Reply to Purcell et al.: Fishers and science agree, rotational harvesting reduces risk and promotes efficiency

Plagányi et al. (1) show that a rotational zone strategy (RZS) applied to sea cucumbers in a multispecies fishery on Australia’s Great Barrier Reef significantly reduces the risk of overall and localized species depletion in the fishery. In addition to implementing limits on catch and minimum size, Plagányi et al. (1) advocate designs of RZS that ensure effort is spread throughout the fishery so that the entire population of patchily distributed species periodically has a chance to grow and breed unfished. Purcell et al. (2) counter that this approach is risky and raise six concerns.

First, whereas we concur that the analogy with rotational harvesting has its limits, RZS is a widely recognized approach in wild fisheries management, from traditional to modern times, and is considered a “concept with origins from rotational harvesting of agricultural crops” (3).

The second claim that declines in white teatfish and prickly redfish have occurred under the RZS is unsubstantiated. Catch alone is not an adequate proxy for abundance, and catch rates (a more reliable index) have increased (1).

Third, based on harvester perceptions, declines in the Canadian sea cucumber fishery are mostly attributed to inadequate license quotas, not the RZS (4). Effective governance is another key challenge facing fisheries (2), but all management measures, such as caps on total catch or effort, can fail in the absence of stakeholder buy-in, which can be enhanced instead by reinforcing key social indicators, such as equity and a sense of self-determination (5).

Fourth, broad statements as to flawed assumptions and data inputs misunderstand that testing model scenarios, such as spatially spreading all harvest effort, provides additional verification of theoretical model results.

Fifth, Purcell et al. (2) consider model results unrealistic because part of one species’ catch is taken from non-RZS zones, but this is irrelevant to the finding that the RZS outperforms constant annual harvest strategies, based on extensive simulation testing across a range of species with different life histories (1).

Sixth, Plagányi et al. (1) acknowledge that they used best available information and data, but model inputs include uncertainty; hence, they included alternative parameter settings and cross-compared with available empirical data. Claims that the model overestimates the resilience of slow-recovering species misunderstand that the model results identify different optimal rotation periods for species with different growth rates, including longer cycles for slow-growing species (see, for example, figure 3 in ref. 1).

Purcell et al. (2) propose that regulating fishing effort to curtail fishing to <5% of virgin biomass annually for allowable species would cost less and afford better protection for biodiversity. However, the associated costs (for necessary baseline and annual surveys, and compliance) would appear to be on a par with the RZS approach. Furthermore, it ignores spatial considerations that are a core issue for relatively sessile species (1, 6). In conclusion, we find no convincing argument as to why an approach that advocates periodically closing spatial areas to fishing so that vulnerable sea cucumbers can grow and breed unfished, while also spreading fishing effort, will be more costly and risky than the counter argument presented.