Does the composition of the demersal fish assemblages in temperate coastal waters change with depth and undergo consistent seasonal changes?

The aim of this study was to determine whether the composition of the demersal fish fauna in coastal marine waters in temperate Australia changes markedly with increasing water depth and distance from the shore and whether the composition of the fish fauna in water depths of 5 to 35 m undergoes cyclic, seasonal changes. Samples of demersal fishes were therefore collected by trawling over the predominantly sandy substrate at nine sites located in water depths of 5 to 15 m or 20 to 35 m and within 20 km of the shore in four regions along ∼200 km on the lower west coast of Australia. The sampling regime involved trawling for fishes at each site at night in seven consecutive seasons between the summer of 1990/1991 and winter of 1992. A total of 72 435 fishes, representing 77 families, 143 genera and 172 species was caught. The compositions of the fish faunas in offshore waters with depths of 5 to 35 m were shown to differ markedly from those previously recorded for nearshore marine waters in the same regions. However, as some species, such as Sillago burrus, S. vittata, S. bassensis and Rhabdosargus sarba, increase in size, they move out from their nursery areas in nearshore waters into deeper and more offshore waters, where spawning occurs. Ordination showed that, in each of the four regions, the composition of the fish fauna in depths of 5 to 15 m differs from that in depths of 20 to 35 m. This difference is attributable to the fact that some species, such as  S. burrus, S. vittata and Upeneichthys lineatus, are far more abundant in depths of 5 to 15 m, whereas other species, such as S. robusta, U. stotti and Lepidotrigla modesta, occur predominantly in depths of 20 to 35 m. However, the samples collected from the single site that was inshore but in deeper water demonstrate that the composition of the fish fauna is influenced by distance from shore as well as by water depth. The compositions of the fish faunas differed with latitude, largely due to the fact that some subtropical species, such as Polyspina piosae, S. burrus and  S. robusta, did not extend down into the more southern regions. Ordination also showed that the composition of the fish faunas at all but one of the nine sites underwent pronounced and consistent cyclic, seasonal changes. This seasonal cyclicity at the different sites was attributable to sequential patterns of immigrations and emigrations by a number of fish species during the course of the year. These seasonal migrations involved, inter alia (1) movements of certain species from their nursery areas into these deeper waters, e.g.  S. bassensis and Scobinichthys granulatus; (2) migrations into and off the sandy areas of the inner continental shelf, e.g. Arnoglossus muelleri; (3) migrations to spawning areas, e.g. Sillago robusta; and (4) movements into areas where detached macrophytes accumulate in winter, e.g. Cnidoglanis macrocephalus and Apogon rueppellii. Received: 21 August 1998 / Accepted: 9 February 1999     

Sequence divergence of mitochondrial DNA indicates cryptic biodiversity in Octopus vulgaris and supports the taxonomic distinctiveness of Octopus mimus (Cephalopoda: Octopodidae)

DNA sequence diversity of octopods was investigated using the mitochondrial cytochrome oxidase III gene (mtCOIII). DNA was obtained from ethanol- or formalin-fixed tissue of 15 specimens belonging to Octopus mimus Gould, 1852, Octopus vulgaris Cuvier, 1797 and Scaeurgus unicirrhus d'Orbigny, 1840, from coastal waters of the Mediterranean (France), the southwestern Atlantic (Brazil), the Caribbean (Costa Rica) and the southeastern/tropical Pacific (north Chile/Costa Rica). A 612 bp fragment of the mtCOIII gene was sequenced and aligned to the orthologous sequences available from northeastern Pacific Octopus species. Possible phylogenetic relationships were reconstructed. The mtCOIII gene tree revealed two main clusters, one comprised O. rubescens, O. dofleini and O. californicus, while the other included all O. vulgaris specimens, O. bimaculatus, O. bimaculoides and O. mimus. With one exception all South American haplotypes including O. bimaculatus and O. bimaculoides appeared as the sister group of the Mediterranean haplotype of O. vulgaris, indicating that most of the South American O. vulgaris specimens investigated would not belong to the recently redescribed species O. vulgaris restricted to the Mediterranean and eastern Atlantic. The treatment of O. mimus as a species distinct from Mediterranean O. vulgaris is supported by a high nucleotide divergence of 12.7%. Based on the mtCOIII gene tree the existence of cryptic species among O. vulgaris-like octopods is suggested. Received: 15 January 1999 / Accepted: 15 October 1999     

