David Ritz | University of Tasmania (original) (raw)
Papers by David Ritz
Mar Biol, 1991
Natural levels of 21°pb:226Ra in otoliths of orange roughy, Hoplostethus atlanticus, from south-e... more Natural levels of 21°pb: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.
Helgolander Wissenschaftliche Meeresuntersuchungen, Jul 1, 1967
Journal of the Marine Biological Association of the United Kingdom, 1997
Journal of the Marine Biological Association of the UK, 2003
Journal of the Marine Biological Association of the United Kingdom, 1970
Journal of the Marine Biological Association of the UK, 2005
Marine Ecology Progress Series, 2001
Marine Biology, 1991
Natural levels of 21°pb:226Ra in otoliths of orange roughy, Hoplostethus atlanticus, from south-e... more Natural levels of 21°pb: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.
Journal of Crustacean Biology, 2002
We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that c... more We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that cellular changes occurring during shrinkage may provide a means for identifying krill that have undergone shrinkage. We compared the muscle tissue of juvenile, adult and shrunken adult krill to identify changes in cell number associated with maturity and shrinkage. Comparison of the absolute number and density of nuclei in abdominal segments of juvenile, adult and shrunken adult krill revealed differences related to maturity and shrinkage. Shrunken adult krill had nearly twice as many nuclei per unit area than adult krill that had not shrunk. This suggests that krill shrink by a reduction in cell volume, rather than cell loss. This simply detected variation in muscle cell nucleus density may be useful in distinguishing shrunken adult krill from juveniles, and contribute to our knowledge of age structure in natural populations.
Advances in Marine Biology, 2011
Aggregations of organisms, ranging from zooplankton to whales, are an extremely common phenomenon... more Aggregations of organisms, ranging from zooplankton to whales, are an extremely common phenomenon in the pelagic zone; perhaps the best known are fish schools. Social aggregation is a special category that refers to groups that self-organize and maintain cohesion to exploit benefits such as protection from predators, and location and capture of resources more effectively and with greater energy efficiency than could a solitary individual. In this review we explore general aggregation principles, with specific reference to pelagic organisms; describe a range of new technologies either designed for studying aggregations or that could potentially be exploited for this purpose; report on the insights gained from theoretical modelling; discuss the relationship between social aggregation and ocean management; and speculate on the impact of climate change. Examples of aggregation occur in all animal phyla. Among pelagic organisms, it is possible that repeated cooccurrence of stable pairs of individuals, which has been established for some schooling fish, is the likely precursor leading to networks of social interaction and more complex social behaviour. Social network analysis has added new insights into social behaviour and allows us to dissect aggregations and to examine how the constituent individuals interact with each other. This type of analysis is well advanced in pinnipeds and cetaceans, and work on fish is progressing. Detailed three-dimensional analysis of schools has proved to be difficult, especially at sea, but there has been some progress recently. The technological aids for studying social aggregation include video and acoustics, and have benefited from advances in digitization, miniaturization, motion analysis and computing power. New techniques permit three-dimensional tracking of thousands of individual animals within a single group which has allowed novel insights to within-group interactions. Approaches using theoretical modelling of aggregations have a long history but only recently have hypotheses been tested empirically. The lack of synchrony between models and empirical data, and lack of a common framework to schooling models have hitherto hampered progress; however, recent developments in this field offer considerable promise. Further, we speculate that climate change, already having effects on ecosystems, could have dramatic effects on aggregations through its influence on species composition by altering distribution ranges, migration patterns, vertical migration, and oceanic acidity. Because most major commercial fishing targets schooling species, these changes could have important consequences for the dependent businesses.
Polar Biology, 1999
We irradiated captive juvenile Euphausia superba in the laboratory with lower than spring surface... more We irradiated captive juvenile Euphausia superba in the laboratory with lower than spring surface levels of ultraviolet-B, ultraviolet-A and photosynthetically active radiation, in order to examine their response in terms of mortality and generalised activity. Levels of photosynthetically active radiation 3±5 times below surface irradiance caused krill to die within a week, while animals in the dark survived. Addition of ultraviolet-B typical of depths up to 15 m were found to signi®cantly accelerate mortality and lead to a drop in activity in all experiments. A drop in activity in krill exposed to ultraviolet-A wavelengths was evident without an increase in mortality. The protein content of animals from various treatments was found not to vary.
