David A. Ritz

Tracing the influence on sediments of organic waste from a salmonid farm using stable isotope analysis

Abstract Stable C isotope measurement in combination with analysis of benthic macrofaunal communities were used to trace the dispersion and effects of solid organic waste derived from a fish cage on marine sediments. The proportion of cage-derived organic C to total organic C in the sediment decreased with increasing distance from a fish cage. A highly organically enriched zone under the cage was characterized by >75% fish cage-derived organic C in the sediment, a semi-enriched zone under the cage was characterizaed characterized by 60% and a lightly enriched zone (> 60 m) by ≈ 40%. Measurable effects of farm organic waste on organisms extended from under the cage to between 10 and 30 m from it. Macrobenthic communities under the cage and 10 m from the cage showed signs of moderate disturbance whereas those from 30 to 150 m appeared undisturbed.

Response to organic enrichment of infaunal macrobenthic communities under salmonid seacages

The response of infaunal macrobenthic communities beneath salmonid seacages to solid organic wastes (food pellets and fish excreta) was tested by means of the ABC-method (abundance, biomass comparison method: Warwick 1986) on samples collected from the SAFCOL seafarm at Badger Cove, southeast Tasmania, from February to July 1988. The intermittent nature of the addition of organic waste allowed us to monitor decline and recovery of the macrofauna, i.e., harvesting of a cage permitted a period of recovery whilst restocking precipitated a decline. The ABC-method proved to be a sensitive indicator of community health. Under a normal feeding regime the macrofaunal community structure indicated a moderately disturbed condition. Only 7 wk after the cage was harvested, species richness had increased markedly and the community adopted an undisturbed condition. Further improvement was apparent 14 wk post-harvest. Similarly, a decline to a moderately disturbed condition was apparent 7 wk after restocking and species richness had declined. No such changes occurred under a cage which contained fish continuously over the same period.

Age determination of orange roughy,Hoplostethus atlanticus (Pisces: Trachichthyidae) using210Pb:226Ra disequilibria

Natural levels of210Pb:226Ra in otoliths of orange roughy,Hoplostethus atlanticus, from south-east Australian waters, were measured to determine fish ages radiometrically. Up to maturity, radiometric age estimates were consistent with a single constant otolith growth rate. Radiometric ages for juveniles were comparable with, but greater than, those obtained in a recent, validated New Zealand study which employed counts of annuli on the surface of otoliths. Beyond maturity, radiometric ages were obtained by modelling with an otolith growth rate set at 45% of the juvenile rate. Radiometric ageing confirms that orange roughy is very slow-growing, with an age at maturity (32 cm standard length, SL) of ~ 32 yr, and is very long-lived, with fish 38 to 40 cm being 77 to 149 yr old. These results have important implications for the management of the fishery.

Effect of habitat complexity and predatory style on the capture success of fish feeding on aggregated prey

When offered a choice between open water and a ‘vegetated’ area of an aquarium (artificial sea grass), mysid ( Paramesopodopsis rufa ) swarms always selected open water. In contrast, juvenile and adult sea horses, Hippocampus abdominalis , which feed by ambush predation, always selected vegetated areas even when their mysid prey occurred in the adjacent clear water. Lunging predators, Australian salmon ( Arripis trutta ), always preferred open water to vegetated regions of the tank. Increasing vegetation density, i.e. from low to medium habitat complexity had a positive effect on capture success of juvenile and adult sea horses feeding on mysid swarms. However, number of attacks by juvenile sea horses, but not adults, was significantly reduced in higher density vegetation. Number of attacks by Australian salmon decreased as vegetation density increased but capture success, nonetheless, increased. Density of vegetation did not significantly affect the number of unsuccessful attacks by adult sea horses. Whereas proportion of unsuccessful attacks increased with vegetation density for juvenile sea horses, it decreased for juvenile salmon. This result is thought to be due to the fact that high vegetation densities disrupt structure of the prey swarm, itself a defensive formation, leaving a large number of more vulnerable small groups. Cohesive escape responses of a large swarm are likely to be a more effective defence against a lunging predator than against an ambush predator whose strategy is to use stealth to avoid provoking escape responses. Dense vegetation will militate against this strategy.

Social aggregation in pelagic invertebrates

Publisher Summary Social aggregations are a familiar sight among aquatic vertebrates: One has only to think of schools of dolphins, pods of whales, flocks of seagulls, and schools of fish. But invertebrates too have evolved social aggregative behaviour though examples may not spring so readily to mind. This chapter summarizes the information regarding social aquatic invertebrates by reference to developing theoretical behavioural ecology and also by comparison with what is known of social aquatic vertebrates, chiefly fish. In proposing ecological hypotheses about the functions of sociality, which applies to the great majority of papers referred to above, it is generally assumed that groups are maintained in order to exploit resources more efficiently than solitary animals. Moreover, resources are taken to include a variety of factors that affect fitness— namely, food, space, mates, spawning sites, and other animals. This chapter illustrates that, social behavior can potentially influence reproductive success in aquatic invertebrates in many ways.

