Northern Spencer Gulf Research Articles

Comparing maximum entropy modelling methods to inform aquaculture site selection for novel seaweed species

Maximum entropy (maxent) modelling is a widely used method for developing species distribution models (SDMs), but default maxent modelling methods can result in overly complex models with poor transferability. Methods suggested to reduce overfitting include increasing regularisation, using only linear and quadratic features, or applying forward selection of predictors using maximum likelihood (ML) methods. We built models using these options to determine environmental suitability within existing aquaculture zones for eight seaweed species, four red (Rhodophyta: Florideophyceae) and four brown (Ochrophyta: Phaeophyceae), that are being investigated for aquaculture in southern Australia. Forward selection models were the most parsimonious, but we encountered failure of ML methods for Pterocladia lucida (Rhodophyta) due to separation. Separation is a known issue for logistic regression and has recently been recognised in maxent models. Separation occurs where a variable, or combination of variables, is a perfect predictor for a binary response, here, species occurrence, and results in ML parameter estimates tending to infinity. One method for obtaining finite parameter estimates under separation is to apply a Cauchy prior distribution for coefficients. We therefore also built models for each species using a Cauchy-prior version of the forward selection method, and found that these models performed similarly to those built with ML methods. Default models achieved marginally higher predictive performance than other options based on training data metrics, but simpler models performed equivalently to, or better than, default models at predicting independent presence-absence test data. Predictive performance using test data varied considerably between species, but the difference in performance between models within each species was generally small. Our results confirm the concern that default maxent models may suffer from over-fitting and poor transferability. Model transferability and interpretability were important for our purpose, hence, based on the principle of parsimony, forward selection models were preferred. We also found that forward selection models retained similar predictive performance to the best model as assessed by each metric, further supporting use of these models. Where ML methods failed due to separation, the use of the Cauchy-prior method was a viable alternative. Predictions for the region of interest (Spencer Gulf, South Australia) were generated using the most parsimonious models, and Solieria robusta (Rhodophyta) showed the highest predicted suitability of the eight candidate species within existing aquaculture zones, especially in northern Spencer Gulf. Predicted suitability was low for the other Rhodophyta considered, while each of the Phaeophyceae showed moderate to high suitability in at least some southern Spencer Gulf aquaculture zones. These model results help to inform selection of the best candidate species and suitable farming areas for future research.

Marine Transgression, Aboriginal Narratives and the Creation of Yorke Peninsula/Guuranda, South Australia

This paper provides new palaeogeographical reconstructions of the Spencer Gulf and Yorke Peninsula/Guuranda region of South Australia for the period ca. 20,000–6000 cal. BP. The rich complexity of traditional Aboriginal (Narungga) sea-level narratives is also explored. These narratives, which form a highly significant system of knowledge, are considered together by the Narungga and non-Narungga authors of the paper to generate a new dialogue about Aboriginal traditions and scientific data. The dialogue between the Narungga knowledge systems and palaeogeographic mapping reveals a strong concordance for events/time slices inclusive of the onset of marine transgression, the creation of northern Spencer Gulf and the formation of some islands—while also establishing that there are deeper complexities inherent within Narungga narratives that demonstrate layers of meaning at multiple scales. The palaeogeographic reconstructions are also considered in relation to the known archaeology and geomorphology of the region as well as new observations arising from this research. Targets for future underwater (and terrestrial) investigation relating to the periods of marine transgression are also outlined.

Evidence for a broad-scale decline in giant Australian cuttlefish (Sepia apama) abundance from non-targeted survey data

Little is known about the population trajectory and dynamics of many marine invertebrates because of a lack of robust observational data. The giant Australian cuttlefish (Sepia apama) is IUCN-listed as Near Threatened because the largest known breeding aggregation of this species in northern Spencer Gulf, South Australia, has declined markedly since the turn of the century. We used by-catch records from long-term trawl surveys to derive abundance data for S. apama and commercial cuttlefish harvest data as a measure of exploitation. Using Bayesian hierarchical models to account for zero-inflation and spatial dependence in these abundance counts, we demonstrated a high probability of broad-scale declines in the density of S. apama, particularly surrounding the primary aggregation site, which supports the recent closure of the entire S. apama fishery in northern Spencer Gulf. Historical harvest data were positively correlated with S. apama density estimated from the trawl surveys, suggesting that the commercial cuttlefish catch tracks the species abundance. Our results also indicated the possibility that the known S. apama breeding grounds might be supplemented by individuals that were spawned elsewhere in northern Spencer Gulf.