Zooplankton responses to hypoxia: behavioral patterns and survival of three species of calanoid copepods

Seasonally recurrent and persistent hypoxic events in semi-enclosed coastal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of < 2.0 ml l⁻¹. Shifts in the distribution patterns of zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investigated. This study assessed interspecific differences in responses to hypoxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), Acartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of moderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l⁻¹, sharp declines in survival at d.o. = 0.9 to 0.6 ml l⁻¹, and 100% mortality with d.o. = 0.5 ml l⁻¹. Adults of L. aestiva and C. hamatus were more sensitive to oxygen depletion: both species experienced significant decreases in survival for d.o. = 1.0 ml l⁻¹. Nauplii of L. aestiva and A. tonsa showed no significant mortality with d.o. = 1.1 to 1.5 ml␣l⁻¹ and d.o. = 0.24 to 0.5 ml l⁻¹, respectively. In addition, experiments investigating behavioral avoidance of moderate to severe hypoxia were carried out for adults of all three species. None of the three species effectively avoided either severely hypoxic (d.o. < 0.5 ml l⁻¹) or moderately hypoxic (d.o. ≈ 1.0 ml l⁻¹) bottom layers in stratified columns. These results suggest that in␣nearshore areas where development of zones of d.o. < 1.0 ml l⁻¹ may be sudden, widespread, or unpredictable, patterns of reduced copepod abundance in bottom waters may be due primarily to mortality rather than avoidance. Received: 31 August 1996 / Accepted: 24 September 1996     

Oceanic diffusion and the pelagic insects Halobates spp. (Gerridae: Hemiptera)

Five pelagic Halobates species occupy a vast area from 40^∘ north to 40^∘ south in the three major oceans. Oceanic diffusion, constantly acting to disperse these insects, must be an important factor in determining their life history and distribution. We investigated the effects of oceanic diffusion on the following aspects of these insects. (1) The estimated radius of a patch of Halobates could be expanded by oceanic diffusion alone from an initial point of origin to 1250 km in 60 d. This distance is about 1/12 of the maximum distributional range of H. micans in the Pacific Ocean. Mutual encounter rates due to oceanic turbulence could be as high as 11 d⁻¹ even at low population densities (100 ind km⁻²). This suggests that individuals from their original habitat could find mates even when they had been carried a long distance. Thus, extensive gene mixing may occur over the whole range of a species' distribution. (2)␣Estimated growth rates are rather low (0.0026 to 0.0079 d⁻¹) compared with those of other insects. However, they are offset by a long life span (over 90 d) and an extended oviposition period (perhaps over 2␣months). Thus, pelagic Halobates spp. appear to have adopted a strategy of slow growth and prolonged longevity to cope with living in an unstable physical environment that is constantly disturbed by storms and winds. Received: 5 February 1995 / Accepted: 30 October 1997     

Relationships between diet and body size, mouth morphology, habitat and movements of six sillaginid species in coastal waters: implications for resource partitioning

The dietary compositions and breadths of sequential 50 mm size classes of the six whiting species found in nearshore (<1.5 m), shallow inner-shelf (5 to 15 m) and/or deep inner-shelf (20 to 35 m) waters of the lower west coast of Australia were determined. Comparisons between the results of principal components analysis of head and mouth dimensions and the dietary compositions of Sillago bassensis, S. vittata, S. burrus, S. schomburgkii, S. robusta and Sillaginodespunctata suggests that any differences in the dietary composition of similar-sized representatives of different species, when they occur in the same habitat, are more likely to be due to differences in foraging behaviour than mouth morphology. Classification, ordination and Schoener's overlap indices showed that, in nearshore waters, the juveniles of Sillago bassensis, which colonise relatively exposed areas, have a different diet to those of the smallest representatives of the other whiting species that occupy more sheltered habitats. S. bassensis consumes mainly amphipods, whereas the smaller representatives of S. vittata, S. burrus, S. schomburgkii and Sillaginodes punctata ingest large volumes of copepods, which are typically abundant in protected nearshore waters. Although the mouth dimensions of S. punctata tend to be smaller than those of Sillago schomburgkii, the larger individuals of the former species ingest greater quantities of larger prey, such as crabs and carid shrimps. As S. bassensis, S. vittata and S. burrus increase in size and migrate out into shallow inner-shelf waters, the latter two species tend to concentrate more on benthic prey, while the former species ingests fauna that is more epibenthic. The largest S. bassensis subsequently migrate out into deep inner-shelf waters, where they co-occur with S. robusta, which is restricted to those waters. In these waters, S. bassensis feeds to a far greater extent on large benthic prey, whereas S. robusta consumes a greater quantity of small epibenthic crustaceans, differences that reflect the far larger lengths of the former species in that region. The above data emphasise that the distribution and ontogenetic movements of the six abundant species of whiting play a major role in facilitating a partitioning of food resources amongst these species found in coastal waters of the lower west coast of Australia. Received: 7 October 1996 / Accepted: 31 January 1997     

