David A. Milton

Sources and uptake of trace metals in otoliths of juvenile barramundi (Lates calcarifer)

Abstract We conducted an experiment to determine if the concentrations of three trace metals (Cu, Sr, and Pb) in the otoliths of juvenile barramundi ( Lates calcarifer ) were related to their concentrations in the water or diet. Fish were kept in freshwater aquaria for 22 days and either fed diet enhanced with 10 μg g −1 Cu, Sr or Pb, or had 10 mg l −1 (Sr and Pb) or 0.5 mg l −1 Cu added to the water. A control group were then transferred to seawater and maintained for a further 83 days. The otoliths were initially ablated for five trace metals (Li, Cu, Sr, Ba and Pb) with LA-ICPMS. Ablation intervals corresponded to between 7 and 11 days of growth. There was no detectable increase in the otolith concentration of any metal for the fish in the enhanced-diet treatments. There was significant uptake of the three metals added to the water (Cu, Sr and Pb). Three (Sr and Pb) or six fish (Cu) were reanalysed with the ICPMS in high-sensitivity mode enabling 5 μm wide ablations to be made at intervals equal to every day or second day during the experiment. These analyses showed that there was considerable variation in uptake rate among individual fish and that the concentration fluctuated widely before an equilibrium concentration was reached after about two weeks exposure. Analysis of the otoliths of the control group of fish that were transferred to seawater showed that the concentration of Li, Sr and Ba in the otoliths varied between the periods of fresh and seawater residency. These variations were related to changes in the concentration of these metals in the water. The fluctuations in Sr concentration were similar to those seen in wild diadromous fish and not as great as those measured in the fish kept in Sr-enhanced water. Sr concentration in the otoliths was strongly negatively correlated with Ba concentration in the experimental fish, but not in wild fish that migrate from the sea into freshwater. Sr/Ca, Ba/Ca and Cu/Ca ratios in the otoliths were positively correlated with water ratios. The partition coefficients of Sr and Ba were similar to published values for marine fish. The concentrations of Li and Pb in the water were below the detection limit in some treatments so no comparisons could be made for these metals. When taken in conjunction with the results of other recent studies of trace metal uptake in otoliths, our data indicate that under experimental conditions, otolith concentrations of several trace metals are related to water concentrations. However, these patterns do not simply extrapolate to the natural environment where the mechanisms involved appear to be more complex.

Species composition, community structure and zoogeography of fishes of mangrove estuaries in the Solomon Islands

Mangrove estuaries in the Solomon Islands are well developed, but are small and isolated from each other by extensive fringing coral reef lagoons. A total of 136 species of fish were recorded from 13 estuaries (6 estuaries in Kolombangara, 3 in New Georgia, 3 in Rendova and 1 in the Florida Group); none contained more than 50 species. Sampling took place during five 3 wk expeditions from 1986 to 1988. The mean biomass of 11.60 g m−2 is comparable with that of similar estuaries in northern Australia. Cluster-analysis revealed two patterns of fish species composition. The first group, in which Gobiidae were the most numerous taxon, inhabited soft, muddy-bottom estuaries. The second group, dominated by Pomacentridae, lived in hard-bottom, log-strewn estuaries. The role of the estuaries as nursery grounds for coral reef species was assessed and found to be insignificant, but they are used as feeding grounds by mobile piscivorous species. The species composition of Solomon Island estuaries was compared with that of other Indo-Pacific estuaries. No endemic species were found and the fauna is typical of such mangrove systems throughout the region. However, several taxa that are common in Australia or New Guinea were not found, notably Ariidae, Centropomidae,Pomadasys, and Sciaenidae. These absentees, and the fish fauna as a whole, are discussed in relation to the position of the Solomon Islands at the western edge of the Pacific Plate, the effects of deep-ocean trenches, the recent geological origin of the islands, and possible methods of colonisation from nearby mangroves in Australia and New Guinea. The importance of larval durations and dispersal to colonisation are discussed in relation to oceanic circulation patterns in the Solomon Sea.