Spatial variation in shape of otoliths for southern garfish Hyporhamphus melanochir – Contribution to stock structure

Otolith morphometrics and shape analyses were used to delineate the sub-population structure and determine the level of mixing of southern garfish in South Australian waters. Adult garfish from the 2+ age class were collected from three sites within each of six regions along the South Australian coast: Northern Gulf St Vincent, Southwest Gulf St Vincent, Kangaroo Island, Northern Spencer Gulf, Southern Spencer Gulf and the West Coast of Eyre Peninsula. This study adopted a multi-stage statistical approach exploring the spatial variation in gross otolith morphology and through elliptical Fourier analysis separately, and in combination, over both broad (regional) and fine (site) spatial scales. Stepwise discriminant function analysis revealed significant separation across both spatial scales; however, overall classification success of garfish to their site of capture was poor (<45%). Despite this, the results indicated some level of structuring within South Australia's southern garfish population, as fish were correctly classified at rates that were 32% better than would be expected by chance. At least six putative regional components with various levels of intermixing were detected based on differences in otolith shape. This result refines the spatial resolution of the fishery and consequently highlights the vulnerability of these regional components to localized depletion, particularly as the resource is currently considered to be over-exploited.

The spawning dynamics of snapper (Chrysophrys auratus) in northern Spencer Gulf, South Australia

This study investigated the inter- and intra-annual variation in the spawning dynamics of snapper (Chrysophrys auratus). Reproductive samples from northern Spencer Gulf, South Australia were collected over three seasons (2005/06, 2006/07 and 2007/08) and were analysed macro and microscopically. Spawning activity was determined by calculating estimates of spawning fraction and batch fecundities. The onset of spawning occurred in November but varied between years and corresponded with times when water temperature was between 18 °C and 20 °C. The length of the spawning season also differed between years. In each year the peak spawning activity occurred during December when fish spawned almost daily. Spawning frequency and relative batch size did not differ between the first two spawning seasons but, in the third season, batch size was considerably greater and spawning fractionally lower. These results are contrasted with a known recruitment of 0+ over the three years of the study.

The use of food resources by 0+ snapper, Chrysophrys auratus, from northern Spencer Gulf, South Australia

Food availability is an important factor in survival and growth of juvenile fish and has been proposed as a major factor shaping the patterns of distribution and abundance of 0+ snapper, Chrysophrys auratus, in New Zealand and Japan. Thus, we tested the hypothesis that diet of 0+ snapper would exhibit spatial variation related to their abundance within a major nursery region. Therefore, we described the stomach contents of 0+ snapper collected from within a nursery region in areas of low and high abundance of 0+ snapper. The data also provided the opportunity to consider sized based differences in diet and temporal feeding patterns. 0+ snapper were generalist carnivores that preyed on a wide range of invertebrates and fish. No evidence of sized based dietary variation was identified but feeding either ceased or was much reduced during the hours of darkness. The major finding was that more polychaetes were present in the stomachs of 0+ snapper in the area of high 0+ snapper abundance than in the areas of low 0+ snapper abundance. The spatial differences in diet observed supports the hypothesis that prey availability is important as a driver of habitat selection by 0+ fish.