Differential increment-deposition rate in embryonic statoliths of the loliginid squid Loligo vulgaris

 Apart from one study that reported growth of less than one increment per day in statoliths of the squid Alloteuthis subulata, most studies so far have presumed that one increment was laid down per day in the statoliths of the squid species they examined. The present study provides evidence of differential daily growth rates in embryonic statoliths of the squid Loligo vulgaris Lamarck, 1798, thus confirming a previous report for A. subulata. Incremental growth rates of L. vulgaris statoliths differ as a function of temperature. Squid embryos were incubated in the laboratory at three temperatures (12.0, 15.5 and 21.1 °C), and tetracycline staining was used to follow statolith growth. This growth slowed in squid exposed to the lowest temperature, but recovered when the squid were returned to warm conditions, indicating statolith adaptation. Statolith growth rate after incubation at 12 °C was 1.3% d⁻¹ and reached 6.1% d⁻¹ for squids exposed to 21.1 °C. Statoliths from embryos incubated at 15.5 °C yielded a rate of 1 increment d⁻¹ and a mean daily growth of 2.2 μm in the dorsal dome area of the statolith. In contrast, the slow growth of statoliths incubated at 12 °C yielded a mean daily growth of only 0.9 μm in the dorsal dome and the readings resulted in a less-than-daily increment-deposition rate. Received: 9 October 1999 / Accepted: 30 March 2000     

Influence of cadmium accumulation and dietary status on fatty acid composition in two colour forms of shore crabs, Carcinus maenas

 The effects of cadmium exposure and dietary status on cadmium accumulation, fatty acid (FA) content and profiles were investigated in two colour forms of the shore crab Carcinus maenas. Groups of shore crabs were either starved or fed with blue mussels, Mytilus edulis, during a 40 d exposure period to 2 or 6 μM Cd²⁺ (as CdCl₂). Starved green individuals accumulated more cadmium in haemolymph and hepatopancreas than did red crabs and green crabs fed during the experiments. In the red colour form, no difference in cadmium accumulation was observed between starved and fed individuals. In both colour forms, hepatopancreas contained more FA than gills and muscle. The FAs often present in the largest amounts in the tissues were 16:0, 16:1ω7, 18:1ω7, 18:1ω9, 20:4ω6, 20:5ω3 and 22:6ω3. However, saturated (SAFA) and mono-unsaturated fatty acids (MUFAs) were dominant in hepatopancreas, whereas poly-unsaturated fatty acids (PUFAs) were dominant in gills and muscles. At the beginning of the experiment, the total FA content in the hepatopancreas was 111.6 mg g⁻¹ (dry weight) for red crabs and 78.4 mg g⁻¹ for green shore crabs. During the experiment, however, the FA content decreased in red crabs. This decrease was more pronounced for starved individuals than for fed individuals. Also, the decrease in FA content was more pronounced in crabs exposed to 6 μM cadmium compared to crabs exposed to 2 μM or crabs not exposed to cadmium. No change in FA content was observed in green shore crabs, irrespective of diet and cadmium exposure. For both colour forms, no change in FA content was observed for gills and muscle. In red crabs, a decrease was observed for all FAs in the hepatopancreas. This decrease, however, was more pronounced for SAFAs and MUFAs than for PUFAs, indicating that the metabolism of FAs during starvation and cadmium exposure is selective. The experiments indicate that green colour forms of shore crabs are more tolerant of natural stress such as starvation and anthropogenic stress, e.g. cadmium exposure, than are red colour forms of shore crabs. Received: 23 September 1999 / Accepted: 29 April 2000     