Journal of the Marine Biological Association of the UK, 2003
Antarctic krill, Euphausia superba, normally live in social aggregations (schools) but rarely agg... more Antarctic krill, Euphausia superba, normally live in social aggregations (schools) but rarely aggregate in laboratory tanks. In order to study the e¡ect of stress on solitary living we tethered krill to wooden skewers and measured heart rate both when they were held isolated from conspeci¢cs and when they were held at normal schooling distances (1 body length). Heart rate did not di¡er signi¢cantly with sex or body size. However, intermoult krill had a signi¢cantly lower heart rate than postmoult animals. When two individuals were held at schooling distance, with one slightly higher in the water column than the other, the heart rate of the higher individual slowed signi¢cantly (106^98 beats min 71 ), while that of the lower individual remained the same. We interpret these results to mean that krill living solitarily are stressed but will respond to neighbouring individuals by decreasing their metabolic rate and saving energy.
Marine Ecology Progress Series, 2000
We compared the lipid stores of coastal and lacustrine populations of the copepod Paralabidocera ... more We compared the lipid stores of coastal and lacustrine populations of the copepod Paralabidocera antarctica over a complete developmental cycle. The life cycle of the coastal population, which was reflected in their pattern of lipid storage, was coupled closely to the growth of sea ice. Nauplii entered the sea ice in early autumn and overwintered predominantly as the NIV stage, with triacylglycerol comprising up to 26% of their dry weight (DW). During their rapid development from stages CI to CIV, much smaller quantities (< 2% DW) of triacylglycerol were present. The population underwent a habitat shift from the ice to the water column at the CIV stage, although adults remained close to the ice-water interface. Adults of both sexes initially contained high amounts of triacylglycerol as a proportion of their total lipid (50% in males, 65% in females), but these stores were depleted over the 3 wk-long period of mating and spawning. The lacustrine population of P. antarctica also accumulated triacylglycerol; however, their life cycle was largely independent of the ice cover on the lake. The CI to CIV stages of the lacustrine population contained more triacylglycerol than the same stages at the coastal site, suggesting that either they experienced short-term periods of starvation due to patchiness in the food supply or that their dietary intake was lipid-rich. Reproduction began 1 mo earlier in the lake population, and the lipid storage patterns of adults indicated a longer reproductive duration. The fatty acid composition of P. antarctica reflected that of particulate matter in the 2 environments, arguing for a relatively simple biochemical pathway in comparison to some of the larger pelagic copepods.
Marine Biology, 2005
Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the ... more Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the lack of data on the metabolic costs associated with swimming. In this study respiration rates and pleopod beating rates were measured at six current speeds. Pleopod beating rates increased linearly with current speed, reaching a maximum of 6 beats s À1 at 17 cm s À1 . There was a concomitant linear increase in respiration rate, from 1.8 mg O 2 g D À1 h À1 at 3 cm s À1 to 8.0 mg O 2 g D À1 h À1 at 17 cm s À1 . The size of the group tested (50, 100 and 300 krill) did not have a significant effect on pleopod beating rates or oxygen consumption (ANCOVA, F=0.264; P>0.05). The cost of transport reached a maximum of 75 J g À1 km À1 at 5 cm s À1 , and then decreased with increasing current speed to 29 J g À1 km À1 . When considered in light of energy budgets for E. superba, these data indicate that the cost of swimming could account for up to 73% of total daily metabolic expenditure during early summer.
Marine Biology, 1997
Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the... more Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/ 1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke-out. Phytoplankton biomass increased again from mid-to late-February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break-out, whereas larval polychaetes and ctenophores dominated after ice break-out. Oncaea curvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C m )3 d )1 ) was recorded on 21 December when O. curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m )3 d )1 ); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February ('5 mg C m )3 d )1 ). The main species responsible for grazing during this period were O. curvata, Oithona similis, Calanoides acutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity.
Journal of Experimental Marine Biology and Ecology, 2000
We hypothesised that Antarctic krill acquire UV-absorbing mycosporine-like amino acids (MAAs) fro... more 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 algae's response to UV exposure. This has implications for transfer of MAAs through marine trophic webs.