Analyzing Variability and the Rate of Decline of Migratory Shorebirds in Moreton Bay, Australia

Estimating the abundance of migratory species is difficult because sources of variability differ substantially among species and populations. Recently developed state-space models address this variability issue by directly modeling both environmental and measurement error, although their efficacy in detecting declines is relatively untested for empirical data. We applied state-space modeling, generalized least squares (with autoregression error structure), and standard linear regression to data on abundance of wetland birds (shorebirds and terns) at Moreton Bay in southeast Queensland, Australia. There are internationally significant numbers of 8 species of waterbirds in the bay, and it is a major terminus of the large East Asian-Australasian Flyway. In our analyses, we considered 22 migrant and 8 resident species. State-space models identified abundances of 7 species of migrants as significantly declining and abundance of one species as significantly increasing. Declines in migrant abundance over 15 years were 43-79%. Generalized least squares with an autoregressive error structure showed abundance changes in 11 species, and standard linear regression showed abundance changes in 15 species. The higher power of the regression models meant they detected more declines, but they also were associated with a higher rate of false detections. If the declines in Moreton Bay are consistent with trends from other sites across the flyway as a whole, then a large number of species are in significant decline.

Diets of lagoon fishes of the Solomon Islands: Predators of tuna baitfish and trophic effects of baitfishing on the subsistence fishery

Abstract Analyses of the stomach contents of 122 species of predators from fringing reef lagoons of the Solomon Islands showed that 45 species ate baitfish species caught commercially. Baitfish (mainly Stolephorus and Spratelloides species) comprised nearly 25% by weight of their combined diets, but in only 28 of these predators were baitfish a major proportion of the diet. To simulate the subsistence fishery, fishing competitions (using drop-lining, spear-fishingand trolling) were held. A total of 183 species of 31 families was recorded. Drop-lining, the predominant technique, contributed most to the overall catch, although trolling was important in one area. Catches from traditional poisoning methods were also monitored. We concluded that most of the fish caught by the subsistence fishery do not eat baitfish. Some pelagic species, mainly Scombridae, are baitfish predators, but are caught only by trolling. Hence, unless there is a marked increase in trolling among subsistence fishermen, there is little evidence that the commercial baitfishery at present directly affects the food chain on which the subsistence reef fishery depends.

Allozyme and morphological variation throughout the geographic range of the tropical shad, hilsa Tenualosa ilisha

Abstract The population structure of the shad, hilsa Tenualosa ilisha, was investigated with both allozymes and morphometric analysis. Fish samples were collected from nine sites within Bangladesh and compared with samples from four other countries that covered the entire species range. Five polymorphic loci, Idh-l, Idh-m, Mdh-l, Mdh-m and Pgm, were interpretable in liver and muscle with starch–gel electrophoresis, although levels of variation were low. No significant differences in allele frequencies were detected within Bangladesh or within the Bay of Bengal (SE India and Myanmar) samples. Significant differences in allele frequencies occurred between Kuwait, Bangladesh and Indonesia. Nine morphological measurements and two meristic counts were compared among the same fish that were collected for genetic analysis. Discriminant function analyses (DFA) of transformed measurement ratios varied widely between sites, even those separated by short distances (

Variable Success in Breeding of the Roseate Tern Sterna dougallii on the Northern Great Barrier Reef

Breeding of Roseate Terns Sterna dougallii on the northern Great Barrier Reef (11°-15°S) was monitored between 1976 and 1994, and they were recorded breeding on the inshore rocky islands in most years, usually starting April-May. Between 50-5000 pairs were recorded breeding each year and indicate that the northern Great Barrier Reef is an important breeding area for Roseate Terns during peak years when large colonies of 1000-5000 pairs formed on low sand cays in the far northern Great Barrier Reef (11°s). On at least three occasions, breeding success was poor in these large colonies, with high mortality of the almost-fledged chicks. Growth in weight and wing length of Roseate Tern chicks was measured at Eagle Island (14°42′S) between 1983 and 1986 in order to assess the influence of food on growth rates and survival. Chicks took up to 35 days to fledge, significantly longer than the ecologically similar, more numerous Black-naped Tern Sterna sumatrana at the same site at the same time and also almost 10 days longer than Roseate Tern chicks from temperate North American colonies. These data suggest that conditions in northern Australia are rarely favourable for the rapid growth and breeding success of Roseate Terns. Pelagic fish populations around inshore islands in tropical Australia may only be able to support limited numbers of Roseate Terns during the breeding season. This makes northeastern Australian populations of this globally threatened species extremely vulnerable to changes in the availability of their prey during the breeding season.

The life history of the tropical shad Tenualosa toli from Sarawak: first evidence of protandry in the Clupeiformes?

