Protection of large predators in a marine reserve alters size-dependent prey mortality

Abstract

Where predator -prey interactions are size-dependent, reductions in predator size owing to fishing has the potential to disrupt the ecological role of top predators in marine ecosystems. In southern California kelp forests, we investigated the size-dependence of the interaction between herbivorous sea urchins and one of their predators, California sheephead (Semicossyphus pulcher). Empirical tests examined how differences in predator size structure between reserve and fished areas affected size-specific urchin mortality. Sites inside marine reserves had greater sheephead size and biomass, while empirical feeding trials indicated that larger sheephead were required to successfully consume urchins of increasing test diameter. Evaluations of the selectivity of sheephead for two urchin species indicated that shorterspined purple urchins were attacked more frequently and successfully than longer-spined red urchins of the same size class, particularly at the largest test diameters. As a result of these size-specific interactions and the higher biomass of large sheephead inside reserves, urchin mortality rates were three times higher inside the reserve for both species. In addition, urchin mortality rates decreased with urchin size, and very few large urchins were successfully consumed in fished areas. The truncation of sheephead size structure that commonly occurs owing to fishing will probably result in reductions in urchin mortality, which may reduce the resilience of kelp beds to urchin barren formation. By contrast, the recovery of predator size structure in marine reserves may restore this resilience, but may be delayed until fish grow to sizes capable of consuming larger urchins.

Experiments reveal limited top‐down control of key herbivores in southern California kelp forests

“…Our experiments incorporated a wider size range of prey than this previous study, including urchins < 35 mm TD. Large sheephead are required to consume the largest urchins (Selden et al 2017), and lobsters in our study more readily consumed small urchins than larger urchins. However, the introduction of refuge-providing red urchins modulated some of the threat of predation for small urchins (Appendix S1: Fig.…”

“…Purple urchin density‐dependent mortality occurred where sheephead were large and abundant, which is common within southern California marine protected areas (MPAs; Hamilton and Caselle , Selden et al. ). However, sheephead and other predators only regulated purple urchins when large red urchins were absent, a condition consistent with fished areas but not typically observed within MPAs (Nichols et al.…”

Interactive Effects of Predator and Prey Harvest on Ecological Resilience of Rocky Reefs

“…Because of the lower predation rate on larger stages, large urchins can persist even if lobster fishing mortality is subsequently reduced. This study adds to recent research incorporating size-structured predation into tests of another emergent property: ecological resilience (see Ling et al 2009, Barnett and Baskett 2015, Marzloff et al 2016, Selden et al 2017 for other examples). S5) and a corresponding shrinkage in the parameter space with bistability (Fig.…”

Scientific Considerations Informing Magnuson–Stevens Fishery Conservation and Management Act Reauthorization

“…First, the scientific advice that grounds fisheries management can be affected by both shifts in productivity and distribution. In addition, population vital rates (e.g., recruitment, growth, mortality) can be directly affected by warming, acidification, and other physical changes, and they may also be indirectly affected by changes in predator-prey overlap and trophic relationships as species shift their distributions at different rates (Friedland 2013;Pershing et al 2015a;Selden et al 2017). Spatial distribution changes can also result in a misalignment with stock area delineations; stock assessments that are based on these delineations may become less representative as the misalignment increases (Link et al 2011).…”

“…Spatial distribution changes can also result in a misalignment with stock area delineations; stock assessments that are based on these delineations may become less representative as the misalignment increases (Link et al 2011). In addition, population vital rates (e.g., recruitment, growth, mortality) can be directly affected by warming, acidification, and other physical changes, and they may also be indirectly affected by changes in predator-prey overlap and trophic relationships as species shift their distributions at different rates (Friedland 2013;Pershing et al 2015a;Selden et al 2017). Estimates of stock productivity and potential productivity may be inaccurate if these effects are not considered, resulting in stock reference points, catch limits, and rebuilding time frames that may need to be adjusted periodically under directional trends in ecosystem conditions (e.g., Mueter et al 2011;Pershing et al 2015b).…”