Disentangling the effects of fishing and environmental forcing on demographic variation in an exploited species

Abstract

Species targeted by fishing often recover in abundance and size within marine protected areas (MPAs) resulting in increased reproductive potential. However, in some situations, concomitant changes in the abundance of predators, competitors, or prey within MPAs, or strong gradients in the surrounding environmental seascape may counteract the purported benefits making it more difficult to predict how species will respond to protection. We used a network of MPAs in California, spanning a large temperature gradient, to investigate the drivers of demographic variability in the commercially important red sea urchin Mesocentrotus franciscanus. We investigated how demographic metrics varied geographically in response to protection, temperature, and the main sea urchin resource, the giant kelp Macrocystis pyrifera. We found significant conservation benefits to this fished sea urchin within MPAs designated six years prior to the beginning of this study. Within MPAs, red sea urchins were generally larger resulting in greater adult biomass density and reproductive biomass density. In addition, kelp density was an important explanatory variable of all red sea urchin demographic traits examined (adult size, gonadosomatic index [GSI], density, adult biomass density, and reproductive biomass density). Kelp density was positively correlated with red sea urchin GSI and adult size, but the relationships with density, adult biomass density, and reproductive biomass density were complex and the directionality changed depending on the region (or environmental setting) examined. Our results demonstrate that kelp, red sea urchin reproduction, and the effects of spatial management on demographic processes are tightly coupled with the oceanographic regime.

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

“…Purple urchin density-dependent mortality occurred where sheephead were large and abundant, which is common within southern California marine protected areas (MPAs; Caselle 2015, Selden et al 2017). 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 2015, Teck et al 2017. While individual spiny lobsters ate relatively few urchins and did not cause direct density-dependent mortality via their functional response, fish predators did induce a region of increasing proportional mortality at low prey densities by aggregating to higher density prey patches.…”

“…, Teck et al. ). While individual spiny lobsters ate relatively few urchins and did not cause direct density‐dependent mortality via their functional response, fish predators did induce a region of increasing proportional mortality at low prey densities by aggregating to higher density prey patches.…”

“…Large red urchins can provide refuge to smaller urchins through a spine canopy refuge (Tegner and Dayton 1977) but are also an alternative prey item for both sheephead and spiny lobsters. Red urchins are also targeted in a fishery and are most abundant in marine reserves (Nichols et al 2015, Teck et al 2017, so their ability to mediate interactions between purple urchins and rocky reef predators may be spatially variable. We discriminated between potential mechanisms underlying the observed mortality patterns by characterizing the functional responses of spiny lobsters and quantifying the aggregative response of fishes foraging on urchins in the field.…”

Evaluating the promise and pitfalls of a potential climate change–tolerant sea urchin fishery in southern California

“…In northern CA, bull kelp ( Nereocystis luetkeana ) declines in 2014 and 2015 forced urchin divers to fish for suitable roe as far north as Alaska. These warm ocean conditions can subsequently affect the gonad production and recruitment of sea urchins into the fishery (Tegner and Dayton, 1991; Arntz et al , 2006; Rogers-Bennett, 2007; Vasquez, 2007; Teck et al , 2017) and may have explained the decrease in S. fragilis gonad production in Fall 2015 (Figure 5b). As these conditions are predicted to become more frequent due to ocean warming (Sweetman et al , 2017), it is critical for stakeholders to consider alternative sources of sea urchins including increased imports, aquaculture, or other alternative food production techniques (McBride, 2005; Takagi et al , 2017).…”

“…Based on the criteria we present, S. fragilis may be a possible viable alternative fishery to supplement the current southern CA red urchin fishery; however, despite previously rejected attempts to commercialize purple urchins, further consideration should be given to S. purpuratus as another potential climate-tolerant species (Kelly et al , 2013). Marketable sea urchin products depend on GIs around 5–15% (McBride, 2005), and thus an abundance of food in the wild is required to meet this standard (Ebert et al , 1999; Teck et al , 2017). Although the gonad weight of S. fragilis gonads was on average 80% lower than M. franciscanus gonads and significantly softer (Figure 7e), the colour and resilience was comparable.…”