Gregory P. Jenkins

The influence of habitat structure on nearshore fish assemblages in a southern Australian embayment: Comparison of shallow seagrass, reef-algal and unvegetated sand habitats, with emphasis on their importance to recruitment

Assemblage structure and juvenile recruitment of fishes was compared amongst three habitats: seagrass, Heterozostera tasmanica (Martens ex Aschers.) den Hartog; reef-algal; and unvegetated sand. Sampling was conducted monthly from October 1993 to March 1994 at three locations in Port Phillip Bay, southern Australia. A fine-mesh seine net was used to sample subtidally at a depth of approximately 0.5 m. Fish assemblages differed primarily between structured habitats and unvegetated sand, and a number of species previously reported to occur in seagrass habitat were also found to utilise reef-algal habitat. Species richness was highest in seagrass and lowest in unvegetated sand with significant differences amongst all habitats. Total abundance varied between habitats depending on location and month examined, but the most common pattern was highest abundance in seagrass and lowest in unvegetated sand, with a significant difference only between seagrass and unvegetated sand. At the level of individual taxa, pipefishes of the genus Stigmatopora showed a strong preference for seagrass habitat from an early juvenile stage. The King George whiting, Sillaginodes punctata (Cuvier and Valenciennes), showed a complex relationship with habitat, occurring on both seagrass and reef-algae immediately after settlement, but, with growth, showing an increasing preference for reef-algae before finally shifting to unvegetated sand approximately 4 months after settlement. Other species previously found as juveniles on seagrass beds also recruited to reef-algal habitats. Strong locality effects were also found, particularly for King George whiting. This variation was unlikely to be related to habitat structure, because macrophyte biomass showed much greater variation within locations than amongst locations. We conclude that while the presence of structure per se is sufficient for the recruitment of many species, some taxa will discriminate amongst habitats based on structural characteristics.

Observational methods used in marine spatial monitoring of fishes and associated habitats: a review

Management areas are used in marine spatial planning to conserve biodiversity of marine ecosystems and to protect fish from fishing pressure. To evaluate the effectiveness of these protected areas, observational techniques are used to determine densities, sizes, biomass, habitat types and distribution of fish species in and around management areas. Two types of observational techniques are used in spatial monitoring: (1) fishery-independent techniques, which include underwater visual census (UVC), underwater video, remote sensing, acoustics, and experimental catch and effort data; and (2) fishery-dependent techniques, which include catch, effort and catch per unit effort data from commercial and recreational fisheries. This review summarises the applications, advantages, disadvantages and biases of each of these observational categories and highlights emerging technologies. The main finding from this review was that a combination of observational techniques, rather than a single method, was the most effective approach to marine spatial monitoring. For example, a combination of hydroacoustics for habitat mapping and UVC or video for fish surveys was one of the most cost-effective and efficient means of obtaining fish-habitat linkages and fish assemblage data. There are also emerging technologies that could increase the precision and efficiency of monitoring surveys. There is a need for continued development of non-intrusive technology for marine monitoring studies.

Potential effects of climate change on Australian estuaries and fish utilising estuaries: a review

Estuaries are especially vulnerable to the impacts of climate change because changes in climatic and hydrologic variables that influence freshwater and marine systems will also affect estuaries. We review potential impacts of climate change on Australian estuaries and their fish. Geographic differences are likely because southern Australian climates are predicted to become warmer and drier, whereas northern regions may see increased precipitation. Environmental factors, including salinity gradients, suspended sediment, dissolved oxygen and nutrient concentrations, will be influenced by changing freshwater input and other climate variables. Potential impacts will vary depending on the geomorphology of the estuary and the level of build-up of sand bars across estuarine entrances. Changes to estuarine fish assemblages will depend on associated changes to salinity and estuarine-mouth morphology. Marine migrants may be severely affected by closure of estuarine mouths, depending on whether species ‘must’ use estuarine habitat and the level of migratory v. resident individuals. Depending on how fish in coastal waters locate estuaries, there may be reduced cues associated with estuarine mouths, particularly in southern Australia, potentially influencing abundance. In summary, climate change is expected to have major consequences for Australian estuaries and associated fish, although the nature of impacts will show significant regional variation.

