Evaluating the Potential to Extract High-Resolution Paleoclimate Information from the Near-Shore New Zealand Molluscan Species Austrovenus stutchburyi

<p>Li, B, Mg, Al, Mn, Cu, Zn, As, Sr, Ba and U/Ca ratios were measured by laser ablation inductively coupled plasma mass spectrometry for 11 modern Austrovenus stutchburyi clams to assess the potential of this molluscan species as a proxy for paleo-ocean temperature and environmental change. A. stutchburyi is an intertidal, infaunal, bivalve, widespread in New Zealand coastal regions and throughout the Quaternary-Pliocene sedimentary rock record. Five individuals from Ligar Bay and Estuary (South Island, New Zealand) were analysed to evaluate the variability between individuals calcifying in similar environmental conditions. A further six individuals were sampled from a range of latitudes (38˚ to 40˚) in the North Island, New Zealand to evaluate variability between individuals from different environments. A strong positive correlation between growth rate and Mg, Al, Mn, Sr, Ba and U/Ca ratios was observed, and a marked negative correlation was found between the same trace element/Ca ratios and ontogenetic age as growth rates slow during the molluscs' life. Thus, biological effects are the primary influence on trace element incorporation in A. stutchburyi. No clear seasonal variations were observed in the Mg and Sr/Ca ratio profiles through A. stutchburyi shells representing time periods of several years. Furthermore, for two shells for which chronologies could be reliably constructed, there were no significant correlations between Mg and Sr/Ca ratios and sea surface temperature. When Mg/Ca ratios were normalised to Sr/Ca ratios in order to eliminate the growth rate effect on trace element incorporation into the mollusc shells, some of the remaining variations appeared to visually correlate positively with sea surface temperature in several sections of a shell. However, a quantitative correlation did not confirm this (r² = 0.012). It is likely that neither Mg nor Sr incorporation into A. stutchburyi shell are primarily thermodynamically controlled. Several coincident Ba/Ca peaks in two of the Ligar Bay shells are most likely caused by environmental processes such as short periods of phytoplankton blooms or elevated seawater Ba/Ca from river flooding. Mn/Ca and U/Ca variations in A. stutchburyi from different coastal sites with different sediment characteristics appeared to be linked to the redox conditions prevailing at an open ocean sand-dominated environment (Ligar Bay) versus tidal mud flat environments (e.g. Miranda). Thus, while A. stutchburyi is unlikely to be a useful archive for past coastal ocean temperatures, it holds considerable promise for tracking past changes in coastal ocean productivity and river run-off, as well as sediment redox conditions.</p>

Evaluating the Potential to Extract High-Resolution Paleoclimate Information from the Near-Shore New Zealand Molluscan Species Austrovenus stutchburyi

<p>Li, B, Mg, Al, Mn, Cu, Zn, As, Sr, Ba and U/Ca ratios were measured by laser ablation inductively coupled plasma mass spectrometry for 11 modern Austrovenus stutchburyi clams to assess the potential of this molluscan species as a proxy for paleo-ocean temperature and environmental change. A. stutchburyi is an intertidal, infaunal, bivalve, widespread in New Zealand coastal regions and throughout the Quaternary-Pliocene sedimentary rock record. Five individuals from Ligar Bay and Estuary (South Island, New Zealand) were analysed to evaluate the variability between individuals calcifying in similar environmental conditions. A further six individuals were sampled from a range of latitudes (38˚ to 40˚) in the North Island, New Zealand to evaluate variability between individuals from different environments. A strong positive correlation between growth rate and Mg, Al, Mn, Sr, Ba and U/Ca ratios was observed, and a marked negative correlation was found between the same trace element/Ca ratios and ontogenetic age as growth rates slow during the molluscs' life. Thus, biological effects are the primary influence on trace element incorporation in A. stutchburyi. No clear seasonal variations were observed in the Mg and Sr/Ca ratio profiles through A. stutchburyi shells representing time periods of several years. Furthermore, for two shells for which chronologies could be reliably constructed, there were no significant correlations between Mg and Sr/Ca ratios and sea surface temperature. When Mg/Ca ratios were normalised to Sr/Ca ratios in order to eliminate the growth rate effect on trace element incorporation into the mollusc shells, some of the remaining variations appeared to visually correlate positively with sea surface temperature in several sections of a shell. However, a quantitative correlation did not confirm this (r² = 0.012). It is likely that neither Mg nor Sr incorporation into A. stutchburyi shell are primarily thermodynamically controlled. Several coincident Ba/Ca peaks in two of the Ligar Bay shells are most likely caused by environmental processes such as short periods of phytoplankton blooms or elevated seawater Ba/Ca from river flooding. Mn/Ca and U/Ca variations in A. stutchburyi from different coastal sites with different sediment characteristics appeared to be linked to the redox conditions prevailing at an open ocean sand-dominated environment (Ligar Bay) versus tidal mud flat environments (e.g. Miranda). Thus, while A. stutchburyi is unlikely to be a useful archive for past coastal ocean temperatures, it holds considerable promise for tracking past changes in coastal ocean productivity and river run-off, as well as sediment redox conditions.</p>

