STOCK ASSESSMENT AND MANAGEMENT MODELS

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Fisheries agencies often experience great difficulty in taking explicit account of the inherent uncertainties attached to key biological parameters when applying many of the mathematical models used in the management of harvested stocks and to assess their status.

CCAMLR has developed models which attempt to incorporate some of the key effects of uncertainty into analysis of biological parameters and into the subsequent management advice provided. To date, CCAMLR has taken a global lead in developing models of this kind and has applied them for target as well as dependent species.  Model outputs are then used to identify management options and these are objectively selected to set catch limits most congruent with the objectives of Article II of the Convention.

CCAMLR’s development of assessment and management models has comprised four distinct phases:

(i) In its early years and in keeping with customary practice at the time, CCAMLR adopted a single (i.e. fishery-targeted) species approach. CCAMLR’s efforts to manage multispecies fisheries and its pursuance of the ecosystem approach led to a need for more realistic and complex management models.

(ii) By 1994, CCAMLR had developed a 'Krill Yield Model' (KYM) to be used in the development of a precautionary catch limit* for the krill fishery. The KYM was developed in such a way as to:

(a) improve the meeting of the Convention’s objectives,

(b) take more explicit account of uncertainty attached to the estimation of krill potential yield, and

(c) apply clear, pre-agreed rules on which to base management decisions (so called decision rules).

In simple terms, the KYM combines estimates of krill biomass (B0) with a factor which takes into account the variability in the species’ life history characteristics (e.g. growth and mortality). In turn the decision rules are applied and a precautionary catch limit is derived. This limit serves to allow for reasonable fisheries catch levels while striving to prevent over fishing which may compromise the biological productivity of the stock concerned. The KYM’s output has been refined as new data have been obtained with the results of the 2000 Synoptic Survey doing much to reduce uncertainty associated with available estimates of krill B0 in Area 48.

(iii) The 'Generalised Yield Model' (GYM) was developed out of the KYM and applies a predominantly similar approach to the CCAMLR fisheries for finfish. Estimates of either current or pre-exploitation biomass, together with estimates of attached biological uncertainties, are used.

 Fluctuations in recruitment (i.e. replenishment of the stock by young animals) are also taken into account. The GYM enables CCAMLR to predict the effects of different levels of catch, even in the absence of direct estimates of stock abundance. Precautionary catch limits can then be calculated. In combination with the development of the GYM, and following the KYM, CCAMLR has also developed decision rules to be used in the management of the toothfish (Dissostichus spp.) fishery.

 (iv) The 'Foraging-Fishery Model' (FFM) aims to formalize descriptions of functional relationships between krill and its predators. Since high krill fishing activity is often located close to, or coincides with, the breeding or foraging areas of krill predators, broad areal estimates of krill biomass do not necessarily indicate the 'availability' of krill to the predator populations concerned. CCAMLR is therefore developing the FFM to assess interactions, as well as the potential overlap (in both space and time) between the fishery and predators foraging for krill.

The multi-species management approaches being developed by CCAMLR have no precedent. In their simplified form the models described above do not require the assessment of a large number of parameter values. However, there is an attached need to derive some estimate of attached uncertainty and this complicates application of the models. Along with an ever-growing data base, such models have contributed greatly to CCAMLR’s development of a strategic and practical approach to describing potential interactions between fisheries, harvested species and other species dependent on, or associated with, those being harvested.

CCAMLR has also embarked upon the development of strategic models to facilitate the setting of scientific priorities as well as on which to develop and base management options.

*As applied by CCAMLR, precautionary catch limits set levels of catch which are considered low enough not to compromise future sustainability of the targeted stocks. They allow some level of fishing so as to provide essential information to be used in assessment of stock yield and in ascertaining the properties of the fishery. CCAMLR Conservation Measure 51-01 is a good example of such a measure and it sets an overall precautionary catch limit for krill in Area 48 as well as a 'trigger level' which will be used to subdivide the overall limit into smaller areas in anticipation of a rapid expansion of the fishery.