Recruitment hotspots in the ecology and management of large predatory fishes on coral reefs

Wen, Colin Kuo-Chang (2012) Recruitment hotspots in the ecology and management of large predatory fishes on coral reefs. PhD thesis, James Cook University.

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Abstract

Large predatory fishes make up the main catch in coral reef fisheries around the globe. However, primarily due to overfishing, populations of these predatory fishes have declined rapidly in the past century. No-take marine reserves have proven to be an effective way to protect reef predators and allow populations of exploited fishes to recover. However, the longterm benefits of reserves can only accrue through the maintenance of recruitment processes. In general, patterns of recruitment and the ecology of juveniles in key predator families such as the Epinephelidae, Serranidae and Lutjanidae are poorly documented. The overall aim of this study was to investigate spatial patterns in the recruitment of three exploited predatory fishes on the Great Barrier Reef in order to understand both the top-down and bottom-up role of recruitment in marine reserve dynamics. That is, to understand how reserves and increases in adult numbers impact on recruitment, and how patterns of recruitment influence the effectiveness of marine reserves. The thesis focuses on the juvenile ecology of coral trout (Plectropomus maculatus), the stripey snapper (Lutjanus carponotatus) and the long-finned rock cod (Epinephelus quoyanus) at the Keppel Island Group, southern Great Barrier Reef.

Chapter 2 examined the effects of reserves on the ecology of the juvenile stages of the three predators, including the direct effects of predation pressure from adults in reserves and nonconsumptive effects on foraging behaviour, including diet and prey selection. I examined differences between reserves and fished areas in recruit abundance, diet, prey availability, and prey selection indices for recruits and juveniles of the 3 predatory fish species. After quantifying recruit abundance in nearshore reef habitats at each of four sites at the Keppel islands (2 reserves and 2 non-reserves), I sampled recruits and juveniles from these same sites and analysed their gut contents to quantify their diets, then quantified prey availability to assess prey selectivity. Recruit abundance was similar between reserves and open areas, indicating that large predators do not directly reduce recruitment. The diets of the three study species did not differ between reserves and open areas, with variation in diets largely explained by fish species and body size. At small sizes, all species consumed high numbers of shrimp (Caridea), but diets diverged with growth: P. maculatus selectively consumed damselfishes (Pomacentridae) and wrasses (Labridae), L. carponotatus focused on gobies (Gobiidae) and crabs (Brachyura), and E. quoyanus primarily targeted crabs (Brachyura). Prey availability and prey selection differed between reserves and open areas, but only for a few categories of cryptic invertebrate prey. Overall, our results produced little evidence that more abundant predators inside reserves influence juvenile feeding ecology. However, recently recruited predators appeared to select a narrow range of invertebrate and small fish prey, and predator populations may therefore be susceptible to the loss of these resources.

Chapter 3 examined the role of the availability of suitable settlement habitat in determining patterns of recruitment and the likely consequences of habitat degradation. Juveniles of all 3 predators exhibited significant habitat selectivity, using specific microhabitats (mostly Acropora corals) disproportionately to their availability, but habitat selectivity was highest for recruits and lowest for adults. There was also, an apparent ontogenetic shift in microhabitat associations for all 3 species, with recruits associated mostly with corymbose Acropora, but adults were more commonly found associated with tabular Acropora. The proportion of P. maculatus (72%) that associated with live corals was higher than for L. carponotatus (68%) and E. quoyanus (44%), but recruits from all three species were found predominantly in structural habitats comprised of live branching corals. Moreover, recruits of all 3 species were found predominantly on patches of live-coral habitat located over loose substrates (sand) rather than consolidated substrata. Densities of recruits were highly variable among locations and among reef zones, but these differences were only partly attributable to availability of microhabitats. Specific reliance on live coral microhabitats has yet to be tested, but these findings show that at least some carnivorous reef fishes strongly associate with live corals (especially during early life-history stages), and may therefore be highly sensitive to increasing coral loss and degradation of reef habitats.