Spatial variation in feeding, prey distribution and food limitation of juvenile flounder Rhombosolea tapirina Günther

Abstract The feeding and prey distribution of juvenile greenback flounder Rhombosolea tapirina Gunther was compared between Swan Bay, a sheltered bay with muddy sediments and a well-developed seagrass-detrital system, and an adjacent area of Port Phillip Bay, a more exposed area with coarser sandy sediments. Meiofauna was dominated by benthic harpacticoid copepods in both bays. Harpacticoids in Port Phillip Bay, however, were mainly interstitial while epibenthic species were dominant in Swan Bay. The diet of juvenile flounder in Swan Bay was almost entirely composed of epibenthic harpacticoids. Juvenile flounder in Port Phillip Bay ate epibenthic harpacticoids, harpacticoid nauplii and gammaridean amphipods; interstitial harpacticoids were apparently not susceptible to flatfish predation. In Swan Bay, there was a rapid increase in the number of prey consumed with fish size but the mean size of prey was constant, reflecting the size distribution of harpacticoids in the environment. At one site in Port Phillip Bay, the pattern was similar except that the increase in prey number with growth was much lower. At a second site, prey number decreased while prey size increased with increasing fish size, due to a replacement of harpacticoid nauplii with much lower numbers of gammaridean amphipods. Consumption of prey in terms of weight increased allometrically with fish size at all sites. The rate was significantly higher (80%) in Swan Bay, suggesting that fish in Port Phillip Bay were food-limited. Gut clearance times for juvenile flounder feeding on epibenthic harpacticoids was estimated to be ≈ 13 h. Feeding rate, in terms of prey number, was approximately three to four times higher in Swan Bay relative to Port Phillip Bay. Although abundance of juvenile flounder in Swan Bay was higher than in Port Phillip Bay, prey abundances were also much higher, and predatory impact was estimated to be insignificant. Impact of predation also appeared insignificant in Port Phillip Bay although more data are needed.

The contributions of habitat structure and larval supply to broad-scale recruitment variability in a temperate zone, seagrass-associated fish

The contribution of habitat structure and larval supply to broad-scale spatial variability in recruitment of a temperate zone, seagrass associated fish, Sillaginodes punctata (Cuvier and Valenciennes), was investigated in Port Phillip Bay, Australia, from September to November, 1994. Replicate artificial seagrass beds were placed at five sites over a 50 km section of coastline, and artificial and adjacent natural seagrass were sampled approximately fortnightly for Sillaginodes punctata recruitment. Significant differences in recruitment amongst sites were apparent for both natural and artificial seagrass. A small but significant effect of habitat was detected where more recruits occurred in artificial relative to natural seagrass at sites with longer plant stems in the natural seagrass. The contribution of larval supply to spatial variability in recruitment was investigated by sampling natural seagrass, and concomitantly sampling the plankton immediately offshore for pre-settlement larvae. There was no significant correlation between larval abundances and recruitment, or between habitat structure and recruitment, over nine sites. We hypothesise that the high spatial variability in recruitment attributable to location is probably related to a combination of factors. These factors may include variation in larval supply, and also variation in the physical exposure of the location that influences mortality and movement of recruits in the early post-settlement stage.

Fish responses to experimental fragmentation of seagrass habitat.

