Estimating dispersal and population connectivity for temperate reef fishes at multiple spatial scales

Curley, Belinda G. (2007) Estimating dispersal and population connectivity for temperate reef fishes at multiple spatial scales. PhD thesis, James Cook University.

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Abstract

Knowledge of scales of dispersal and levels of population connectivity is critical for understanding population dynamics and effective management of reef fishes. These processes are important for effective design of Marine Protected Areas (MPAs) particularly if they are to generate ‘spillover’ and ‘recruitment effects’. Despite this, empirical data across appropriate spatial and temporal scales are limited. This is the first study to focus on dispersal and population connectivity for temperate reef fishes in central NSW, Australia, at scales relevant to the implementation of MPAs (100's m - 100's km). The study provides: (1) empirical data on the localised benefits of small MPAs relative to the mobility of exploited reef fishes; (2) baseline data on the utility of different methods (microsatellite markers and otolith chemistry) for determining levels of population connectivity, and potential scales of benefits of MPAs to unprotected areas. Work on microsatellite markers compared population genetic structure in species which span the post-settlement dispersal potentials of reef fishes in this region, and provides a benchmark for understanding general mechanisms which govern gene flow, and population connectivity, in central NSW.

The response of exploited reef fishes to the establishment of small MPAs (≤ 0.2 km2), was investigated relative to knowledge of post-settlement movement. Two established MPAs were surveyed: Cabbage Tree Bay (CTB) a 2.5 year old ‘no-take’ MPA, and Gordon's Bay (GB) a 12.5 year old MPA closed to spear fishing only. Abundances and sizes of four ‘sedentary’ and three ‘mobile’ fishes within each MPA were compared with three control locations at six times over two years. Temporal variation in abundances suggested that MPAs did not encompass the movement of most species, with the exception of two ‘sedentary’ species (Cheilodactylus fuscus and Achoerodus viridis). However, generalizations could not be made between estimated mobility, duration of protection and MPA response. Densities of legal-sized C. fuscus were 2.8-times higher and fish were larger within GB relative to controls. Legal C. fuscus were more abundant in shallow areas of GB indicating that spear fishing influences local depth distributions. Surprisingly, mean densities of legal-sized ‘mobile’ Acanthopagrus australis were 2.6-times higher in CTB relative to controls, with a similar trend for GB, and for Girella tricuspidata in CTB. Response of ‘mobile’ species to protection was indicative of pre-existing differences between MPAs and controls, immigration rather than recruitment of fish, and/or intraspecific variation in movement. The lack of detectable effect for all other species and differential response between MPAs were attributed to mobility relative to the scale of MPAs, inadequate protection of habitats or depths, population recovery time, and partial protection versus 'no-take' status of MPAs. Overall results emphasise that small MPAs can have significant ecological value, even for highly mobile species. Importantly, as MPAs become smaller their location relative to habitat and depth, local aggregations, recruitment 'hotspots', adjacent habitats, and existing fishing pressure is critical in determining responses and rates of recovery.