The salinity response of an inverse estuary to climate change &amp; desalination

Abstract Just as runoff primarily governs the tidally-averaged salinity at a point within a classical estuary, this paper examines the extent to which the salinity of an inverse or negative estuary (one that experiences salinities higher than the adjacent ocean) responds to loss of freshwater. The hypothesis that ’the mean salinity of an inverse estuary varies predictably from the effects of net freshwater loss, and that the mean salinity at any time reflects the integration of these effects during a constant, measurable prior period’ is tested using observations spanning 27 years from Spencer Gulf in South Australia. It is shown that, on any day, the mean salinity of the whole (∼500 km2) of northern Spencer Gulf (NSG), deduced from broad oceanographic surveys on fourteen separate occasions between 1982 and 2009, mirrors the integration of net evaporation for a prior period of approximately six months. Linear regression (with a correlation coefficient of 0.89) indicated that a sustained change of one millimetre per day of net evaporation caused the mean salinity of NSG to change by 0.23. It is also shown that the regression relationship deduced using only 1980s data, remained entirely consistent with new data collected in 2008 and 2009, despite substantial longer-term trends during the intervening decades. Since net evaporation is essentially a continuous variable (measured approximately daily) its use as a proxy measure for mean salinity permits deductions about NSG’s long-term salinity behaviour. For example, the Gulf is inferred to reach its annual maximum salinity three months after the summertime net evaporation maximum, and the annual salinity cycle is almost exactly symmetric between rising and falling phases. This behaviour is surprisingly consistent with a simple one-dimensional model of inverse estuarine salt dynamics. Assuming that desalination, as it affects the mean salinity of the whole of NSG, is equivalent to evaporation in its removal of freshwater, estimates of the changes of NSG mean salinity based on proposed desalination rates, and based on projections of local climate change effects in 2030 and 2070, are provided. Finally, each of the principal oceanographic mechanisms that contributes to the efflux of salt from northern Spencer Gulf is examined to show that anticipated perturbations (both natural and anthropogenic) are not expected to perturb the dynamics beyond the Gulf’s ability to respond in the manner described.

Variation in productivity of a key snapper, Chrysophrys auratus, fishery related to recruitment and fleet dynamics

Abstract Temporal variation in production of the Chrysophrys auratus (Forster) fishery of Northern Spencer Gulf, South Australia, between 1984 and 2009 was investigated using commercial fishery statistics and estimates of age and size structures from market sampling. Fishery production was divisible into two periods, i.e. from 1984 to early 2000s and the middle to late 2000s. During the former, cycles in production were relatable to variable fishable biomass. Age structures that displayed considerable differences in age class strength reflected interannual variation in juvenile recruitment, which resulted in high variability in the strength of year classes entering the fishery. Periodic strong year classes were the major influence on fishable biomass as they passed through the population over numerous years. For the latter period, fishery statistics were conflicting as catches and effort were relatively low, whilst the estimates of catch per unit effort (CPUE) were consistently high. These statistics reflect a considerable change to the structure of the fishery that involved a reduction in number of fishers and fishing effort. The fishery statistics may indicate hyperstability, with CPUE estimates remaining high despite declining biomass because of the aggregative behaviour of the fish and the efficiency of remaining fishers.

Spatial variation in parasite abundance: evidence of geographical population structuring in southern garfish Hyporhamphus melanochir

Southern garfish Hyporhamphus melanochir were examined for metazoan parasites from nine sites in three regions (Spencer Gulf, Gulf St Vincent and northern Kangaroo Island) in South Australia to document parasite assemblages, identify candidate species suitable for use as biological tags and investigate spatial variation in parasite abundance. Four ectoparasite and 10 endoparasite species were identified representing Cestoda, Trematoda, Monogenea, Nematoda, Acanthocephala, Copepoda and Isopoda. Lernaeenicus hemirhamphi, Micracanthorhynchina hemirhamphi, Mothocya halei and Philometra sp. were suggested for 'permanent' biological markers. Multivariate discriminant function analysis showed that most sites could be distinguished based on differences in parasite abundance. Four endoparasites (Conohelmins sp., Hysterothylacium sp., M. hemirhamphi and Philometra sp.) were most important for site characterization. Limited spatial variation in permanent endoparasite abundance among localities in northern Spencer Gulf provided evidence for a distinct northern Spencer Gulf population with little interregional mixing. In contrast, considerable spatial variation in permanent endoparasite abundance between localities sampled off Kangaroo Island implied limited local movement and suggested H. melanochir may comprise a metapopulation structure. These results largely align with recent evidence from otolith chemistry that indicates fine-scale geographical population structuring in South Australian waters.

Environmental effects of yellowtail kingfish aquaculture in South Australia

The culture of yellowtail kingfish Seriola lalandi in South Australia is one of the most rapidly growing aquaculture sectors in Australia. To date, there is a paucity of information on the environmental impacts of this industry, due to its relatively small size in comparison to other industry sectors. Here, we report on a study examining the response of a range of environmental variables to yellowtail kingfish aquaculture in Fitzgerald Bay, northern Spencer Gulf, South Australia. The clearest response occurred in several chemical variables, with an 81% increase in ammonia con- centrations adjacent to cages relative to controls, increased sediment organic carbon (7 to 46%) and increased porewater phosphorus in the sediments (80 to 3077%). While there were also statistically significant effects on infaunal and epifaunal assemblages, results for both groups were equivocal because of high small-scale variability. For the infauna, assemblages at the sites furthest from cages tended to be similar to those adjacent to cages, while those at intermediate distances differed. No impacts were detected on phytoplankton, or on seagrasses (although the latter only occurred >250 m from any aquaculture cage). The lack of clear responses in the biotic datasets is probably related to the relatively low production levels (<2000 t yr -1 at the time of the study), although the responses in the chemical variables provide a valuable early warning as production increases.