Heavy metal contamination and physiological variability in the Brazilian mangrove crabs Ucides cordatus and Callinectes danae (Crustacea: Decapoda)

 Physiological studies were made on the crabs Ucides cordatus (L.) and Callinectes danae sampled from populations living in “polluted” mangroves on the southeast littoral of Brazil. Analysis of Cu, Cd, Zn, and Fe of sediments and crab tissues showed interspecific differences in tissue concentrations, and significantly higher levels of Cu, Cd, and Zn in “polluted” populations compared to “unpolluted” crabs living in uncontaminated mangrove in the same geographical area. Individuals of both species from the polluted site showed significantly greater capacities for regulating blood osmotic concentrations at low salinity (9‰). However, U. cordatus showed a reduced hypo-regulatory ability in 34‰S. Differences in ionoregulation were also seen. “Polluted”C. danae showed significantly higher Na/ K-ATPase levels in posterior gills compared to “unpolluted” crabs. Oxygen consumption rates (M˙ _O2) were elevated in U. cordatus, but depressed in C. danae from the “polluted” population. Individuals of both species from this site showed significantly lower O:N ratios, mainly because of an increased net efflux of ammonia. Adenylate energy charge (AEC) values of muscle and hepatopancreas in “unpolluted” and “polluted” populations of both species were not significantly different. These physiological differences are discussed in relation to the known acute physiological and metabolic effects of heavy metals in crustaceans, and interpretated in the light of possible adaptive changes following long-term exposure to contamination. Received: 6 August 1999 / Accepted: 22 June 2000     

Seasonal variations in hepatopancreas fatty acid profiles of two colour forms of shore crabs, Carcinus maenas

 The relationship between intermoult duration (coloration), sex, size and seasonal variations in fatty acid (FA) profiles was studied in a population of shore crabs, Carcinus maenas, inhabiting the Isefjord, Denmark. For male shore crabs, the total hepatopancreas FA content was high in July and December (12.7 to 16.0 mg g⁻¹ dry weight, dw) but lower in May and September (7.3 to 10.0 mg g⁻¹). This indicates that male shore crabs are in relatively good condition before winter, when the crabs migrate off shore, but in relatively poor condition when they return to shallow waters during spring. The hepatopancreas FA content also decreased over the mating season. After the mating season the hepatopancreas FA content of males had decreased to approximately 60% of that prior to the mating season. Female shore crabs had significantly higher hepatopancreas FA levels than males in May (11.7 mg g⁻¹ dw), September (12.6 mg g⁻¹ dw) and December (17.9 mg g⁻¹ dw) but lower levels in July (9.5 mg g⁻¹ dw). This indicates that the spawning season is the most energy-demanding part of the female reproductive cycle. For all seasons, the hepatopancreas FA content of green shore crabs was significantly higher than that of red shore crabs. For both colour forms, the amount of polyunsaturated fatty acids (PUFAs) was significantly higher than that of saturated fatty acids (SAFAs) and monounsaturated fatty acids (MUFAs), with the relative proportion of PUFAs increasing when the total hepatopancreas FA content decreased. For both genders and colour forms, the most dominating SAFA was palmitic acid (16:0). Palmitoleic acid (16:1ω7), vaccenic acid (18:1ω7) and oleic acid (18:1ω9) were the three MUFAs found in highest concentrations. The most dominating PUFA was eicosapentaenoic acid (EPA, 20:5ω3). Docosahexaenoic acid (DHA, 22:6ω3) and arachidonic acid (AA, 20:4ω6) were also abundant in all samples. The results demonstrated that season, sex, size and intermoult duration all influence the amount of FAs present in the hepatopancreas of shore crabs. Received: 23 September 1999 / Accepted: 29 April 2000     

Carbon and nitrogen exchange between sandy beach clams (Donax serra) and kelp beds in the Benguela coastal upwelling region