Microsatellite markers were developed to provide information on population connectivity at scales ≤ 400 km for reef fishes with low (Parma microlepis) and high post-settlement dispersal capabilities (G. tricuspidata). It was hypothesized that P. microlepis would exhibit spatial genetic structure and a significant pattern of isolation-by-distance (IBD) at these scales, whereas G. tricuspidata would not. Genetic differentiation at seven microsatellite loci in P. microlepis, and six loci in G. tricuspidata were examined across multiple spatial scales. P. microlepis was collected from; sites (separated by 1-2 km), nested within locations (separated by 10-50 km), nested within three regions (separated by 70-80 km). G. tricuspidata were collected from a subset of the locations sampled for P. microlepis. This included five locations (separated by 50-60 km) spanning three sampling regions (separated by 70-100 km). There was no evidence that post-settlement dispersal capabilities influenced genetic structure. Broad-scale genetic homogeneity and lack of IBD was well supported for both species. The proportion of the total genetic variation attributable to differences among sampling regions, locations or sites was effectively zero (e.g. ΦPT ≤ 0.003 and RST ≤ 0.004). The geographic distribution of genetic diversity and the high polymorphism (P. microlepis, HE 0.21-0.95; G. tricuspidata, HE 0.65-0.97) was indicative of high mutation rates, large effective population sizes, and high rates of gene flow. Genetic homogeneity for fishes and invertebrates in central NSW suggests that gene flow important to genetic structure is driven by factors influencing pre-settlement dispersal such as the East Australian Current (EAC) and habitat continuity. Thus, genetic homogeneity is likely in other exploited reef fishes in this region which have similar pre-settlement durations (≥ 2 weeks). Scales of genetic homogeneity may not reflect demographically relevant dispersal distances. However, it does imply that populations of P. microlepis and G. tricuspidata are well connected from an evolutionary perspective and have large effective population sizes. This reduces the genetic risks associated with natural or anthropogenic declines in local populations. Furthermore, genetic diversity across spatial scales ≤ 400 km could be conserved within small MPAs as 99-100% of the total genetic variation for both species was represented within 1-2 km of reef. Future studies using genetics to determine population connectivity of reef fishes in central NSW should focus on species with very low dispersal capabilities, small population sizes, short life spans, and whose habitats are rare or patchily distributed along-shore.

The use of otolith chemistry as a natural tag requires the presence of differences in the aquatic environment that translate into differences in otolith chemistry. Consequently, most studies focus on populations distributed across large environmental gradients and spatial scales. This study examined spatial variation in otolith chemistry of the territorial damselfish P. microlepis at fine spatial scales in an exclusively marine environment. Solution-based inductively coupled plasma-mass spectrometry was used to measure the integrated otolith chemistry of individual fish, reflective of average environmental differences among regions (separated by 70-80 km), locations within regions (separated by 10-50 km), and between sites within locations (separated by 1-2 km). Mean concentrations of Sr/Ca, Ba/Ca, Mg/Ca, Mn/Ca, Cu/Ca, and Zn/Ca and multi-element signatures varied among regions, locations and sites. Fine scale differences accounted for the majority of the variability in the data and there was a trend for unique chemistries at some sites and locations. Multi-element signatures were good spatial discriminators, with 75-80% of fish correctly classified to the regions in which they were collected. It was difficult to establish simple causal relationships for variation in individual elements. However, regional multielement signatures were highly correlated with the behaviour of the EAC which delivers water masses varying in chemistry, temperature and salinity to the different regions. Results demonstrate that the magnitude of environmental variability within open coastal regions such as central NSW facilitates the use of otolith chemistry for determining population connectivity of reef fishes at scales < 100's km.

The thesis provides clear implications for management of reef fishes in central NSW, testable hypotheses, and priorities for future research. Overall results demonstrate the ecological value of small MPAs for protecting reef fishes of varying mobility, as well as population genetic diversity representative of broader-spatial scales. The determination of scales of 'spillover' of eggs, larvae and adults remains the greatest challenge. This study suggests that levels of gene flow will limit the utility of microsatellite markers for providing information on population connectivity for most reef fishes in central NSW. Given this, a combination of otolith chemistry, artificial tags, and modelling are the most promising techniques for future studies. Such studies should focus on species which demonstrated localised responses to MPA (e.g. C. fuscus, G. tricuspidata, and A. australis).

Item ID: 2014
Item Type: Thesis (PhD)
Keywords: dispersal, population connectivity, reef fishes, Marine Protected Areas, population dynamics, central NSW, Australia, gene flows, Cabbage Tree Bay, Gordon's Bay, otolith chemistry, Girella tricuspidata
Date Deposited: 01 Dec 2008 00:08
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
06 BIOLOGICAL SCIENCES > 0604 Genetics > 060411 Population, Ecological and Evolutionary Genetics @ 0%
06 BIOLOGICAL SCIENCES @ 0%
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