Stock discrimination of Southern Garfish (Hyporhamphus melanochir) by stable isotope ratio analysis of otolith aragonite

Stable isotope analysis (δ18O and δ13C) of otolith aragonite was used to infer ontogenetic patterns of movement, delineate sub-population structure, and determine the mixing of Southern Garfish (Hyporhamphus melanochir) in South Australian waters. Adult garfish from the 2+ age class were collected from three sites within each of six regions along the South Australian coast: Northern Gulf St. Vincent, South West Gulf St. Vincent, Kangaroo Island, Northern Spencer Gulf, South West Spencer Gulf and the West Coast of Eyre Peninsula. Significant spatial variation was detected in mean δ18O and δ13C values among sites within regions and as a function of fish age. Salinity, rather than sea surface temperature, appeared to account for regional variation in δ18O, whereas variation in diet and metabolic processes were suggested to account for observed differences in δ13C. Otolith oxygen and carbon stable isotope values indicated that the South Australian Southern Garfish fishery is comprised of multiple, regional, population components that persist through time. Both the West Coast and Northern Spencer Gulf regions constitute distinct population components that exhibit little inter-regional mixing and therefore may be considered as discrete management units. The South West Spencer Gulf region may also be considered a separate population component, however, its level of connectivity with Gulf St. Vincent is uncertain. With the exception of juvenile garfish from Kangaroo Island, there was no clear regional separation within Gulf St. Vincent.

A Record of Holocene Benthic Foraminifera in Vibrocore SG#279, Spencer Gulf, South Australia

Core SG#279 recovered 1.1 m of Holocene bioclastic sediment from northern Spencer Gulf, South Australia. Benthic foraminifera preserved in these sediments provide proxies for the postglacial marine transgression. Species that are characteristic of tidal dominated, coastal seagrass meadows, Cribrobulimina mixta + Nubecularia lucifuga + Peneroplis planatus + Discorbis dimidiatus decrease in numbers upcore confirming their potential to signify a rising sea. Massilina milletti, which favours the deeper waters of northern Spencer Gulf, increases in abundance upcore, thus also signifying the transgression. Quinqueloculina lamarckiana, which positively correlates with increasing water depth in northeastern Gulf St Vincent, similarly provides evidence of the transgression. Maximum water depth is indicated for the core sediments at 40 cm. Data based on the distributions and relative numbers of Elphidium crispum and Elphidium macelliforme were not in accord with those of the other proxy species. It is concluded that the Elphidium ratio is unreliable when applied to sediments deposited at depths of <15 m. Above 40 cm in the core, the distribution of M. milletti provides evidence of marine regression that has been attributed to hydroisostatic uplift of northern Spencer Gulf. The foraminifera provide no credible evidence of any late Holocene sea level rise.

Isolation of additional polymorphic tri‐ and tetranucleotide microsatellite loci for the giant Australian cuttlefish (Sepia apama)

AbstractWe isolated 10 polymorphic microsatellite loci for the giant Australian cuttlefish, Sepia apama, from a genomic library enriched for (AAC)n and (AAAG)n repetitive elements. In the nine loci that reliably amplified, the number of alleles ranged from four to 12 per locus with observed heterozygosity ranging from 0.343 to 0.926. These and a previously developed set of six loci will be useful for analysis of genetic structure of populations and determining input to a massive seasonal breeding aggregation in northern Spencer Gulf, Australia.