Understanding the consequences of habitat fragmentation has come mostly from comparisons of patchy and continuous habitats. Because fragmentation is a process, it is most accurately studied by actively fragmenting large patches into multiple smaller patches. We fragmented artificial seagrass habitats and evaluated the impacts of fragmentation on fish abundance and species richness over time (1 day, 1 week, 1 month). Fish assemblages were compared among 4 treatments: control (single, continuous 9-m(2) patches); fragmented (single, continuous 9-m(2) patches fragmented to 4 discrete 1-m(2) patches); prefragmented/patchy (4 discrete 1-m(2) patches with the same arrangement as fragmented); and disturbance control (fragmented then immediately restored to continuous 9-m(2) patches). Patchy seagrass had lower species richness than actively fragmented seagrass (up to 39% fewer species after 1 week), but species richness in fragmented treatments was similar to controls. Total fish abundance did not vary among treatments and therefore was unaffected by fragmentation, patchiness, or disturbance caused during fragmentation. Patterns in species richness and abundance were consistent 1 day, 1 week, and 1 month after fragmentation. The expected decrease in fish abundance from reduced total seagrass area in fragmented and patchy seagrass appeared to be offset by greater fish density per unit area of seagrass. If fish prefer to live at edges, then the effects of seagrass habitat loss on fish abundance may have been offset by the increase (25%) in seagrass perimeter in fragmented and patchy treatments. Possibly there is some threshold of seagrass patch connectivity below which fish abundances cannot be maintained. The immediate responses of fish to experimental habitat fragmentation provided insights beyond those possible from comparisons of continuous and historically patchy habitat.

Evaluating the impact of predation by fish on the assemblage structure of fishes associated with seagrass (Heterozostera tasmanica) (Martens ex Ascherson) den Hartog, and unvegetated sand habitats

The role of fish predation in structuring assemblages of fish over unvegetated sand and seagrass was examined using enclosure and exclusion cages to manipulate the abundance of predatory fish from November 1998 to January 1999. In our exclusion experiment, piscivorous fish were excluded from patches of unvegetated sand and seagrass to measure how they altered abundances of small fishes, i.e., fish <10 cm in length. Habitats from which piscivorous fish were excluded contained more small fish than those with partial cages, which in turn contained more fish than uncaged areas. These patterns were consistent between unvegetated sand and seagrass areas, although the relative differences between predator treatments varied with habitat. Overall, small fish were more abundant in unvegetated sand than seagrass. Atherinids and syngnathids were the numerically dominant families of small fish and varied in complex ways amongst habitats and cage treatments. The abundance of atherinids varied inconsistently between cage treatments through time. Only during the final two sampling times did the abundance of atherinids vary significantly across cage treatments. Syngnathids were strongly associated with seagrass and were significantly more abundant in caged than uncaged habitats. In our enclosure experiment, five individuals of a single species of transient piscivorous fish, Western Australian salmon (Arripidae: Arripis truttacea Cuvier), were enclosed in cages to provide an estimate of the potential for this species to impact on small fish. The abundance of small fish varied significantly between cage treatments. Small fish were more abundant in enclosure cages and exclusion cages than uncaged areas; however, there was no difference in the abundance of small fish in enclosure cages and partial cages, and no difference between exclusion cages and partial cages. These patterns were consistent amongst habitats. Atherinids and syngnathids were again the numerically dominant families of small fish; atherinids varied more with cage structure while syngnathids did not vary statistically between cages, blocks (locations within which a single replicate of each cage treatment was applied) or habitats. Dietary analysis of caged A. truttacea demonstrated the potential for this species to influence the assemblage structure of small fish through predation - atherinids were consumed more frequently in unvegetated sand than seagrass, and syngnathids were consumed only in seagrass, where they are most abundant. Observations of significant cage or predation effects depended strongly on the time at which sampling was undertaken. In the case of the atherinids, no predation or cage effects were observed during the first two sampling times, but cage effects and predation effects strongly influenced abundances of fish during the third and fourth sampling times, respectively. Our study suggests that transient piscivorous fish may be important in structuring assemblages of small fish in seagrass and unvegetated sand, and seagrass beds may provide a refuge to fishes. But the importance of habitat complexity and predation, in relation to the potentially confounding effects of cage structure, depends strongly on the time at which treatments are sampled, and the periodicity and multiplicity of sampling should be considered in future predation studies.