Mar Biol, 1991
Natural levels of 21°pb:226Ra in otoliths of orange roughy, Hoplostethus atlanticus, from south-e... more Natural levels of 21°pb: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.
Helgolander Wissenschaftliche Meeresuntersuchungen, Jul 1, 1967
Journal of the Marine Biological Association of the United Kingdom, 1997
Journal of the Marine Biological Association of the UK, 2003
Journal of the Marine Biological Association of the United Kingdom, 1970
Journal of the Marine Biological Association of the UK, 2005
Marine Ecology Progress Series, 2001
Marine Biology, 1991
Natural levels of 21°pb:226Ra in otoliths of orange roughy, Hoplostethus atlanticus, from south-e... more Natural levels of 21°pb: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.
Journal of Crustacean Biology, 2002
We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that c... more We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that cellular changes occurring during shrinkage may provide a means for identifying krill that have undergone shrinkage. We compared the muscle tissue of juvenile, adult and shrunken adult krill to identify changes in cell number associated with maturity and shrinkage. Comparison of the absolute number and density of nuclei in abdominal segments of juvenile, adult and shrunken adult krill revealed differences related to maturity and shrinkage. Shrunken adult krill had nearly twice as many nuclei per unit area than adult krill that had not shrunk. This suggests that krill shrink by a reduction in cell volume, rather than cell loss. This simply detected variation in muscle cell nucleus density may be useful in distinguishing shrunken adult krill from juveniles, and contribute to our knowledge of age structure in natural populations.
Advances in Marine Biology, 2011
Aggregations of organisms, ranging from zooplankton to whales, are an extremely common phenomenon... more Aggregations of organisms, ranging from zooplankton to whales, are an extremely common phenomenon in the pelagic zone; perhaps the best known are fish schools. Social aggregation is a special category that refers to groups that self-organize and maintain cohesion to exploit benefits such as protection from predators, and location and capture of resources more effectively and with greater energy efficiency than could a solitary individual. In this review we explore general aggregation principles, with specific reference to pelagic organisms; describe a range of new technologies either designed for studying aggregations or that could potentially be exploited for this purpose; report on the insights gained from theoretical modelling; discuss the relationship between social aggregation and ocean management; and speculate on the impact of climate change. Examples of aggregation occur in all animal phyla. Among pelagic organisms, it is possible that repeated cooccurrence of stable pairs of individuals, which has been established for some schooling fish, is the likely precursor leading to networks of social interaction and more complex social behaviour. Social network analysis has added new insights into social behaviour and allows us to dissect aggregations and to examine how the constituent individuals interact with each other. This type of analysis is well advanced in pinnipeds and cetaceans, and work on fish is progressing. Detailed three-dimensional analysis of schools has proved to be difficult, especially at sea, but there has been some progress recently. The technological aids for studying social aggregation include video and acoustics, and have benefited from advances in digitization, miniaturization, motion analysis and computing power. New techniques permit three-dimensional tracking of thousands of individual animals within a single group which has allowed novel insights to within-group interactions. Approaches using theoretical modelling of aggregations have a long history but only recently have hypotheses been tested empirically. The lack of synchrony between models and empirical data, and lack of a common framework to schooling models have hitherto hampered progress; however, recent developments in this field offer considerable promise. Further, we speculate that climate change, already having effects on ecosystems, could have dramatic effects on aggregations through its influence on species composition by altering distribution ranges, migration patterns, vertical migration, and oceanic acidity. Because most major commercial fishing targets schooling species, these changes could have important consequences for the dependent businesses.
Polar Biology, 1999
We irradiated captive juvenile Euphausia superba in the laboratory with lower than spring surface... more We irradiated captive juvenile Euphausia superba in the laboratory with lower than spring surface levels of ultraviolet-B, ultraviolet-A and photosynthetically active radiation, in order to examine their response in terms of mortality and generalised activity. Levels of photosynthetically active radiation 3±5 times below surface irradiance caused krill to die within a week, while animals in the dark survived. Addition of ultraviolet-B typical of depths up to 15 m were found to signi®cantly accelerate mortality and lead to a drop in activity in all experiments. A drop in activity in krill exposed to ultraviolet-A wavelengths was evident without an increase in mortality. The protein content of animals from various treatments was found not to vary.