Antarctic krill (Euphausia superba) acquire a UV-absorbing mycosporine-like amino acid from dietary algae

We hypothesised that Antarctic krill acquire UV-absorbing mycosporine-like amino acids (MAAs) from dietary algae, which produce MAAs in response to ultraviolet (UV) irradiation. To test this hypothesis, we grew cultures of Phaeocystis antarctica that had been grown under either photosynthetically active radiation (PAR, 400-750 nm) plus UV irradiation (UVR, 280-400 nm), or else PAR-only. Algae grown under PAR-only produced high concentrations of porphyra-334, whereas additional UVR caused formation of high concentrations of mycosporine-glycine:valine and lower concentrations of porphyra-334. Krill were fed with either of these two cultures on eight occasions over 63 days. A third group was starved for the duration of the experiment. Animals were analysed after 36 and 63 days for MAA content. Remaining animals from all treatments were starved for a further 35 days and analysed to examine MAA retention characteristics. Our findings are that krill acquired different MAAs from dietary algae depending on the light conditions under which the algae were grown. Specifically, krill fed algae grown under PAR-only had higher concentrations of porphyra-334 than starved krill. Conversely, krill fed algae grown under PAR with additional UVR had high body concentrations of mycosporine-glycine:valine. MAA concentrations in starved krill remained static throughout the experiment. However, long term starvation (35 days) caused levels of certain acquired MAAs to decline. From this we can infer that MAA concentrations in krill are dependent on the MAA content of phytoplankton, and therefore the algaes response to UV exposure. This has implications for transfer of MAAs through marine trophic webs.

Fouling, surface bacteria and antibacterial agents of four bryozoan species found in Tasmania, Australia

Extracts from four species of bryozoans, found in Tasmanian coastal waters, have been demonstrated to exhibit selective antibacterial activity. The four species showed gradations in fouling by encrusting organisms and differential surface bacterial numbers. The two chemically defended species Amathia wilsoni and Orthoscuticella ventricosa had the most active antibacterial extracts and the lowest levels of fouling. In contrast, extracts of Cellaria pilosa and Bugularia dissimilis, which have no known secondary metabolites, had weak antibacterial properties, and colonies showed large numbers of encrusting fouling organisms. Counts of bacteria over the surfaces of the four species revealed that O. ventricosa had the lowest numbers; B. dissimilis and C. pilosa had higher numbers while A. wilsoni had the highest counts, but the most patchy distribution.

Metal interaction during accumulation by the mussel Mytilus edulis planulatus

Mytilus edulis planulatus (Lamarck) were collected from Howden, South-east Tasmania in autumn 1981. Interaction effects of cadmium, copper and zinc during accumulation by mussels exposed for ten days to all three metals simultaneously were examined in a series of experiments in which each metal was tested at three concentrations. In general, interaction effects were most evident at the highest concentrations tested (20 μg l-1 Cd; 20 μg l-1 Cu; 200 μg l-1 Zn) and led to a reduction in the accumulation of cadmium and an increase in that of copper and zinc. More specifically, high levels of zinc caused a decrease in cadmium uptake and an increase in copper accumulation. The presence of copper resulted in depressed cadmium accumulation while zinc accumulation increased. Cadmium tended to enhance zinc accumulation, but copper accumulation was only affected to any great extent when zinc was also present.

Changes in carbon and hydrogen stable isotope ratios of macroalgae and seagrass during decomposition

Abstract Stable isotope ratios of carbon and hydrogen were determined for six algae and one seagrass species during a 60-day decomposition experiment. Changes in the carbon ratios were in the order of 1‰. However, significant isotopic changes were observed for hydrogen in all species and in most cases these changes were substantial. These changes probably reflect differential leaching of isotopically distinct compounds.

Production of the euphausiid Nyctiphanes australis in Storm Bay, south-eastern Tasmania

The euphausiid Nyctiphanes australis Sars is an important food item for a wide range of seabirds and commercially exploited fish in coastal waters of south-eastern Tasmania. During most of the year, N. australis forms the major component of the zooplankton biomass in Storm Bay, particularly during the summer months when it swarms in dense aggregations. Production was calculated using methods devised for individuals with continuous recruitment. Estimates of production, integrated for the whole of Storm Bay (approx 18.5km3) were from 78.3 to 84.8 mg m-3 yr1. The P:B ratio obtained for the period December 1979 to March 1981 was about 14. The production of exuviae was also calculated based on certain assumptions about moulting rate and dry weight of the moult, supported by some experimental data. The production of exuviae was 41.03 mgm-3yr-1 or 7.6 times the mean biomass. This gives a total production of flesh and exuviae of 2 212 tonnes dry weight per year for Storm Bay.