Resource distribution influences positive edge effects in a seagrass fish

According to conceptual models, the distribution of resources plays a critical role in determining how organisms distribute themselves near habitat edges. These models are frequently used to achieve a mechanistic understanding of edge effects, but because they are based predominantly on correlative studies, there is need for a demonstration of causality, which is best done through experimentation. Using artificial seagrass habitat as an experimental system, we determined a likely mechanism underpinning edge effects in a seagrass fish. To test for edge effects, we measured fish abundance at edges (0-0.5 m) and interiors (0.5-1 m) of two patch configurations: continuous (single, continuous 9-m2 patches) and patchy (four discrete 1-m2 patches within a 9-m2 area). In continuous configurations, pipefish (Stigmatopora argus) were three times more abundant at edges than interiors (positive edge effect), but in patchy configurations there was no difference. The lack of edge effect in patchy configurations might be because patchy seagrass consisted entirely of edge habitat. We then used two approaches to test whether observed edge effects in continuous configurations were caused by increased availability of food at edges. First, we estimated the abundance of the major prey of pipefish, small crustaceans, across continuous seagrass configurations. Crustacean abundances were highest at seagrass edges, where they were 16% greater than in patch interiors. Second, we supplemented interiors of continuous treatment patches with live crustaceans, while control patches were supplemented with seawater. After five hours of supplementation, numbers of pipefish were similar between edges and interiors of treatment patches, while the strong edge effects were maintained in controls. This indicated that fish were moving from patch edges to interiors in response to food supplementation. These approaches strongly suggest that a numerically dominant fish species is more abundant at seagrass edges due to greater food availability, and provide experimental support for the resource distribution model as an explanation for edge effects.

Elements of habitat complexity that influence harpacticoid copepods associated with seagrass beds in a temperate bay

The influence of habitat structure on abundance and taxonomic richness of epibenthic harpacticoid copepods in seagrass beds of Port Phillip Bay, Australia was investigated using artificial seagrass plants. The density and length of artificial seagrass plants was manipulated at three sites over two sampling times. Results for artificial plants were also compared with controls without plants. The presence of habitat structure in the form of artificial seagrass resulted in a significant increase in harpacticoid abundance at all sites and taxonomic richness at one site. In terms of artificial seagrass treatments, higher blade density resulted in higher harpacticoid abundance, but blade length and surface area had no significant effect. Taxonomic richness did not vary amongst artificial seagrass treatments. At the site where taxonomic richness was increased in the presence of artificial seagrass, rarefaction showed that the result was consistent with a passive increase related to increased sample size. In contrast, although abundances in artificial seagrass were significantly higher than in controls at the other two sites, the taxonomic richness was similar to controls, suggesting that the full range of taxa available was represented in control samples. This study shows that structural aspects of complexity can have importance beyond the simple provision of complexity in the form of increased surface area of habitat, and may depend on the scale examined. Further, the study emphasises the importance of spatial and temporal replication of experiments to give generality to results.

Age and growth of co-occurring larvae of two flounder species, Rhombosolea tapirina and Ammotretis rostratus

Daily growth increments on otoliths were used to age larvae of the pleuronectid fluonders Rhombosolea tapirina Günther and Ammotretis rostratus Günther, collected from Port Phillip Bay, Victoria, Australia, in winter 1984. Daily formation of growth increments was confirmed for R. tapirina by examining the growth of the marginal increment on otoliths of larvae collected over two 24h periods in winter 1985. The first distinctive growth increment was laid down approximately 5 d after hatching, at the onset of external feeding. Growth of flounder larvae was exponential from an early feeding stage to notochord flexion at approximately 30 d after hatching. The specific growth rate was very similar for the two species, at slightly over 4% of standard length per day. Predicted absolute growth rate of R. tapirina larvae increased from approximately 0.10 mm d-1 in early feeding larvae to approximately 0.23 mm d-1 in flexion-stage larvae, compared with 0.12 to 0.28 mm d-1 for A. rostratus larvae of equivalent ages. Exponential models did not adequately describe growth of first-feeding larvae, which was slower than predicted. Growth in the field was faster than that recorded for the same species in the laboratory at higher water temperatures and prey abundances. Otolith growth accelerated markedly in relation to growth in length at the beginning of metamorphosis, causing a significant alteration in the morphology of growth increments, and eventually leading to the cessation of production of visible increments.