Depletion of predatory fish by fishing in a ­temperate reef ecosystem leads to indirect effects on prey, but not to lower trophic levels

Abstract

Indirect effects of fishing extending 2 or more trophic levels have been observed in a range of ecosystems, but they are not ubiquitously present, and identifying the circumstances in which they occur (or do not occur) is a key challenge for ecologists. We compared a fully protected sanctuary (no fishing allowed) with 2 other types of management zones (one in which all types of fishing were allowed and another in which only recreational line fishing was allowed) at Rottnest Island, Australia. We predicted that fishing would result in lower abundance of predatory fish in the 2 fished zones. We further predicted that, if indirect effects of fishing are important, there would be lower predation on sea urchins and turbinid gastropods, higher abundance of these herbivorous invertebrates and lower biomass of macroalgae in both types of fished zones. The abundance of predatory fish and predation on the sea urchin Heliocidaris erythrogramma were higher in the sanctuary than in fished zones. Predation on the sea urchin Centrostephanus tenuispinus was higher and densities of H. erythrogramma were lower in the sanctuary, but the magnitude of the difference in these measures (and, therefore, the results of statistical tests) was variable among years. However, predation on turbinid gastropods, densities of C. tenuispinus and turbinid gastropods, and the biomass of macroalgae did not differ significantly among management zones. The pattern of higher abundance of predatory fish and higher rates at which prey were consumed in the sanctuary provide evidence for the indirect effects of fishing on the abundance of H. erythrogramma. However, there was no evidence that these indirect effects extend to a second species of sea urchin, or to turbinid gastropods, and no evidence that the effects lead to broad changes in the state of the ecosystem.

A small no-take marine sanctuary provides consistent protection for small-bodied by-catch species, but not for large-bodied, high-risk species

“…These results are comparable to Kleczkowski et al (2008) for O. lineolatus and to Cook and Vanderklift (2011) for C. auricularis. Both these by-catch species are protogynous hermaphrodites (Hutchins and Swainston, 2003;Smallwood et al, 2006) and appear to be more vulnerable to fishing at a larger size, as it is generally the dominant males that are caught and sometimes retained by recreational fishers in the region (Pers.…”

“…Proportion of reef cover sourced or estimated* from Harvey (2009 the established no-take area would be significantly and relatively consistently different from comparable fished areas and that the assemblage biomass composition and size-structure of the newly established no-take areas would gradually become more significantly different with time. In particular, populations of large-bodied high-risk species and smaller-bodied by-catch species would be consistently greater inside the single established no-take area across years of sampling, consistent with the studies by Kleczkowski et al (2008) and Cook and Vanderklift (2011).…”

“…There were significant differences in the length distribution of C. auricularis inside and outside the established sanctuary zone Table 3 Maximum length and fisheries status of species of interest, including fishes thought to prey on sea urchins* from Cook and Vanderklift (2011 when sampled with stereo-BRUVs (MS 1,23 = 27088, F = 4.1, P 0.042) and stereo-DOVs (MS 1,22 = 50762, F = 4.62, P = 0.034), with no effect of Year. A greater proportion of smaller individuals in the fished areas was found, and a greater proportion of larger individuals in the no-take area (Fig.…”

“…In 2003, Kleczkowski et al (2008, using downward facing baited remote underwater video (BRUV), found significantly greater biomass of two large-bodied high-risk fished species, the endemic West Australian dhufish (Glaucosoma hebraicum, 500% greater) and the breaksea cod (Epinephelides armatus, 320% greater), and a significantly higher abundance of male southern maori wrasse (Ophthalmolepis lineolatus, 230% greater), a smaller-bodied by-catch species and protogynous hermaphrodite (Hutchins and Swainston, 2003;Smallwood et al, 2006), compared to fished areas. Using underwater visual census (UVC), between , Cook and Vanderklift (2011 observed higher abundances of fish species thought to prey upon sea urchins within the established sanctuary zone in particular male western king wrasse (Coris auricularis), which is another smallbodied by-catch species and protogynous hermaphrodite ( Table 1, Smallwood et al, 2006), compared to fished areas. Subsequent surveys of the established sanctuary zone in 2006-2007 by Bellchambers et al (2008), using horizontally facing stereo-BRUV systems did not observe any differences in the abundance or biomass of these species, although this study was not specifically designed to compare the fish populations inside and outside this sanctuary zone.…”

“…The current study will focus on the Rottnest Island Marine Reserve, as this area contains the oldest no-take sanctuary zones in south-western Australia: the Kingston Reef and Parker Point sanctuary zones (established in 1988). The Kingston Reef sanctuary zone has been the focus of the majority of published research from the region (Bellchambers et al, 2008;Cook and Vanderklift, 2011;Kleczkowski et al, 2008). Rottnest Island is a very popular location for recreational fishing within the Perth metropolitan region (Smallwood et al, 2006) and also includes three newly created notake areas (established in 2007, including the extension of Parker Point), that cover a greater range of depths (0-15 m) than any other existing no-take areas in the region (Table 1).…”