Carbon consumption and nitrogen requirements were estimated for populations of the sandy beach bivalve Donax serra on nine beaches of the west coast of South Africa. Subtidal populations composed mainly of adult clams were responsible for the bulk of standing stock (3538 g C m⁻¹), annual carbon consumption (13 444 g C m⁻¹ yr⁻¹), faeces production (6478 g C m⁻¹ yr⁻¹ ) and nitrogen regeneration (2525 g N m⁻¹ yr⁻¹). Kelp detritus, bacteria and kelp consumers' faeces available in the water column surpass several times the carbon and nitrogen requirements of intertidal and subtidal clam populations. Individual Donax serra pop ulations, in turn, may regenerate up to 3.2% of the total nitrogen requirements of all primary producers from kelp beds and 14% of the requirements of phytoplankton. These high standing stocks of clams are presumably supported mainly by organic matter originating from kelp which, in contrast to phytoplankton, is in constant supply and comprises the largest proportion of the annual production of particulate organic matter on this coast. Wide and shallow continental shelves with gentle slopes probably limit the penetration of upwelled waters to the nearshore waters, decreasing the influence of external inputs and increasing the importance of internal flows of nutrients and carbon within the nearshore zone. In this context, sandy beaches, rocky shores and kelp beds may be more closely interlinked compartments of a larger ecosystem encompassing the whole nearshore than traditionally thought. Received: 28 August 1996 / Accepted: 7 October 1996     

Observations of copepod feeding and vertical distribution under natural turbulent conditions in the North Sea

We present results of simultaneous measurements of turbulent-dissipation rate, zooplankton vertical distribution and copepod gut pigments in the northern North Sea. Analysis shows that some, but not all, copepods (by species, sex and stage) exhibit significant dependence on turbulence in respect to vertical distribution and feeding rate. Oithona similis (female and copepodite stages) exhibits an avoidance of the surface layer when turbulence is strong there. For the range of turbulence (10⁻⁷ to 10⁻³m² s⁻³) and ambient chlorophyll concentration (0.5–0.8 μg l⁻¹) encountered, Calanus spp. and Metridia lucens exhibited a significant negative response in feeding-rate index with increasing turbulence. Centropages typicus and Pseudocalanus spp. also exhibited a negative response but of less significance. Received: 12 October 2000 / Accepted: 11 December 2000     

Allozyme variation in global populations of striped mullet, Mugil cephalus (Pisces: Mugilidae)

The striped mullet, Mugil cephalus Linnaeus, 1758, is one of the few species of marine shore fish with a worldwide circumtropical distribution. Because of this distribution and the dependency of M. cephalus on coastal waters during various phases of its life cycle, as well as nearshore living habits, questions have been raised regarding levels of genetic divergence and gene flow among transoceanic populations. To cast more light on this, allozyme variation at 27 presumptive gene loci was investigated in ten globally diverse populations. The observed heterozygosity ranged from 0.018 (Hawaii) to 0.081 (Florida), averaging 0.050. The proportion of polymorphic loci showed a similar trend. Several populations were characterised by fixed allelic differences. Estimated gene diversities were very high, the allele frequency variation among populations was found to be 68%; genetic distances reached 0.242, with an average of 0.117. Estimated rates of gene flow were high among Mediterranean populations (Nm = 7.26), and between Mediterranean and East Atlantic populations (Nm= 2.86), but extremely low between non-contiguous populations within the Indian, Pacific and Atlantic Oceans, where Nm ranged from 0.03 to 0.05. Received: 7 April 1997 / Accepted: 4 February 1998     

Spatial and temporal pattern in seagrass community composition and productivity in south Florida

