Guenther CM et al. (2012), Trophic cascades induced by lobster fishing are...

ECB-ART-42666

PLoS One 2012 Jan 01;711:e49396. doi: 10.1371/journal.pone.0049396.

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Trophic cascades induced by lobster fishing are not ubiquitous in southern California kelp forests.

Guenther CM , Lenihan HS , Grant LE , Lopez-Carr D , Reed DC .

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Fishing can trigger trophic cascades that alter community structure and dynamics and thus modify ecosystem attributes. We combined ecological data of sea urchin and macroalgal abundance with fishery data of spiny lobster (Panulirus interruptus) landings to evaluate whether: (1) patterns in the abundance and biomass among lobster (predator), sea urchins (grazer), and macroalgae (primary producer) in giant kelp forest communities indicated the presence of top-down control on urchins and macroalgae, and (2) lobster fishing triggers a trophic cascade leading to increased sea urchin densities and decreased macroalgal biomass. Eight years of data from eight rocky subtidal reefs known to support giant kelp forests near Santa Barbara, CA, USA, were analyzed in three-tiered least-squares regression models to evaluate the relationships between: (1) lobster abundance and sea urchin density, and (2) sea urchin density and macroalgal biomass. The models included reef physical structure and water depth. Results revealed a trend towards decreasing urchin density with increasing lobster abundance but little evidence that urchins control the biomass of macroalgae. Urchin density was highly correlated with habitat structure, although not water depth. To evaluate whether fishing triggered a trophic cascade we pooled data across all treatments to examine the extent to which sea urchin density and macroalgal biomass were related to the intensity of lobster fishing (as indicated by the density of traps pulled). We found that, with one exception, sea urchins remained more abundant at heavily fished sites, supporting the idea that fishing for lobsters releases top-down control on urchin grazers. Macroalgal biomass, however, was positively correlated with lobster fishing intensity, which contradicts the trophic cascade model. Collectively, our results suggest that factors other than urchin grazing play a major role in controlling macroalgal biomass in southern California kelp forests, and that lobster fishing does not always catalyze a top-down trophic cascade.

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Genes referenced: LOC100887844 tefl

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	Figure 1. Map of study area along 55 km of Southern California coast.Black dots mark annual transect sites from July 2001 through July 2008 for the Santa Barbara Coastal LTER. Grey polygons mark the 8 trapping areas around LTER sites where commercial lobster fishermen reported daily effort and catch. Mean polygon area was 1.23 km2 with the largest being 2.8 km2 and the smallest 0.44 km2. Abbreviations and numbers of transects sampled at each site are as follows (AHND = Arroyo Hondo, 2 transects; AQUE = Arroyo Quemado, 6 transects; NAPL = Naples, 7 transects; IVEE = Isla Vista, 2 transects; GOLB = Goleta Beach 2 transects; ABUR = Arroyo Burro, 2 transects; MOHK = Mohawk, 2 transects; CARP = Carpinteria, 7 transects).
	Figure 2. Relationships among lobster catch (a proxy for lobster abundance), urchins, and macroalgae.Lobster catch [legal-sized (≥83 mm carapace length) lobsters only] in number of individuals caught in traps km−2, urchin density, and total macroalgal biomass at each of the eight sites from 2001–2008. Macroalgae consisted of non-calcareous species, including giant kelp Macrocystis pyrifera. Data for lobster catch were not recorded in 2005 for Goleta Bay, and in 2001 and 2002 for Carpinteria Reef. Note the differences in scale for urchin density at Carpinteria Reef.
	Figure 3. Relationships among trophic levels and lobster fishing intensity.Data represent the mean values for the eight sites shown in Figure 2, each datum pooled across all eight years (2001–2008) (excluding the missing few data described in Figure 2). (A) Urchin density as a function of fishing intensity, measured as the mean daily number of lobster traps pulled (“sampled”) per km−2 of each fishing polygon (see Figure 1). (B) Total macroalgal biomass as a function of urchin density. (C) Macroalgal biomass as a function of fishing intensity.

References [+] :

Arkema, Direct and indirect effects of giant kelp determine benthic community structure and dynamics. 2009, Pubmed