CCAMLR

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.