Science Centres: Fisheries

What is the best basis for determining the total allowable catch for a fishery?

The conventional approach is to conduct regular or periodic stock assessments and then, based on possible results of different scenarios, make a management decision.

Under an Operational Management Procedure (OMP), however, stakeholders define a range of objectives that they wish to achieve (i.e., healthy stock size, good catch rates, minimal risk of poor outcomes). A range of alternative decision rules are then scientifically evaluated to determine the ‘best’ rule, and this rule is used to set the allowable catch.

Better rules for rock lobster

Fisheries scientists have been working with the National Rock Lobster Management Group (NRLMG) to develop and refine an OMP for the southern substock of red rock lobster.

The southern substock comprises two South Island quota management areas, CRA 7 (Otago) and CRA 8 (Fiordland, Foveaux Strait, Stewart Island). In the 1990s, it was in poor shape. The NRLMG used the OMP to develop a decision rule that would rebuild the stock to safer and more productive levels. This resulted in quota cuts of 20% in 1999 and again in 2001, but now the stock has started to rebuild.

Scientists from NIWA, in collaboration with scientists from Trophia and Starrfish, have been working with the NRLMG and other stakeholders to produce a set of objectives for the fishery. These objectives have guided how the decision rules should be evaluated.

To develop the ‘best’ rule, hundreds of alternative harvest strategies were developed, and thousands of test simulations conducted. All of the rules were evaluated against the agreed objectives, and involved trade-offs, for example, between the time to rebuild the fishery and the annual lobster catch. It took close consultation between the scientists, the NRLMG, and other stakeholders to decide on the final rule.

The new rule involves an intricate calculation, but it appears to be paying dividends. Last year, the available commercial catch increased by 6%, and the stock is expected to reach the target level well within the target timeframe of 10 to 14 years.

The same team of scientists has also explored alternative OMP approaches for other rock lobster fisheries – and, in addition to the southern substock management procedure, two decision rules have been tested on an annual basis since 1997.