Evidence for multiple year classes of the giant Australian cuttlefish Sepia apama in northern Spencer Gulf, South Australia

Giant Australian cuttlefish form a mass spawning aggregation at a single site in northern Spencer Gulf (NSG) in South Australia every austral winter. Samples of cuttlefish were collected from this region over three consecutive years. Analysis of regular growth increments in the cuttlebones of these individuals, revealed a polymorphism in growth pattern for both sexes. Three distinct “bone patterns” were identified based on the variation in increment widths over the lengths of the bones. All bones analysed conformed to one of the three bone patterns, and the increment width patterns were consistent between years. Interpretation of the patterns, suggested that Sepia apama have two alternative life cycles. The first involves rapid juvenile growth during the first summer after hatching, with maturity reached within 7–8 months. These individuals return to spawn in their first year as small individuals. The second life cycle involves much slower juvenile growth during the first summer, with maturity deferred until their second year, when they return to spawn as much larger individuals. Thus, the age compositions of populations of S. apama in the NSG appear to consist of two year classes for both sexes.

A Tagging Study on Yellowtail Kingfish (Seriola Lalandi) and Samson Fish (S. Hippos) in South Australian Waters

SummaryWild yellowtail kingfish (Seriola lalandi Valenciennes, 1833) and Samson fish (S. hippos Günther, 1876) were tagged with nylon-headed, single-barbed dart tags between December 1, 2004 and December 31, 2006 in Spencer Gulf and offshore from the west coast of Eyre Peninsula, South Australia. Two-hundred and forty-one S. lalandi and 73 S. hippos were tagged. Twenty-four S. lalandi were recaptured and the maximum distance between capture points was 130 km and the maximum time at liberty was 442 days. Two S. hippos were recaptured, of which one was at liberty for a maximum of 378 days. Both S. hippos were recaptured at the original capture site. Recapture results indicate that large S. lalandi remain in, or return to, northern Spencer Gulf. This region may be important for aggregations of large, reproductively mature S. lalandi. One large S. lalandi tagged at Port Augusta was recaptured near Fitzgerald Bay, indicating that wild fish may move past S. lalandi sea-cage farms in this region. This course of movement may provide opportunities for disease and parasite interactions between wild and farmed fish. An additional outcome of this research was the residual impact that it has had on sustainable fishing practices in South Australia with increased recreational fisher participation in the tag and release of S. lalandi and S. hippos.

Foraminiferal record of the postglacial (Holocene) marine transgression and subsequent regression, northern Spencer Gulf, South Australia

Core SG120 recovered 3.65 m of Quaternary sediment from a northern, shallow-water environment of Spencer Gulf, a marine embayment into the southern continental margin of Australia. Previous investigations had revealed that the upper interval 0 – 148 cm is Holocene marine bioclastic sediment, and that the lower Late Pleistocene interval 250 – 365 cm, with its carbonate palaeosol, had a similar marine origin. However, the age and origin of the interval 148 – 250 cm remained subject to ambiguous interpretation. Re-examination of core SG120, employing detailed foraminiferal analysis, has revealed that this middle unit records the earliest sedimentation associated with the postglacial marine transgression into the northern gulf. These basal Holocene sediments, which incorporated broken, corroded and carbonate-encrusted tests from the underlying palaeosol, together with tests of more pristine appearance, were deposited in a shallow-water, seagrass sandflat environment similar to those in coastal settings of the modern gulf. The lithological change at 148 cm has therefore been reinterpreted as a facies change related to increasing water depth. Radiocarbon analyses of fossil molluscs support this interpretation and reveal that marine transgression, at the site of SG120, was initiated prior to 8600 y cal BP. Selected species of foraminifers (Nubecularia lucifuga, Massilina milletti, Peneroplis planatus, Discorbis dimidiatus, Elphidium crispum and E. macelliforme) together reveal a consistent record of the final stages of the transgression with maximum water depth indicated at a core depth of 90 cm. Subsequent regression, which has been attributed to the combined effects of hydroisostatic uplift and sediment aggradation, is equally recorded by the foraminiferal assemblages.

