...area, perimeter, circularity, and rectangularity), as well as by Fourier analysis to capture the finer details of otolith shape. Variables were compared among four regions of the GBR separated by hundreds of kilometers, as well as among three reefs within each region, hundreds of meters to tens of kilometers apart. The temporal stability in otolith structure was examined by comparing two cohorts of fully recruited four-year-old P. leopardus collected two years before and two years after a significant disturbance in the southern parts of the GBR caused by a large tropical cyclone in March 1997. Results indicated the presence of at least two stocks of P. leopardus, although the structure of each stock varied depending on the cohort considered. The results highlight the importance of incorporating data from several years in studies using otolith morphology to discriminate temporary and possibly misleading signals from those that indicate persistent spatial structure in stocks. We conclude that otolith morphology can be used as an initial step to direct further research on groups of P. leopardus that have lived at least a part of their life in different environments.

**********

Marine fish populations are generally distributed over large geographical ranges in a heterogeneous environment. Variable physical and biological processes may restrict the exchange of dispersive larvae and adults between areas within a population's species range, resulting in groups of individuals that are phenotypically or genetically distinguishable. Genetic and environmental processes may also have variable effects on the productive capacity (e.g., growth and reproduction) of individuals in different areas. Such variations may be directly measurable with variable life history characteristics, but also indirectly measurable with phenotypic characteristics, such as meristic and morphological characteristics.

Groups of individuals with different genetic or phenotypic characteristics can be defined as separate stocks. Although the precise definition of a stock has been widely debated (see reviews by MacLean and Evans, 1981; Begg, 1998; Booke, 1999), its ultimate meaning should depend on the management objective related to its use. If the management objective is to protect the genetic diversity of a species, for example, genetic information should be sought. If the purpose is to prevent over-fishing and localized depletion, information about life history characteristics is required. This information is needed because groups with different life history characteristics may respond differently to fishing pressure and therefore have different vulnerabilities to over-fishing (Cole, 1954; Adams, 1980; Jennings et al., 1998).

Variations in morphological characteristics of otoliths have proved useful for identifying stocks for a range of temperate marine fishes (e.g., Bird et al., 1986; Castonguay et al., 1991; Smith, 1992; Campana and Casselman, 1993; Friedland and Reddin, 1994; Begg et al., 2001; Smith et al., 2002), but have not been examined for tropical reef fishes. Differences in morphological characteristics between putative stocks indicate that the stocks have spent some periods of their lives in different environments (Begg et al., 1999; Cadrin, 2000) and therefore have the potential to develop different life history characteristics. Otolith morphological characteristics used as indicators of stock separation generally fall within one of three categories. The first category includes the traditional one-dimensional linear measurements of size-related attributes, such as otolith length and width (e.g., Begg and Brown, 2000; Bolles and Begg, 2000) and distances between specific features on the otolith (e.g., Turan, 2000). Internal otolith measurements, such as nucleus length (e.g., Messeih, 1972; Neilson et al., 1985) and width of hyaline bands or increments (e.g., Begg et al., 2001) also fall within this category. The second category comprises two-dimensional size measurements, such as area, perimeter (e.g., Campana and Casselman, 1993; Begg and Brown, 2000; Bolles and Begg, 2000) and different shape indices, including circularity and rectangularity (e.g., Friedland and Reddin, 1994; Begg and Brown, 2000; Bolles and Begg 2000, Tuset et al., 2003). A third, more recent morphological technique examines the two-dimensional outline of otolith shape using Fourier analysis (e.g., Bird et al., 1986; Smith, 1992; Campana and Casselman, 1993; Begg and Brown, 2000; Smith et al., 2002). Fourier analysis produces a series of cosine and sine curves from the coordinates of a traced outline which, when added together, describe the outline of the traced form. The cosine and sine curves can be defined mathematically in a series of Fourier descriptors and used as variables to compare otolith shapes among individuals or potential stocks (Christopher and Waters, 1974; Younker and Ehrlich, 1977).

Plectropomus leopardus (common coral trout) (also known as leopard coralgrouper, FishBase (1)) is the most important commercially and recreationally harvested reef fish on the Great Barrier Reef (GBR), Australia (Mapstone et al. (2); Williams (3)). Plectropomus leopardus comprises between 35% and 50% of the commercial reef line catch annually (Mapstone et al. (2)) and in 2004 a total allowable commercial catch (TACC) of 1300 t was implemented. Regional (hundreds of km) or inter-reef (hundreds to thousands of m) variations have been demonstrated in some life history characteristics of P. leopardus on the GBR (Begg et al., 2005), such as differences in density (Ayling et al. (4)), reproductive strategies (Adams, 2002), size and age (Russ et al., 1995; Lou et al., 2005), and mortality (Russ et al., 1995, 1998; Mapstone et al., 2004). Current management arrangements (such as TACC, fish size limits, gear restrictions, recreational bag limits, and spatial and temporal closures), however, do not incorporate the localized or regional spatial structure in the life history characteristics of P. leopardus or any other exploited species on the GBR.

The overall aim of this study therefore was to examine the use of otolith morphology for determining the stock structure of P. leopardus on the GBR. We investigated the broad spatial scale of P. leopardus by comparing aspects of otolith morphology among fish collected from four regions of the GBR, separated by hundreds of kilometers (north to south). Otolith structure was also assessed at finer spatial scales, among P. leopardus collected from neighboring reefs separated by hundreds of meters to tens of kilometers, within each of the four regions. In addition, because temporal variation in otolith shape could confound the spatial information if samples were taken from only one time, we also compared otolith morphological characteristics from two cohorts of P. leopardus with non-overlapping life histories either side of a significant environmental disturbance that affected the southern half of the GBR (the large and persistent Cyclone Justin in March 1997). Spatially variable effects of the Cyclone, such as a significant drop in temperature and salinity in large parts of the GBR (AIMS (5)), provided us with a unique opportunity to test the temporal stability of spatial patterns in otolith morphology.

Methods and data analysis

Background

Common coral trout (Plectropomus leopardus) were collected as part of the Cooperative Research Centre for the Great Barrier Reef World Heritage Area (CRC Reef) Effects of Line Fishing (ELF) experiment (Campbell et al., 2001; Mapstone et al., 2004). The ELF experiment, which began in 1995 and concluded in 2006, monitored line-caught fish populations from a group of six neighboring reefs in each of four regions extending over 7[degrees] of latitude along the GBR (Fig. 1; Mapstone et al. (6), 1996, 1997, 2004; Davies et al. (7)). At the start of the experiment, four reefs in each region had been closed to fishing for 10-12 years under GBR Marine Park Zoning Plans (zoned Marine National Park B, MNP-B) and two reefs in each region had been open to fishing (zoned as General Use, GU). Two of the reefs closed to fishing remained closed during the experiment, other than to the annual research line fishing surveys. The other two closed reefs were each subjected to one year of fishing, in 1997 and 1999, after which they were closed again. The two reefs in each region that had historically been open to fishing were subjected to increased fishing pressure for one year (i.e., were temporarily opened). These reefs were then closed for five years before reverting to their original zoning status (GU).

[FIGURE 1 OMITTED]

All reefs were sampled each year in the austral spring (October-December) to coincide with the peak spawning period of the main target species, P. leopardus. Each reef was divided into six approximately equal-size, contiguous blocks, and sampled on a single day on each sampling occasion. Standardized commercial reef line fishing effort was distributed uniformly...

NOTE: All illustrations and photos have been removed from this article.



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