SynopsisSharp declines in catches prompted a detailed study of the commercially and culturally important ‘terubok’ Tenualosa toli, which lives in the fast-flowing, turbid estuaries and adjacent shallow coastal waters of Sarawak. Its reproduction, diet, age and growth were investigated. An absence of small females and large males, together with histological data showing transitional gonads, suggest that T. toli is a protandrous hermaphrodite. Ageing based on otoliths indicates that individuals may not live more than about two years. Male fish spawn towards the end of their first year, change sex (transitional gonads were recorded in fish from 14 to 31 cm SL) and spawn as females in their second year. Spawning takes place in the middle reaches of estuaries and females deposit all their eggs at once. Fecundity is linearly related to fish length but shows significant seasonal and site variations. Hermaphroditism is discussed in relation to possible environmental and isolating mechanisms. T. toli is a zooplanktivore eating mainly calanoid copepods. Laser-ablation inductively-coupled plasma mass spectroscopy of trace elements across otoliths revealed that the species does not move into full seawater or freshwater, but completes its life cycle in estuarine and adjacent coastal waters. Therefore T. toli populations in each estuary and adjacent coastal waters may be relatively isolated from one another, and hence particularly susceptible to overfishing.

Rapid population decline in migratory shorebirds relying on Yellow Sea tidal mudflats as stopover sites

Migratory animals are threatened by human-induced global change. However, little is known about how stopover habitat, essential for refuelling during migration, affects the population dynamics of migratory species. Using 20 years of continent-wide citizen science data, we assess population trends of ten shorebird taxa that refuel on Yellow Sea tidal mudflats, a threatened ecosystem that has shrunk by >65% in recent decades. Seven of the taxa declined at rates of up to 8% per year. Taxa with the greatest reliance on the Yellow Sea as a stopover site showed the greatest declines, whereas those that stop primarily in other regions had slowly declining or stable populations. Decline rate was unaffected by shared evolutionary history among taxa and was not predicted by migration distance, breeding range size, non-breeding location, generation time or body size. These results suggest that changes in stopover habitat can severely limit migratory populations.

Continental-scale decreases in shorebird populations in Australia

Abstract Decreases in shorebird populations are increasingly evident worldwide, especially in the East Asian—Australasian Flyway (EAAF). To arrest these declines, it is important to understand the scale of both the problem and the solutions. We analysed an expansive Australian citizen-science dataset, spanning the period 1973 to 2014, to explore factors related to differences in trends among shorebird populations in wetlands throughout Australia. Of seven resident Australian shorebird species, the four inland species exhibited continental decreases, whereas the three coastal species did not. Decreases in inland resident shorebirds were related to changes in availability of water at non-tidal wetlands, suggesting that degradation of wetlands in Australias interior is playing a role in these declines. For migratory shorebirds, the analyses revealed continental decreases in abundance in 12 of 19 species, and decreases in 17 of 19 in the southern half of Australia over the past 15 years. Many trends were strongly associated with continental gradients in latitude or longitude, suggesting some large-scale patterns in the decreases, with steeper declines often evident in southern Australia. After accounting for this effect, local variables did not explain variation in migratory shorebird trends between sites. Our results are consistent with other studies indicating that decreases in migratory shorebird populations in the EAAF are most likely being driven primarily by factors outside Australia. This reinforces the need for urgent overseas conservation actions. However, substantially heterogeneous trends within Australia, combined with declines of inland resident shorebirds indicate effective management of Australian shorebird habitat remains important.

Survival rates of sea snakes caught by demersal trawlers in northern and eastern Australia

This paper examines the survival rates of sea snakes caught in research and commercial trawls in the main fishing grounds of northern and eastern Australia. Observers recorded whether the snake was alive or dead when it was landed on deck, and (if possible) its species, its length and weight, the duration of the trawl and the total weight of the catch. To estimate long-term survival, captured sea snakes were kept in sea water in drums on a large research vessel and monitored after 1, 6, 12, and 24 h then daily for up to 4 days. Up to 28% of all sea snakes landed on deck from commercial prawn trawling were found to be dead. Both duration of the trawl and catch weight had significant (P<0.001) positive effects on within-trawl (short-term) mortality of these sea snakes. About 30% of all sea snakes from 30 min duration trawls died during the 4-day survival experiment, with Hydrophis ornatus showing the highest mortality (< 50% alive after 96 h in a tank). Hydrophis elegans appeared to have the lowest mortality rate after capture (94% alive after 96 h). Overall, mortality from research and commercial prawn trawling was estimated as 48.5% of snakes caught. Visible damage to sea snakes by trawling was relatively low (5.4%). The introduction of Bycatch reduction devices, such as square mesh and fisheye windows, should improve the survival of sea snakes by reducing the catch weight and the number of sea snakes being caught.