We document the distribution and abundance of seagrasses, as well as the intra-annual temporal patterns in the abundance of seagrasses and the productivity of the nearshore dominant seagrass (Thalassia testudinum) in the south Florida region. At least one species of seagrass was present at 80.8% of 874 randomly chosen mapping sites, delimiting 12,800 km² of seagrass beds in the 17,000-km² survey area. Halophila decipiens had the greatest range in the study area; it was found to occur over 7,500 km². The range of T. testudinum was almost as extensive (6,400 km²), followed by Syringodium filiforme (4,400 km²), Halodule wrightii (3,000 km²) and Halophila engelmanni (50 km²). The seasonal maxima of standing crop was about 32% higher than the yearly mean. The productivity of T. testudinum was both temporally and spatially variable. Yearly mean areal productivity averaged 0.70 g m⁻²day⁻¹, with a range of 0.05–3.29 g m⁻² day⁻¹. Specific productivity ranged between 3.2 and 34.2 mg g⁻¹ day⁻¹, with a mean of 18.3 mg g⁻¹ day⁻¹. Annual peaks in specific productivity occurred in August, and minima in February. Integrating the standing crop for the study area gives an estimate of 1.4 × 1011 g T. testudinum and 3.6 × 10¹⁰ g S. filiforme, which translate to a yearly production of 9.4 × 10¹¹ g T. testudinum leaves and 2.4 × 10¹¹ g S. filiforme leaves. We assessed the efficacy of rapid visual surveys for estimating abundance of seagrasses in south Florida by comparing these results to measures of leaf biomass for T. testudinum and S. filiforme. Our rapid visual surveys proved useful for quantifying seagrass abundance, and the data presented in this paper serve as a benchmark against which future change in the system can be quantified. Received: 30 January 2000 / Accepted: 24 July 2000     

Are mangroves and seagrasses sources of organic carbon for penaeid prawns in a tropical Australian estuary? A multiple stable-isotope study

The vertical distribution of the larvae of shelf-dwelling fish species that spawn in the NW Mediterranean Sea in spring was studied in relation to environmental data. Two sampling cycles were carried out at fixed stations on the continental shelf in May and June 1992. Three patterns of larval vertical distribution for the various taxa represented in the samples were observed. The larvae of most species (e.g. Boops boops, Diplodus sargus) were mainly located in the surface layer (10 m), others (e.g. Arnoglossus sp.) had broader distributions in the upper 40 m of the water column, and but a few (e.g. Gobiidae) were present in large concentrations at greater depths. The vertical distribution patterns of the various species showed no variations, despite high hydrographic variability during the study. The vertical distribution of only a few species (e.g. Arnoglossus sp., Crystallogobius linearis and Engraulis encrasicolus) varied over the diel cycle. The possible influence of the vertical distribution of fish larvae on their horizontal distribution patterns is discussed. Received: 10 March 1997 / Accepted: 4 April 1997     

Reproduction in the seagrass Zostera novazelandica on intertidal platforms in southern New Zealand

This study investigates the reproductive periodicity and reproductive output of the seagrass Zostera novazelandica on two intertidal reefs. Peak numbers of flowering shoots occurred during March (late summer) of two years at both sites and no flowering shoots occurred during the winter months of July to September. There were greater numbers of flowering shoots in seagrass patches in the low intertidal zone (up to an average of 55 per 0.1 m²) compared to the middle (up to 20 per 0.1 m²) and upper (up to 9 per 0.1 m²) zones, and about three times greater reproductive output in patches associated with tidepools compared to those not bordering tidepools. The average number of inflorescences per shoot was 3.1 (±0.25) at one site vs 1.2 (±0.08) at the other, and showed a progressive decrease from the lower shore to the upper shore. Patches associated with tidepools had twice the number of inflorescences per shoot (2.8 ± 0.24) than patches not bordering tidepools (1.5 ± 0.16). The number of flowering shoots was highly correlated with leaf width, leaf length, and ramet density, while the leaf-area index decreased from the lower shore to the upper shore. The reproductive effort of plants, as measured by the percent biomass invested in flowering shoots during peak reproduction, was significantly different between sites, tidepool associations, and shore level. For all the variables measured, there was considerable spatial variation, with significant interaction terms between most factors investigated. In laboratory experiments, more inflorescences were produced at light intensities of 30 and 300 μE m⁻² s⁻¹ than at 100 μE m⁻² s⁻¹. At a salinity of 17‰, 1.5 × the number of flowers was produced than at 33‰, while none was formed at 70‰. Plants cultured at 5 °C had about three times the number of inflorescences than those at 15 °C, while none was formed at 25 °C. Received: 25 June 1997 / Accepted: 24 September 1997     

Species composition, biomass and vertical distribution of micronekton over the mid-slope region off southern Tasmania, Australia

