Stock Assessments

US AMLR Stock Assessments

Stock assessments to estimate the status and productivity of biological populations are a central tool in the management of the harvesting of living resources. These methods are used to determine sustainable catch levels that can be demonstrated to meet long-term management criteria.

2013 krill net JWalshThe role of AMLR in assessing the sustainability of the fisheries in the CCAMLR convention area is two-fold. First, AMLR scientists participate with other CCAMLR member nation scientists in working group discussion and review of assessed, exploratory, and research fisheries in the CCAMLR convention area using existing quantitative tools. The eventual goal of this work is to establish that all allowable fisheries removals for each fished stock are sustainable. Second, AMLR scientists participate in the development of new assessment frameworks that can incorporate additional ecosystem and food web considerations and data into the formal models used to establish catch limits.

Traditionally, fisheries stock-assessment is based on single-species models, which for the most part consider the productivity of a fished stock in isolation from the rest of its environment. The effect of the environment enters traditional models indirectly through such parameters as natural mortality (often estimated as a single constant representing an average value over all ages and years) or recruitment (sometimes correlated with environmental data but often estimated as an independent value or time series).

Conventional stock assessments use as data such measured quantities as total catches, age- or length-structure of the catches, and possibly other indices such as catch-per-unit-effort (CPUE) or non-fishery data from research surveys, to estimate the parameters of a quantitative model for the fishery system. These single-species frameworks for estimating model parameters and derived quantities of interest have a well-developed statistical foundation but ignore much of the ecological detail that would be required to recognize ecological phase transitions such as regime change or shifts in the scale of variability that could affect long term productivity or resilience.

Catch limits for the three most important (by weight and by economic value) harvested species in the CCAMLR convention area, Antarctic krill (Euphausia superba), Patagonian toothfish (Dissostichus eleginoides) and Antarctic toothfish (Dissostichus mawsoni) are established using single-species models. The CCAMLR "precautionary principle" defines these catch limits in terms of changes that could be reversible within two or three decades if harvesting ceased.

Krill harvest targets are established using the "Generalized Yield Model" (GYM), a simulation framework developed over several decades. Toothfish catch limits in most assessed areas are set using the "CASAL" integrated modeling framework, although limits for exploratory or research fisheries in areas without formal assessments may be set using GYM, comparative-CPUE, or similar approaches.

All of these modeling approaches are simplifications (see Figures 1 and 2 below). The goal is to capture the important aspects of a system in a quantitative way so that the likely effects of alternative management options can be estimated and understood.

Figure 1. Distributions of krill lengths measured in net trawls during 19 years of January and February surveys in four areas* of the Antarctic Peninsula.2014 Krill size


Figure 2. Model distributions of krill lengths for January and February in four areas of the Antarctic Peninsula produced from a stock assessment.

2014 Krill size figure 2

For additional information on stock assessments prepared by the AERD, contact Doug Kinzey.

*The four AMLR study areas are Elephant Island (EI), Joinville, South and West, as shown below:

2014 WAMLR survey grid