlation, combined with the average CPUE over the four years and multiplying by the total fishable area in Subarea 48.3 we arrived at an estimate of total toothfish biomass of 55,666t. Clearly this method requires development, but it does hold out some possibility of potential as an alternative assessment method for toothfish.

We examine two alternatives to the survey recruitment/GY method for estimating toothfish abundance. An age structured production model (ASPM) is constructed and tuned to the CPUE series. The fit of the model to the CPUE series is not good, and there are several unexplained problems. The first is that the CPUE in the early 1990s did not decline as would be expected as the resource was initially fished quite heavily. The second is that when a decline was seen, it was much more abrupt than would be predicted by the model. Thirdly, all models predict a continuing decline, not the increase in CPUE that actually has been seen since 1996. Fitting the model to partial CPUE series, or assuming an unknown additional level of IUU fishing, improves the fit of the model but does not fully explain the initial lack of early decline, the abrupt decline between 1995 and 1996 and the subsequent increase in CPUE. Toothfish have been tagged at South Georgia since 2000, and it is possible now to examine the usefulness of tagging data to estimate population size. We use two equations, the Jolly-Seber unbiased method and a simple M- and tagging mortality adjusted Petersen estimator. This latter analysis suggests biomass in 2002 was between 78,000 and 114,000 t.

We reanalysed 16 groundfish surveys at South Georgia to determine whether recruitment data used by the Working Group for toothfish assessments in 1999 and later were correct. We found several unexplainable errors. A new, corrected, series of recruitment estimates was derived using new procedures and 6 strata (3 depth strata each at Shag Rocks and South Georgia). It is easy to spot, visually, the modal progression of peaks in the length densities corresponding to cohorts assessed by sequential surveys. Length based analysis of these confirms the growth rate expected from the CCAMLR growth equation for South Georgia. Although it is easy to spot cohorts, the size of individual cohorts can not be consistently determined from the survey data. The expected reduction in cohort density with time at a log-scale rate of -0.16 (natural mortality) was evident in only very few cohort plots, even those which were particularly obvious as visual modal progressions, and even when the data were trimmed to remove suspect surveys. We conclude that survey-based estimates of recruitment biomass are not reliable input data for assessment models using the GY approach at South Georgia.

These are the technical specifications of the software Fish Heaven: version 2.1.5. The specifications include a description of the object structure of the model, the process structure of the model and a mathematical description of the model.

At its 2003 meeting, WG-FSA recommended that investigations be undertaken to determine whether the method for estimating catches from illegal, unregulated or unreported (IUU) fishing developed by Agnew and Kirkwood (2002) for Subarea 48.3 could be applied to other CCAMLR areas (SC-CAMLR-XXII, Annex 5, paragraph 3.18).
The results of these simulations suggest that more information on IUU fleet dynamics, particularly evasion tactics, will be necessary to devise an observation strategy for estimating the number of IUU fishing days, particularly for areas where patrolling may be relatively infrequent or prone to evasion. Also, the assessment method may need to be refined to take these issues into account.
An important consideration for the future will be the inevitable tension for fishery patrol vessels between detecting IUU activity, thereby estimating the number of IUU fishing days, and deterring such activity. For assessment purposes, this highlights a general problem that the method of observation has a strong effect on the behaviour of the system (of the illegal fishing cruises). It may be necessary to develop other forms of observations to facilitate this task.