The ecological basis for microhabitat selection was investigated in chapter 4. Here I used a combination of field-based sampling and aquarium-based experiments to establish trade-offs between shelter requirements versus prey selection in microhabitat selection by larval coral trout (mostly, Plectropomus maculatus). Coral trout show strong affinity for structural microhabitats (e.g. live or dead colonies of Acropora), but the underlying habitat (sand versus consolidated reef substratum) further influences patterns of microhabitat use. Field-based surveys revealed that live coral habitats support higher densities of potential prey species compared to dead corals. Furthermore structural microhabitats on sand have higher densities of prey (especially crustaceans) compared to comparable microhabitats on consolidated carbonate substrata. In the absence of prey, juvenile coral trout did not distinguish between live versus dead corals, but both these microhabitat were preferred over rubble, macroalgae and sand. In aquarium-based studies of prey use, juvenile coral trout consumed prey fishes that associate with non-coral habitats (e.g. Eviota zebrina) and mid water species (e.g. Aioliops tetrophthalmus), but did not consume those fishes with an obligate association with live corals. Our results suggest that studies of microhabitat preferences should consider both the structure and location of specific microhabitats. It is presumed the structural microhabitats are essential for evading predators, while occupation of live corals positioned over sandy substrata maximises accessibility to a diverse array of potential prey fishes and crustaceans.

Chapter 5 examined the role of recruitment in explaining the magnitude of reserve-effects for adults of the 3 predators. The rationale was that long-term increases in reserves can only be sustained if there is adequate juvenile recruitment, but patterns of recruitment inside and outside reserves have seldom been quantified. I hypothesised that the effectiveness of reserves depends on whether or not they contain "recruitment hotspots" (or sites that contain a disproportionate abundance of juveniles). To test this, I used an orthogonal sampling design to compare the abundance of sub-adults and adults of three predatory fishes at both reserve and fished reefs, with and without recruitment hotspots. For P. maculatus and L. carponotatus, adult densities were 2-3 times greater in reserves with recruitment hotspots, compared with reserves without hotspots or fished areas, which were all similar. The abundance of sub-adults was primarily explained by the presence of recruitment hotspots, not reserve status. Compared with reserves, the size-distributions of P. maculatus and L. carponotatus were truncated at the minimum size limit (MSL) for all fished populations, regardless of recruitment patterns. My results suggest that identifying recruitment hotspots could be a valuable addition to reserve selection criteria, particularly for reserves targeting large exploited species using common recruitment areas.

This thesis has provided valuable new information on the ecology of the juvenile stages of important predators on the Great Barrier Reef, highlighting the critical importance of juvenile diet and habitat, and recruitment hotspots. While adult stages of these predators appear to have relatively little impact on juveniles, the juvenile ecology may be a key determinant of patterns in adult abundance and the effectiveness of marine reserves. I conclude that a greater focus on recruitment would greatly benefit conservation and fisheries management for those species examined in this study and thus contribute to improving the design and implementation of marine reserves.

Item ID: 31083
Item Type: Thesis (PhD)
Keywords: coral trout; Epinephelus quoyanus; Keppel Island Group; Lutjanus carponotatus; no take marine reserves; Plectropomus maculatus; recruit abundance; recruitment hotspots; reserve design; reserve effectiveness; rock cod; southern GBR; southern Great Barrier Reef; stripey snapper
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Leis JM, Piola RF, Hay AC, Wen C, Kan KP (2009) Ontogeny of behaviour relevant to dispersal and connectivity in the larvae of two non-reef demersal, tropical fish species. Marine and Freshwater Research 60:211-223.

Wen, C.K-C. , Pratchett, M.S., Shao, K-T., Kan, K-P., and Chan, B.K.K. (2010) Effects of habitat modification on coastal fish assemblages. Journal of Fish Biology, 77 (7). pp. 1674-1687.

Date Deposited: 26 Feb 2014 23:11
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 34%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050211 Wildlife and Habitat Management @ 33%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070403 Fisheries Management @ 33%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 34%
83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 33%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 33%
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