The influence of commercial harvesting on parasite infection in the bivalve Austrovenus stutchburyi

Commercial harvesting often reduces densities of target species, potentially affecting transmission of parasites within the population. Rates of parasitic infection not only impact the hosts directly, but can result in wider ecosystem effects through influences on host behaviour. Commercial harvesting may therefore have important ecological consequences beyond the direct effects of removal of biomass. Clams (Austrovenus stutchburyi) are commonly infected by trematode parasites (Acanthoparyphium spp. and Curtuteria australis) and have been commercially harvested in Otago, New Zealand, since 1983. To assess the effects of harvesting on the abundance of trematodes in A. stutchburyi, the number of parasites per clam was compared in harvested and unharvested areas. The mean number of parasites per clam was 36% higher at commercially harvested sites, suggesting that harvesting enhances the local transmission of parasites. Harvesters may be able to reduce their influence on parasite infection levels in clam populations by harvesting less intensively and allowing more time between harvests for clam biomass to regenerate.

Predictability of cockle (Austrovenus stutchburyi) population trends in New Zealand’s northern North Island

Population monitoring programmes are used to gain knowledge about system functioning or about the response of a system's state to management measures. The current study provided an assessment of comprehensive monitoring data of intertidal bivalve populations in northern New Zealand. In this region, a number of bivalve species are targeted in recreational and customary fisheries, including cockles (Austrovenus stutchburyi) that are one of the main target species. To monitor the sustainability of cockle populations, regular population surveys have been conducted since the 1990s at different northern New Zealand sites, although the sampling frequency has varied greatly across sites. Furthermore, a general lack of fishing data for the target populations prevents a systematic assessment of management measures and population trends within a fishery assessment framework. In view of these shortcomings, the current study provided a systematic phenomenological assessment of the survey data to make general recommendations about the effectiveness of management and for guiding prioritisation and design for future surveys. The assessment was based on a Bayesian state-space model that simultaneously analysed monitoring data from 21 incomplete survey time-series between 1999-2000 and 2014-15. The model showed that the predictability of estimated large-size cockle densities was highly site-specific. Across all sites, the predictability of density trends could be used to guide the prioritisation of sites to be included in the survey, or to adjust the sampling frequency at individual sites. Based on the current analysis, existing management measures in the form of fishery closures at some of the sites appear to be effective in allowing recovery of depleted cockle populations. Only those sites that had management measures in place showed an increase in large cockle densities. In the absence of fishing data, adaptive management and targeted intervention could be used to further assess the efficacy of management measures.

Predictability of cockle (Austrovenus stutchburyi) population trends in New Zealand’s northern North Island

Population monitoring programmes are used to gain knowledge about system functioning or about the response of a system's state to management measures. The current study provided an assessment of comprehensive monitoring data of intertidal bivalve populations in northern New Zealand. In this region, a number of bivalve species are targeted in recreational and customary fisheries, including cockles (Austrovenus stutchburyi) that are one of the main target species. To monitor the sustainability of cockle populations, regular population surveys have been conducted since the 1990s at different northern New Zealand sites, although the sampling frequency has varied greatly across sites. Furthermore, a general lack of fishing data for the target populations prevents a systematic assessment of management measures and population trends within a fishery assessment framework. In view of these shortcomings, the current study provided a systematic phenomenological assessment of the survey data to make general recommendations about the effectiveness of management and for guiding prioritisation and design for future surveys. The assessment was based on a Bayesian state-space model that simultaneously analysed monitoring data from 21 incomplete survey time-series between 1999-2000 and 2014-15. The model showed that the predictability of estimated large-size cockle densities was highly site-specific. Across all sites, the predictability of density trends could be used to guide the prioritisation of sites to be included in the survey, or to adjust the sampling frequency at individual sites. Based on the current analysis, existing management measures in the form of fishery closures at some of the sites appear to be effective in allowing recovery of depleted cockle populations. Only those sites that had management measures in place showed an increase in large cockle densities. In the absence of fishing data, adaptive management and targeted intervention could be used to further assess the efficacy of management measures.