Journal of the Marine Biological Association of the UK, 2003
Antarctic krill, Euphausia superba, normally live in social aggregations (schools) but rarely agg... more Antarctic krill, Euphausia superba, normally live in social aggregations (schools) but rarely aggregate in laboratory tanks. In order to study the e¡ect of stress on solitary living we tethered krill to wooden skewers and measured heart rate both when they were held isolated from conspeci¢cs and when they were held at normal schooling distances (1 body length). Heart rate did not di¡er signi¢cantly with sex or body size. However, intermoult krill had a signi¢cantly lower heart rate than postmoult animals. When two individuals were held at schooling distance, with one slightly higher in the water column than the other, the heart rate of the higher individual slowed signi¢cantly (106^98 beats min 71 ), while that of the lower individual remained the same. We interpret these results to mean that krill living solitarily are stressed but will respond to neighbouring individuals by decreasing their metabolic rate and saving energy.
Marine Ecology Progress Series, 2000
We compared the lipid stores of coastal and lacustrine populations of the copepod Paralabidocera ... more We compared the lipid stores of coastal and lacustrine populations of the copepod Paralabidocera antarctica over a complete developmental cycle. The life cycle of the coastal population, which was reflected in their pattern of lipid storage, was coupled closely to the growth of sea ice. Nauplii entered the sea ice in early autumn and overwintered predominantly as the NIV stage, with triacylglycerol comprising up to 26% of their dry weight (DW). During their rapid development from stages CI to CIV, much smaller quantities (< 2% DW) of triacylglycerol were present. The population underwent a habitat shift from the ice to the water column at the CIV stage, although adults remained close to the ice-water interface. Adults of both sexes initially contained high amounts of triacylglycerol as a proportion of their total lipid (50% in males, 65% in females), but these stores were depleted over the 3 wk-long period of mating and spawning. The lacustrine population of P. antarctica also accumulated triacylglycerol; however, their life cycle was largely independent of the ice cover on the lake. The CI to CIV stages of the lacustrine population contained more triacylglycerol than the same stages at the coastal site, suggesting that either they experienced short-term periods of starvation due to patchiness in the food supply or that their dietary intake was lipid-rich. Reproduction began 1 mo earlier in the lake population, and the lipid storage patterns of adults indicated a longer reproductive duration. The fatty acid composition of P. antarctica reflected that of particulate matter in the 2 environments, arguing for a relatively simple biochemical pathway in comparison to some of the larger pelagic copepods.
Marine Biology, 2005
Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the ... more Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the lack of data on the metabolic costs associated with swimming. In this study respiration rates and pleopod beating rates were measured at six current speeds. Pleopod beating rates increased linearly with current speed, reaching a maximum of 6 beats s À1 at 17 cm s À1 . There was a concomitant linear increase in respiration rate, from 1.8 mg O 2 g D À1 h À1 at 3 cm s À1 to 8.0 mg O 2 g D À1 h À1 at 17 cm s À1 . The size of the group tested (50, 100 and 300 krill) did not have a significant effect on pleopod beating rates or oxygen consumption (ANCOVA, F=0.264; P>0.05). The cost of transport reached a maximum of 75 J g À1 km À1 at 5 cm s À1 , and then decreased with increasing current speed to 29 J g À1 km À1 . When considered in light of energy budgets for E. superba, these data indicate that the cost of swimming could account for up to 73% of total daily metabolic expenditure during early summer.
Marine Biology, 1997
Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the... more Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/ 1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke-out. Phytoplankton biomass increased again from mid-to late-February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break-out, whereas larval polychaetes and ctenophores dominated after ice break-out. Oncaea curvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C m )3 d )1 ) was recorded on 21 December when O. curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m )3 d )1 ); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February ('5 mg C m )3 d )1 ). The main species responsible for grazing during this period were O. curvata, Oithona similis, Calanoides acutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity.
Journal of Experimental Marine Biology and Ecology, 2000
We hypothesised that Antarctic krill acquire UV-absorbing mycosporine-like amino acids (MAAs) fro... more 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 algae's response to UV exposure. This has implications for transfer of MAAs through marine trophic webs.