The results from this study indicate that adjacent to Tasmania, striped trumpeter constitute a single homogenous stock. Although theFST values between the Tasmanian samples are
large enough to represent isolation in an ecological if not evolutionary sense, considering the scale of the fixation index is from 0 to 1, with the lower the value the less the genetic difference, therefore it is possible that if the sample sizes were increased these values could become significant and the finding of a homogenous stock would be compromised. Data from striped trumpeter larval dispersal modeling (Tracey and Hobday, unpublished
data) indicate that the extended larval dispersal coupled with the oceanographic features
common to Tasmania create a situation where there is a high probability of significant mixing of larvae from isolated spawning grounds adjacent to Tasmania. This supports the case for homogeneity of the Tasmania population.
A similar finding indicating the absence of sub-structuring across southern Australia based on genetic analysis has been reported for N. macropterus (Grewe et al., 1994), al- though there is conjecture as to this finding. An otolith microchemistry study suggested a more complex population structure around the same area (Thresher et al., 1994). The discrepancy between these two findings has been ascribed to the relative sensitivities of the two techniques to the rates of exchange between the two populations (Thresher et al., 1994).
Identifying stock structure is an essential component to understanding a population’s ability to maintain viability under fishing pressure. The apparent homogenous structure of the population adjacent to Tasmania would indicate that a single stock management strat- egy is appropriate for this species. However, the caveat as to the robustness of the findings based solely on genetic analysis would warrant further study to confirm this finding. This could include analysis of: sequence data from the entire control region, sequence data from another coding region of the mitochondrial genome, a nuclear marker or even a divergent
5.4. Discussion
technique such as otolith microchemistry. All of these techniques have potential to further resolve the phylogenetic relationship between the Tasmanian sample locations. However, given the small-scale of the striped trumpeter fishery adjacent to Tasmania it would be difficult to warrant management in the traditional sense as anything other than a single stock based on genetic divergence alone. If it was shown that the life history parameters of the discrete sub-populations were different then separate stock assessments would be war- ranted and potentially alternate size limits, etc could be imposed to regulate exploitation of the sub-populations.
Genetic data relating to the structure of marine population on a species-by-species ba- sis is important from both an ecological and a conservation perspective. This information enhances the design of modern fisheries management techniques such as spatially imple- mented marine reserves. In addition, only by gaining knowledge about the population structures of a greater number of marine species can we make ecologically useful general- izations concerning their population dynamics and key issues such as the impact of pelagic larval duration on the broader ecology of the oceans.
Chapter
6
Shallow inshore reefs as juvenile
habitat for a temperate fish
displaying an ontogenic
depth-stratified migration
Abstract. An understanding of critical life history requirements, including habitat utilisation, is im-
portant when managing fish populations. Striped trumpeter (Latris lineata) support small, but iconic commercial and recreational fisheries in Tasmania, characterised by exceptional recruitment vari- ability and apparent ontogenic habitat preferences. Using otolith microchemistry we estimated the comparative contribution to the adult population of juvenile striped trumpeter from shallow inshore habitats. Juvenile striped trumpeter from a strong recruitment pulse (1993 cohort) were collected at age two from inshore reefs and as adults at age six from deeper offshore reefs around the coast of Tasmania. Natural variations were identified in the concentrations of lithium and strontium within the incremental structure of the observed otoliths. Discriminant analysis suggested that 70% of adults sampled originated from an inshore juvenile habitat, 13% were from deeper reefs and 17% could not be statistically allocated with confidence. These results emphasise the importance of shallow in- shore reefs for this species by quantifying the disproportionate contribution of juveniles to the adult biomass from these areas.
6.1
Introduction
Understanding the links between different life history stages and habitat utilisation is es- sential for the effective management and the sustainable utilization of aquatic resources, particularly when juvenile and/or nursery habitats are distinct from adult habitats. There are many examples of coastal fish species that utilise discrete habitats, such as estuaries, seagrass meadows, kelp forests and reef systems, as juveniles before being recruited to adult populations (for review see Gillanders et al., 2003). An understanding of the spa- tial complexities and habitat utilisation involved in successful recruitment is particularly important for species that display strong recruitment variability, a characteristic of many temperate marine species (Rothschild, 1986).
Striped trumpeter (Latris lineata) is a demersal teleost that is widely distributed through- out the southern hemisphere including Australia, New Zealand and several isolated island locations in the Indian, Pacific and Atlantic Oceans (Tracey et al., 2006). Aquaculture trials have found that for the first nine months of life striped trumpeter are pelagic lar- vae/paperfish, before undertaking a significant morphological change thought to coincide with settlement to demersal reefs based on the observation of juveniles in the field (Tracey unpublished data). Evidence from fishery and scientific sampling in southeast Australia suggest that throughout the juvenile phase striped trumpeter have a distinct preference for shallow inshore rocky reefs (<50 m in depth) (Tracey & Lyle, 2005). As striped trumpeter approach maturity at approximately five years of age, they move offshore to reef areas on the upper edge of the continental shelf at depths ranging from 80 – 250 m (Tracey & Lyle, 2005). The affinity of juveniles to these inshore habitats around Tasmania was most evident through 1995-1998 when juvenile striped trumpeter were unusually abundant in inshore gillnet catches, this cohort was back-calculated to a particularly strong recruitment pulse spawned in 1993 (Tracey & Lyle, 2005).
These observations pose the question of whether inshore reefs do in fact represent a crucial habitat in the life history of striped trumpeter or whether juveniles also recruit to
6.2. Material and Methods
the adult population from other areas, including deeper reefs.
Determining previous juvenile habitats of individual adults using conventional tagging techniques is difficult because of the small size of larvae and juveniles, high rates of mor- tality at early life history stages, and the large numbers that need to be tagged in order to recover a sufficient sample size (Gillanders, 2005). Otolith microchemistry is perhaps the most successful method available to date to back-classify individuals to particular juvenile nursery areas (Gillanders & Kingsford, 2000; Forrester & Swearer, 2002). The method depends on environmental gradients between locations producing differences in the suite of elements incorporated into the otoliths of individuals from each of the environments.
Our underlying hypothesis is that the apparent depth-based distribution by life history stage displayed by striped trumpeter will be reflected in otolith microchemistry such that the relative importance of the inshore reefs as nursery habitats can be evaluated.
In this study a single cohort was monitored at various times over a seven year pe- riod with microchemical changes in the otolith structure associated with movement from shallow coastal reefs to deeper offshore reefs. The specific objective was to estimate the proportion of adults from the 1993 cohort that could be linked back to inshore reefs as juveniles, and hence evaluate the significance of shallow, inshore reefs as juvenile habitat for striped trumpeter.