Seasonal sampling was carried out based on day/night, vertically stratified tows (100 or 125 m strata) in the upper 900 m of the water column over the mid-slope commercial fishing grounds south of Tasmania. A large midwater trawl (105 m² mouth area) was used with an opening/closing cod-end. Subtropical convergence and subtropical species dominated the fauna, but many less abundant, more widely-distributed species were also present. Fishes, which contributed 89% of micronekton biomass and 135 of 178 species, were dominated by the Myctophidae (48% biomass and 48 species). Twenty micronekton species made up 80% of the total biomass. Overall, the micronekton fish biomass in this region was 2.2 g m⁻² wet weight. A pronounced day/night shift in the distribution of biomass was attributable to diel migratory species. During the day, <0.2% of the total micronekton biomass was found in 0 to 300 m; most biomass was below 400 m, with peaks at 400 to 525 m and 775 to 900 m. At night, 53% of the biomass was found in 0 to 300 m, with progressively less in each deeper stratum. The vertical ranges of individual species typically exceeded 400 to 500 m during the day and night and were non-coincident, although nyctoepipelagic migrators were concentrated in the surface 200 m at night. Distinct epipelagic, lower and upper mesopelagic assemblages were identified, and patterns of epipelagic migration, limited migration and non-migration were categorised for species from each of the lower and upper mesopelagic assemblages. The vertical distribution of these assemblages was coincident with the primary water masses: subantarctic mode water (∼250 to 600 m) and antarctic intermediate water (below ∼700 m). The flux of migrating micronekton, estimated at 0.94 to 3.36 g C m⁻² yr⁻¹ to the lower mesopelagic and 1.14 to 4.06 g C m⁻² yr⁻¹ to the upper mesopelagic, appeared to be considerably outweighed by the consumption needs of aggregated mid-slope benthopelagic predators. We suggest that advection of mesopelagic prey in antarctic intermediate water may sustain aggregated populations of orange roughy (Hoplostethus atlanticus) and other predators on the micronekton in mid-slope depths at this site. Received: 2 April 1997 / Accepted: 21 August 1997     

Kinetic measurements of metal accumulation in two marine macroalgae

 We measured the uptake kinetics of four metals (Cd, Cr, Se and Zn) in two marine macroalgae (the green alga Ulva lactuca and the red alga Gracilaria blodgettii). Metal uptake generally displayed a linear pattern with increasing exposure time. With the exception of Cr, which exhibited comparable uptake rate constants at different concentrations, uptake rate constants of Cd, Se and Zn decreased with increasing metal concentration, indicating that the seaweeds had a higher relative uptake at lower metal concentration. Uptake of Cd and Zn was higher in U. lactuca than in G. blodgettii, whereas uptake of Cr and Se was comparable between the two species. Only Cd and Zn uptake in U. lactuca was significantly inhibited by dark exposure. A decrease in salinity from 28 to 10‰ enhanced the uptake of Cd, Cr, Se and Zn in U. lactuca 1.9-, 3.0-, 3.6-, and 1.9-fold, respectively. In G. blodgettii, Cd uptake increased twofold when salinity was decreased from 28 to 10‰, whereas uptake of Cr and Zn was not significantly affected by salinity change. The calculated depuration rate constants of metals in U. lactuca were 0.01 d⁻¹ for Cd, 0.05 to 0.08 d⁻¹ for Cr, 0.14 to 0.16 d⁻¹ for Se, and 0.12 to 0.15 d⁻¹ for Zn, and were relatively independent of the metal body burden in the algae. The predicted bioconcentration factor was 3 × 10⁴ for Cd, 2 × 10³ for Cr, 40 to 150 for Se, and 1 to 2 × 10⁴ for Zn in U. lactuca. Our kinetic study suggested that U. lactuca would be a good biomonitor of Cr and Zn contamination in coastal waters. Received: 14 September 1998 / Accepted: 29 May 1999     

Effects of municipal piers on the growth of juvenile fishes in the Hudson River estuary: a study across a pier edge

