Giant kelp Macrocystis pyrifera is a foundational species that forms a 3-dimensional habitat and supports numerous high-value fisheries species. Constant grazing of kelp holdfasts by overabundant sea urchins causes catastrophic ecological and economic impacts on rocky reefs worldwide. Overgrazing creates urchin barrens that persist for decades in the absence of ecological forcing that would shift the ecosystem back to a kelp-dominated state. Annual surveys of kelp forest and urchin barren sites in the Southern California Bight were performed from 2011 to 2020 to assess changes in kelp forest communities as a result of restoration efforts through sea urchin culling. However, that time period also encompassed a sea urchin mass mortality event. Following drastic reductions of sea urchin densities, rocky reefs returned to a kelp-dominated state within approximately 6 mo and remained stable through the remainder of the study. Benthic cover, fish, and kelp and macroinvertebrate communities inside former urchin barrens became more similar to that of kelp forest reference sites and continued to do so for the next 5 yr. Giant kelp density increased significantly compared to existing kelp forests, while benthic indicators of urchin dominance (i.e. crustose coralline algae and bare rock cover) decreased. Kelp restoration through sea urchin culling essentially mimics sea urchin mass mortality events. If culling can produce similar declines in urchin density, it may be a viable management tool to rapidly restore persistent urchin barrens at moderate spatial scales, while a mass mortality event can drive recovery of kelp forest communities at more extensive spatial scales.
1. Marine protected areas (MPAs) can be effective tools for marine resource management. However, despite evidence of the positive effects of MPAs, such as increases in body sizes of organisms targeted by fisheries, there is often heterogeneity in biological outcomes among them.2. Because fishing can drastically impact fish populations, the goal of this study was to determine if the levels of exploitation prior to protection could predict variation in the magnitude of MPA effects.3. Using a diver-operated stereo-video camera system, we compared sizes of fishes targeted by anglers within seven MPAs spanning the Southern California Bight to nearby comparison areas (non-MPAs). We used fine-scale data on pre-closure fishing pressure to test responses of fishes to protection along a gradient of exploitation.4. Fish size responded to protection in proportion to pre-closure fishing pressure, with MPAs in areas with high pre-closure fishing having greater responses in average lengths and size distributions than those in areas with low fishing pressure.5. This response was evident in species heavily targeted by recreational fishing, but not in species that were not targeted in fisheries.6. Synthesis and applications. Pre-closure fishing pressure of an area can impact the efficacy of MPAs, and, when available, should be considered when predicting and evaluating MPA performance. Prioritizing heavily exploited areas for protection when implementing spatial management tools could maximize ecological outcomes. K E Y W O R D Sbody size, fishing effects, fishing pressure, marine protected areas, MPA design, Paralabrax clathratus, Semicossyphus pulcher, stereo-video
Resource managers often outline discrete goals when implementing marine protected areas (MPAs), such as increases in organism abundance, biomass, or body size, to evaluate MPA success and thus inform future management decisions. Understanding which biological indicators are most sensitive to protection is essential for adaptive management, and while density is a commonly used metric to monitor MPAs, previous work has pointed to a lack of reliability of this metric to effectively evaluate MPAs, particularly during initial stages of protection. To determine which biological indicators of MPA success were evident 5 yr after protection began, we examined the differences in populations inside vs. outside MPAs for 3 fishes targeted by anglers—kelp bass Paralabrax clathratus, barred sand bass P. nebulifer, and California sheephead Semicossyphus pulcher—and 4 non-targeted species. We used stereo-video to determine the average body size, biomass, and density of each species in 7 MPAs in Southern California that were paired with nearby non-MPA sites. Responses of targeted species were detectable after 5 yr of protection in some MPAs. In most but not all MPAs, individuals were larger compared to outside MPAs for kelp bass and California sheephead. Biomass and density of targeted species appeared to be less sensitive to MPA effects. As expected, non-targeted species did not show evidence of MPA effects. Similar to past studies, our findings indicate that the choice of response variable and species studied may influence the perceived efficacy of MPAs, and we emphasize that increases in length may be a particularly sensitive indicator of effective protection.
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