Modulation of habitat-based conservation plans by fishery opportunity costs: a New Caledonia case study using fine-scale catch data

Abstract

Numerous threats impact coral reefs and conservation actions are urgently needed. Fast production of marine habitat maps promotes the use of habitat-only conservation plans, where a given percentage of the area of each habitat is set as conservation objectives. However, marine reserves can impact access to fishing grounds and generate opportunity costs for fishers that need to be minimized. In New Caledonia (Southwest Pacific), we used fine-scale fishery catch maps to define nineteen opportunity costs layers (expressed as biomass catch loss) considering i) total catches, ii) target fish families, iii) local marine tenure, and iv) gear type. The expected lower impacts on fishery catch when using the different cost constraints were ranked according to effectiveness in decreasing the costs generated by the habitat-only scenarios. The exercise was done for two habitat maps with different thematic richness. In most cases, habitat conservation objectives remained achievable, but effectiveness varied widely between scenarios and between habitat maps. The results provide practical guidelines for coral reef conservation and management. Habitat-only scenarios can be used to initiate conservation projects with stakeholders but the costs induced by such scenarios can be lowered by up to 50-60% when detailed exhaustive fishery data are used. When using partial data, the gain would be only in the 15-25% range. The best compromises are achieved when using local data.

Figure 1. View and maps of the study area.

Left panel: view of the Voh-Koné-Pouembout coral reefs and lagoons. The white lines show the extent of the study area. Right and Central panels: habitat maps used for this study, Millennium habitat map and Detailed habitat map respectively. Legends are not shown here given their length (23 and 106 classes) but the differences in levels of details are visible.

Figure 2. Map of Total Catch for the Voh-Koné-Pouembout area.

Example data included in the fishery atlas used to compute opportunity costs (From Guillemot and Léopold [21]).

Figure 3. Flow chart showing how the different metrics were obtained.

Habitat map and fishery Catch (or production) data are in input; then different conservation designs can be computed, with or without cost optimization. The various metrics (see text for details) can be computed and provide different outputs relative to the effectiveness of the different scenarios. The “Habitat only design” icon is repeated for clarity, to avoid crossing lines in the chart.

Figure 4. Examples of conservation designs, with best solutions given by Marxan and selection frequencies of planning units.

For the two different habitat maps, the changes in proposed reserve locations are shown between a habitat-only scenario and a cost-based scenario, based here on Total Catch data (see Figure 2). High frequencies and best solutions are in strong agreement in some cases (panels A and B; panels G and H). When they differ in specific areas, it is because of habitats particularly abundant, such as the deep lagoon, that offer flexibility in the selection of management units (panels C and D), or because of lack of prioritary units (as shown also in low contrasted frequencies) (panels E and F).

Figure 5. Comparison of E and E₁ achieved between the two habitat maps and the various cost-based scenarios. Negative values <−100% (see Table 5 , Gerreidae family) are forced to −80%.