The growth rates of two fish species, the winter flounder Pseudopleuronectes americanus (Walbaum) (19.3 to 42.6 mm total length, TL) and the tautog Tautogaonitis (Linnaeus) (23.9 to 55.9 mm TL), were used to evaluate habitat quality under and around municipal piers in the Hudson River estuary, USA. Growth rates were measured in a series of 10 d field caging-experiments conducted at two large piers in the summers of 1996 and 1997. Cages (0.64 m²) were deployed along␣transects that stretched from underneath the piers to beyond them, encompassing the pier edge (the transitional zone between the pier interior and the outside). Growth in weight (G _w ) was determined at five locations along the transect, 40 m beneath the pier, 20 m beneath the pier, at the pier edge, 20 m beyond the pier edge, and 40 m beyond. Under piers, mean growth rates of winter flounder and tautogs were negative (xˉG _W = −0.02 d⁻¹), and rates were comparable to laboratory-starved control fishes (xˉG _W = −0.02 d⁻¹). In contrast, mean growth rates at pier edges and in open waters beyond piers were generally positive (xˉG _W ranged from −0.001 to +0.05 d⁻¹), with growth at pier edges often being more variable and less rapid than at open-water sites. Analyses of stomach contents upon retrieval of caged fishes revealed that dry weights of food were generally higher among fishes caged at open-water stations (xˉ range = 0.02 to 0.72 mg dry wt) than at pier-edge (xˉ range = 0.01 to 0.54 mg) or under-pier (xˉ range = 0.03 to 0.11 mg) stations, although it was apparent that benthic prey were available at all stations on the transect. Our results indicate poor feeding conditions among fishes caged under piers, and suboptimal foraging among fishes caged at pier edges. Inadequate growth rates can lead to higher rates of mortality, and, based on these and other earlier experiments, we conclude that under-pier environments are poor-quality habitats for some species of juvenile fishes. Received: 12 March 1998 / Accepted: 9 November 1998     

Zooplankton growth rates: the influence of size and resources in tropical marine copepodites

Growth rates were determined for copepodites of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total copepodite development time was as short as 4 to 5 d. Mean instantaneous growth rates (g) ranged from as high as 1.2 d⁻¹ to as low as 0.1 d⁻¹. In general, cyclopoid copepods appeared to grow more slowly than calanoids of the same size. Enhancement of resources by nutrient addition caused a 32% increase in growth rates in experiments from a mesotrophic site, but only a 17% increase at a more eutrophic site. Additionally, copepodites at both sites showed faster development and generally larger size at stage in response to nutrient addition. Growth rates were positively related to chlorophyll concentration in the >2 μm size-fraction. A significant relationship of growth rate to body size (r ² = 0.45) emerged across a wide range of trophic status, but it was confounded with resource availability. It appears that growth in tropical copepod copepodites may be frequently limited by resources in a size-dependent manner. Received: 30 May 1997 / Accepted: 13 May 1998     

Effect of temperature on statolith growth of the European squid Loligo vulgaris during early life

Over the past decade, statolith interpretation has resulted in a major advance in our knowledge of squid population-dynamics, but the way in which environmental conditions affect the statolith increment-deposition ratio remains virtually unknown. The object of the present study was to determine the effect of temperature on this process, using tetracycline marks to validate statolith growth in Loligo vulgaris Lamarck, 1798 under rearing conditions equivalent to severe winter (11 °C) and summer (19 °C) temperature regimes. Tetracycline marking was performed every 10 d (at 10, 20, 30, 40, 50 and 60 d of age). The newly hatched squid paralarvae were slightly smaller in summer than those hatched in winter. Survival rates were similar in both cultures, but growth rates (wet mass) of summer squids were double those in winter. At hatching, statoliths were already longer in the summer squids, and growth rates were 2% d⁻¹ as opposed to 0.9% d⁻¹ for winter statoliths. For the dorsal dome area of the statolith, where more increment counts were made, statolith growth was of 3.25 μm d⁻¹ in summer, and daily increment deposition was confirmed in 87% of the statoliths. The slow growth of statoliths at winter temperatures yielded a mean growth of 1.1 μm d⁻¹– insufficient to discern the increments using light microscopy. Subsequent SEM observation enabled only 21% of the winter statoliths to be read; these also indicated a deposition rate of one increment d⁻¹. Since the life span of L. vulgaris is ≃1 yr, squids will experience at least one winter during their life cycle, and this might be visible on the statolith. Received: 28 June 1999 / Accepted: 20 December 1999