Paradeontacylix godfreyi n. sp. (Digenea: Sanguinicolidae) from the heart of wild Seriola lalandi (Perciformes: Carangidae) in southern Australia

Paradeontacylix godfreyi n. sp. (Digenea: Sanguinicolidae) is described from the heart of wild yellowtail kingfish, Seriola lalandi Valenciennes, 1833, collected near Port Augusta, northern Spencer Gulf, South Australia. One specimen of P. godfreyi was also collected from the heart of a single wild specimen of S. lalandi captured near Killarney, Victoria. Paradeontacylix godfreyi is distinguished from other species in the genus by a combination of morphological characters including the shape and number of posterior tegumental spines, the number of rows of tegumental spines along the ventral body margin, the maximum number of marginal tegumental spines per row, the number of testes and the extent of the testicular field. Comparisons are made with a Paradeontacylix sp. collected from the heart of wild Samson fish, S. hippos Günther, 1876 from Greenly Island, South Australia and from the heart of wild S. lalandi from Killarney, Victoria. We also document a new host record for P. sanguinicoloides McIntosh, 1934 from the heart of wild S. hippos from Greenly Island, South Australia. The importance of determining potential intermediate hosts for Paradeontacylix species in relation to South Australian S. lalandi aquaculture is discussed.

Ontogenetic shifts in the distribution and reproductive patterns of Australian anchovy (Engraulis australis) determined by otolith microstructure analysis

Upwelling systems can provide ideal conditions for survival and growth of larval fishes. However, these unstable environments are highly dispersive and mortalities can be high. Anchovies (Engraulis spp.) utilise a range of behavioural strategies to increase egg and larval survival. Ontogenetic shifts in distribution have been proposed for Australian anchovy (Engraulis australis), but no supporting otolith-based age studies have been published to date. The present study used otolith increment counts to age larval, juvenile and adult Australian anchovy, which spawn throughout South Australian gulf and shelf waters. Only fish up to 1 year of age occurred in the northern Spencer Gulf, where eggs and larvae in high densities are retained in a nursery area favourable to anchovies. The southern gulf area was inhabited by 1, 2 and 3 year olds. Older anchovy were found in offshore shelf waters, where upwelling occurs and larval survivorship may be high. We hypothesise that Australian anchovy move offshore with age to utilise the wide range of environments that provide suitable spawning and nursery areas for this species.

Molecular and morphological analyses of the cuttlefish Sepia apama indicate a complex population structure

The giant Australian cuttlefish Sepia apama Gray, 1849 annually forms a massive and unique spawning aggregation in northern Spencer Gulf, South Australia, which has attracted commercial fishing interests in recent years. However, many basic life-history characteristics of S. apama are unknown, and anecdotal evidence suggests that there is more than one species. The present study assessed the population structure and species status of S. apama using data from allozyme electrophoresis, microsatellite loci, nucleotide sequences of the mitochondrial COXIII gene, multivariate morphometrics and colour patterns. Analyses of allozyme and microsatellite allele frequencies revealed two very divergent but geographically separated populations consisting of specimens from the east coast and southern Australia. However, the presence of a heterozygote in a putative contact zone between the east coast and southern Australia suggested that these populations were not reproductively isolated. Mitochondrial haplo-types seem to have introgressed further north into the contact zone than have nuclear alleles. Differences in colour patterns that previously had been attributed anecdotally to different geographic populations were, in fact, correlated with sexual dimorphism. These data are most consistent with S. apama being one species the populations of which were geographically isolated in the past (historical vicariance) and have come into secondary contact. Comparison of microsatellite allele frequencies among four South Australian samples indicated significant deviations from panmixia. South Australian samples were also reliably diagnosed by means of multivariate morphometrics. Significant differences in mantle length were observed among populations.

Dynamics in 0+ recruitment and early life history for snapper (Pagrus auratus, Sparidae) in South Australia

The population dynamics of snapper (Pagrus auratus) relate to interannual variation in 0+ recruitment. The focus of the present study was to improve our understanding of this variation for snapper in northern Spencer Gulf, South Australia. The objectives of this study were to describe the distribution and abundance of 0+ juveniles annually from 2000 to 2002 and to determine early life history characteristics through retrospective analysis of otolith microstructure from captured fish. The environmental characteristics differed between years, with 2001 being extremely hot and 2002 the coldest summer recorded. Sampling was undertaken in April after spawning and settlement were complete. The estimates of recruitment were relatively low, suggesting a complex relationship with sea surface temperature. Each year new recruits occurred at the same places that were bare and flat with muddy substratum, suggesting nursery areas are actively selected by the recruits. The sagittae from juveniles had clear daily increments and settlement mark, allowing age and presettlement duration to be determined. Growth rates varied considerably both within and between years, and were related to water temperature, which affected the presettlement duration. Based on spawn dates, successful recruitment resulted from specific periods through the reproductive season that appeared to relate to the specific water temperature regimen in each year.